Protective effect of quercetin against in vitro erythrocyte rheology alterations produced by arsenic.

Humans are exposed to heavy metals such as arsenic (As), through contaminated food and drinking water. The effect of As on RBC membrane is one of the most important biological effects. In a previous work, we have studied the AsVin vitro effect on erythrocytes biophysical properties discovering alterations regarding aggregability deformability, cell morphology, membrane fluidity and osmotic response. We have also observed that the presence of the metal produces an oxidative stress in RBCs that might be the origin of rheological impairment. In the present work we analyzed RBCs rheological properties associated with membrane fluidity and lipid peroxidation in presence of As and quercetin (Qc). From our results we can conclude that RBCs treatment with Qc is efficient to prevent the impairment of the mechanical properties of the cell membrane produced by the As, through oxygen reactive agents in the membrane structure, mainly on the lipids. This protective effect is observed in the preservation of the erythrocytes rheological properties and consequently in the maintenance of an appropriate blood flow, specially in the small vessels in the peripheral circulation.

Arsenic intoxication, a hemorheologic view.

Arsenic (As) is a toxic semi-metal of wide distribution in nature. People living in regions where drinking water contains large quantities of arsenic, have an unusually high likelihood of developing blood-vessel diseases, but little is known about the mechanisms involved, i.e. the blood rheologic alterations that would contribute to the circulatory obstruction. Erythrocytes are the main target cells for arsenic compounds systemically absorbed and their cell membrane is the first place against the toxic. In this paper we have examined the in vitro effect of arsenic (As(V)) on the rheologic properties of human erythrocytes in relation with membrane fluidity and internal microviscosity. According to our present results, As(V) treatment produces oxidative degradation of membrane lipids and alteration of internal microviscosity. These red blood cells (RBCs) membrane and cytoplasmic structural damage consequently alters RBCs rheologic properties: an alteration of the RBCs discoid shape to stomatocytes, a diminution of erythrocyte deformability and an enhancement of osmotic fragility and cell aggregability. These effects impaired blood fluid behaviour that contribute to obstruct peripheral circulation and provides anemia, both clinic evidences typical of arsenic cronic intoxication.

In vitro effect of aluminium upon erythrocyte membrane properties.

The link between aluminium (Al(III)) and a range of disorders in organisms (plants and animals including human beings) has been stated in diverse studies. As regards as human beings in particular, there are numerous studies on this metal's toxicity in relation to pathological processes. Only few references to the metal's effect upon cell rheological properties can be found. In this study, we present evidence for alterations in the rheological properties of cells as consequence of the Al(III)'s interaction with human red blood cell membrane. Al(III) could damage membrane functions of the red blood cell by favouring lipid peroxidation reactions due to the presence of Fe(II) as an initiator. The metal's effect on lipid bilayer, and probably on the cytoskeleton as well, would constitute the cause for the impaired erythrocyte rheology.

Dynamic modules and heterogeneity of function: a lesson from tyrosine kinase receptors in endothelial cells.

An important unresolved issue related to tyrosine kinase receptor signaling pathways is the lack of specificity of the molecular effectors involved. The specificity of the biological responses that are nevertheless elicited may be explained by differences in activation thresholds, as well as by temporal (transient versus sustained) and topographical aspects of receptor activation. On the basis of recent lessons from endothelial cells, we argue that an additional strategy can be adopted to generate specificity, i.e. tyrosine kinase receptors may form distinct signaling modules with other transmembrane proteins, such as adhesive receptors, to elicit different biological programs in stimulated cells.

X-ray structure of junctional adhesion molecule: structural basis for homophilic adhesion via a novel dimerization motif.

Junctional adhesion molecules (JAMs) are a family of immunoglobulin-like single-span transmembrane molecules that are expressed in endothelial cells, epithelial cells, leukocytes and myocardia. JAM has been suggested to contribute to the adhesive function of tight junctions and to regulate leukocyte trans migration. We describe the crystal structure of the recombinant extracellular part of mouse JAM (rsJAM) at 2.5 A resolution. rsJAM consists of two immunoglobulin-like domains that are connected by a conformationally restrained short linker. Two rsJAM molecules form a U-shaped dimer with highly complementary interactions between the N-terminal domains. Two salt bridges are formed in a complementary manner by a novel dimerization motif, R(V,I,L)E, which is essential for the formation of rsJAM dimers in solution and common to the known members of the JAM family. Based on the crystal packing and studies with mutant rsJAM, we propose a model for homophilic adhesion of JAM. In this model, U-shaped JAM dimers are oriented in cis on the cell surface and form a two-dimensional network by trans-interactions of their N-terminal domains with JAM dimers from an opposite cell surface.

Pores in the sieve and channels in the wall: control of paracellular permeability by junctional proteins in endothelial cells.

