Small GTPases and regulation of cadherin dependent cell-cell adhesion

Insights into the post-translational modification and its emerging role in shaping the tumor microenvironment

More and more in-depth studies have revealed that the occurrence and development of tumors depend on gene mutation and tumor heterogeneity. The most important manifestation of tumor heterogeneity is the dynamic change of tumor microenvironment (TME) heterogeneity. This depends not only on the tumor cells themselves in the microenvironment where the infiltrating immune cells and matrix together forming an antitumor and/or pro-tumor network. TME has resulted in novel therapeutic interventions as a place beyond tumor beds. The malignant cancer cells, tumor infiltrate immune cells, angiogenic vascular cells, lymphatic endothelial cells, cancer-associated fibroblastic cells, and the released factors including intracellular metabolites, hormonal signals and inflammatory mediators all contribute actively to cancer progression. Protein post-translational modification (PTM) is often regarded as a degradative mechanism in protein destruction or turnover to maintain physiological homeostasis. Advances in quantitative transcriptomics, proteomics, and nuclease-based gene editing are now paving the global ways for exploring PTMs. In this review, we focus on recent developments in the PTM area and speculate on their importance as a critical functional readout for the regulation of TME. A wealth of information has been emerging to prove useful in the search for conventional therapies and the development of global therapeutic strategies.

Distinct influence of the anthracycline derivative doxorubicin on the differentiation efficacy of mESC-derived endothelial progenitor cells

Cardiotoxicity is a highly relevant, because often life-threatening, adverse effect of doxorubicin (Doxo)-based anticancer therapy. Here, we investigated the Doxo-response of cardiovascular stem/progenitor cells employing a mouse embryonic stem cell (mESC)-based in vitro differentiation model. Endothelial progenitor cells revealed a pronounced Doxo sensitivity as compared to mESC, differentiated endothelial-like (EC) and cardiomyocyte-like cells (CM) and CM progenitors, which rests on the activation of senescence. Doxo treatment of EC progenitors altered protein expression of individual endothelial markers, actin cytoskeleton morphology, mRNA expression of genes related to mitochondrial functions, autophagy, apoptosis, and DNA repair as well as mitochondrial DNA content, respiration and ATP production in the surviving differentiated EC progeny. By contrast, LDL uptake, ATP-stimulated Ca²⁺ release, and cytokine-stimulated ICAM-1 expression remained unaffected by the anthracycline treatment. Thus, exposure of EC progenitors to Doxo elicits isolated and persistent dysfunctions in the surviving EC progeny. In conclusion, we suggest that Doxo-induced injury of EC progenitors adds to anthracycline-induced cardiotoxicity, making this cell-type a preferential target for pharmacoprotective and regenerative strategies.

Epithelial-Mesenchymal Transitions during Neural Crest and Somite Development

Epithelial-to-mesenchymal transition (EMT) is a central process during embryonic development that affects selected progenitor cells of all three germ layers. In addition to driving the onset of cellular migrations and subsequent tissue morphogenesis, the dynamic conversions of epithelium into mesenchyme and vice-versa are intimately associated with the segregation of homogeneous precursors into distinct fates. The neural crest and somites, progenitors of the peripheral nervous system and of skeletal tissues, respectively, beautifully illustrate the significance of EMT to the above processes. Ongoing studies progressively elucidate the gene networks underlying EMT in each system, highlighting the similarities and differences between them. Knowledge of the mechanistic logic of this normal ontogenetic process should provide important insights to the understanding of pathological conditions such as cancer metastasis, which shares some common molecular themes.

