MCB - 48 - Cell-Cell & Cell-Matrix Adhesion

What receptor families are involved in transient cell adhesion? (There are 5).

1. Cadherins 2. Selectins 3. Ig Superfamily 4. Integrins 5. Proteoglycans

Describe how diapedesis occurs (not the mechanism, just the general steps).

1. A white cell is floating through the blood stream. 2. A white cell's surface proteins come into contact with the surface proteins of an endothelial cell. 3. The endothelial cell passes on a signal to enter the tissue. 4. The white cell begins to roll along the surface of the endothelium. As it rolls, it interacts with the surface proteins of the endothelial cells. 5. Interactions between surface proteins slow the white cell. 6. The white cell stops on the endothelial surface. 7. The white cell begins extravasation (diapedesis). 8. The white cell migrates out of the blood stream into connective tissue. 9. Bacteria get roundhouse kicked through large pieces of glass.

Describe the mechanism for diapedesis.

1. The leukocyte circulates freely. P-selectin is held within an endothelial vesicle. 2. Cytokines from damaged or infected cells diffuse to the endothelial cell and signals the endothelial cell to express P-selectin. P-selectin is expressed on the endothelial cell surface. 3. P-selectin's ligand happens to be a surface protein (carbohydrate) of the leukocyte. It sticks to the leukocyte. 4. Successive interactions with P-selectins cause the leukocyte to slow down, then begin rolling on the endothelial surface (a series of p-selectins are almost like anti-lock breaks). 5. PAF (platelet activating factor) activates integrin by binding to its receptor on the surface of the leukocyte. The binding of PAF starts a series of events that leads to the phosphorylation of integrin. This phosphorylation of the integrin changes its structure, which activates it. 6. In its new conformation, integrin can interact with the cell adhesion molecules of the endothelial cells (ICAMs). This brings the leukocyte to a full and complete stop. 7. Now the leukocyte finds a junction between two endothelial cells and begins breaking-down the cadherins between them. The white cell squeezes through the opening in the cadherins and the cadherins re-seal behind the white cell. 8. The white cell is outside of the blood stream.

Why are veins an ideal location for white blood cells to migrate out of the blood stream? (We thought of 3).

1. Veins have relatively thin walls. 2. Blood pressure is low in veins. 3. Venous diameter is larger than capillary diameter.

How large is the space between adjacent cells involved in a zonula adherens?

15-20 nanometers.

What is a connexin?

A connexin is a component of a connexons, which is a component of a gap junction. Six connexins are required to form on connexon.

What is a connexon?

A connexon is composed of six connexins. When a connexon attaches two cells, it is a gap junction.

Describe the ultrastructure of the macula adherens.

A dark-line surrounded by a large region of electron density. See the black and white image above.

What is a focal contact (focal adhesion)?

A focal adhesion is a stable connection between a cell and the extra cellular matrix (very comparable to a hemidesmosome). The difference between a focal adhesion and a hemidesmosome is that, while hemidesmosomes attach to intermediate filaments, focal adhesions attach to the actin skeleton.

In terms of cell adhesion molecules, what is a "fuzzy plaque?"

A fuzzy plaque refers to the region of electron density near a zonula adherens.

What is a hemidesmosome?

A hemidesmosome is a stable connection between a cell and the extra cellular matrix. A hemidesmosome makes this connection via intermediate filaments (cell to basal lamina).

What is the difference between a desmosome and a hemidesmosome?

A hemidesmosome is exactly like a desmosome with one exception: instead of attaching a cell to another cell, it attaches a cell to the underlying tissue. Both desmosomes and hemidesmosomes use intermediate filaments in their attachments. Additionally, hemidesmosomes use integrins (alpha6beta4) rather than desmogleins/desmocollins.

What is a junctional adhesions molecule?

A junctional adhesion molecule (JAM) is a member of the Ig superfamily. JAM works with claudin. That's all that Dr. Yee had to say about them.

What is a macula adherens?

A macula adherens is a stable junction between two cells that is very comparable to the zonula adherens. The difference between macula and zonula adherens is that that macula adherens connects the intermediate filaments of two cells while zonula adherens connects the actin skeletons of two cells.

Review: What is epidermolysis bullosa?

A mutation in keratin 14, leading to blistering in the epidermis.

What is a zonula adherens?

A zonula adherens is a stable junction between two cells. Zonula adherens links the actin cytoskeletons of adjacent cells to each other.

What is a zonula occludens?

A zonula occludens is a stable, tight junction. Zonula occludens occuldes the paracellular (between two cells) pathway. Zonula occludens separates the basal-lateral membrane from the apical membrane, which limits lateral diffusion.

