Interstitial cell migration: integrin-dependent and alternative adhesion mechanisms

Adhesion and migration are integrated cell functions that build, maintain and remodel the multicellular organism. In migrating cells, integrins are the main transmembrane receptors that provide dynamic interactions between extracellular ligands and actin cytoskeleton and signalling machineries. In parallel to integrins, other adhesion systems mediate adhesion and cytoskeletal coupling to the extracellular matrix (ECM). These include multifunctional cell surface receptors (syndecans and CD44) and discoidin domain receptors, which together coordinate ligand binding with direct or indirect cytoskeletal coupling and intracellular signalling. We review the way that the different adhesion systems for ECM components impact cell migration in two- and three-dimensional migration models. We further discuss the hierarchy of these concurrent adhesion systems, their specific tasks in cell migration and their contribution to migration in three-dimensional multi-ligand tissue environments.     

Rapid expression and activation of MMP-2 and MMP-9 upon exposure of human breast cancer cells (MCF-7) to fibronectin in serum free medium

Interactions between tumour cells and the extracellular matrix (ECM) strongly influence tumour development, affecting cell survival, proliferation and migration. Many of these interactions are mediated through a family of cell surface receptors named integrins. Fibronectin and its integrin receptors play important roles in tumour development. The alpha5beta 1 integrin interacts with the central cell adhesive region of fibronectin and requires both the RGD and synergy sites for maximal binding. Matrix metalloproteinases (MMPs) are a family of zinc dependent endopeptidases. They are capable of digesting the different components of the ECM and basement membrane. The ECM gives structural support to cells and plays a central role in cell adhesion, differentiation, proliferation and migration. Binding of ECM to integrins modulates expression and activity of the different MMPs. Our experimental findings demonstrate that cultivation of human breast cancer cells, MCF-7, in serum free medium in the presence of fibronectin upregulates the activity of MMP-2 and MMP-9. Blocking of alpha5beta 1 integrin with anti-alpha5 monoclonal antibody inhibits the fibronectin-induced MMP activation response appreciably. This strongly indicates alpha5beta 1 mediated signalling events in activation of MMP-2 and MMP-9. Phosphorylation of FAK and PI-3 kinase and the nuclear translocation of ERK and NF-kappaB upon fibronectin binding demonstrate possible participation of the FAK/PI-3K/ERK signalling pathways in the regulation of MMP-2 activity.     

The oligodendrocyte precursor mitogen PDGF stimulates proliferation by activation of alpha(v)beta3 integrins

Central nervous system development requires precise and localized regulation of neural precursor behaviour. Here we show how the interaction between growth factor and integrin signalling pathways provides a mechanism for such precision in oligodendrocyte progenitor (OP) proliferation. While physiological concentrations of platelet-derived growth factor (PDGF) were not in themselves sufficient to promote OP proliferation, they did so on extracellular matrix (ECM) substrates that bind alpha(v)beta3 integrin. Upon PDGF-AA exposure and alpha(v)beta3 engagement, a physical co-association between both receptors was demonstrated, confirming the interaction between these signalling pathways. Furthermore, we found that PDGFalphaR stimulated a protein kinase C-dependent activation of integrin alpha(v)beta3, which in turn induced OP proliferation via a phosphatidylinositol 3-kinase-dependent signalling pathway. These studies establish a mechanism by which OP proliferation is dependent on the availability of both an ECM ligand and a mitogenic growth factor. Growth factor- mediated integrin activation is the critical integrative step in proliferation signalling, and ensures that the response of neural precursor cells to long-range cues can be regulated by their cellular neighbours, allowing precise control of cell behaviour during development.     

Oxidation of glycosaminoglycans by free radicals and reactive oxidative species: A review of investigative methods

Abstract

Glycosaminoglycans, in particular hyaluronan (HA), and proteoglycans are components of the extracellular matrix (ECM). The ECM plays a key role in the regulation of cellular behaviour and alterations to it can modulate both the development of human diseases as well as controlling normal biochemical processes such as cell signalling and pro-inflammatory responses. For these reasons, in vitro fragmentation studies of glycosaminoglycans by free radicals and oxidative species are seen to be relevant to the understanding of in vivo studies of damage to the ECM. A wide range of investigative techniques have therefore been applied to gain insights into the relative fragmentation effects of several reactive oxidative species with the ultimate goal of determining mechanisms of fragmentation at the molecular level. These methods are reviewed here.     

