Lecture 16
Cell Junctions and the extracellular matrix
The key features of cell junctions and the extracellular matrix are best illustrated by considering two broad categories of tissues that are found in all animals:
Connective tissues:
Such as bone or tendon, are formed from an extracellular matrix produced by cells that are distributed sparsely in the matrix.
It is the matrix, rather than the cells, that bears most of the mechanical stress to which the tissue is subjected.
Direct attachments between one cell and another are relatively rare, but the cells have important attachments to the matrix.
These cell-matrix junctions link the cytoskeleton to the matrix, allowing the cells to move through the matrix and monitor changes in its mechanical properties.
Epithelial tissues:
Such as the lining of the but or the epidermal covering of the skin, cells are tightly bound together. into sheets called epithelia.
The extracellular matrix is less pronounced, consisting mainly of a thin mat called the basal lamina underlying the sheet.
Within the epithelium, cells are attached to each other directly by cell-cell junctions, where cytoskeletal filaments are anchored, transmitting stresses across the interiors of the cell, from adhesion to adhesion site.
Two types of anchoring junctions link the cytoskeletons of adjacent epithelial cells:
Adherens junctions: anchorage sites for actin filaments
Desmosomes: anchorage sites for intermediate filaments.
Two additional types of anchoring junctions link the cytoskeleton of the epithelial cells to the basal lamina:
Actin-linked cell-matrix junctions: anchor actin filaments to the matrix
Hemidesmosomes: anchor intermediate filaments to it.
Other various cell junctions
Tight junctions: hold the cells closely together near the apex, sealing the gap between the cells and thereby preventing molecules from leaking across the epithelium.
Gap junction: allows the passage of small water-soluble molecules from cell to cell. They are channels that bridge two cells.
Tight junctions contain strands of transmembrane adhesion proteins
When tight junctions are visualized, they are seen as a branching network of sealing strands that completely encircles the apical end of each cell in the epithelial sheet.
Each strand 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 adhere directly to one another to occlude the intercellular space.
The main transmembrane proteins forming these are the claudins, which are essential for tight-junction formation and function.
Normal tight junctions also contain a second major transmembrane protein called occludin, which is not essential for the assembly or structure of the tight junction, but is important for limiting junctional permeability.
The prototypical examples of adherens junctions occur in epithelia, where they often form a continuous adhesion belt just beneath the apical face of the epithelium, encircling each of the interacting cells in the sheet.
Its most obvious feature is a contractile bundle of actin filaments running along the cytoplasmic surface of the junctional plasma membrane.
Cadherins for a diverse family of adhesion molecules
Cadherins take their name from their dependence on Ca2+ ions
Removing Ca2+ from the extracellular medium causes adhesions mediated by cadherins to come apart.
E-cadherin is present in many types of epithelial cells; N-cadherin on nerve, muscle, and lens cells; and P-cadherin on cells in the placenta and epidermis.
Cadherins mediate homophilic adhesion
Anchoring junctions between cells are usually symmetrical.
The binding between cadherins is generally homophilic (like-to-like): cadherin molecules of a specific subtype on one cell bind to cadherin molecules of the same or closely related subtype on adjacent cells.
Homophilic binding occurs at the N-terminal tips of the cadherin molecules.
Each cadherin unit forms a more-or-less rigid unit, joined to the next cadherin domain by a hinge.
Ca2+ ions bind to sites near each hinge and prevent it from flexing, so that the whole string of cadherin domains behaves as a rigid and slightly curved rod.
When Ca2+ is removed, the hinges can flex, and the structure becomes floppy. At the same time, the conformation at the N-terminus is thought to change slightly, weakening the binding affinity for the matching cadherin molecule on the opposing cell.
Cadherins typically bind to their partners with relatively low affinity.
Strong attachments result from the formation of many weak bonds in parallel.
When binding to oppositely oriented partners on another cell, cadherin molecules are often clustered side-to-side with many other cadherin molecules on the same cell.