Regulation of the endothelial barrier function: a filum granum of cellular forces, Rho-GTPase signaling and microenvironment

Although the endothelium is an extremely thin single-cell layer, it performs exceedingly well in preventing blood fluids from leaking into the surrounding tissues. However, specific pathological conditions can affect this cell layer, compromising the integrity of the barrier. Vascular leakage is a hallmark of many cardiovascular diseases and despite its medical importance, no specialized therapies are available to prevent it or reduce it. Small guanosine triphosphatases (GTPases) of the Rho family are known to be key regulators of various aspects of cell behavior and studies have shown that they can exert both positive and negative effects on endothelial barrier integrity. Moreover, extracellular matrix stiffness has now been implicated in the regulation of Rho-GTPase signaling, which has a direct impact on the integrity of endothelial junctions. However, knowledge about both the precise mechanism of this regulation and the individual contribution of the specific regulatory proteins remains fragmentary. In this review, we discuss recent findings concerning the balanced activities of Rho-GTPases and, in particular, aspects of the regulation of the endothelial barrier. We highlight the role of Rho-GTPases in the intimate relationships between biomechanical forces, microenvironmental influences and endothelial intercellular junctions, which are all interwoven in a beautiful filigree-like fashion.

Obligatory roles of filamin A in E-cadherin-mediated cell-cell adhesion in epidermal keratinocytes

BACKGROUND

Extracellular Ca(2+) (Cao(2+))-induced E-cadherin-mediated cell-cell adhesion plays a critical role in promoting differentiation in epidermal keratinocytes. Our previous studies show that the calcium-sensing receptor (CaR) regulates keratinocyte cell-cell adhesion and differentiation via Rho A-mediated signaling. CaR forms a protein complex with Rho A, guanine nucleotide exchange factor Trio, and a cytoskeletal actin-binding protein, filamin A, at the cell-cell junctions in response to elevated Cao(2+) levels. Filamin A has the ability to interact directly with CaR, Trio, and Rho and mediate CaR-dependent signaling events.

OBJECTIVE

This study was conducted to investigate the roles of filamin A and Trio in regulating Cao(2+)-induced Rho activation and intercellular adhesion.

METHODS

Expression of filamin A and Trio in keratinocytes was inhibited by siRNA. Its effects on Cao(2+)-dependent junction formation and adhesion complex formation were evaluated by fluorescence immunostaining and immunoprecipitation. Endogenous Rho activity and expression of keratinocyte differentiation markers were also examined. The significance of the physical interactions of filamin A with Trio and Rho was assessed in dominant-negative inhibition studies.

RESULTS

Inhibiting filamin A expression blocked the formation of CaR-Rho A-Trio-E-cadherin protein complex. Knockdown of filamin A or Trio inhibited Cao(2+)-induced membrane localization and activation of Rho A, formation of the E-cadherin-catenin adhesion complex, and keratinocyte terminal differentiation. Expressing dominant-negative peptides disruptive to the endogenous filamin-Trio, filamin-Rho, and CaR-filamin interactions suppressed the formation of adherens junctions.

CONCLUSION

Through physical interactions with CaR, Trio and Rho, filamin A generates a scaffold for organizing a signaling complex that promotes E-cadherin-mediated cell-cell adhesion and keratinocyte differentiation.

Antagonistic activities of Rho and Rac GTPases underlie the transition from neural crest delamination to migration

BACKGROUND

Neural crest progenitors arise as epithelial cells and then undergo a transition into mesenchyme that generates motility. Previously, we showed that active Rho maintains crest cells in the epithelial conformation by keeping stress fibers and membrane-bound N-cadherin.

RESULTS

While Rho disappears from cell membranes upon delamination, active Rac1 becomes apparent in lamellipodia of mesenchymal cells. Loss of Rac1 function at trunk levels inhibited NC migration but did not prevent cell emigration that is associated with N-cadherin downregulation and G1/S transition. Furthermore, inhibition of Rho stimulated premature Rac1 activity and consequent formation of lamellipodia, leading to NC migration. To examine whether timely migration influences cell fate, Rac1 activity was transiently inhibited to delay dispersion of early NC cells that generate neural derivatives, and its activity was restored by the time of melanoblast migration. Even if confronted with a melanocytic environment, late-dispersing progenitors colonized sensory ganglia where they generated neurons and glia.

CONCLUSIONS

In the context of crest delamination and migration, activities of Rho and Rac are differential, sequential, and antagonistic. Furthermore, transient inhibition of Rac1 that delays the onset of crest dispersion raises the possibility that the fate of trunk neural progenitors might be restricted prior to migration.

