Adhesive Interactions Between Cells Flashcards
What is fibronectin?
- an ECM protein of connective ECM
What is the role of transglutaminase with fibronectin?
Cross-links fibronectin dimers into multimers that form fibrils around cells
What is the role of the RGD motif in fibronectin?
- 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
How were receptors for ECM proteins found?
- 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
What proteins are receptors for ECM proteins?
Integrins
What is the structure of integrins?
- Heterodimers with alpha and beta subunits
- Each subunit has varioud exrtracellular domains as well as s single TM domain and a short cytoplasmic domain
What do the extracellular domains of integrins bind to for function?
Divalent ions
What is the function of integrins?
Bind ECM proteins outside the cell and link to the actin cytoskeleton inside the cell
How are the alpha and beta chains of integrins bound together?
Held together by non-covalent bonds
How can integrins bind different ECM proteins?
Via different heterodimers
Where is the ECM binding cleft of RGD-binding integrins?
Between alpha and beta subunits
How is the ECM connected to the cytoskeleton through integrins?
- Connects ECM through TM helix
- Through TM integrin to FAK to cytoskeleton
What happens when Talin is recruited?
- 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
How do active integrins set up intracellular signals?
- 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
How can intracellular kinases or 2nd messengers be activated by integrins?
- Either through activated proteins that bind to Talin or Kindlin
- Or by proteins that bind to the integrin alpha subunit cytoplasmic domain
What is laminin a major component of?
- Basement membrane
- focal adhesions
- Hemidesmosome
What are the 4 laminin binding integrins?
- α3β1
- α6β1
- α7β1
- α6β4
Where are laminin binding integrins α3β1, α6β1, and α7β1 found?
In focal adhesions and linked to the cytoskeleton
Where is α6β4 laminin binding integrin found?
In hemidesmosomes and linked to keratin intermediate filaments
What are genetic skin blistering disorders called?
Epidermolysis bullosa
What causes epidermolysis bullosa?
- Mutations in laminin chains or alpha6 or beta4 integrin subunits that disrupt hemidesmosomes
- This leads to epithelial cell detachment and chronic skin blisters
What is the function of the dytroglycan complex?
- 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
What causes musclar dystrophy?
Mutations in components of dystroglycan complex
How do mutations in components of dystroglycan complex cause musclar dystrophy?
- 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
When the loosening of integrins to matrix components occurs what happens to molecular signalling complexes?
They fall apart on either side of membrane
Concentration of what in extracellular medium affects the binding of integrins to matrix ligands?
Mg2+ and Ca2+
What disease does a lack of alpha subunits of integrins cause?
Muscular dystrophy
What disease does a lack of beta 2 subunits of integrins cause?
Leukocyte adhesion deficiency
What does disruption of beta3 subunits of integrins cause?
Defective clotting and excessive bleeding
What happens to integrin structure when they are inactive?
- 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
What happens to the structure of integrins when active?
- 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
How can activation of integrin proteins occur?
- Outside in activation
- Inside out activation
How does outside in activation of integrins occur?
- 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
How does inside out activation of integrins occur?
- 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
Give an example of a cytoplasmic adaptor protein for integrin
Talin
How can integrins form strong adhesions?
- Once activated integrins cluster together to make dense plaque so many integrins are anchored to cytoskeletal filaments
What does assembly of mature cell-matrix junctional complexes rely on?
Recruitment of dozens of scaffolding and signalling proteins
How can the Ras/MAP kinase pathway be activated?
- Either through conventional cell signalling
- or through integrins
Cells usually need both to proliferate
How can Talin help organise actin filaments?
- 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