Exchange of solutes and ions between the luminal and abluminal compartments of the circulation is critically dependent on the barrier properties of the vascular endothelium. Transport of solutes and fluids occurs along the transcellular and paracellular pathways that are mediated by intracellular vesicles and intercellular junctions, respectively. Although the ability of endothelial cells to dynamically regulate permeability has long been recognized, the precise mechanism and the signaling pathways involved have not been fully elucidated. Finally, current definition of the complex molecular composition of intercellular junctions is expected to explain the difference in permeability between diverse segments of the circulation and possibly to highlight the existence of specific junctional channels. The properties of junctional adhesion molecule-1 (JAM-1) and vascular endothelial cadherin (VE-cadherin), two transmembrane components of interendothelial junctions, are described in detail.

Interendothelial junctions and their role in the control of angiogenesis, vascular permeability and leukocyte transmigration.

Endothelial cell-cell junctions play an important role in vascular hemostasis. The two junctional proteins VE-cadherin and JAM-1 are localized at adherens and tight junctions, respectively. VE-cadherin is only expressed by endothelial cells, suggesting that it can exert cell specific function. Absence of VE-cadherin or blocking of its adhesive activity prevents a normal organization of new vascular structures, suggesting that VE-cadherin may be a molecular target of antiangiogenic therapy. In addition, the ability of permeability-increasing agents and adherent leukocytes to modify VE-cadherin/catenin organization may be related to a role in the control of vascular permeability and leukocyte infiltration. JAM-1 is an integral membrane protein expressed in endothelial and epithelial cells. Its extracellular domain can dimerize and bind homophilically. The intracellular domain (and in particular a PDZ-binding motif) enables JAM-1 to interact with structural and signaling proteins. Study of the molecular interactions of JAM-1 may help explain mechanisms of JAM-mediated function, such as control of paracellular permeability and leukocyte transmigration.

Effect of tetracaine chlorhydrate on the mechanical properties of the erythrocyte membrane.

Pharmacologically active agents that locate in the cell membrane are useful tools to investigate the interactions taking place between its molecular components. In the present work, the effect of tetracaine chlorhydrate (Tc) on the membrane mechanical properties of intact and desialated erythrocytes was studied. Our results evince the complex interaction between the drug and the membrane structures. The effect of Tc on erythrocyte shape suggests that this drug locates in the inner hemilayer of the lipid bilayer. Since Tc also modifies osmotic fragility and mechanical properties ascribed to the cytoskeleton, it can be inferred that the lipid bilayer has an effect on the rheology of the membrane, in a direct or indirect way, in this case through the close interaction with the structural proteins. Moreover, our results support the hypothesis of a second localization of the drug in the membrane, i.e., as monovalent cations intercalated among the glycocalix sialic endings, where it generates an effect superimposed on that produced from its typical site in the lipid bilayer.

Phosphorylation of a conserved integrin alpha 3 QPSXXE motif regulates signaling, motility, and cytoskeletal engagement.

Integrin alpha 3A cytoplasmic tail phosphorylation was mapped to amino acid S1042, as determined by mass spectrometry, and confirmed by mutagenesis. This residue occurs within a "QPSXXE" motif conserved in multiple alpha chains (alpha 3A, alpha 6A, alpha 7A), from multiple species. Phosphorylation of alpha 3A and alpha 6A did not appear to be directly mediated by protein kinase C (PKC) alpha, beta, gamma, delta, epsilon, zeta, or mu, or by any of several other known serine kinases, although PKC has an indirect role in promoting phosphorylation. A S1042A mutation did not affect alpha 3-Chinese hamster ovary (CHO) cell adhesion to laminin-5, but did alter 1) alpha 3-dependent tyrosine phosphorylation of focal adhesion kinase and paxillin (in the presence or absence of phorbol 12-myristate 13 acetate stimulation), and p130(CAS) (in the absence of phorbol 12-myristate 13 acetate stimulation), 2) the shape of cells spread on laminin-5, and 3) alpha 3-dependent random CHO cell migration on laminin-5. In addition, S1042A mutation altered the PKC-dependent, ligand-dependent subcellular distribution of alpha 3 and F-actin in CHO cells. Together, the results demonstrate clearly that alpha 3A phosphorylation is functionally relevant. In addition, the results strongly suggest that alpha 3 phosphorylation may regulate alpha 3 integrin interaction with the cytoskeleton.

Association of junctional adhesion molecule with calcium/calmodulin-dependent serine protein kinase (CASK/LIN-2) in human epithelial caco-2 cells.