Catenins link cadherins to the actin cytoskeleton
The intracellular domains of cadherins interact with filaments of the cytoskeleton: actin at adherens junctions and intermediate filaments at desmosomes.
These cytoskeletal linkages are essential for efficient cell-cell adhesion, as cells that lack their cytoplasmic domains cannot stably hold cells together.
The linkage of cadherins to the cytoskeleton is indirect and depends on adaptor proteins that assemble on the cytoplasmic tail of the cadherin.
At adherens junctions, the cadherin tail binds β-catenin and p120-catenin. A third protein called α-catenin interacts with β-catenin.
Remember that β-catenin is also the transcription factor for the Wnt pathway
Cadherin-dependent cell-cell adhesion guides the organization of developing tissue
Cadherins mediate highly selective recognition, enabling cells of a similar type to stick together and stay segregated from other types of cells.
Experiments were done in which amphibian embryos were dissociated into single cells, and then those cells were mixed up and allowed to reassociate.
The dissociated cells often reassembled into structures resembling those of the original embryo.
These experiments revealed that selective cell-cell recognition systems make cells of the same differentiated tissue preferentially adhere to one another.
Cadherins play a crucial part in these cell-sorting processes during development.
Homophilic binding of cadherins controls these processes of tissue segregation.
Cells in culture can sort themselves out according to the type and level of cadherins they express. For example, cells expressing N-cadherin will sort out from cells expressing E-cadherin.
Cells expressing different amounts of the same cadherin will also sort out. The cells expressing high levels adhere more strongly and end up internally.
In vertebrate embryo, changes in cadherin expression are seen when the neural tube forms and pinches off from the overlying ectoderm.
Neural tube cells lose E-cadherin and acquire other cadherins, including N-cadherin, while the cells in the overlying ectoderm continue to express E-cadherin.
E-cadherin and N-cadherin want to push each other away, which is what causes the neural tube to close.
Desmosomes give epithelia mechanical strength
Desmosomes are structurally similar to adherens junctions but contain specialized cadherins that link to intermediate filaments instead of actin filaments.
Their main function is to provide mechanical strength.
Desmoglein and desmocollin are the cadherin-family membranes that serve as the transmembrane receptors.
Desmosomes typically appear as button-like spots of adhesion, riveting the cells together.
Inside the cell, the bundles of ropelike intermediate filaments that are anchored to the desmosomes form a structural framework of great tensile strength.
Plakoglobin and desmoplakin serve as the intracellular anchoring proteins.
The importance of desmosomes is demonstrated by some forms of the potentially fatal skin disease pemphigus.
Affected individuals make antibodies against one of their own desmosomal cadherin proteins.
These antibodies bind to and disrupt the desmosomes that hold their epidermal cells together.
This results in severe blistering of the skin, with leakage of body fluids into the loosened epithelium.
Integrins are transmembrane heterodimers that link the extracellular matrix to the cytoskeleton
Focal adhesion: actin-linked cell-matrix adhesion anchors actin filaments in cell to extracellular matrix
Integrins are the receptors that cluster at focal adhesions.
An integrin molecule is composed of two noncovalently associated glycoprotein subunits called α and β. Both span the cell membrane.
The extracellular domains bind to specific amino acid sequence motifs in extracellular matrix proteins.
The intracellular portion of an integrin dimer binds to a complex of several different proteins, which together form a linkage to actin filaments in the cytoskeleton.
A large adaptor protein called talin is a component of the linkage in many cases.
Vinculin marks focal adhesions where bundles of actin filaments terminate at the plasma membrane, connecting to receptors that bind the extracellular matrix.
Vinculin binds talin at the end of the actin filaments.
Humans contain many different β-chain genes and α-chain genes, dimerized in different combinations.
Each integrin dimer has distinct properties and functions.
The binding of integrins to their matrix ligands is affected by the concentration of a divalent cation (like Ca2+).
This reflects the presence of divalent cation-binding domains in the α and β subunits.
