Module 24 - Cell Junctions

How do we maintain the transcellulat transport?

1) The sets of transport proteins must not migrate between the two membrane domains. 2) spaces between epithelial cells must be tightly sealed.

How is the permeability of gap junctions rapidly and reversibly modulated?

1) changes in the cytosolic pH 2) cytosolic concentrations of calcium ions. 3) voltage differences between the two connected cells.

What are two types of desmosome cadherins?

1) desmoglein 2) desmocollin

What two anchoring junctions have intergins as transmembrane proteins?

1) focal adhesions 2)hemidesmosomes

transcellular transport of nutrients depends on what two different sets of membrane transport proteins?

1) the apical surface of the epithelial cell (the surface facing the lumen) and actively transports selected molecules from the gut into the cell. 2)the basolateral surface of the cell and allows the same molecules to leave the cell by facilitated diffusion into the extracellular fluid on the other side of the epithelium.

What are the anchoring junctions dependent upon?

1) whether these are formed between cells or between cells and the ECM (2) which cytoskeleton the junctions communicate with.

What are the four types of anchoring junctions?

1)adherens junctions 2)desmosomes 3)focal adhesions 4)hemidesmosomes

What do adherens junctions create?

A continuous actin network that could contract with the help of myosins.

What do gap (communication) junctions allow?

Allow for chemical and electrical communication between adjacent cellular cytoplasms through diffusion of signals.

Where do actin filaments attach?

Attach to the β subunit of integrins through adapter proteins (e.g., vinculin)

What is the difference between desmosomes and adherens junctions?

Connect cells through the intermediate filaments such as keratin and desmin.

How do connexon pores vary in size and polarity?

Depending on the connexin proteins

How does mesenchymal stem cells (MSCs) differentiate?

Depends on the substrate.

What happens to Mesenchymall stems cells if it binds to a substrate of greater stiffness

Develop into muscle cells,

What happens to Mesenchymal Stem Cells if it binds to a substrate of high stiffness?

Differentiate into bone cells.

What happens to Mesenchymal stem cells when it binds a soft substrate?

Differentiate into nerve cells.

What do the seals not allow?

Do not allow water and solute diffusion from the extracellular compartment on one side of the cell sheet to the other side.

True/False Gap junction channels are also open?

False flip between open and closed states.

What do adherens junctions look like and where are they found?

Frequently have a band- or belt-like appearance. The belt is positioned near the apical surface

When are tight junctions permeable to small molecules?

In the epithelium lining of the small intestine Highly permeable to inorganic ions, such as Na+,

What do hemidesmomsomes increase and how?

Increase the overall rigidity of epithelial tissues. ' By interconnecting the intermediate filaments with the fibers of the basal lamina.

what determines the survival of cells in focal adhesions?

Intracellular signaling from the binding of ECM substrates

What do integrins in epithelial cells bind to?

Laminin

Compare and Contrast cadherins in desmosomes & classical cadherins in adherens junctions.

Similar: by connecting the cytoskeletons of adjacent cells Difference: desmosome cadherins gives tissues structural integrity and enable them to resist stress.

What do hemidesmosomes appear like?

Spot-like. Have the appearance of half desmosomes.

How do gap junctions form?

The connexon complexes stretch across the cell membrane two adjacent cell connexons interact

What happens to the intermediate filaments from desmosomes?

The intermediate filaments attach to membrane-associated proteins to form a plaque on the cytoplasmic face of the plasma membrane.

WHat is the purpose of hemidesmosomes?

These link the cells to the ECM Example: link between the basal laminae and epithelia.

When happens to focal adhesions when a cell moves or enters mitosis?

They are easily disassembled when a cell is stimulated to move or enter mitosis.

Why do epithelial cells transiently alter their tight junctions?

To permit an increased flow of solutes and water through breaches in the junctional barriers.

True/ False Tight junctions are cell-to-cell junctions?

True cell-to-cell junctions.

What are tight junctions composed of?

a network of sealing strands that encircles the apical end of each cell in an epithelial sheet.

What do gap junctions allow?

allow for the transfer of chemical and electrical signals between cells.

What is laminin?

an ECM protein in the basal lamina

What is the function of adherens junctions?

anchor the actin filaments of neighboring cells.

What causes blistering on the skin in bullous pemphigoid?

antibodies to hemidesmosomes result in detachment of the lower layer of epidermis from the basement membrane.

What do desmosomes appear like and where?

are "spot-like", occur only at a single location between adjacent cells

What are the purpose of desmosomes?

attach cells to cells.

What is the purpose of focal adhesions

attach cells to the ECM. connect the cytoskeletal actin to the ECM components fibronectin or collagen.

What is the purpose of clasical Cadherins?

attach to these cytoplasmic plaques, pass through the membranes and bind to cadherins coming through the membranes of the adjacent cells.

How integrins secure cell-to-ECM adhesions?

bind either actin or intermediate filaments.

How do cadherins secure cell-to-cell adhesions?

binding either actin filaments or intermediate filaments

What allows integrin clustering at the cell surface?

binding of integrin to the ECM components induces conformation changes in the cytoplasmic domain of integrin allow binding to cytoskeletal actin.

How are tight junctions built?

built by the transmembrane proteins claudins and occludins that bind actin filaments.

What are the cell adhesion molecules that mediate the transmembrane link?

cadherins.

What are the adaptor proteins in the cells of the heart, muscle and epidermis tisses

catenins

What do focal adhesions enable?

cell locomotion.

What do gap (communication) junctions mediate?

cell-to-cell interactions

What are gap junctions?

communicating junctions

What do hemidesmosomes connect to?

connect to intermediate filaments in the cytoplasm Ex: keratin filaments

What makes up a gap junction?

connexons

What is the purpose of anchoring junctions?

couple neighboring cells to each other by interacting with components of the cytoskeleton made of intermediate filaments and actin a subgroup links the cells to the ECM

Why is paracellular transport important?

in the absorption of amino acids and monosaccharides from the intestinal lumen

What do adherens junctions facilitate

folding and bending of entire epithelial cell sheets

What proteins are associated with the integrin cytoplasmic domain?

kinases FAK and SRC.

What do tight junctions limit?

limit the diffusion of some membrane proteins and lipids between the apical and basolateral domains of the plasma membrane seal neighboring cells together.

What are tight junctions impermeable to?

macromolecules

Where are anchoring junctions found and why?

most abundant in skin and heart tissues skin and heart tissues are subject to mechanical stress

What are tight junctions?

occluding or zonulae occludentes

What is the purpose of tight junctions?

physical barriers between cells

What is another role of focal adhesions?

play a role in cell survival.

Where are focal adhesions found

present at discrete sites

What is another role of gap junctions?

signaling in the brain, differentiation of retinal and skin cells.

What is connexon?

six connexin proteins that form a cylinder with a pore in the center

Why must the spaces between epithelial cells be tightly sealed in the lumen?

so that the transported molecules cannot diffuse back into the gut lumen through these spaces.

Why are gap junctions important?

synchronizes the contractions of both heart muscle cells and the smooth muscle cells responsible for the peristaltic movements of the intestine.

What is the difference between hemidesmosomes and desmosomes?

their transmembrane proteins are integrins, rather than cadherins.

What are the three types of cell junctions?

tight junctions, anchoring junctions gap junctions

Where are adheren junctions found?

tissues under mechanical stress such as heart, muscle and epidermis.

What are the integrins of focal adhesions?

transmembrane linkers

What causes a low-molecular weight tracer to no pass through a plasma membrane

usually does not pass through the tight junction.

What are anchoring junctions?

zonulae adherentes