TEM4 is a junctional RhoGEF required for cell-cell adhesion, monolayer integrity, and barrier function

111

Endothelial cell-cell junctions maintain a restrictive barrier that is tightly regulated to allow dynamic responses to permeability-inducing angiogenic factors, as well as to inflammatory agents and adherent leukocytes. The ability of these stimuli to transiently remodel adherens junctions depends on Rho-GTPase-controlled cytoskeletal rearrangements. How the activity of Rho-GTPases is spatio-temporally controlled at endothelial adherens junctions by guanine-nucleotide exchange factors (GEFs) is incompletely understood. Here, we identify a crucial role for the Rho-GEF Trio in stabilizing junctions based around vascular endothelial (VE)-cadherin (also known as CDH5). Trio interacts with VE-cadherin and locally activates Rac1 at adherens junctions during the formation of nascent contacts, as assessed using a novel FRET-based Rac1 biosensor and biochemical assays. The Rac-GEF domain of Trio is responsible for the remodeling of junctional actin from radial into cortical actin bundles, a crucial step for junction stabilization. This promotes the formation of linear adherens junctions and increases endothelial monolayer resistance. Collectively, our data show the importance of spatiotemporal regulation of the actin cytoskeleton through Trio and Rac1 at VE-cadherin-based cell-cell junctions in the maintenance of the endothelial barrier.

Section: Discussionmentioning

confidence: 73%

A local VE-cadherin/Trio-based signaling complex stabilizes endothelial junctions through Rac1

Timmerman

Heemskerk

Kroon

et al. 2015

111

“…5). A previous study has shown that depletion of TEM4, a Rho GEF, impairs cell-cell junction formation in MDCK cells and reduces the amount of activated RhoA (Ngok et al, 2013). Thus, the balance in the activity of Rac and Rho as a result of activation of Rho or inactivation of Rac might be necessary for the maintenance of adherens junctions.…”

Section: Discussionmentioning

confidence: 97%

FilGAP, a Rho–ROCK-regulated GAP for Rac, controls adherens junctions in MDCK cells

Shinichiro

Tsutsumi

Zuinen

et al. 2015

Rho family small GTPases are essential for the formation of adherens junctions in epithelial cells. Here, we found that FilGAP (also known as ARHGAP24), a Rac-specific Rho GTPase-activating protein, promoted the formation of adherens junctions in Madin-Darby canine kidney (MDCK) cells. Knockdown of FilGAP by siRNA stimulated the disassembly and migration of MDCK cells induced by hepatocyte growth factor (HGF). By contrast, forced expression of FilGAP induced accumulation of E-cadherin at adherens junctions. Endogenous FilGAP colocalized with E-cadherin at adherens junctions, and depletion of FilGAP reduced the amount of E-cadherin expressed at the surface. The Rac GAP domain of FilGAP was necessary for the suppression of cell scattering induced by HGF. In agreement with this, siRNA-mediated knockdown of both Rac1 and FilGAP suppressed cell scattering induced by HGF. Forced expression of Rho kinase (ROCK, of which there are two isoforms ROCK1 and ROCK2) induced the accumulation of E-cadherin at the adherens junction, and depletion of FilGAP prevented the accumulation of E-cadherin. Moreover, wild-type FilGAP but not a non-phosphorylatable FilGAP mutant rescued the accumulation of E-cadherin at adherens junctions. These results suggest that FilGAP might regulate cell-cell adhesion through inactivation of Rac downstream of Rho-ROCK-signaling in MDCK cells.

“…A number of RhoA GEFs have been identified at apical junctions. Specifically, Ect2, Syx, p114RhoGEF (p114), TEM4 and ARHGEF11 (GEF11) promote the localization and activity of RhoA at the ZA to support junction integrity (Ngok et al, 2012;Ratheesh et al, 2012;Terry et al, 2011;Itoh et al, 2012;Ngok et al, 2013). In contrast, GEF-H1-regulated RhoA activity is negatively controlled to prevent junction disassembly (Aijaz et al, 2005;Guillemot et al, 2008;Samarin et al, 2007).…”

Section: The Activation Of Rho Family Gtpases Is Spatio-temporally mentioning

confidence: 99%

Establishment of epithelial polarity – GEF who's minding the GAP?

Ngok

Lin

Anastasiadis

2014

Self Cite

Cell polarization is a fundamental process that underlies epithelial morphogenesis, cell motility, cell division and organogenesis. Loss of polarity predisposes tissues to developmental disorders and contributes to cancer progression. The formation and establishment of epithelial cell polarity is mediated by the cooperation of polarity protein complexes, namely the Crumbs, partitioning defective (Par) and Scribble complexes, with Rho family GTPases, including RhoA, Rac1 and Cdc42. The activation of different GTPases triggers distinct downstream signaling pathways to modulate protein–protein interactions and cytoskeletal remodeling. The spatio-temporal activation and inactivation of these small GTPases is tightly controlled by a complex interconnected network of different regulatory proteins, including guanine-nucleotide-exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine-nucleotide-dissociation inhibitors (GDIs). In this Commentary, we focus on current understanding on how polarity complexes interact with GEFs and GAPs to control the precise location and activation of Rho GTPases (Crumbs for RhoA, Par for Rac1, and Scribble for Cdc42) to promote apical–basal polarization in mammalian epithelial cells. The mutual exclusion of GTPase activities, especially that of RhoA and Rac1, which is well established, provides a mechanism through which polarity complexes that act through distinct Rho GTPases function as cellular rheostats to fine-tune specific downstream pathways to differentiate and preserve the apical and basolateral domains. This article is part of a Minifocus on Establishing polarity. For further reading, please see related articles: ‘ERM proteins at a glance’ by Andrea McClatchey (J. Cell Sci. 127, [098343]). ‘Integrins and epithelial cell polarity’ by Jessica Lee and Charles Streuli (J. Cell Sci. 127, [146142]).