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Lecture 6

DEV2011: Lecture 6 summary

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LECTURE 6 5 Cell Junctions: A cell junction is a type of structure that exists within the tissue of some multicellular organisms. Cell junctions consist of multiprotein complexes that provide contact between neighboring cells or between a cell and the extracellular matrix. Cell junctions are especially abundant in epithelial tissues. Cell junctions are especially important in enabling communication between neighboring cels via specialized proteins called communication junctions. Cell junctions are also important in reducing stress placed upon cells. In vertebrates, there are 3 major types of cell junctions: - Adherens junctions, desmosomes and hemidesmosomes (anchoring junctions) - Gap junctions (communicating junction) - Tight junctions (occluding junctions) Anchoring junctions: Cells within tissues and organs must be anchored to one another and attached to components of the extracellular matrix. Cells have developed several types of junctional complexes to serve these functions, and in each case, anchoring proteins extend through the plasma membrane to link cytoskeletal proteins in one cell to cytoskeletal proteins in neighbouring cells as well as to proteins in the extracellular matrix. Junction Cytoskeletal Transmembrane Ties Cell To: Anchor Linker Desmosomes Intermediate Cadherin Other Cells filaments Hemidesmosomes Intermediate Integrins EC Matrix filaments Adherens Actin filaments Cadherin/Intergrins Other cells/EC Junctions matrix Anchoring-type junctions not only hold cells together but also provide tissues with structural cohesion. These junctions are most abundant in tissues that are subject to constant mechanical stress such as skin and heart. Desmosomes: Desmosomes can be visualized as rivets through the plasma membrane of adjacent cells. Intermediate filaments composed of keratin or desmin are attached to membrane-associated attachment proteins that form a dense plaque on the cytoplasmic face of the membrane. Cadherin molecules form the actual anchor by attaching to the cytoplasmic plaque, extending through the membrane and binding strongly to cadherins coming through the membrane of the adjacent cell. Hemidesmosomes: Hemidesmosomes form rivet-like links between cytoskeleton and extracellular matrix components such as the basal laminae that underlie epithelia. Like desmosomes, they tie to intermediate filaments in the cytoplasm, but in contrast to desmosomes, their transmembrane anchors are integrins rather than cadherins. Adherens Junctions: Adherens junctions share the characteristic of anchoring cells through their cytoplasmic actin filaments. Similarly to desmosomes and hemidesmosomes, their transmembrane anchors are composed of cadherins in those that anchor to other cells and integrins in those that anchor to extracellular matrix. There is considerable morphologic diversity among adherens junctions. The band-type of adherens junctions is associated with bundles of actin filaments that also encircle the cell just below the plasma membrane. Spot-like adherens junctions help cells adhere to extracellular matrix both in vivo and in vitor where they are called adhesion plaques. The cytoskeletal actin filaments that tie into adherens junctions are contractile proteins and in addition to providing an anchoring function, adherens junctions are through to participate in folding and bending of epithelial cell sheets. Communicating (GAP) Junctions: Communicating junctions, or gap junctions allow for direct chemical communication between adjacent cellular cytoplasm through diffusion without contact of the extracellular fluid. This is possible due to six connexin proteins, interacting to form a cylinder with a pore in the centre. This protrudes across the cell memebrane, and when 2 adjacent cell connexons interact, they form the gap junction channel. Connexon pores cary in size, polarity, and when 2 adjacent cell connexons interact, they form the gap junction channel. Whilst variation in gap junction channels do occur, their structure remains relatively standard, and this interaction ensures efficient communication without the escape of molecules of ions to the extracellular fluid. Tight Junctions: Found in vertebrate epithelia, tight junctions act as barriers that regulate the movement of water and solutes between epithelial layers. Tight junctions are classified as a paracellular barrier, which is defined as not having directional discrimination, however movement is largely dependent upon solute size and charge. Physiological pH plays a part in the selectivity of solutes passing through tight junctions with most tight junctions being slightly selective for cations. Tight junctions present in different types of epithelia are selective for solutes for differing size, charge, and polarity. Cell Adhesion Molecule (CAM): Cell adhesion molecules are proteins located on the cell surface involved in binding with other cells or with the extracellular matrix in the process called cell adhesion. Essentially, cell adhesion molecules help cells stick to each other and to their surroundings. These proteins are typically transmembrane receptors and are composed of three domains: - and intracellular domain that interacts with the cytoskeleton - a transmembrane domain - an extracellular domain that interacts either with other CAMs of the same kind (hemophilic binding) or with other CAMS or the extracellular matrix (heterophilic binding). Families of CAMs: Most of the CAMs belong to four protein families: - Ig (immunoglobulin) superfamily (IgSF CAMs) - the integrins - the cadherins - the selectins Calcium-dependent: - IgSF CAMs Calcium-depedent: - Integrins - Cadherins - Selectins IgSF CAMs: Immunoglobulin superfamily CAMs (IgSF CAMs) are either hemophilic or heterophilic and bind integrins or different IgSF CAMs. Integrins: Integrins are one of the major classes of receptors within the ECM and mediate cell-ECM interactions with collagen, fibrinogen, fibronectin and vitronecti. Integrins provide essential links between the extracellular enviro
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