Rac1, but not RhoA, signaling protects epithelial adherens junction assembly during ATP depletion

Manipulation of microvillar proteins during Salmonella enterica invasion results in brush border effacement and actin remodeling

Enterocyte invasion by the gastrointestinal pathogen Salmonella enterica is accompanied by loss of brush border and massive remodeling of the actin cytoskeleton, leading to microvilli effacement and formation of membrane ruffles. These manipulations are mediated by effector proteins translocated by the Salmonella Pathogenicity Island 1-encoded type III secretion system (SPI1-T3SS). To unravel the mechanisms of microvilli effacement and contribution of SPI1-T3SS effector proteins, the dynamics of host-pathogen interactions was analyzed using live cell imaging (LCI) of polarized epithelial cells (PEC) expressing LifeAct-GFP. PEC were infected with S. enterica wild-type and mutant strains with defined defects in SPI1-T3SS effector proteins, and pharmacological inhibition of actin assembly were applied. We identified that microvilli effacement involves two distinct mechanisms: i) F-actin depolymerization mediated by villin and ii), the consumption of cytoplasmic G-actin by formation of membrane ruffles. By analyzing the contribution of individual SPI1-T3SS effector proteins, we demonstrate that SopE dominantly triggers microvilli effacement and formation of membrane ruffles. Furthermore, SopE via Rac1 indirectly manipulates villin, which culminates in F-actin depolymerization. Collectively, these results indicate that SopE has dual functions during F-actin remodeling in PEC. While SopE-Rac1 triggers F-actin polymerization and ruffle formation, activation of PLCγ and villin by SopE depolymerizes F-actin in PEC. These results demonstrate the key role of SopE in destruction of the intestinal barrier during intestinal infection by Salmonella.

Epidermal growth factor attenuates blood-spinal cord barrier disruption via PI3K/Akt/Rac1 pathway after acute spinal cord injury

After spinal cord injury (SCI), disruption of blood–spinal cord barrier (BSCB) elicits blood cell infiltration such as neutrophils and macrophages, contributing to permanent neurological disability. Previous studies show that epidermal growth factor (EGF) produces potent neuroprotective effects in SCI models. However, little is known that whether EGF contributes to the integrity of BSCB. The present study is performed to explore the mechanism of BSCB permeability changes which are induced by EGF treatment after SCI in rats. In this study, we demonstrate that EGF administration inhibits the disruption of BSCB permeability and improves the locomotor activity in SCI model rats. Inhibition of the PI3K/Akt pathways by a specific inhibitor, LY294002, suppresses EGF‐induced Rac1 activation as well as tight junction (TJ) and adherens junction (AJ) expression. Furthermore, the protective effect of EGF on BSCB is related to the activation of Rac1 both in vivo and in vitro. Blockade of Rac1 activation with Rac1 siRNA downregulates EGF‐induced TJ and AJ proteins expression in endothelial cells. Taken together, our results indicate that EGF treatment preserves BSCB integrity and improves functional recovery after SCI via PI3K‐Akt‐Rac1 signalling pathway.

Reactive oxygen species in inflammation and tissue injury

Abstract Reactive oxygen species (ROS) are key signaling molecules that play an important role in the progression of inflammatory disorders. An enhanced ROS generation by polymorphonuclear neutrophils (PMNs) at the site of inflammation causes endothelial dysfunction and tissue injury. The vascular endothelium plays an important role in passage of macromolecules and inflammatory cells from the blood to tissue. Under the inflammatory conditions, oxidative stress produced by PMNs leads to the opening of inter-endothelial junctions and promotes the migration of inflammatory cells across the endothelial barrier. The migrated inflammatory cells not only help in the clearance of pathogens and foreign particles but also lead to tissue injury. The current review compiles the past and current research in the area of inflammation with particular emphasis on oxidative stress-mediated signaling mechanisms that are involved in inflammation and tissue injury.