Striatal neurons expressing full-length mutant huntingtin exhibit decreased N-cadherin and altered neuritogenesis

The expanded CAG repeat that causes striatal cell vulnerability in Huntington's disease (HD) encodes a polyglutamine tract in full-length huntingtin that is correlated with cellular [ATP] and [ATP/ADP]. Since striatal neurons are vulnerable to energy deficit, we have investigated, in Hdh CAG knock-in mice and striatal cells, the hypothesis that decreased energetics may affect neuronal (N)-cadherin, a candidate energy-sensitive adhesion protein that may contribute to HD striatal cell sensitivity. In vivo, N-cadherin was sensitive to ischemia and to the effects of full-length mutant huntingtin, progressively decreasing in Hdh(Q111) striatum with age. In cultured striatal cells, N-cadherin was decreased by ATP depletion and STHdh(Q111) striatal cells exhibited dramatically decreased N-cadherin, due to decreased Cdh2 mRNA and enhanced N-cadherin turnover, which was partially normalized by adenine supplementation to increase [ATP] and [ATP/ADP]. Consistent with decreased N-cadherin function, STHdh(Q111) striatal cells displayed profound deficits in calcium-dependent N-cadherin-mediated cell clustering and cell-substratum adhesion, and primary Hdh(Q111) striatal neuronal cells exhibited decreased N-cadherin and an abundance of immature neurites, featuring diffuse, rather than clustered, staining for N-cadherin and synaptic vesicle markers, which was partially rescued by adenine treatment. Thus, mutant full-length huntingtin, via energetic deficit, contributes to decreased N-cadherin levels in striatal neurons, with detrimental effects on neurite maturation, strongly suggesting that N-cadherin-mediated signaling merits investigation early in the HD pathogenic disease process.

The calcium-sensing receptor-dependent regulation of cell-cell adhesion and keratinocyte differentiation requires Rho and filamin A

Extracellular Ca(2+) (Ca(2+)(o)) functioning through the calcium-sensing receptor (CaR) induces E-cadherin-mediated cell-cell adhesion and cellular signals mediating cell differentiation in epidermal keratinocytes. Previous studies indicate that CaR regulates cell-cell adhesion through Fyn/Src tyrosine kinases. In this study, we investigate whether Rho GTPase is a part of the CaR-mediated signaling cascade regulating cell adhesion and differentiation. Suppressing endogenous Rho A expression by small interfering RNA (siRNA)-mediated gene silencing blocked the Ca(2+)(o)-induced association of Fyn with E-cadherin and suppressed the Ca(2+)(o)-induced tyrosine phosphorylation of β-, γ-, and p120-catenin and formation of intercellular adherens junctions. Rho A silencing also decreased the Ca(2+)(o)-stimulated expression of terminal differentiation markers. Elevating the Ca(2+)(o) level induced interactions among CaR, Rho A, E-cadherin, and the scaffolding protein filamin A at the cell membrane. Inactivation of CaR expression by adenoviral expression of a CaR antisense complementary DNA inhibited Ca(2+)(o)-induced activation of endogenous Rho. Ca(2+)(o) activation of Rho required a direct interaction between CaR and filamin A. Interference of CaR-filamin interaction inhibited Ca(2+)(o)-induced Rho activation and the formation of cell-cell junctions. These results indicate that Rho is a downstream mediator of CaR in the regulation of Ca(2+)(o)-induced E-cadherin-mediated cell-cell adhesion and keratinocyte differentiation.

The TRPV4 channel contributes to intercellular junction formation in keratinocytes

Transient receptor potential vanilloid 4 (TRPV4) channel is a physiological sensor for hypo-osmolarity, mechanical deformation, and warm temperature. The channel activation leads to various cellular effects involving Ca(2+) dynamics. We found that TRPV4 interacts with beta-catenin, a crucial component linking adherens junctions and the actin cytoskeleton, thereby enhancing cell-cell junction development and formation of the tight barrier between skin keratinocytes. TRPV4-deficient mice displayed impairment of the intercellular junction-dependent barrier function in the skin. In TRPV4-deficient keratinocytes, extracellular Ca(2+)-induced actin rearrangement and stratification were delayed following significant reduction in cytosolic Ca(2+) increase and small GTPase Rho activation. TRPV4 protein located where the cell-cell junctions are formed, and the channel deficiency caused abnormal cell-cell junction structures, resulting in higher intercellular permeability in vitro. Our results suggest a novel role for TRPV4 in the development and maturation of cell-cell junctions in epithelia of the skin.