Cell signalling through thromboxane A2 receptors

Thromboxane A2 receptors (TPs) are widely distributed among different organ systems and have been localized on both cell membranes and intracellular structures. Following the initial cloning of this receptor class from human placenta, the deduced amino acid sequence predicted seven-transmembrane spanning regions, four extracellular domains and four intracellular domains, making TP a member of the seven-transmembrane G-protein-coupled receptor (GPCR) super family. A single gene on chromosome 19p13.3 leads to the expression of two separate TP isoforms: TPalpha which is broadly expressed in numerous tissues, and a splice variant termed TPbeta which may have a more limited tissue distribution. Mutagenesis, photoaffinity labelling, and immunological studies have indicated that the ligand binding domains for this receptor may reside in both the transmembrane (TM) and extracellular regions of the receptor protein. In addition, separate studies have provided evidence that this receptor can couple to at least four separate G protein families. As a consequence, TP signalling has been shown to result in a broad range of cellular responses including phosphoinositide metabolism, calcium redistribution, cytoskeletal arrangement, integrin activation, kinase activation, and the subsequent nuclear signalling events involved in DNA synthesis, cell proliferation, cell survival and cell death. While activation of these different signalling cascades can all derive from TP stimulation, the relative signalling preference for a given cascade appears to be both tissue and cell specific. Finally, separate studies have indicated that TP signalling capacity can be both down-regulated by protein kinase activation and up-regulated by GPCR cross-signalling. Thus, the multitude of signalling events which derive from TP activation can themselves be modulated by endogenous cellular messengers.     

Role of the extracellular matrix in cell-cell signalling: paracrine paradigms

The plant extracellular matrix (ECM) is complex and diverse, and is involved in cell-cell communication in a wide range of developmental, reproductive and pathogenic processes. Characterisation of integral ECM components is leading to improved understanding of their roles in signalling. Interactions between the extracellular domains of plant plasma membrane receptor kinases and their ligands are potentially regulated by the properties of the ECM. Several of these interactions, for example those involving the S-locus receptor kinase, are being characterised in some detail. Non-protein constituents are also implicated in regulating the movement of signalling molecules in the ECM, which is associated with developmental patterning. In contrast to the situation in animal cells, cytoskeleton-integrin-ECM signalling complexes appear not to be dominant features of signal transduction in plant cells. Nevertheless, structural adhesions between the plasma membrane and cell wall are important for a variety of functions.     

Mitogenic signal transduction by integrin- and growth factor receptor-mediated pathways

Engagement of cells with the extracellular matrix (ECM) proteins is crucial for various biological processes, including cell adhesion, spreading, proliferation, differentiation, migration, apoptosis, and gene induction, contributing to maintenance of tissue integrity, embryogenesis, wound healing, and the metastasis of tumor cells (Hynes, 2002b; Juliano, 2002). The engagement involves cell adhesion mediated by integrins, a large family of cell adhesion receptors that are transmembrane glycoproteins which bind to ECM or to counter-receptors on neighbor cells. In this review, the molecular basis of signaling mediated by integrins and their collaboration with growth factor receptors will be discussed, based on recent observations. Although other cell adhesion receptors including cadherins, selectins, syndecans, and the immunoglobulin superfamily of cell adhesion molecules (IgCAMs) can play important roles or be involved in these processes, we suggest readers refer to recent outstanding reviews on them (Barclay, 2003; Brummendorf and Lemmon 2001; Panicker et al. 2003).     

Tenectin is a novel αPS2βPS integrin ligand required for wing morphogenesis and male genital looping in Drosophila

Morphogenesis of the adult structures of holometabolous insects is regulated by ecdysteroids and juvenile hormones and involves cell-cell interactions mediated in part by the cell surface integrin receptors and their extracellular matrix (ECM) ligands. These adhesion molecules and their regulation by hormones are not well characterized. We describe the gene structure of a newly described ECM molecule, tenectin, and demonstrate that it is a hormonally regulated ECM protein required for proper morphogenesis of the adult wing and male genitalia. Tenectin's function as a new ligand of the PS2 integrins is demonstrated by both genetic interactions in the fly and by cell spreading and cell adhesion assays in cultured cells. Its interaction with the PS2 integrins is dependent on RGD and RGD-like motifs. Tenectin's function in looping morphogenesis in the development of the male genitalia led to experiments that demonstrate a role for PS integrins in the execution of left-right asymmetry.     

Integrin diversity brings specificity in mechanotransduction

Cells sense and respond to the biochemical and physical properties of the extracellular matrix (ECM) through adhesive structures that bridge the cell cytoskeleton and the surrounding environment. Integrin‐mediated adhesions interact with specific ECM proteins and sense the rigidity of the substrate to trigger signalling pathways that, in turn, regulate cellular processes such as adhesion, motility, proliferation and differentiation. This process, called mechanotransduction, influenced by the involvement of different integrin subtypes and their high ECM–ligand binding specificity, contributes to the cell‐type‐specific mechanical responses. In this review, we describe how the expression of particular integrin subtypes affects cellular adaptation to substrate rigidity. We then explain the role of integrins and associated proteins in mechanotransduction, focusing on their specificity in mechanosensing and force transmission.     