The Rac1/JNK pathway is critical for EGFR-dependent barrier formation in human airway epithelial cells

The airway epithelial barrier provides defenses against inhaled antigens and pathogens, and alterations of epithelial barrier function have been proposed to play a significant role in the pathogenesis of chronic airway diseases. Although the epidermal growth factor receptor (EGFR) plays roles in various physiological and pathological processes on the airway epithelium, the role of EGFR on barrier function in the airway remains largely unknown. In the present study, we assessed the effects of EGFR activation on paracellular permeability in airway epithelial cells (AECs). EGFR activation induced by the addition of EGF increased transepithelial electrical resistance (TER) in AECs. An EGFR-blocking antibody eradicated the development of TER, paracellular influx of dextran, and spatial organization of tight junction. Moreover, the effects of EGFR activation on paracellular permeability were eradicated by knockdown of occludin. To identify the EGFR signaling pathway that regulates permeability barrier development, we investigated the effects of several MAP kinase inhibitors on permeability barrier function. Pretreatment with a JNK-specific inhibitor, but not an ERK- or p38-specific inhibitor, attenuated the development of TER induced by EGFR activation. Rac1 is one of the upstream activators for JNK in EGFR signaling. Rac1 knockdown attenuated the phosphorylation of JNK activation and EGFR-mediated TER development. These results suggest that EGFR positively regulates permeability barrier development through the Rac1/JNK-dependent pathway.

N-cadherin is depleted from proximal tubules in experimental and human acute kidney injury

Ischemia remains the most common cause of acute kidney injury (AKI). Decreased intercellular adhesion and alterations in adhesion molecules may contribute to the loss of renal function observed in AKI. In the present study, we evaluated the distribution of adhesion molecules in the human kidney and analyzed their expression in human and experimental AKI. Specimens of human kidneys obtained from patients with and without AKI were stained for the cell adhesion molecules E-cadherin, N-cadherin and beta-catenin. Experimental AKI in rats was induced by renal artery clamping. Immunostaining and immunoblotting were carried out for E-cadherin, N-cadherin and beta-catenin. Proximal tubule cells from opossum kidneys (OKs) were used to analyze the effect of chemical hypoxia (ATP depletion) in vitro. In the adult human kidney, N-cadherin was expressed in proximal tubules, while E-cadherin was expressed in other nephron segments. beta-Catenin was expressed in both proximal and distal tubules. In human AKI and in ischemic rat kidneys, N-cadherin immunostaining was depleted from proximal tubules. There was no change in E-cadherin or beta-catenin. In vitro, OK cells expressed N-cadherin only in the presence of collagen, and ATP depletion led to a depletion of N-cadherin. Collagen IV staining was reduced in ischemic rat kidneys compared to controls. The results of the study suggest that N-cadherin may play a significant role in human and experimental AKI.

Homocysteine promotes vascular smooth muscle cell migration by induction of the adipokine resistin

Adipokines may represent a mechanism linking insulin resistance to cardiovascular disease. We showed previously that homocysteine (Hcy), an independent risk factor for cardiovascular disease, can induce the expression and secretion of resistin, a novel adipokine, in vivo and in vitro. Since vascular smooth muscle cell (VSMC) migration is a key event in vascular disease, we hypothesized that adipocyte-derived resistin is involved in Hcy-induced VSMC migration. To confirm our hypothesis, Sprague-Dawley rat aortic SMCs were cocultured with Hcy-stimulated primary rat epididymal adipocytes or treated directly with increasing concentrations of resistin for up to 24 h. Migration of VSMCs was investigated. Cytoskeletal structure and cytoskeleton-related proteins were also detected. The results showed that Hcy (300-500 microM) increased migration significantly in VSMCs cocultured with adipocytes but not in VSMC cultured alone. Resistin alone also significantly increased VSMC migration in a time- and concentration-dependent manner. Resistin small interfering RNA (siRNA) significantly attenuated VSMC migration in the coculture system, which indicated that adipocyte-derived resistin mediates Hcy-induced VSMC migration. On cell spreading assay, resistin induced the formation of focal adhesions near the plasma membrane, which suggests cytoskeletal rearrangement via an alpha(5)beta(1)-integrin-focal adhesion kinase/paxillin-Ras-related C3 botulinum toxin substrate 1 (Rac1) pathway. Our data demonstrate that Hcy promotes VSMC migration through a paracrine or endocrine effect of adipocyte-derived resistin, which provides further evidence of the adipose-vascular interaction in metabolic disorders. The migratory action exerted by resistin on VSMCs may account in part for the increased incidence of restenosis in diabetic patients.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Requirements for proximal tubule epithelial cell detachment in response to ischemia: role of oxidative stress