N-cadherin acts in concert with Slit1-Robo2 signaling in regulating aggregation of placode-derived cranial sensory neurons

Vertebrate cranial sensory ganglia have a dual origin from the neural crest and ectodermal placodes. In the largest of these, the trigeminal ganglion, Slit1-Robo2 signaling is essential for proper ganglion assembly. Here, we demonstrate a crucial role for the cell adhesion molecule N-cadherin and its interaction with Slit1-Robo2 during gangliogenesis in vivo. A common feature of chick trigeminal and epibranchial ganglia is the expression of N-cadherin and Robo2 on placodal neurons and Slit1 on neural crest cells. Interestingly, N-cadherin localizes to intercellular adherens junctions between placodal neurons during ganglion assembly. Depletion of N-cadherin causes loss of proper ganglion coalescence, similar to that observed after loss of Robo2, suggesting that the two pathways might intersect. Consistent with this possibility, blocking or augmenting Slit-Robo signaling modulates N-cadherin protein expression on the placodal cell surface concomitant with alteration in placodal adhesion. Lack of an apparent change in total N-cadherin mRNA or protein levels suggests post-translational regulation. Co-expression of N-cadherin with dominant-negative Robo abrogates the Robo2 loss-of-function phenotype of dispersed ganglia, whereas loss of N-cadherin reverses the aberrant aggregation induced by increased Slit-Robo expression. Our study suggests a novel mechanism whereby N-cadherin acts in concert with Slit-Robo signaling in mediating the placodal cell adhesion required for proper gangliogenesis.

Cell-cell interactions in the endocrine pancreas

Cell-cell communication within any given tissue is an important aspect of correct organ function. The islets of Langerhans forming the endocrine pancreas are composed of alpha-, beta-, delta-, epsilon- and PP-cells, and interactions between these cells are required for fine-tuning glucose homeostasis of the body. The endocrine cells communicate through homotypic or heterotypic cell-cell adhesion, or in a paracrine fashion, and this communication is involved in the regulated secretion of islet hormones. This review discusses how islet hormones, secreted molecules and ions influence the endocrine cells and how cell adhesion molecules such as neural cell adhesion molecule, cadherins, connexin-36, Eph receptors and ephrin ligands, as well as extracellular matrix proteins, modulate pancreatic islet function.

A negative modulatory role for rho and rho-associated kinase signaling in delamination of neural crest cells

Background

Neural crest progenitors arise as epithelial cells and then undergo a process of epithelial to mesenchymal transition that precedes the generation of cellular motility and subsequent migration. We aim at understanding the underlying molecular network. Along this line, possible roles of Rho GTPases that act as molecular switches to control a variety of signal transduction pathways remain virtually unexplored, as are putative interactions between Rho proteins and additional known components of this cascade.

Results

We investigated the role of Rho/Rock signaling in neural crest delamination. Active RhoA and RhoB are expressed in the membrane of epithelial progenitors and are downregulated upon delamination. In vivo loss-of-function of RhoA or RhoB or of overall Rho signaling by C3 transferase enhanced and/or triggered premature crest delamination yet had no effect on cell specification. Consistently, treatment of explanted neural primordia with membrane-permeable C3 or with the Rock inhibitor Y27632 both accelerated and enhanced crest emigration without affecting cell proliferation. These treatments altered neural crest morphology by reducing stress fibers, focal adhesions and downregulating membrane-bound N-cadherin. Reciprocally, activation of endogenous Rho by lysophosphatidic acid inhibited emigration while enhancing the above. Since delamination is triggered by BMP and requires G1/S transition, we examined their relationship with Rho. Blocking Rho/Rock function rescued crest emigration upon treatment with noggin or with the G1/S inhibitor mimosine. In the latter condition, cells emigrated while arrested at G1. Conversely, BMP4 was unable to rescue cell emigration when endogenous Rho activity was enhanced by lysophosphatidic acid.

