Adhesive Interactions Between Cells

Question 1

What is fibronectin?

Answer 1

• an ECM protein of connective ECM

Question 2

What is the role of transglutaminase with fibronectin?

Answer 2

Cross-links fibronectin dimers into multimers that form fibrils around cells

Question 3

What is the role of the RGD motif in fibronectin?

Answer 3

• RDG motif is essential for cells to attach to fibronectin or to cell-binding fragment of fibronectin
• Exposed position of RGD makes it accessible for binding to cell surface receptors

Question 4

How were receptors for ECM proteins found?

Answer 4

• Made affinity column from ECM protein fragment and identify ligand
• Measure cell attachment to purified ECM proteins and develop monoclonal antibodies which block attachment
• autoantibodies that recognise antigens in cell membranes used to identify protein target of antibodies

Question 5

What proteins are receptors for ECM proteins?

Answer 5

Integrins

Question 6

What is the structure of integrins?

Answer 6

• Heterodimers with alpha and beta subunits
• Each subunit has varioud exrtracellular domains as well as s single TM domain and a short cytoplasmic domain

Question 7

What do the extracellular domains of integrins bind to for function?

Answer 7

Divalent ions

Question 8

What is the function of integrins?

Answer 8

Bind ECM proteins outside the cell and link to the actin cytoskeleton inside the cell

Question 9

How are the alpha and beta chains of integrins bound together?

Answer 9

Held together by non-covalent bonds

Question 10

How can integrins bind different ECM proteins?

Answer 10

Via different heterodimers

Question 11

Where is the ECM binding cleft of RGD-binding integrins?

Answer 11

Between alpha and beta subunits

Question 12

How is the ECM connected to the cytoskeleton through integrins?

Answer 12

• Connects ECM through TM helix
• Through TM integrin to FAK to cytoskeleton

Question 13

What happens when Talin is recruited?

Answer 13

• Other intracellular cytoskeletal and signalling proteins are recruited to form a focal adhesion
• This activates intracellular kinase cascades and other 2nd messengers and physically links the integrin to cytoskeleton

Question 14

How do active integrins set up intracellular signals?

Answer 14

• Chemical signalling via intracellular kinases and 2nd messengers
• Responses by mechanical forces are exerted through the physical binding of ECM protein to integrin and integrin to cytoskeleton

Question 15

How can intracellular kinases or 2nd messengers be activated by integrins?

Answer 15

• Either through activated proteins that bind to Talin or Kindlin
• Or by proteins that bind to the integrin alpha subunit cytoplasmic domain

Question 16

What is laminin a major component of?

Answer 16

• Basement membrane
• focal adhesions
• Hemidesmosome

Question 17

What are the 4 laminin binding integrins?

Answer 17

• α3β1
• α6β1
• α7β1
• α6β4

Question 18

Where are laminin binding integrins α3β1, α6β1, and α7β1 found?

Answer 18

In focal adhesions and linked to the cytoskeleton

Question 19

Where is α6β4 laminin binding integrin found?

Answer 19

In hemidesmosomes and linked to keratin intermediate filaments

Question 20

What are genetic skin blistering disorders called?

Answer 20

Epidermolysis bullosa

Question 21

What causes epidermolysis bullosa?

Answer 21

• Mutations in laminin chains or alpha6 or beta4 integrin subunits that disrupt hemidesmosomes
• This leads to epithelial cell detachment and chronic skin blisters

Question 22

What is the function of the dytroglycan complex?

Answer 22

• Cell attachment to laminin in the basement membrane of skeletal muscle
• Withstands changes in mechanical force as muscles contract and relax
• Allows continuous association of the muscle with surrounding basement membrane

Question 23

What causes musclar dystrophy?

Answer 23

Mutations in components of dystroglycan complex

Question 24

How do mutations in components of dystroglycan complex cause musclar dystrophy?

Answer 24

• Disrupts integrity of complex
• Results in loss of mechanical force balance and loss of link to cytoskeleton and signalling pathways
• Causes cell death and disrupts muscle tissue

Question 25

When the loosening of integrins to matrix components occurs what happens to molecular signalling complexes?

Answer 25

They fall apart on either side of membrane

Question 26

Concentration of what in extracellular medium affects the binding of integrins to matrix ligands?

Answer 26

Mg²⁺ and Ca²⁺

Question 27

What disease does a lack of alpha subunits of integrins cause?

Answer 27

Muscular dystrophy

Question 28

What disease does a lack of beta 2 subunits of integrins cause?

Answer 28

Leukocyte adhesion deficiency

Question 29

What does disruption of beta3 subunits of integrins cause?

Answer 29

Defective clotting and excessive bleeding

Question 30

What happens to integrin structure when they are inactive?

Answer 30

* External segments of the integrin dimer are folded together in a compact structure that cannot bind matrix proteins * Cytoplasmic tails hooked together to prevent interaction with cytoskeletal linker proteins

Question 31

What happens to the structure of integrins when active?

Answer 31

* 2 subunits are unhooked at the membrane to expose intracellular binding sites for cytoplasmic adaptor proteins * External domains unfold and extend to expose high affinity matrix binding site

Question 32

How can activation of integrin proteins occur?

Answer 32

* Outside in activation * Inside out activation

Question 33

How does outside in activation of integrins occur?

Answer 33

* Binding of an external matrix protein can drive integrins into high affinity active state * Binding sites for cytoplasmic adaptor proteins are exposed on tail of beta chain * Adaptor binding leads to actin filament attachment * Force can be applied at point of cell attachment

Question 34

How does inside out activation of integrins occur?

Answer 34

* Intracellular signals stimulate proteins to interact with beta chain and compete with alpha chain of integrin for its binding site * When protein binds it blocks beta chain from linking * Alpha and beta chains spring apart

Question 35

Give an example of a cytoplasmic adaptor protein for integrin

Answer 35

Talin

Question 36

How can integrins form strong adhesions?

Answer 36

* Once activated integrins cluster together to make dense plaque so many integrins are anchored to cytoskeletal filaments

Question 37

What does assembly of mature cell-matrix junctional complexes rely on?

Answer 37

Recruitment of dozens of scaffolding and signalling proteins

Question 38

How can the Ras/MAP kinase pathway be activated?

Answer 38

* Either through conventional cell signalling * or through integrins Cells usually need both to proliferate

Question 39

How can Talin help organise actin filaments?

Answer 39

* Has a binding site for actin regulatory protein viculin * these binding sites are exposed when domains are unfolded by unstretching talin * When actin is pulled it unstretches Talin and exposes vinculin binding sites to recruit and organise additional actin filaments