Sublethal renal ischemia induces tubular epithelium damage and kidney dysfunction. Using NRK-52E rat proximal tubular epithelial cells, we have established an in vitro model, which includes oxygen and nutrients deprivation, to study the proximal epithelial cell response to ischemia. By means of this system, we demonstrate that confluent NRK-52E cells lose monolayer integrity and detach from collagen IV due to: (i) actin cytoskeleton reorganization; (ii) Rac1 and RhoA activity alterations; (iii) Adherens junctions (AJ) and Tight junctions (TJ) disruption, involving redistribution but not degradation of E-cadherin, beta-catenin and ZO-1; (iv) focal adhesion complexes (FAC) disassembly, entangled by mislocalization of paxillin and FAK dephosphorylation. Reactive oxygen species (ROS) are generated during the deprivation phase and rapidly balanced at recovery involving MnSOD induction, among others. The use of antioxidants (NAC) prevented FAC disassembly by blocking paxillin redistribution and FAK dephosphorylation, without abrogating AJ or TJ disruption. In spite of this, NAC did not show any protective effect on cell detachment. H(2)O(2), as a pro-oxidant treatment, supported the contribution of ROS in tubular epithelial cell-matrix but not cell-cell adhesion alterations. In conclusion, ROS-mediated FAC disassembly was not sufficient for the proximal epithelial cell shedding in response to sublethal ischemia, which also requires intercellular adhesion disruption.

aPKC-PAR complex dysfunction and tight junction disassembly in renal epithelial cells during ATP depletion

Renal ischemia and in vitro ATP depletion result in disruption of the epithelial tight junction barrier, which is accompanied by breakdown of plasma membrane polarity. Tight junction formation is regulated by evolutionarily conserved complexes, including that of atypical protein kinase C (aPKC), Par3, and Par6. The aPKC signaling complex is activated by Rac and regulated by protein phosphorylation and associations with other tight junction regulatory proteins, for example, mLgl. In this study, we examined the role of aPKC signaling complex during ATP depletion and recovery in Madin-Darby canine kidney cells. ATP depletion reduced Rac GTPase activity and induced Par3, aPKCzeta, and mLgl-1 redistribution from sites of cell-cell contact, which was restored following recovery from ATP depletion. Zonula occludens (ZO)-1 and Par3 phosphorylation was reduced and association of aPKCzeta with its substrates Par3 and mLgl-1 was stabilized in ATP-depleted Madin-Darby canine kidney cells. ATP depletion also induced a stable association of Par3 with Tiam-1, a Rac GTPase exchange factor, which explains how aPKCzeta and Rac activities were suppressed. Experimental inhibition of aPKCzeta during recovery from ATP depletion interfered with reassembly of ZO-1 and Par3 at cell junctions. These data indicate that aPKC signaling is impaired during ATP depletion, participates in tight junction disassembly during cell injury and is important for tight junction reassembly during recovery.

Rac1 and RhoA as regulators of endothelial phenotype and barrier function in hypoxia-induced neonatal pulmonary hypertension

Hypoxia is a common cause of persistent pulmonary hypertension in the newborn (PPHN), a condition associated with endothelial dysfunction and abnormal pulmonary vascular remodeling. The GTPase RhoA has been implicated in the pathogenesis of PPHN, but its contribution to endothelial remodeling and function is not known. We studied pulmonary artery endothelial cells (PAECs) taken from piglets with chronic hypoxia-induced pulmonary hypertension and from healthy animals and analyzed the roles of Rho GTPases in the regulation of the endothelial phenotype and function under basal normoxic conditions, acute hypoxia, and reoxygenation. The activities of RhoA, Rac1, and Cdc42 were correlated with changes in the endothelial cytoskeleton, adherens junctions, permeability, ROS production, VEGF levels, and activities of transcription factors hypoxia-inducible factor (HIF)-1alpha and NF-kappaB. Adenoviral gene transfer was used to express dominant-negative GTPases, kinase-dead p21-activated kinase (PAK)-1, and constitutively activated Rac1 in cells. PAECs from pulmonary hypertensive piglets had a stable abnormal phenotype with a sustained reduction in Rac1 activity and an increase in RhoA activity, which correlated with an increase in actin stress fiber formation, increased permeability, and a decrease in VEGF and ROS production. Cells from pulmonary hypertensive animals were still able to respond to acute hypoxia. They also showed high activities of HIF-1alpha and NF-kappaB, likely to result from changes in the activities of Rho GTPases. Activation of Rac1 and its effector PAK-1 as well as inhibition of RhoA restored the abnormal phenotype and permeability of hypertensive PAECs to normal.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