Conclusion

Rho-GTPases, through Rock, act downstream of BMP and of G1/S transition to negatively regulate crest delamination by modifying cytoskeleton assembly and intercellular adhesion.

Focal adhesion kinase regulates cell-cell contact formation in epithelial cells via modulation of Rho

Focal Adhesion Kinase (FAK) is a non-receptor tyrosine kinase that plays a key role in cellular processes such as cell adhesion, migration, proliferation and survival. Recent studies have also implicated FAK in the regulation of cell-cell adhesion. Here, evidence is presented showing that siRNA-mediated suppression of FAK levels in NBT-II cells and expression of dominant negative mutants of FAK caused loss of epithelial cell morphology and inhibited the formation of cell-cell adhesions. Rac and Rho have been implicated in the regulation of cell-cell adhesions and can be regulated by FAK signaling. Expression of active Rac or Rho in NBT-II cells disrupted formation of cell-cell contacts, thus promoting a phenotype similar to FAK-depleted cells. The loss of intercellular contacts in FAK-depleted cells is prevented upon expression of a dominant negative Rho mutant, but not a dominant negative Rac mutant. Inhibition of FAK decreased tyrosine phosphorylation of p190RhoGAP and elevated the level of GTP-bound Rho. This suggests that FAK regulates cell-cell contact formation by regulation of Rho.

Microtubule dynamics and Rac-1 signaling independently regulate barrier function in lung epithelial cells

Cadherin-mediated cell-cell adhesion controls the morphology and function of epithelial cells and is a critical component of the pathology of chronic inflammatory disorders. Dynamic interactions between cadherins and the actin cytoskeleton are required for stable cell-cell contact. Besides actin, microtubules also target intercellular, cadherin-based junctions and contribute to their formation and stability. Here, we studied the role of microtubules in conjunction with Rho-like GTPases in the regulation of lung epithelial barrier function using real-time monitoring of transepithelial electrical resistance. Unexpectedly, we found that disruption of microtubules promotes epithelial cell-cell adhesion. This increase in epithelial barrier function is accompanied by the accumulation of beta-catenin at cell-cell junctions, as detected by immunofluorescence. Moreover, we found that the increase in cell-cell contact, induced by microtubule depolymerization, requires signaling through a RhoA/Rho kinase pathway. The Rac-1 GTPase counteracts this pathway, because inhibition of Rac-1 signaling rapidly promotes epithelial barrier function, in a microtubule- and RhoA-independent fashion. Together, our data suggest that microtubule-RhoA-mediated signaling and Rac-1 control lung epithelial integrity through counteracting independent pathways.

Microtubules regulate disassembly of epithelial apical junctions

Background

Epithelial tight junction (TJ) and adherens junction (AJ) form the apical junctional complex (AJC) which regulates cell-cell adhesion, paracellular permeability and cell polarity. The AJC is anchored on cytoskeletal structures including actin microfilaments and microtubules. Such cytoskeletal interactions are thought to be important for the assembly and remodeling of apical junctions. In the present study, we investigated the role of microtubules in disassembly of the AJC in intestinal epithelial cells using a model of extracellular calcium depletion.

Results

Calcium depletion resulted in disruption and internalization of epithelial TJs and AJs along with reorganization of perijunctional F-actin into contractile rings. Microtubules reorganized into dense plaques positioned inside such F-actin rings. Depolymerization of microtubules with nocodazole prevented junctional disassembly and F-actin ring formation. Stabilization of microtubules with either docetaxel or pacitaxel blocked contraction of F-actin rings and attenuated internalization of junctional proteins into a subapical cytosolic compartment. Likewise, pharmacological inhibition of microtubule motors, kinesins, prevented contraction of F-actin rings and attenuated disassembly of apical junctions. Kinesin-1 was enriched at the AJC in cultured epithelial cells and it also accumulated at epithelial cell-cell contacts in normal human colonic mucosa. Furthermore, immunoprecipitation experiments demonstrated association of kinesin-1 with the E-cadherin-catenin complex.