Where, in the process of diapedesis, does inside-out signaling occur? Describe the signal.

After the endothelial cell expresses PAF (platelet activating factor), the PAF binds to its receptor on the leukocyte. This binding leads to a series of events that end in the phosphorylation of integrin. When the integrin is phosphorylated, it changes shape and function. Signals within the cell lead to an effect on the integrin that is outside the cell.

What are alpha6beta4 integrins?

Alpha6beta4 integrins are the particular type of integrin associated with hemidesmosomes. Basically, relative to a desmosome, integrin takes the place of desmoglein or desmocollin.

What is pemphigus foliaceus?

An auto-immune disease in which antibodies are made for desmoglein 1. This leads to the breakdown of desmosomes, the opening of a space between the ECM and the epidermis. The space fills with fluid and you have large blisters.

What is pemphigus vulgaris?

An auto-immune disease in which antibodies are made for desmoglein 3. This leads to the breakdown of desmosomes, the opening of unnatural spaces. The spaces fill with fluid and large blisters form.

What is bullous pemphigoid?

An autoimmune disease leading to the formation of antibodies against BP230 (serves as an attachment protein) or collagen type 17 in hemidesmosomes. Bullous pemphigoid is another epidermal blistering disease, but the fluid accumulation occurs between the epidermis and dermis - not in the stratum spinosum.

What is the general result of autoimmune diseases that affect desmosomes in the skin?

Blistering. Horrible nasty blistering. These also commonly lead to chronic infections.

How are the apical and basal lateral membranes of an epithelial cell kept separate from each other?

By zonula occludens or tight junctions. Beside not letting anything through, this is another important role of tight junctions.

Mutations in connexin 46 and 50 lead to what kinds of symptoms? Why (briefly)?

Cataracts. Basically, the eye loses its ability to remove metabolites from the lens, leading to cataracts.

What is the role of catenin in the zonula adherens of two epithelial cells?

Catenins serve to attach cadherins to the actin cytoskeleton of a cell. In the case of e-cadherin, catenin within each of the two epithelial cells, links the cadherin with their respective cell's actin cytoskeleton.

Among cadherins, what is a cis interaction? A trans interaction?

Cis interaction - this refers to the connection of two cadherins on one cell surface into a homodimer. Trans interaction - this refers to the connection of two homodimers from the surfaces of adjacent cells into one cis/trans unit. The process of trans interaction is described as "zipping."

What is claudin?

Claudin is a family of proteins (differs in different locations) that forms channels for ions and water through the tight junction. Basically, the purpose of the claudin is to provide selective permeability through the paracellular space. Claudin is like a really small gate in the occludin fence.

Mutations in connexin 26 leads to what sympoms? Why (briefly)?

Congenital deafness. This is because of the abnormal recirculation of ions in the ear.

Cadherins like desmoglein and desmocollin are associated with which type of stable cell-to-cell adhesion?

Desmosomes (macula adherens).

What is diapedesis? What types of cells engage in diapedesis?

Diapedesis is the process by which cells (particularly white blood cells) leave blood vessels to enter tissue. All leukocytes use diapedesis to leave the blood stream and enter tissues.

What is the role of e-cadherin in the zonula adherens of two epithelial cells.

E-cadherin is basically a transmembrane protein. Two adjacent cells both have e-cadherins on their cell surfaces. Both grab an intercellular calcium, and the two cells are linked.

True or false: a gap junction can only have one connexon between two cells.

False. Dr. Yee indicated that gap junctions can include many, many connexins and connexons.

True or false: at a tight junction, two cells are completely contiguous with each other - - like to books pushed together with pressure.

False. In reality, at a tight junction, there is an intricate network of ridges and grooves. The surfaces of the cells are not flat against each other. It's almost like a basketball net squished between two books.

True or false: integrins only mediate connections between a cell and the extracellular matrix.

False. Sometimes integrins can mediate cell-to-cell attachments as well.

True or false: a zonula occludens is an absolute barrier: stuff is not going to get through.

False. While the zonula occludens does block most things (most of the time) there are a few things that can "sneak" through. Specifically, Dr. Yee mentioned water and ions (often through specific pores).

What is the most common ligand for integrins in focal adhesions?

Fibronectin.

What is epidermolytic plantopalmar keratoderma?

Fragmentation of the epidermis on the palms and soles of the feet. This happens due to a mutation in keratin 9.

What are gap junctions?