RalA interacts with ZONAB in a cell density-dependent manner and regulates its transcriptional activity

Ral proteins are members of the Ras superfamily of small GTPases and are involved in signalling pathways for actin cytoskeleton remodelling, cell cycle control, cellular transformation and vesicle transport. To identify novel RalA effector proteins, we used the reverse Ras recruitment system and found that RalA interacts with a Y-box transcription factor, ZO-1-associated nucleic acid-binding protein (ZONAB), in a GTP-dependent manner. The amount of the RalA-ZONAB complex increases as epithelial cells become more dense and increase cell contacts. The RalA-ZONAB interaction results in a relief of transcriptional repression of a ZONAB-regulated promoter. Additionally, expression of oncogenic Ras alleviates transcriptional repression by ZONAB in a RalA-dependent manner. The data presented here implicate the RalA/ZONAB interaction in the regulation of ZONAB function.

Intestinal barrier dysfunction by enteropathogenic Escherichia coli is mediated by two effector molecules and a bacterial surface protein

The human intestinal pathogen, enteropathogenic Escherichia coli (EPEC), causes diarrhoeal disease by a mechanism that is dependent on the injection of effector proteins into the host cell. One effector, EspF, is reported to be required for EPEC to disrupt tight junction integrity of intestinal cells and increase the paracellular movement of molecules, which is likely to contribute to diarrhoea. Here, we show that not one but three EPEC-encoded factors play important roles in this process. Thus, the Map (Mitochondria-associated protein) effector is shown to: (i) be as essential as EspF for disrupting intestinal barrier function, (ii) be able to function independently of EspF, (iii) alter tight junction structure and (iv) mediate these effects in the absence of mitochondrial targeting. Additionally, the outer membrane protein Intimin is shown to be crucial for EspF and Map to disrupt the intestinal barrier function. This function of Intimin is completely independent of its interaction with its known receptor Tir, revealing a physiologically relevant requirement for Intimin interaction with alternative receptor(s). This work demonstrates that EPEC uses multiple multifunctional proteins to elicit specific responses in intestinal cells and that EPEC can control the activity of its injected effector molecules from its extracellular location.

Rho GTPases: potential candidates for anticancer therapy

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

Role of Na-K-ATPase in the assembly of tight junctions

Na-K-ATPase, also known as the sodium pump, is a crucial enzyme that regulates intracellular sodium homeostasis in mammalian cells. In epithelial cells Na-K-ATPase function is also involved in the formation of tight junctions through RhoA GTPase and stress fibers. In this review, a new two-step model for the assembly of tight junctions is proposed: step 1, an E-cadherin-dependent formation of partial tight junction strands and of the circumferential actin ring; and step 2, active actin polymerization-dependent tethering of tight junction strands to form functional tight junctions, an event requiring normal function of Na-K-ATPase in epithelial cells. A new role for stress fibers in the assembly of tight junctions is proposed. Also, implications of Na-K-ATPase function on tight junction assembly in diseases such as cancer, ischemia, hypomagnesemia, and polycystic kidney disease are discussed.

Rho GTPases show differential sensitivity to nucleotide triphosphate depletion in a model of ischemic cell injury