We report here that junctional adhesion molecule (JAM) interacts with calcium/calmodulin-dependent serine protein kinase (CASK), a protein related to membrane-associated guanylate kinases. In Caco-2 cells, JAM and CASK were coprecipitated and found to colocalize at intercellular contacts along the lateral surface of the plasma membrane. Association of JAM with CASK requires the PSD95/dlg/ZO-1 (PDZ) domain of CASK and the putative PDZ-binding motif Phe-Leu-Val(COOH) in the cytoplasmic tail of JAM. Temporal dissociation in the junctional localization of the two proteins suggests that the association with CASK is not required for recruiting JAM to intercellular junctions. Compared with mature intercellular contacts, junction assembly was characterized by both enhanced solubility of CASK in Triton X-100 and reduced amounts of Triton-insoluble JAM-CASK complexes. We propose that JAM association with CASK is modulated during junction assembly, when CASK is partially released from its cytoskeletal associations.

The molecular organization of endothelial junctions and their functional role in vascular morphogenesis and permeability.

We review here our work on the molecular and functional organization of endothelial cell-to-cell junctions. The first part of the review is dedicated to VE-cadherin, characterized by our group few years ago. This protein is a member of the large family of transmembrane adhesion proteins called cadherins. It is endothelial cell specific and plays a major role in the organization of adherens junctions. Inactivation of VE-cadherin gene or in vivo truncation of its cytoplasmic tail leads to a lethal phenotype due to the lack of correct organization of the vasculature in the embryo. We found that the defect was due to apoptosis of endothelial cells, which became unresponsive to the survival signal induced by vascular endothelial cell growth factor. Our data indicate that VE-cadherin may act as a scaffolding protein able to associate vascular endothelial cell growth factor receptor and to promote its signaling. In the second part of the review we consider another protein more recently discovered by us and called junctional adhesion molecule (JAM). This protein is a small immunoglobulin which is located at tight junctions in the endothelium and in epithelial cells. Evidence is discussed indicating that JAM takes part in the organization of tight junctions and modulates leukocyte extravasation through endothelial intercellular junctions in vitro and in vivo. The general role of tight junctions in endothelial cells is also discussed.

Homophilic interaction of junctional adhesion molecule.

Junctional adhesion molecule (JAM) is an integral membrane protein that belongs to the immunoglobulin superfamily, localizes at tight junctions, and regulates both paracellular permeability and leukocyte transmigration. To investigate molecular determinants of JAM function, the extracellular domain of murine JAM was produced as a recombinant soluble protein (rsJAM) in insect cells. rsJAM consisted in large part of noncovalent homodimers, as assessed by analytical ultracentrifugation. JAM dimers were also detected at the surface of Chinese hamster ovary cells transfected with murine JAM, as evaluated by cross-linking and immunoprecipitation. Furthermore, fluid-phase rsJAM bound dose-dependently solid-phase rsJAM, and such homophilic binding was inhibited by anti-JAM Fab BV11, but not by Fab BV12. Interestingly, Fab BV11 exclusively bound rsJAM dimers (but not monomers) in solution, whereas Fab BV12 bound both dimers and monomers. Finally, we mapped the BV11 and BV12 epitopes to a largely overlapping sequence in proximity of the extracellular amino terminus of JAM. We hypothesize that rsJAM dimerization induces a BV11-positive conformation which in turn is critical for rsJAM homophilic interactions. Dimerization and homophilic binding may contribute to both adhesive function and junctional organization of JAM.

Interaction of junctional adhesion molecule with the tight junction components ZO-1, cingulin, and occludin.

Junctional adhesion molecule (JAM) is an integral membrane protein that has been reported to colocalize with the tight junction molecules occludin, ZO-1, and cingulin. However, evidence for the association of JAM with these molecules is missing. Transfection of Chinese hamster ovary cells with JAM (either alone or in combination with occludin) resulted in enhanced junctional localization of both endogenous ZO-1 and cotransfected occludin. Additionally, JAM was coprecipitated with ZO-1 in the detergent-insoluble fraction of Caco-2 epithelial cells. A putative PDZ-binding motif at the cytoplasmic carboxyl terminus of JAM was required for mediating the interaction of JAM with ZO-1, as assessed by in vitro binding and coprecipitation experiments. JAM was also coprecipitated with cingulin, another cytoplasmic component of tight junctions, and this association required the amino-terminal globular head of cingulin. Taken together, these data indicate that JAM is a component of the multiprotein complex of tight junctions, which may facilitate junction assembly.

Endothelial adhesion molecules in the development of the vascular tree: the garden of forking paths.

In the past, year targeted null mutation studies have further supported the concept that endothelial cell-matrix and cell-cell adhesion is involved in the formation and maintenance of the network of branched tubes within the vascular tree. In addition, recent results derived from the closely related experimental system of branching tubulogenesis in epithelial cells may provide an appealing model for endothelial biology.

Vascular endothelial (VE)-cadherin: only an intercellular glue?