Conclusion

Our data suggest that microtubules play a role in disassembly of the AJC during calcium depletion by regulating formation of contractile F-actin rings and internalization of AJ/TJ proteins.

Requirement of Rac activity for maintenance of capillary endothelial barrier properties

Our previous experiments indicated that GTPases, other than RhoA, are important for the maintenance of endothelial barrier integrity in both intact microvessels of rats and mice and cultured mouse myocardial endothelial (MyEnd) cell monolayers ( J Physiol 539: 295–308, 2002). In the present study, we inhibited the endothelial GTPase Rac by Clostridium sordellii lethal toxin (LT) and investigated the relation between the degree of inhibition of Rac by glucosylation and increased endothelial barrier permeability. In rat venular microvessels, LT (200 ng/ml) increased hydraulic conductivity from a control value of 2.5 ± 0.6 to 100.8 ± 18.7 × 10–7cm·s–1·cmH2O–1after 80 min. In cultured MyEnd cells exposed to LT (200 ng/ml), up to 60% of cellular Rac was glucosylated after 90 min, resulting in depolymerization of F-actin and interruptions of junctional distribution of vascular endothelial cadherin (VE-cadherin) and β-catenin as well as the formation of intercellular gaps. To understand the mechanism by which inhibition of Rac caused disassembly of adherens junctions, we used laser tweezers to quantify VE-cadherin-mediated adhesion. LT and cytochalasin D, an actin depolymerizing agent, both reduced adhesion of VE-cadherin-coated microbeads to the endothelial cell surface, whereas the inhibitor of Rho kinase Y-27632 did not. Stabilization of actin filaments by jasplakinolide completely blocked the effect of cytochalasin D but not of LT on bead adhesion. We conclude that Rac regulates endothelial barrier properties in vivo and in vitro by 1) modulation of actin filament polymerization and 2) acting directly on the tether between VE-cadherin and the cytoskeleton.

Rho1 interacts with p120ctn and α-catenin, and regulates cadherin-based adherens junction components in Drosophila

Rho GTPases are important regulators of cellular behavior through their effects on processes such as cytoskeletal organization. Here we show interactions between Drosophila Rho1 and the adherens junction components α-catenin and p120ctn. We find that while Rho1 protein is present throughout the cell, it accumulates apically, particularly at sites of cadherin-based adherens junctions. Cadherin and catenin localization is disrupted in Rho1 mutants, implicating Rho1 in their regulation. p120ctn has recently been suggested to inhibit Rho activity through an unknown mechanism. We find that Rho1 accumulates in response to lowered p120ctn activity. Significantly, we find that Rho1 binds directly to α-catenin and p120ctn in vitro, and these interactions map to distinct surface-exposed regions of the protein not previously assigned functions. In addition, we find that both α-catenin and p120ctn co-immunoprecipitate with Rho1-containing complexes from embryo lysates. Our observations suggest that α-catenin and p120ctn are key players in a mechanism of recruiting Rho1 to its sites of action.

Phosphatidylinositol 3-kinase controls human intestinal epithelial cell differentiation by promoting adherens junction assembly and p38 MAPK activation

The signaling pathways mediating human intestinal epithelial cell differentiation remain largely undefined. Phosphatidylinositol 3-kinase (PI3K) is an important modulator of extracellular signals, including those elicited by E-cadherin-mediated cell-cell adhesion, which plays an important role in maintenance of the structural and functional integrity of epithelia. In this study, we analyzed the involvement of PI3K in the differentiation of human intestinal epithelial cells. We showed that inhibition of PI3K signaling in Caco-2/15 cells repressed sucrase-isomaltase and villin protein expression. Morphological differentiation of enterocyte-like features in Caco-2/15 cells such as epithelial cell polarity and brush-border formation were strongly attenuated by PI3K inhibition. Immunofluorescence and immunoprecipitation experiments revealed that PI3K was recruited to and activated by E-cadherin-mediated cell-cell contacts in confluent Caco-2/15 cells, and this activation appears to be essential for the integrity of adherens junctions and association with the cytoskeleton. We provide evidence that the assembly of calcium-dependent adherens junctions led to a rapid and remarkable increase in the state of activation of Akt and p38 MAPK pathways and that this increase was blocked in the presence of anti-E-cadherin antibodies and PI3K inhibitor. Therefore, our results indicate that PI3K promotes assembly of adherens junctions, which, in turn, control p38 MAPK activation and enterocyte differentiation.

