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BIO CH19.docx

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Kenneth Yip

BIO CH.19 Cell Junctions, CellAdhesion, and the Extracellular Matrix • Most fundamental interactions between cells are those that hold the cells together. This can occur through, cell-cell junctions, or even by extracellular materials that they excrete. • Junctions create pathways for communications.Adhesions can control the shape of the cell. • 2 ways in which stresses can be transmitted across a multicellular structure. One strategy depends on the extracellular matrix. The other strategy depends on the strength of the cytoskeleton inside the cells and on cell-cell adhesions. • In plants the strength is due to the ECM. In animals, both architectural strategies are used. • Connective tissue: Any supporting tissue that lies between other tissues and consists of cells embedded in a relatively large amount of extracellular matrix. Includes bone, cartilage, and loose connective tissue. Rich in collagen. The matrix, rather than cells, that bears most of the stress. • Animal tissues tend to fall into 2 categories: connective tissue and epithelial tissue. • Epithelial tissue: Includes the lining of the gut or the epithermal covering the skin. Often includes the basal lamina. • Epithelia: Sheet of cells covering the outer surface of a structure or lining a cavity. • Epithelial cells connected to the basal lamina by anchors. • 4 main junctions can be distinguished. • Anchoring junction: Cell junction attaches cells to neighbouring cells or to the ECM. • Occluding junction: type of cell junction that seals cells together in an epithelium, forming a barrier through which even small molecules cannot pass – making the cell sheet an impermeable (or selectively permeable) membrane. • Channel-forming junction: Cell-cell junction that links the cytoplasm of adjacent cells and provides a passageway for small molecules and ions to pass from cell to cell. In animal tissues, composed of connexin or innexin proteins. In plants, a similar function is performed by plasmodesmata. • Signal-relaying junction: Complex type of cell-cell junction that allows signals to be relayed from one cell to another across their plasma membranes at sites of cell-to-cell contact. Typically includes anchorage proteins as well as proteins mediating signal transduction. • For signal-relaying junctions, the cell membranes must be held in contact with one another for the ligands to activate the receptors. Cadherins and Cell-CellAdhesion • Tight junction, adherin junctions, and desmosomes make up the junctional complex. • Tight junctions seal gaps between epithelial cells. Type of occluding junction. • Cell-cell anchoring junctions include adherens and desmosomes.Adherens junctions connect actin filament bundles in one cell with that in the next cell. Desmosomes connect intermediate filaments in one cell to those in the next cell. • Channel forming junctions, an example would be gap junctions that allow the passage of small water-soluble molecules from cell-to-cell. • Cell-matrix anchoring junctions would include actin-linked cell-matrix adhesions that anchors actin filaments in cell to the ECM as well as hemidesmosomes that anchor intermediate filaments in a cell to the ECM. • Adherens junction: Cell junction in which the cytoplasmic face of the plasma membrane is attached to actin filaments. Examples include adhesion belts linking adjacent epithelial cells and focal contacts on the lower surface of cultures fibroblasts. • Desmosome: Anchoring cell-cell junction, usually formed between 2 epithelial cells. Characterized by dense plaques of protein into which intermediate filaments in the 2 adjoining cells insert. • At each of the 4 types of anchoring junctions, the central role is played by transmembrane adhesion proteins that span the membrane, with one end linking to the cytoskeleton inside the cell and the other end linking to other structures outside it. • Cadherin: Member of the large cadherins superfamily of transmembrane adhesion proteins. Mediates hemophilic Ca2+ dependent cell-cell adhesion in animal tissues. • Integrin: Transmembrane adhesion protein that is involved in the attachment of cells to the ECM and to each other. • Within each family of cadherins or integrins there is specializations. Some cadherins link to actin and form adherens junctions, while others link to intermediate filaments and form desmosomes. Cadherins Mediate Ca2+ -Dependent Cell-CellAdhesion inAllAnimals • So far, cadherins have been present in all multicellular animals whose genomes have been sequenced. • Some fungi and plants lack cadherins. • The cadherins take their name from their dependence on Ca2+ ions: removing them from the extracellular medium causes adhesions mediated by cadherins to come adrift. • After adhesions are broken, they can be re-formed if put back into a normal medium. • Cadherin is the main adhesion molecules holding cells together in early embryonic tissues. The Cadherin Superfamily in Vertebrates Includes Hundreds of Different Proteins, Including Many with Signaling Functions • E-cadherins are present mainly on epithelial cells, P-cadherins are present mainly in the placenta and epidermis, and N-cadherins are mainly present on nerve, muscle, and lens cells. • These are classical cadherins, they are closely related in sequence throughout their extracellular and intracellular domains. • They are also important for signalling. • Alarge number of nonclassical cadherins are more distantly related in sequence. Cadherins Mediate HomophilicAdhesion • Homophilic binding: Binding between molecules of the same kind, especially those involved in cell-cell adhesion. • Homophilic binding as in actin binds to actin, intermediate filaments bind to intermediate filaments. • Binding occurs at N-terminal tips of cadherin molecules (furthest from the membrane.) Bind through insertion of knob into pocket. • The binding allows for the cadherin domain to remain a rigid, slightly curved, rod. When Ca2+ is removed, the hinges can flex, and the binding becomes sloppy. Cadherins that lose their Ca2+ get rapidly degraded. • Cadherins bind each other with
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