Data collected during the past years indicate that AJ- and more specifically VE-cadherin play an important role in endothelial cell biology. VE-cadherin may transfer information intracellularly through interaction with a complex network of cytoskeletal and signaling molecules. Expression of VE-cadherin is required for the control of vascular permeability and vascular integrity. In addition, the molecule may exert a morphogenetic role modulating the capacity of endothelial cells to organize into tubular-like structures. VE-cadherin presents many structural and sequence homologies to the other members of the family and apparently binds to the same intracellular molecules. However, remarkably, VE-cadherin may transfer specific signals to endothelial cells to modulate their functional reactivity.

Molecular structure and functional role of vascular tight junctions.

Interendothelial tight junctions regulate paracellular permeability and maintain cell polarity. The assembly and remodeling of tight junctions are examined, focusing on the molecular interactions between tight junction components and their functional role in endothelial biology. The molecular structures of two subcellular organelles related to tight junctions, the intercalated disks in cardiomyocytes and the slit diaphragms in glomerular podocytes, are discussed.

Divalent cations and ligands induce conformational changes that are highly divergent among beta1 integrins.

Here we show striking differences in conformational regulation among beta1 integrins. Upon manganese stimulation, a beta1 epitope defined by monoclonal antibody (mAb) 9EG7 was induced strongly (on alpha4beta1), moderately (on alpha5beta1), weakly (on alpha2beta1), or was scarcely detectable (on alpha6beta1 and alpha3beta1). Comparable results were seen for the beta1 epitope defined by mAb 15/7. Likewise, soluble ligands caused strong (alpha4beta1), moderate (alpha5beta1), weak (alpha2beta1, alpha6beta1), or minimal (alpha3beta1) induction of the 9EG7 epitope. Exchange or deletion of alpha chain cytoplasmic tails did not alter Mn2+-induced 9EG7 epitope levels. Upon removal of calcium by EGTA or EDTA, the hierarchy of 9EG7 epitope induction was similar (alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), except that EGTA reduced rather than induced 9EG7 expression on alpha4beta1. Thus in contrast to other beta1 integrins, calcium uniquely supports constitutive expression of the 9EG7 epitope on alpha4beta1. Likewise, calcium supported vascular cell adhesion molecule-stimulated 9EG7 appearance on alpha4beta1, whereas calcium inhibited ligand-induced 9EG7 epitope on other integrins. Constitutive expression of 9EG7 on alpha4beta1 was eliminated by a D698E mutation in alpha4, suggesting that Asp-698 may play a key role in maintaining atypical alpha4beta1 response to calcium. In conclusion, our results (i) demonstrate that mAb such as 9EG7 and 15/7 have limited diagnostic utility as reporters of ligand or Mn2+ occupancy for beta1 integrins, (ii) indicate pronounced differences in conformational flexibilities (alpha4beta1 > alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), (iii) allow us to hypothesize that beta1 integrins may differ markedly in conformation-dependent inside-out signaling, and (iv) have uncovered an atypical alpha4beta1 response to calcium that requires alpha4 Asp-698.

Are changes in integrin affinity and conformation overemphasized?

The activation of integrin-type adhesion receptors might result in the increased affinity of the receptor for ligand. In addition, the activated receptor might display new epitopes, which are increasingly monitored in clinical settings. Here, we highlight examples of integrin 'activation' that is not accompanied by enhanced ligand binding. Also, we emphasize that the dominant integrin conformational changes occur not with 'activation', but after integrins have already bound ligand.

Effects of an amphipathic drug on the rheological properties of the cell membrane.

Sodium thiopental, as other amphiphilic molecules, interacts with the membrane by inserting into the lipid bilayer and causing alterations of the membrane properties such as curvature and hypotonic lysis resistance. But can it modify the mechanical properties of the membrane? In the present work it was observed that sodium thiopental affected the membrane rheological properties by improving erythrocyte deformability; this effect resulted from a reduction of both the elastic modulus and surface viscosity. In erythrocytes devoid of sialic acid after treatment with neuraminidase, sodium thiopental membrane concentration was significantly higher than in normal cells, suggesting that drug access to the lipid bilayer be facilitated by the absence of the steric and electrostatic barrier of the glycocalyx negative charges. From a rheological point of view, desialated and normal cells showed the same response to the anesthetic as regards elastic modulus but in opposite direction if surface viscosity was considered. This finding supports the hypothesis that sodium thiopental molecules enter the bilayer of desialated cells in a higher proportion, as compared to the normal erythrocyte, promoting a disorganization that results in a greater inner friction. The changes in the rheological parameters, triggered by sodium thiopental, could be attributed to the bilayer contribution to the membrane mechanical properties, either directly or through interaction between the bilayer and the cytoskeleton.