Structure and control of a cell-cell adhesion complex associated with spermiation in rat seminiferous epithelium

Spermiation, the release of late spermatids from the Sertoli cell, is disrupted by a number of toxicants. Control of the spermiation process, and the proteins that interact to adhere mature spermatids to Sertoli cells, is poorly understood. In these studies we used immunohistochemistry, coimmunoprecipitation/Western blotting, and mass spectrometry to refine an earlier model of sperm adhesion proposed by our laboratory. We have identified specific proteins linked together as part of a multiprotein complex, as well as several additional proteins (cortactin, ERK1/2, and 14-3-3 zeta) that may be functioning in both structural and signal transduction roles. The current and prior data suggest that protein phosphorylation is central to the control of spermiation. We also present and characterize an in vitro tubule culture system that allowed functional testing of the spermiation model by pharmacologic manipulation, and yielded data consistent with the importance of protein phosphorylation in spermiation.

Oxidant stress and endothelial cell dysfunction

Reactive oxygen species (ROS) are generated at sites of inflammation and injury, and at low levels, ROS can function as signaling molecules participating as signaling intermediates in regulation of fundamental cell activities such as cell growth and cell adaptation responses, whereas at higher concentrations, ROS can cause cellular injury and death. The vascular endothelium, which regulates the passage of macromolecules and circulating cells from blood to tissues, is a major target of oxidant stress, playing a critical role in the pathophysiology of several vascular diseases and disorders. Specifically, oxidant stress increases vascular endothelial permeability and promotes leukocyte adhesion, which are coupled with alterations in endothelial signal transduction and redox-regulated transcription factors such as activator protein-1 and nuclear factor-kappaB. This review discusses recent findings on the cellular and molecular mechanisms by which ROS signal events leading to impairment of endothelial barrier function and promotion of leukocyte adhesion. Particular emphasis is placed on the regulation of cell-cell and cell-surface adhesion molecules, the actin cytoskeleton, key protein kinases, and signal transduction events.

Signaling from E-cadherins to the MAPK pathway by the recruitment and activation of epidermal growth factor receptors upon cell-cell contact formation

E-cadherins are well characterized cell surface molecules expressed in epithelial cells, which play a major role in cell adhesion through the establishment of calcium-dependent homophilic interactions at sites of cell-cell contacts. They are also integral components of morphogenetic programs controlling the maintenance of the structural and functional integrity of epithelia. Accumulated evidence indicates that the E-cadherin-mediated cell adhesion system is highly regulated from inside the cells by a number of intracellular signaling pathways. Recently available information suggests that E-cadherins may also play a role in the transduction of signals from the outside of the cell to the cytoplasm. However, the nature of the biochemical routes regulated by E-cadherins is still largely unknown. In this study, we set out to explore the possibility that E-cadherins may regulate the activity of MAPK, a key signaling pathway involved in cell fate decisions, upon the formation of cell-cell contacts among neighboring cells. By using an immortalized non-tumorigenic keratinocyte cell line, HaCat, as a model system, we provide evidence that the assembly of calcium-dependent adherens junctions leads to a rapid and remarkable increase in the state of activation of MAPK and that this event is mediated by E-cadherins. Furthermore, we found that E-cadherins stimulate the MAPK pathway through the ligand-independent activation of epidermal growth factor receptors and the consequent activation of a biochemical route leading to the stimulation of MAPKs. These findings suggest that E-cadherins can initiate outside-in signal transducing pathways through the engagement of tyrosine kinase receptors for epidermal growth factor, thus providing a novel molecular mechanism whereby these cell adhesion molecules may ultimately control the fate of normal and transformed epithelial cells.

