Cell Junctions PDF

Cell Junctions Cell Biology Cell Junctions Cell junctions perform the function of mediating cellular adhesion A tissue’s proper functioning depends critically on how the individual cells are arranged within it and the nature of physical connections between cells....

Cell Junctions Cell Biology Cell Junctions Cell junctions perform the function of mediating cellular adhesion A tissue’s proper functioning depends critically on how the individual cells are arranged within it and the nature of physical connections between cells. Cell-cell junctions Cell-matrix junctions Cell Biology Cell Junctions Cell junctions can be characterized by The visible structure under the microscope Proteins involved in the junction Cell Biology Cell Junctions Three functional groups of cell junctions… Occluding junctions Anchoring junctions Communicating junctions Cell Biology Occluding junctions Seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other. Form a selectively permeable barrier across epithelial cell sheets 1. Tight junctions (vertebrates only) 2. Septate junctions (invertebrates mainly) Cell Biology Occluding Junctions-Tight Junctions Unique to vertebrates Predominantly composed of two proteins called claudins and occludins They tightly hold the cells against each other A tight junction is a water-tight seal between two adjacent animal cells that prevents materials from leaking between the cells Cell Biology Occluding Junctions-Tight Junctions Each tight junction is composed of a long row of transmembrane adhesion proteins embedded in each of the two interacting plasma membranes. The extracellular domains of these proteins join directly with one another to occlude the intercellular space Cell Biology Occluding Junctions-Tight Junctions  Typically found in epithelial tissues that line internal organs and cavities  Ex 1: The tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space  They serve as selectively permeable barriers, separating fluids on either side that has a different chemical composition.  This function requires that the adjacent cells be sealed together by occluding junctions. Cell Biology  Occluding Junctions-Tight Junctions Ex 2: The epithelial cells lining the small intestine form a barrier that keeps the gut contents in the gut cavity Yet, the cells must transport selected nutrients across the epithelium from the lumen into the extracellular fluid that permeates the connective tissue on the other side These nutrients then diffuse into small blood vessels to provide nourishment to the organism. This depends on two sets of membrane-bound membrane transport proteins. Cell Biology  Occluding Junctions-Tight Junctions One set is confined to the epithelial cell surface facing the lumen (apical) and actively transports selected molecules into the cell from the gut. The other set is confined to the basolateral surfaces of the cell, and it allows the same molecules to leave the cell by facilitated diffusion into the extracellular fluid. Cell Biology Occluding Junctions-Tight Junctions To maintain this directional transport, the spaces between epithelial cells must be tightly sealed, so that the transported molecules cannot diffuse back through these spaces The tight junctions between epithelial cells perform this function. Cell Biology Occluding Junctions-Tight Junctions Tight junctions seal neighbouring cells together All tight junctions are impermeable to macromolecules Permeability to small molecules varies greatly in different epithelia. Ex: Tight junctions in the epithelium lining of small intestine are 10,000 times more permeable to inorganic ions, such as Na+ than the tight junctions in the epithelium lining of urinary bladder. These differences are because of tight junction proteins that form the junctions. Cell Biology Occluding Junctions - Septate Junctions The main occluding junction in invertebrates Plasma membranes are joined by proteins that are arranged in parallel rows with a regular periodicity The proteins found in these junctions in different systems/organisms have different names Ex: In Drosophila - Neurexin protein Cell Biology Anchoring Junctions Appear to have been the first to evolve. Primitive forms of anchoring junctions can even be found in a primitive organisms like sponges Found in all animal species. They mechanically attach the cytoskeleton of a cell to the cytoskeletons of other cells or to the extracellular matrix. Cell Biology Anchoring Junctions Actin filament attachment sites 1. cell-cell junctions (adherens junctions) 2. cell-matrix junctions (focal adhesions) Intermediate filament attachment sites 1. cell-cell junctions (desmosomes) 2. cell-matrix junctions (hemidesmosomes) Cell Biology Anchoring Junctions The lipid bilayer is delicate and cannot transmit large forces from cell to cell or from cell to extracellular matrix. Anchoring junctions solve the problem by forming a strong membrane- spanning structure that is