Gap junction are stable connections between two cells. These connections are mediated by connexins and connexons. These allow intercellular communication (between the attached cells).

What is the difference between a hemidesmosome and a focal adhesion?

Hemidesmosomes and focal adhesions are very similar. There are two main differences: 1. Hemidesmosomes use intermediate filaments and focal adhesions use actin. 2. Hemidesmosomes attach epithelial cells to basal lamina and focal adhesions attach other kinds of cells to the extracellular matrix.

What are the two mechanisms that attach cells to the extracelluar matrix?

Hemidesmosomes and focal adhesions.

Which type of interaction, homophilic or heterophilc, does the selectin family of proteins undergo?

Heterophilic. One selectin on a cell will tend to bind to a different kind of molecule (typically carbohydrates) on a different cell. Give me some sugah!

Which type of interaction, homophilic or heterophilc, does the integrin family of proteins undergo?

Heterophilic. This means that integrin from one cell will connect to a different molecule from a different cell. Integrins are usually involved in linking a cell to the extra cellular matrix; however, there are a few cases in which integrins link cells together.

What is heterotypic adhesion? How does it differ from heterophilic binding?

Heterotypic adhesion occurs when two cells of different types adhese or connect to each other. Heterophilic binding, on the other hand, is when two proteins of different types attach to each other (Jack Yee attaches to Joe Knezitek).

What is homophilic binding? How does it differ from homotypic adhesion?

Homophilic binding is when two proteins of the same type attach to each other (two Jack Yee proteins for example). Homotypic adhesion refers to the connection of two cells of similar cell type.

Which type of interaction, homophilic or heterophilic, does the cadherin family undergo?

Homophilic. A cadherin on one cell will link to a cadherin on a neighboring cell.

Which type of interaction, homophilic or heterophilc, does the Ig-superfamily of proteins undergo?

Homophilic. This means that one Ig family protein will tend to connect to another Ig family protein (on a different cell).

What is the ligand for the alpha6beta4 integrin?

Laminin. Laminin is involved in linking collagen and hemidesmosome connection to the ECM (via integrin).

What is epidermolytic hyperkeratosis?

Mutations in keratin 1 and 10 that lead to disease. Symptoms begin as blisters and become really thick skin (hyperkeratosis).

In bullous pemphigoid, in which epidermal layer does fluid accumulate, giving rise to blisters?

Not really in an epidermal layer at all. This occurs between the basal lamina and the epidermis. In other words, this occurs below the stratum spinosum (which is where most of the blistering diseases cause fluid to accumulate).

What is occludin?

Occludin is a transmembrane protein that contributes to the paracellular barrier and the barrier between the basal and apical membranes (preventing lateral diffusion). Occludin is like a fence.

What are the three major transmembrane proteins involved in tight junctions?

Occludin, claudin, and junctional adhesion molecule (JAM).

What are selectins?

Selectins are cell surface proteins that attach to carbohydrates. Selectins are important in cell signaling. Selectins are heterophilic - - a selectin attaches to a molecule (usually carbohydrates) that are not selectins.

What kinds of molecules can move through gap junctions?

Small ones (less than 1200 daltons). This includes things like ions, etc. and also more important things like cAMP and cGMP. Because of these kinds of connections, groups of cells can be called "metabolically coupled."

Describe the "ingredients" of a gap junction.

So... Six connexins form one connexon. Connexon's from two cells that connect to each other are called a gap junction.

Specific to integrins, etc. what is inside-out signaling?

Something happens inside the cell that changes integrins affinity for its ligand.

In terms of the cell membrane, what does it mean to stop lateral diffusion? Relate zonula occludens to limiting lateral diffusion.

Stopping lateral diffusion means that the lipid bi-layer in one part of the plasma membrane cannot communicate with the lipid bi-layer in another. In the case of zonula occludens, the apical membrane does not communicate (exchange lipids with) the basal-lateral membrane.

The black arrows in this image are pointing at what structures (what is their name)?

Terminal bars. Originally, they thought these things were filled with inter-cellular cement. The cement thing is false - - it's actually cell adhesion molecules (maybe zonula adherens and occludens?)

In epithelium, what produces the basal lamina?

The epithelial cells themselves. It is important to note that the basal lamina is like a continuous sheet that separates the epithelium from the underlying connective tissue.

What is the ligand of integrin in hemidesmosomes? What particular integrin is involved with hemidesmosomes?

The ligand of integrin in laminin. Alpha6beta4 integrin is involved with hemidesmosomes.

Histologically, how can you distinguish a macula adherens from a zonula adherens?