E-cadherin-mediated cell-cell attachment activates Cdc42

E-cadherin is a transmembrane protein that mediates Ca(2+)-dependent cell-cell adhesion. Cdc42, a member of the Rho family of small GTPases, participates in cytoskeletal rearrangement and cell cycle progression. Recent evidence reveals that members of the Rho family modulate E-cadherin function. To further examine the role of Cdc42 in E-cadherin-mediated cell-cell adhesion, we developed an assay for active Cdc42 using the GTPase-binding domain of the Wiskott-Aldrich syndrome protein. Initiation of E-cadherin-mediated cell-cell attachment significantly increased in a time-dependent manner the amount of active Cdc42 in MCF-7 epithelial cell lysates. By contrast, Cdc42 activity was not increased under identical conditions in MCF-7 cells incubated with anti-E-cadherin antibodies nor in MDA-MB-231 (E-cadherin negative) epithelial cells. By fusing the Wiskott-Aldrich syndrome protein/GTPase-binding domain to a green fluorescent protein, activation of endogenous Cdc42 by E-cadherin was demonstrated in live cells. These data indicate that E-cadherin activates Cdc42, demonstrating bi-directional interactions between the Rho- and E-cadherin signaling pathways.

Dendritic cells: new roles for Cdc42 and Rac in antigen uptake?

In dendritic cells, macropinocytosis is downregulated during maturation and antigen presentation. New studies indicate a key role for Rho-family GTPases in this regulation, but what these proteins are actually doing remains a mystery.

Deposition of laminin 5 by keratinocytes regulates integrin adhesion and signaling

Deposition of laminin 5 over exposed dermal collagen in epidermal wounds is an early event in repair of the basement membrane. We report that deposition of laminin 5 onto collagen switches adhesion and signaling from collagen-dependent to laminin 5-dependent. Ligation of laminin 5 by integrin alpha(6)beta(4) activates phosphoinositide 3-OH-kinase (PI3K) signaling. This activation allows for adhesion and spreading via integrin alpha(3)beta(1) on laminin 5 independent of RhoGTPase, a regulator of actin stress fibers. In contrast, adhesion and spreading on collagen via alpha(2)beta(1) is Rho-dependent and is inhibited by toxin B, a Rho inhibitor. Deposition of laminin 5 and ligation of alpha(6)beta(4) increases PI3K-dependent production of phosphoinositide di- and triphosphates, PI3K activity, and phosphorylation of downstream target protein c-Jun NH(2)-terminal kinase. Conversely, blocking laminin 5-deposition with brefeldin A, an inhibitor of vesicle transport, or with anti-laminin 5 monoclonal antibodies abolishes the PI3K-dependent spreading mediated by alpha(3)beta(1) and phosphorylation of c-Jun NH(2)-terminal kinase. Studies with keratinocytes lacking alpha(6)beta(4) or laminin 5 confirm that deposition of laminin 5 and ligation by alpha(6)beta(4) are required for PI3K-dependent spreading via alpha(3)beta(1). We suggest that deposition of laminin 5 onto the collagen substratum, as in wound repair, enables human foreskin keratinocytes to interact via alpha(6)beta(4) and to switch from a RhoGTPase-dependent adhesion on collagen to a PI3K-dependent adhesion and spreading mediated by integrin alpha(3)beta(1) on laminin 5.

The E-cadherin-catenin complex in tumour metastasis: structure, function and regulation

E-cadherin and the associated catenin complex have been recognised as performing a key role in cell adhesion. Loss of cell adhesion is seen as a key step in the cascade leading to tumour metastasis. The ability of both extra- and intracellular factors to regulate E-cadherin-mediated cell adhesion in physiological processes has provided insight into both the interactions of the E-cadherin-catenin complex, and possible mechanisms utilised by tumours in the process of metastasis. The interaction of the E-cadherin-catenin complex with various regulating factors, their effect on cell signalling pathways, and the relationship with the metastatic potential of tumours are reviewed.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.