bound inside the cell to the tension-bearing filaments of the cytoskeleton These junctions are found in tissues subject to mechanical stress Ex: muscle and skin epithelium Cell Biology Anchoring Junctions Adherens junctions and desmosomes hold cells together and are formed by transmembrane adhesion proteins that belong to the cadherin family. Focal adhesions and hemidesmosomes bind cells to the extracellular matrix and are formed by transmembrane adhesion proteins of the integrin family. On the intracellular side of the membrane adherens junctions and focal adhesions serve as connection sites for actin filaments Desmosomes and hemidesmosomes serve as connection sites for intermediate filaments Cell Biology Anchoring Junctions - Adherens junctions Based on the protein cadherin, which is a Ca2+ dependent adhesion molecule. Cadherin is a transmembrane protein with an extracellular domain that can interact with the extracellular domain of a cadherin in an adjacent cell to join the cells together. Adherens junctions are found in animals ranging from jellyfish to vertebrates. Cell Biology Anchoring Junctions - Desmosomes Found only in animals A cadherin-based junction unique to vertebrates. They contain the cadherins desmocollin and desmoglein, which interact with intermediate filaments of cytoskeletons. Through desmosomes, the intermediate filaments of adjacent cells are linked into a net that extends throughout the many cells of a tissue These connections support tissues against mechanical stress. Ex: the skin, heart, and muscles. Cell Biology Anchoring Junctions - Desmosomes Cell Biology  Anchoring Junctions - Focal Adhesions Enable cells to get a hold on the extracellular matrix through integrins that link to actin filaments in the cell. The extracellular domains of transmembrane integrin proteins bind to a protein component of the extracellular matrix Intracellular domains bind indirectly to bundles of actin filaments via the intracellular anchor proteins talin, α-actinin, filamin, and vinculin Cell Biology Anchoring Junctions - Hemidesmosomes Resemble desmosomes morphologically and in connecting to intermediate filaments Hemidesmosomes connect the basal surface of an epithelial cell to the underlying extracellular matrix The extracellular domains of the integrins bind to a laminin protein in the basal lamina Act as rivets to distribute tensile or shearing forces through an epithelium. Cell Biology Anchoring Junctions - Hemidesmosomes Cell Biology Communicating Junctions Mediate the passage of chemical or electrical signals from one interacting cell to its partner. 1. Gap junctions 2. Chemical synapses 3. Plasmodesmata (plants only) Cell Biology Communicating Junctions Multicellular organisms have communicating junctions. These junctions allow communication between cells by diffusion through small openings. Communicating junctions permit small molecules or ions to pass from one cell to the other. In animals - gap junctions In plants - plasmodesmata Cell Biology Communicating Junctions - Gap Junctions Found in both invertebrates and vertebrates. In invertebrates they are formed by proteins known as pannexins. Vertebrates have pannexin-base gap junctions as well as junctions formed by proteins called connexins. In each case, a structure is formed by complexes of six identical transmembrane proteins Cell Biology Communicating Junctions - Gap Junctions The proteins are arranged in a circle to create a channel through the plasma membrane that protrudes from the cell surface. Gap junction forms when the connexons of two cells align perfectly, creating an open channel that spans the plasma membranes of both cells. Cell Biology Communicating Junctions - Gap Junctions Gap junctions develop when a set of six proteins (connexins) in the plasma membrane arrange themselves in a connexon. When the connexon's pores in adjacent animal cells align, a channel between the two cells forms. Gap junctions are particularly important in cardiac muscle. The electrical signal for the muscle to contract efficiently through gap junctions Cell Biology Communicating Junctions - Gap Junctions Gap junctions provide passageways large enough to permit small substances, such as simple sugars and amino acids, to pass from one cell to the next. But are small enough to prevent the passage of larger molecules, such as proteins. Gap junction channels are dynamic structures that can open or close in response to a variety of factors, including Ca2+ and H+ ions. Cell Biology Plasmodesmata In plants, cell walls separate every cell from all others. Cell–cell junctions occur only at holes or gaps in the walls, where the plasma membranes of adjacent cells can meet one another. Cytoplasmic connections that form across the touching plasma membranes are called plasmodesmata Plasmodesmata connect their cytoplasm, and enable transport of materials from cell to cell, and thus throughout the plant Cell Biology Thank You! Cell Biology