Ringing the alarm bells: signalling and apoptosis in influenza virus infected cells

Small RNA viruses such as influenza viruses extensively manipulate host-cell functions to support their replication. At the same time the infected cell induces an array of defence mechanisms to fight the invader. These processes are mediated by a variety of intracellular signalling cascades. Here we will review the current knowledge of functional kinase signalling and apoptotic events in influenza virus infected cells and how these viruses have learned to misuse these cellular responses for efficient replication.     

Mesenchymal stem cells,neural lineage potential,heparan sulfate proteoglycans and the matrix

Along with the tri-lineage of bone, cartilage and fat, human mesenchymal stem cells (hMSCs) retain neural lineage potential. Multiple factors have been described that influence lineage fate of hMSCs including the extracellular microenvironment or niche. The niche includes the extracellular matrix (ECM) providing structural composition, as well as other associated proteins and growth factors, which collectively influence hMSC stemness and lineage specification. As such, lineage specific differentiation of MSCs is mediated through interactions including cell–cell and cell–matrix, as well as through specific signalling pathways triggering downstream events. Proteoglycans (PGs) are ubiquitous within this microenvironment and can be localised to the cell surface or embedded within the ECM. In addition, the heparan sulfate (HS) and chondroitin sulfate (CS) families of PGs interact directly with a number of growth factors, signalling pathways and ECM components including FGFs, Wnts and fibronectin. With evidence supporting a role for HSPGs and CSPGs in the specification of hMSCs down the osteogenic, chondrogenic and adipogenic lineages, along with the localisation of PGs in development and regeneration, it is conceivable that these important proteins may also play a role in the differentiation of hMSCs toward the neuronal lineage. Here we summarise the current literature and highlight the potential for HSPG directed neural lineage fate specification in hMSCs, which may provide a new model for brain damage repair.     

Cytokine receptors and signal transduction

Cytokines are important regulators of hemopoiesis which exert their actions by binding to specific, high affinity, cell surface receptors. In the past several years, molecular cloning of these receptors has revealed a new superfamily referred to as the hemopoietic growth factor receptors. Members of this family are defined by a 200 amino acid conserved domain; however, it has become increasingly apparent that another characteristic of these receptors is the shared usage of a common signalling subunit among subgroups in this family. The shared signalling component explains the functional redundancy of many cytokines; however, the mechanism by which these receptors transduce a signal across the membrane is not yet clear. Studies into cytokine action have shown that many of the events that occur in response to ligand stimulation are similar to those observed for the better characterized intrinsic tyrosine kinase receptors. Thus, although the cytokine receptors do not possess intrinsic tyrosine kinase activity, these observations have led to a model of cytokine signal transduction adapted from the signalling mechanisms described for the tyrosine kinase receptors.     

Syndecans play dual roles as cell adhesion receptors and docking receptors

Syndecan are a family of cell surface heparan sulfate proteoglycans that act as cell surface receptors. Most cell surface receptors have a limited number and type of ligand interactions, responding only to the binding of (a) specific ligand(s). In contrast, syndecans can interact with various numbers and types of ligands, and thus play more diverse roles than others. Various syndecan functions have not yet been fully classified and categorized, but we herein review previous studies suggesting that syndecans play dual function as cell surface receptors by acting as both adhesion receptors and docking receptors. Through this dual regulatory function, syndecans are capable of regulating both intra- and extracellular activities, potentially altering a variety of cell behaviors.     

Integrin receptors: the dynamic modulators of endometrial function

Cell-cell and cell-extracellular matrix (ECM) interactions play a critical role in various developmental processes, including differentiation, proliferation and migration of cells. ECM proteins can influence cellular function thus creating a complex feedback mechanism. The adhesion of cells to each other, their ECM proteins and endothelial surfaces is mediated by a variety of membrane proteins collectively known as adhesion molecules. Adhesion molecules have been further divided into five subfamilies, the integrins, the selectins, the cadherins, the mucins and the immunoglobulin superfamily. Members of the integrin family of cell surface adhesion receptors are important mediators of cell-ECM contact. Integrin receptors are alpha beta heterodimers with a transmembrane segment, a short cytoplasmic domain and a large extracellular domain. The role of integrins in reproduction has been established. Several reasons make these molecules very attractive due to their constant involvement from egg to birth. They participate in sperm-egg interaction, fertilization, implantation and placentation in many species including humans. Integrins provide signals to individual cells essential for growth and development of different tissues. In the present review, we describe (1) the regulatory pathways for controlling expression of integrins in the endometrium, (2) various biomarkers and their role in endometrial function, (3) reproductive disorders in women related to aberrant integrin expression in the endometrium and (4) the functional significance of integrins available from gene knockout studies.     

