cell interactions and extracellular matrix 4

integrins

• α and β subunits (heterodimers)

• only in higher developed organisms (not prokaryotes, plants, or fungi)

integrins in C. elegans

• two α subunits and one β subunit

• form two integrins

integrins in vertebrates

• 18 α and 8 β subunits

• form 24 heterodimers

mammalian integrin receptor family

• cell-matrix adhesion - recognize extracellular matrix molecules

  • RGD receptors

  • collagen receptors

  • laminin receptors

• cell-cell adhesion - recognize IG superfamily counter receptors

  • leukocyte-specific receptors

integrin heterodimers

• type I transmembrane protein (N-terminus in ECM)

• connected via intracellular anchor proteins to actin

  • except α6β4 - intermediate filaments)

• Ca²⁺ and Mg²⁺ in head domains involved in ligand binding

• extracellular spacer domains allow separation from membrane

• α chain contains furin cleavage that provides flexibility

  • re-connected by disulfide bond

• β chain contains cysteine-rich domains

integrin-binding sequences on ECM proteins

• RGD motif

  • arginine-glycine-aspartic acid

• involves frequently either D or E (glutamic acid) residue

  • negative charge at third position is important for ligand binding

  • negatively charged residue interacts with Ca²⁺ in head domain

• present in fibronectin, fibrinogen, some collagens, and more

synergy sites

• accessory sites for receptor binding

• helps stabilize integrin binding

fibronectin synergy site

• Fn10 contains RGD motif - binds β integrin head domain

• Fn9 (synergy site) - binds α subunit binding domain

integrin activation for cell aggregation

• αIIbβ3 integrin mediates blood platelet aggregation via binding to fibrinogen

• activation is necessary, occurs via exposure to collagen IV and/or thrombin at site of injury

• activated αIIbβ3 integrin binds fibrinogen, causing fibrin polymer assembly and blood clot formation

• mutations in αIIbβ3 causing bleeding disorders (improper blood clot formation)

• constitutive activation would lead to blood clot formation in blood vessels → heart attack and stroke

integrin activation

• unbound integrins diffuse freely in plasma membrane

• some integrins are not constitutively active, often on the cell surface in inactive, non-ligand binding state

• activation can occur via extracellular ligand binding or from within the cell via integrin cytoplasmic tail

importance of integrin activation

• cell aggregation (adhesion of circulating cells to soluble matrix)

• cell-cell adhesion (leukocytes and inflammation)

outside-in integrin activation

1. regulated via extracellular ligand

2. binding to the bent inactive conformation (weak interaction)

   • binds if a lot of ligand is present

3. straightening of α and β chains

   • conformational change

4. extracellular ligand binds much stronger

5. conformational change in β subunit head domain

6. separation of the entire α and β subunits

7. active integrin

inside-out integrin activation

1. talin in closed, inactive (self-binding) state

2. talin activated by either phosphoinositol-4,5-bisphosphate (lipid in plasma membrane) or calpain (protease), head domain is available for binding

   • PI4,5P leads to open, active form

   • calpain leads to cleaved, active form

3. activated talin head binds to αIIbβ3 β subunit

4. disrupts ionic salt bridge between α and β subunits

5. separation of α and β subunits

6. active integrin

disintegrins

• large group of snake venom proteins containing a number of cysteine residues and RGD sequence

• more than 40 identified

• small proteins, 40-100 amino acids long, with high homology

• block functions of some integrins

  • αIIbβ3 - fibrin interaction

  • αvβ3 - matrix protein interactions and thus formation of blood vessels (angiogenesis)

  • eristostatin, echistatin, kistrin block platelet aggregation

integrin clustering

• low to moderate affinity, high avidity

• ligand binding promotes lateral diffusion and redistribution to focal complexes

• β1 integrins clustered in focal adhesions at ends of actin filaments (highly dynamic)

integrin-mediated fibronectin assembly

1. soluble fibronectin dimer connected by disulfide bonds bound to α5β1 integrin

2. focal adhesion kinase, vinculin, and paxillin bind α5β1 integrin

3. insoluble fibronectin network formed by self-assembly of more fibronectin molecules, aligned with intracellular actin filaments

cytochalasin

• agent that disassembles actin filaments

• disassembly of actin filaments → fibronectin fibers also become disorganized