The divalent cations can influence both the affinity and the specificity of the binding of an integrin to its extracellular ligands.
Hemidesmosomes are very similar to focal adhesions in that integrins act as the transmembrane receptor and the anchor is to the extracellular matrix.
Unlike focal adhesions (but like desmosomes), the intracellular attachment is to intermediate filaments.
Selectins mediate transient cell-cell adhesions in the bloodstream
Selectins are cell-surface carbohydrate-binding proteins (lectins) that mediate a variety of transient cell-cell adhesion interactions in the bloodstream.
Their main role is governing the traffic of white blood cells into normal lymphoid organs and any inflamed tissues.
White blood cells live a nomadic life, roving between the bloodstream and the tissue, and this necessitates special adhesive behavior.
The selectins control the binding of white blood cells to the endothelial cells lining blood vessels, thereby enabling the blood cells to migrate out of the bloodstream into a tissue.
Each selectin is a transmembrane protein with a conserved tectin domain that binds to a specific oligosaccharide on another cell.
Endothelial cells will express oligosaccharides that are recognized by specific selectins on a cell type (white blood cells, blood platelets, etc.)
There is a weak interaction, so the cell will loiter and become trapped. They tend to “roll” down the selectins.
Selectins do not act alone; they collaborate with integrins, which strengthen the binding of the blood cells to the endothelium.
Selectins and integrins act in sequence to let white blood cells leave the bloodstream and enter tissues.
The selectins mediate a weak adhesion because the binding of the lectin domain of the selectin to its carbohydrate ligand is of low affinity.
This allows the blood cell to roll along the surface of the blood vessel, propelled by the flow of blood.
The rolling continues until the blood cell activates its integrins, which will latch onto specific molecules on the endothelial cells.
Once it has attached this way, the white blood cell escapes from the bloodstream into the tissue by crawling out of the blood vessel between adjacent endothelial cells.
The extracellular matrix is made and oriented by the cells within int
The macromolecules that constitute the extracellular matrix are mainly produced locally by cells in the matrix.
In most connective tissues, the matrix macromolecules are secreted by cells called fibroblasts.
The extracellular matrix is constructed from three major classes of macromolecules:
Glycosaminoglycans (GAGs)
Fibrous proteins, which are primarily members of the collagen family
A large class of non collagen glycoproteins
Glycosaminoglycan (GAG) chains occupy large amounts of space and form hydrated gels
GAGs are unbranched polysaccharide chains composed of repeating disaccharide units.
Because there are sulfate or carboxyl groups on most of their sugars, GAGs are highly negatively charged.
They are the most anionic molecules produced by animal cells.
Because of that, they attract Na+ ions that, in turn, suck up tons of water.
GAGs tend to adopt highly extended conformations that occupy a high volume relative to their mass
This is because polysaccharide chains are too stiff to fold.
This means they form hydrated gels even at low concentrations.
Hyaluronan, for example, is enormous. One molecule can take up the space equivalent to a bacteria.
Proteoglycans are composed of GAG chains covalently linked to a core protein
All GAGs are covalently attached to protein as proteoglycans, which are produced by most animal cells.
The core protein is usually a large transmembrane protein that the GAGs are attached to.
Proteoglycans are clearly distinguished from other glycoproteins by the nature, quantity, and arrangement of their sugar side chains
By definition. at least one of the sugar side chains of a proteoglycan must be a GAG.
Aggrecan is an example of a proteoglycan.
Major component of cartilage
Molecules can assemble with hyaluronan in cartilage matrix to form aggregates that are as big as a bacterium.
Collagens are the major proteins of the extracellular matrix
Collagens are a family of fibrous proteins found in all multicellular animals.
They are secreted in large quantities by connective tissue cells.
As a major component of skin and bone, collagens are the most abundant protein in mammals, where they constitute 25% of the total protein mass.
The primary feature of a typical collagen molecule its long, stiff, triple-stranded helical structure, in which three collagen polypeptide chains, called α chains, are wound around one another in a ropelike superhelix.