Protease-activated receptors: a means of converting extracellular proteolysis into intracellular signals

Protease-activated receptors (PARs) mediate cellular responses to a variety of extracellular proteases. The four known PARs constitute a subgroup of the family of seven-transmembrane domain G protein-coupled receptors and activate intracellular signalling pathways typical for this family of receptors. Activation of PARs involves proteolytic cleavage of the extracellular domain, resulting in formation of a new N terminus, which acts as a tethered ligand. PAR-1, -3, and -4 are relatively selective for activation by thrombin whereas PAR-2 is activated by a variety of proteases, including trypsin and tryptase. Recent studies in mice genetically incapable of expressing specific PARs have defined roles for PAR-1 in vascular development, and for PAR-3 and -4 in platelet activation, which plays a fundamental role in blood coagulation. PAR-1 has also been implicated in a variety of other biological processes including inflammation, and brain and muscle development. Responses mediated by PAR-2 include contraction of intestinal smooth muscle, epithelium-dependent smooth muscle relaxation in the airways and vasculature, and potentiation of inflammatory responses. The area of PAR research is rapidly expanding our understanding of how cells communicate and control biological functions, in turn increasing our knowledge of disease processes and providing potential targets for therapeutic intervention.     

Clustering of adhesion receptors following exposure of insect blood cells to foreign surfaces

Cell-mediated immune responses of insects involve interactions of two main classes of blood cells (hemocytes) known as granular cells and plasmatocytes. In response to a foreign surface, these hemocytes suddenly transform from circulating, non-adherent cells to cells that interact and adhere to each other and the foreign surface. This report presents evidence that during this adhesive transformation the extracellular matrix (ECM) proteins lacunin and a ligand for peanut agglutinin (PNA) lectin are released by granular cells and bind to surfaces of both granular cells and plasmatocytes. ECM protein co-localizes on cell surfaces with the adhesive receptors integrin and neuroglian, a member of the immunoglobulin superfamily. The ECM protein(s) secreted by granular cells are hypothesized to interact with adhesion receptors such as neuroglian and integrin by cross linking and clustering them on hemocyte surfaces. This clustering of receptors is known to enhance the adhesiveness (avidity) of interacting mammalian immune cells. The formation of ring-shaped clusters of these adhesion receptors on surfaces of insect immune cells represents an evolutionary antecedent of the mammalian immunological synapse.     

Integrin-mediated regulation of neurovascular development,physiology and disease

The mammalian central nervous system (CNS) is comprised of billions of neurons and glia that are intertwined with an elaborate network of blood vessels. These various neural and vascular cell types actively converse with one another to form integrated, multifunctional complexes, termed neurovascular units. Cell-cell communication within neurovascular units promotes normal CNS development and homeostasis, and abnormal regulation of these events leads to a variety of debilitating CNS diseases. This review will summarize (i) cellular and molecular mechanisms that regulate physiological assembly and maintenance of neurovascular units; and (ii) signaling events that induce pathological alterations in neurovascular unit formation and function. An emphasis will be placed on neural-vascular cell adhesion events mediated by integrins and their extracellular matrix (ECM) ligands. I will highlight the role of a specific adhesion and signaling axis involving αvβ8 integrin, latent transforming growth factor β’s (TGFβ’s), and canonical TGFβ receptors. Possible functional links between components of this axis and other signal transduction cascades implicated in neurovascular development and disease will be discussed. In summary, comprehensively understanding the pathways that regulate bidirectional neural-vascular cell contact and communication will provide new insights into the mechanisms of neurovascular unit development, physiology and disease.     

Matrix metalloproteinase stromelysin-3 in development and pathogenesis

The extracellular matrix (ECM) serves as a medium for cell-cell interactions and can directly signal cells through cell surface ECM receptors, such as integrins. In addition, many growth factors and signaling molecules are stored in the ECM. Thus, ECM remodeling and/or degradation plays a critical role in cell fate and behavior during many developmental and pathological processes. ECM remodeling/degradation is, to a large extent, mediated by matrix metalloproteinases (MMPs), a family of extracellular or membrane-bound, Zn2+-dependent proteases that are capable of digesting various proteinaceous components of the ECM. Of particular interest among them is the MMP11 or stromelysin-3, which was first isolated as a breast cancer associated protease. Here, we review some evidence for the involvement of this MMP in development and diseases with a special emphasis on amphibian metamorphosis, a postembryonic, thyroid hormone-dependent process that transforms essentially every organ/tissue of the animal.