The macula adherens has a much darker line between the cells (whereas zonula adherens has a white-ish space). Additionally, macula adherens has a larger region of electron density around the cell junction.

What causes gap junctions to open and close? Which component of the gap junction is directly involved in opening and closing?

The presence of certain signals, particularly calcium, causes the connexins of the gap junction to change conformation. This can open and close a 2.5 nm pore. Because of the action of these pores, cells can be connected while retaining their individuality.

In pemphi-type diseases, the body often makes antibodies against certain components of the desmosome. In what layer of the epidermis would pemphus-like blisters arise in these diseases?

The spinous layer or stratum spinosum.

What is the purpose of the ZO proteins?

These proteins connect occludin and claudin to the actin cytoskeleton.

What is paraneoplastic pemphigus?

This is an auto-immune disease that causes blistering and is often found with existing malignancies. It causes blistering of the mouth, lips, and esophagus.

Specific to integrins, etc. what is outside-in signaling?

This is when an adhesion molecule outside the cell (laminin for example) can cause the activation of a pathway within the cell. When Dr. Hansen taught about cyclins, etc. she mentioned that cells need adhesions to be able to enter the cell cycle. Maybe they play some factor (via outside-in signaling) in internal pathway activation.

Very generally, how do cells communicate with each other?

Through the proteins, etc. that cells have on their surface. These cell surface markers react with the markers of other cells through homophilic and heterophilic reactions, allowing cells to identify other cells of the same type, as well as cells of different types.

What is the difference between a transcellular pathway and a paracellular pathway?

Transcellular - something passes into one side of the cell (basal end) moves through the cell, then passes out the other side of the cell (apical end). The opposite is also true. Paracelluar - something passes between two cells. This is not likely to happen if a zonula occludens exists in the paracellular space.

True or false: as a consequence of desmosomes, mechanical force exerted on one cell is transferred to other, connected cells.

True.

True or false: gap junctions open and close depending on cell conditions and the presence of certain signals.

True. Connexins open and close in response to signaling. Calcium is important in this type of signaling.

True or false: in two adjacent cells with a gap junction, each of the cells donates half of each connexon that forms the gap junction.

True. I would imagine that there is an extraordinarily horrible pathway to explain how that junk happens.

True or false: the zipper connections between cadherins on two adjacent cells are calcium dependent.

True. If you take calcium away, these connections will tend to break.

True or false: integrins are selective.

True. Integrins can only attach to certain types of extracellular matrix molecules.

True or false: the binding of integrin to its ligand can activate intracellular signal pathways.

True. This is also called outside-in signaling.

True or false: keratin is the only kind of intermediate filament involved in desmosomes and hemidesmosomes.

True. This is, at least partially, because desmosomes and hemidesmosomes function in epithelial cells (where keratin is a very, very common intermediate filament).

How does the cholera toxin cause diarrhea (we outlined two things)?

Two things: 1. Cholera toxin acts on ZO proteins to loosen the tight junctions in bowel in epithelium (making it easier for water to flow into the intestinal lumen). 2. Cholera turns a Gs protein on constituitively, which activates adenylate cyclase, which produces cAMP, which causes electrolytes to flow out of the cell into the intestinal lumen. Water follows these electrolytes into the lumen. 3. You crap your pants like a boss.

What is the role of vinculin and alpha actinin in the zonula adherens of two epithelial cells?

Vinculin and alpha actinin serve the purpose (like catenin) to link cadherins to the actin cytoskeleton. Specifically, vinculin connects catenin to actin and alpha-actinin connects actin filaments to each other.

What is the most important difference between zonula adherens and macula adherens?

Whereas zonula adherens connects the actin cytoskeletons of two cells via cadherins, catenins, etc. macula adherens connects the intermediate (keratin in epithelial cells) filaments of two cells via desmoglein, desmocollin, etc.

Where do white blood cells migrate out of blood vessels into body tissues?

White cells leave the circulatory system through simple, thin-walled vessels. The best blood vessels for white cells to leave the circulatory system are post-capillary venules.

What do we mean when we compare zonula adherens and macula adherens to spot welds and actin adherens belts?

Zonula adherens - the actin cytoskeletal components attached by a zonula adherens is like a belt wrapping around the entire cell. The zonula adherens, then, is like the connection between the two loops in the number "8." Macula adherens - the intermediate filaments to which macula adherens attaches is more like a rope that stretches through the cell. The macula adherens is often described as a "spot weld" between two cells.

Classic cadherins like E-cad, N-cad, and P-cad are associated with which type of stable cell-to-cell adhesion?

Zonula adherens.