Cell- cell interaction

Question 1

What is the importance of cell-cell interaction

Answer 1

1) Maintains tissue shape
2) Holds cell together
3) organise different cell types
4)Transmit mechanical forces
5) Allow cells to sense + respond to mechanical forces

Question 2

What are the two ways that cells are held together ?

Answer 2

1) Direct cell-cell interaction
2) Indirect interactions via ECM

Question 3

List the types of cell junctions

Answer 3

1) Tight junctions
2) Adherens junction
3) desmosome
4) gap junction

Question 4

What are the adhesion proteins found in the cell junction

Answer 4

1) Transmembrane adhesion protein : One end links to the cytoskeleton the other end links to external contents

2) Intracellular adaptor proteins : These proteins link the adhesion proteins to actin and intermediate filaments

Question 5

Give an example of transmembrane adhesion proteins

Answer 5

1) Cadherins -> cell-cell adhesion

2) Intergrins -> cell - ECM adhesion

3) Ig superfamily -> cell - cell adhesion

Question 6

Where are the tight junctions located

• What cytoskeleton does it attatch to
• What is the function
  -State the adhesive proteins found in this junction

Answer 6

• In the epithelial sheets
• The actin filament ( indirectly)
• 1) Create paracellu;ar barrier which seals cells together
  2) Prevent leakage
  3) maintains polarity
  4) Seperates apical vs basalateral membranes

1) Claudins = Forms a seal
2) Occludin = Regulates permeability
3) Tricellulin = Seals triple cell junction

Question 7

Adherens Junction
- Describe
- What cytoskeleton is it linked to
- What force is it subjected to
- What is its function

Answer 7

• These are clusters of cadherins
• Linked to the actin filament
• Subjected to pulling forces via myosin II activity

1) Coordinates adhession and contraction
2) Enable epithelial sheet folding

Question 8

Desmosomes
- Describe
- What cytoskeleton does it attatch to
- What are their key adopters
- State the function

Answer 8

These are similar to adherens junctions: have specialised cadherins
- Bind to intermediate filaments

1) Plakoglobin
2) Plakophilin
3) Desmoplakin

• They resist mechanical stress

Question 9

Gap junction
- Describe
- What cytoskeleton does it link to
- What is able to pass through
- Describe its function

Answer 9

• Connects cells via their cytoplasm
• Gap is about 2- 4 nm
• None
  Channels are formed through
  connexins (vertebrates)
  innexins (invertebrates)
• ions
• metabolites
• signalling molecules
• It plays a role in electrical coupling
• Metabolic coordiantion

Question 10

What are cadherins
- List the different types of cadherins

Answer 10

These are ca 2+ dependent adhesion proteins

E - Cadherins -> Epithelial cadherins
N - Cadherins -> Nerve cadherins
P- Cadherins -> Placenta/ Epidermis
Desmosomes

Question 11

Discuss the cadherin binding mechansim

Answer 11

Cadherins are homophilic: They only bind to each other
- The binding occurs at the N-Terminal tips
- Cadherins when binded alone have a weak bond and are flexible

• Ca2+ binds at the hinges and makes the structure more rigid and allows for strong ligand binding. Changes in Ca2 + allows for the assembly aand dissassembly of cadehrins

Question 12

How do cadherins bind to the actin filament ? and how do the joints mature?
Note: Adherens junction + desmosomes contain a bunch of cadherins

Answer 12

• Cadherins link to the actin filament via catinin
• Beta catenin and p120 catenin -> binds to cadherin tail
• Alpha catenin is linked to vinculin which is linked to the actin filament

1) Cadherins cluster and this activates Rac
2) Results in actin protrusion
3) More cadherins are recruited and contact zone enlarges
4) Rac off Rho on
5) Actin reorganises into contractile bundle and strong joints form

Question 13

Describe the mechanotransduction at the adherens junction

Answer 13

1) The cadherins and catenins cluster together to connect the cell cytoskeletons together
2) Myosin II which pulls on the actin filament will generate tension
3) This will casue the adhesion proteins @ the joints to stretch
4) The stretching exposes cryptic binding sites
5) This results in the recruitment of vinculin
6) Vinculin will then attatch more actin filaments to the adherens joints
7) This will then make the joints strengthen in repsonse to a force