EFB325 Cell Physiology

Cell-cell junctions

In the process of building a multicellular organism, cells are attached to each other in specific ways in the process of building tissues and organs. The organization of cells into "sheets" is also very important in the process of vertebrate development. Through development and also as tissues are maintained in a mature organism, cells express informational and junction molecules on their surface, so that cells recognize and adhere to like cells. In this lecture, we will explore the molecules involved in forming cell-cell adhesion and mechanical junctions.

Cell-cell junctions can be classified into three functional types

• occluding junctions: function to seal cells together in an epithelial sheet
• adhesive junctions: mechanically attach cells to one another or to the ECM
• communicating junctions: allow direct passage of chemical or electrical signals from cell to cell

Occluding junctions

Tight junctions function to form a seal between adjacent cells and also define the apical and basal sides of an epithelial cell

• direct connections between PMs of adjacent cells formed by membrane proteins (claudins and occludins)
• especially important in intestine, bladder, glands
• tight junction segregates active and passive glucose transporters for directional glucose uptake from the lumen of the gut (see Figure 12-15 on page 400)

Adhesive junctions

• hold cells in fixed position
• prominent in epithelial tissues
• associated with cytoskeleton

Adherens junctions and desmosome junctions rely on associations between cadherin proteins

• cadherins cross the plasma membrane and are linked to the cytoskeleton by linker proteins (in this respect, they function like the integrins)
• different cell types produce different types of cadherin proteins
• a cadherin protein will interact only with an identical type of cadherin (homophilic binding)

Adherens junctions join actin bundles in one cell to actin bundles in an adjoining cell

• cadherins linked to actin microfilaments extend out of the cell and bind to cadherins of an adjoining cell
• adherens junctions are often in a continuous belt around the cell=adhesion belt
• can also be in a bundle
• found in heart and thin cell layers covering organs or lining cavities
• the actin microfilaments can contract, thus adherens junctions can be involved in coordinating invaginations during development

Desmosome junctions link the intermediate filaments of adjoining cells

• cadherins are linked to keratin intermediate filaments inside the cell, then extend across the PM to associate with identical cadherins of an adjacent cell
• the cadherins and keratin filaments are anchored to a dense mixture of attachment proteins called a plaque, making them function like snaps between cells
• abundant in skin, heart, neck of uterus where they are needed to withstand mechanical stress
• also important in maintaining cell position during development

Hemidesmosomes link keratin intermediate filaments to the basal lamina

• integrins are linked to keratin filaments inside the cell and extend across the PM to bind to laminin in the basal lamina - NOTE - these are connections between an epithelial cell and the ECM, NOT cel-to-cell junctions

Communicating junctions

• direct connections between the cytoplasms of adjoining cells can allow exchange of small molecules as signals

Gap junctions form channels across the PMs of adjoining animal cells

• a hexamer complex of proteins (called a connexon) in each of the two adjacent cells are aligned, forming an aqueous channel between them
• the cell can regulate the opening of the channel, but rapid diffusion of small molecules and ions is usually allowed
• common in smooth & cardiac muscle
• blocking gap junctions can disrupt development

Plasmodesmata connect nearly all plant cells to their neighboring cells

• tube-like channel through the cell wall that allows the movement of metabolites, ions, hormones, even proteins and RNA between adjacent cells
• continuous connection of the cytoplasms of adjoining cells
• central channel is formed by the desmotubule (extension of the ER)
• desmotubule diameter can vary from 20-200 nm
• desmotubule is surrounded by cytoplasm-annulus
• pore through the cell wall is lined with PM
• function is very similar to that of gap junctions in animal cells

Plasmodesmata also allow the movement of viruses through the plant

• proteins made by plant viruses can change the diameter of the pore, allowing the virus to move between cells
• researchers are studying this mechanism to try to prevent the movement of virus, to make virus-resistant plants

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