Rho GTPases are critical for actin cytoskeletal regulation, and alterations in their activity may contribute to altered cytoskeletal organization that characterizes many pathological conditions, including ischemia. G protein activity is a function of the ratio of GTP-bound (active) to GDP-bound (inactive) protein, but the effect of altered energy metabolism on Rho protein activity has not been determined. We used antimycin A and substrate depletion to induce depletion of intracellular ATP and GTP in the kidney proximal tubule cell line LLC-PK10 and measured the activity of RhoA, Rac1, and Cdc42 with GTPase effector binding domains fused to glutathione S-transferase. RhoA activity decreased in parallel with the concentration of ATP and GTP during depletion, so that by 60 min there was no detectable RhoA-GTP, and recovered rapidly when cells were returned to normal culture conditions. Dissociation of the membrane-actin linker ezrin, a target of RhoA signaling, from the cytoskeletal fraction paralleled the decrease in RhoA activity and was augmented by treatment with the Rho kinase inhibitor Y27632. The activity of Cdc42 did not decrease significantly during depletion or recovery. Rac1 activity decreased moderately to a minimum at 30 min of depletion but then increased from 30 to 90 min of depletion, even as ATP and GTP levels continued to fall. Our data are consistent with a principal role for RhoA in cytoskeletal reorganization during ischemia and demonstrate that the activity of Rho GTPases can be maintained even at low GTP concentrations.

Testing effects of signal transduction pathways on cadherin junctional complex assembly using quantitative image analysis

Cadherin adhesion molecules function in numerous cell biological processes that influence embryo development, normal cell physiology, and pathophysiology of many disease processes. Cadherins nucleate the assembly of the adherens junction, a cell-to-cell adhesion plaque that is prominent in simple epithelial cells and found in many cell types. Numerous cell biological approaches have been used to study this interesting class of molecules. Here, we outline methodology used in our studies of junctional complexes to examine effects of signaling molecules on assembly mechanisms. This is a quantitative method that allows the investigator to test the combined effect of two different signaling processes to determine whether these two signals act in concert within the same pathway. We discuss how this method could be generalized to other studies to examine consequences of various experimental manipulations on the assembly of cellular structures.

Differential regulation of junctional complex assembly in renal epithelial cell lines

Several signaling pathways that regulate tight junction and adherens junction assembly are being characterized. Calpeptin activates stress fiber assembly in fibroblasts by inhibiting SH2-containing phosphatase-2 (SHP-2), thereby activating Rho-GTPase signaling. Here, we have examined the effects of calpeptin on stress fiber and junctional complex assembly in Madin-Darby canine kidney (MDCK) and LLC-PK epithelial cells. Calpeptin induced disassembly of stress fibers and inhibition of Rho GTPase activity in MDCK cells. Interestingly, calpeptin augmented stress fiber formation in LLC-PK epithelial cells. Calpeptin treatment of MDCK cells resulted in a displacement of zonula occludens-1 (ZO-1) and occludin from cell-cell junctions and a loss of phosphotyrosine on ZO-1 and ZO-2, without any detectable effect on tight junction permeability. Surprisingly, calpeptin increased paracellular permeability in LLC-PK cells even though it did not affect tight junction assembly. Calpeptin also modulated adherens junction assembly in MDCK cells but not in LLC-PK cells. Calpeptin treatment of MDCK cells induced redistribution of E-cadherin and beta-catenin from intercellular junctions and reduced the association of p120ctn with the E-cadherin/catenin complex. Together, our studies demonstrate that calpeptin differentially regulates stress fiber and junctional complex assembly in MDCK and LLC-PK epithelial cells, indicating that these pathways may be regulated in a cell line-specific manner.

Rho GTPases and the regulation of endothelial permeability

Endothelial permeability depends on the integrity of intercellular junctions as well as actomyosin-based cell contractility. Rho GTPases have been implicated in signalling by many vasoactive substances including thrombin, tumour necrosis factor alpha (TNF-alpha), bradykinin, histamine, lysophosphatidic acid (LPA), vascular endothelial growth factor (VEGF), and hepatocyte growth factor (HGF). Two Rho family GTPases, Rho and Rac, have emerged as key regulators acting antagonistically to regulate endothelial barrier function: Rho increases actomyosin contractility, which facilitates breakdown of intercellular junctions, whereas Rac stabilizes endothelial junctions and counteracts the effects of Rho. In this review, we present evidence for the opposing effects of these two regulatory proteins and discuss links between them and other key signalling molecules such as cyclic AMP (cAMP), cyclic GMP (cGMP), phosphatidylinositide 3-kinases (PI3Ks), mitogen-activated protein kinases (MAPKs), and protein kinases C (PKCs). We also discuss strategies for targeting Rho GTPase signalling in therapies for diseases involving altered endothelial permeability.