ECM & Cell Adhesion

Question 1

What is the extracellular matrix?

Answer 1

the complex network of secreted proteins and carbohydrates that fills the space between cells

Question 2

What is the basal lamina?

Answer 2

A layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits
- provides structure to both the endo and epithelial cells

Question 3

What secretes the macromolecules that make up the extracellular matrix in the connective tissue?

Answer 3

Fibroblasts

Question 4

What are the functions of the extracellular matrix?

Answer 4

• space filler
• tissue organisers
• protective
• cell migration
• regulation of cell function
• provide nutrients

Question 5

What are the 3 main components of the ECM?

Answer 5

1. fibrous proteins
   - structural (collagen)
   - adhesive (laminin, fibronectin, collagen)
2. Glycosaminoglycans
3. Proteoglycans

Question 6

What is collagen?

Answer 6

• an insoluble, extracellular glycoprotein
• most abundant protein in the body
• essential structural component of all connective tissues (bone, skin etc)
• good for resisting stretching forces

Question 7

What are GAG’s?

Answer 7

Glycosaminoglycans

• polysaccharide unbranched chains of repeating units
• highly negatively charged
• inflexible and adopt extended structures that occupy a large volume
• form gels by sucking in water via osmosis due to it being highly negatively charged
• good for resisting compressive forces
• usually covalently linked to proteins to form proteoglycans
• have <300 sugar chains

Question 8

Name an example of a GAG.

Answer 8

Hyaluranon

• large molecule of up to 25,000 repeating disaccharide units
• globular protein
• cheap and easy to make packing material
• not a typical GAG

Question 9

What is a proteoglycan?

Answer 9

a macromolecule consisting of a protein bonded to GAGs, present especially in connective tissue.

Question 10

What is the structure of typical proteoglycan Aggrecan?

Answer 10

aggrecan is important in knee joints

• attacked to the core is many GAG side chains
• short (<300 sugars)
• can form aggregates
• helps resist forces found in joints and cartilage

Question 11

What do adhesive glycoproteins do?

Answer 11

Most adhesive glycoproteins bind cells through cell surface integrin receptors in conjunction with other cell surface receptors, and interact with other extracellular matrix proteins to form an intensive matrix network.

Question 12

Which knock-out mice experiments show the importance of adhesive glycoproteins?

Answer 12

Mouse which has a knock down for laminin
- They are born
- But have muscular dystrophy phenotype
- Weak muscles
Mouse which has a knock down for fibronectin
- dies in day 10 in embryo due to vascular defects
Mouse which has a knock down for fibrinogen
- mice are born but failure to breed
- clotting problems

Question 13

What are integrins?

Answer 13

Cell surface adhesion receptors

- very abundant on most cells

Question 14

What is the structure of integrins?

Answer 14

Always formed in a heterodimer conformation
- α and β subunit
- Each subunit has a single trans-membrane domain
- Need to bind to cations Mg or Ca
- Short cytoplasmic tails 
α subunit:
- 18 different α subunits
3-4 divalent cation binding domains
- single trans-membrane domain
- short cytoplasmic tails
β subunit:
- 8 different β subunits
- short cytoplasmic tails

Question 15

What are the principles of the structure of integrins?

Answer 15

• always an alpha with a beta
• some beta subunits can interact with multiple alpha subunits
• some beta subunits can only bind to one alpha

Question 16

What do the β 2 integrins do?

Answer 16

beta subunits can interact with multiple alpha subunits

• bind to other cell surface proteins
• involved in cell-cell attachment not cell matrix attachment like the other ones

Question 17

Where does the integrin bind to in Fibronectin?

Answer 17

Fibronectin Type 3 repeat no.10

• repeat no.10 has an exposed loop which contains a 3 amino acid seq
• Arg, Gly, Asp
• integrin recognises and binds to the loop

Question 18

Which integrin is involved with inside out signalling in platelets?

Answer 18

alpha 2 beta 3 integrin

• normally circulating integrin is inactive (does not bind to fibrinogen)
• platelets are activated by thrombin via GPCR
• this activates the integrin such that the platelet can now bind to fibrin and contribute to the clotting response

Question 19

How do you activate an integrin?

Answer 19

Involves a conformational change, events happening inside the cell triggers the change in shape

Question 20

What is inside-out and outside-in signalling?

Answer 20

inside-out:
- events inside the cell affects what happens with the integrins
outside-in:
- events outside the cell affecting what happens with the integrins

Question 21

How is milk duct formation a part of outside-in signalling?

Answer 21

1. Start out with resting gland
2. After pregnancy alveoli forms
3. Cross-section of alveoli
   - Single sheath of epithelial cells
   - Myoepithelial cells are contractile
   - Contracting of the structure expels the milk

Question 22

How do integrins signal?

Answer 22

Extracellular matrix protein binds to the integrin
They interact to actin filaments via filamin
- clusters form