30 August 2006
Cell Motility
Dr. Robert Crissman, Ph.D.



Importance of Cell Mobility


  • Directed cell movement
  • Importance of motility:
    • Wandering cells must get to sites of infection
    • Cells must migrate during embryology and normal development
    • Important in repair of wounds (wound healing)
    • Important part of cancer spread throughout body

Mechanisms of Cell Adhesion


  • Before a cell can move, it must remove itself from attachments to other cells and the extracellular matrix
  • Cell-to-cell homophilic attachments typically use cadherins
  • Cell-to-matrix heterophilic attachments typically use integrins

Junctional Attachments

  • Stronger attachments than non-junctional attachments
  • Cell-to-cell:
    • Tight junctions
    • Adhesion belts (cadherins)
    • Desmosomes (cadherins)
    • Gap junctions
  • Cell-to-matrix adhesion
    • Hemidesmosomes (integrin)
    • Focal attachments (integrins)

Non-junctional

  • Weaker attachment than junctional attachments
  • Cell-to-cell:
    • Cadherins
    • Ig-like CAMs (cell adhesion molecules
    • Integrins
    • Selectins
  • Cell-to-matrix:
    • Integrins
    • Proteoglycan surface molecules
    • Transmembrane surface molecules (syndycan, fibroglycan)

Stages of Cell Motility


  • How does a cell know where to leave blood vessel to reach a site of infection?
  • How do cells move through the extracellular matrix?

Extravasation

  • Attaching to the endothelial lining
  • Migration of cell through capillary wall
  • Four steps:
    • Activation
      • Cytokines activate endothelial cells from Ab-Ag interaction or release by mast cells
      • Causes translocation of P-selectin vesicles and exocytosis to cell surface within seconds
      • Production of PAF (platelet activating factor) by endothelium
    • Trapping
      • P-selectin temporally binds to selectin receptor on surface of leukocyte
        • Hydrogen bond
      • Easily broken, causing cell rolling, binding more and more selectin and slowing cell down
      • PAF activates PAF receptor on leukocyte that causes conformational change in integrin on surface of leukocyte
    • Adhesion
      • Integrin of leukocyte binds to ICAM (intracellular adhesion molecule) of endothelial cell.
      • Leukocyte now tightly attached and cell thins and spreads out over capillary surface
        • Leukocyte adhesion deficiency:
          • Improperly produced integrin
          • Leukocytes cannot adhere and cannot effectively migrate out of blood vessel
    • Migration
      • Flattened cell reaches out with lamellipodia or pseudopodia and sticks down between cell, grabbing basement membrane and pull itself through
      • Leukocyte secretes metalloproteases to break junctional complex

Focal Contact

  • Transmembrane protein Integrin
    • Cytosolic side linked to cytoskeleton filaments of actin via linker proteins talin, vinculin, and alpha actinin
    • Exterior side attaches to laminin, collagen IV, tenacin, or fibronectin

Integrin

  • Transmembrane protein
  • Consists of α and β heterodimers
    • 14 types of α subunits
    • 8 types of β subunits
    • Different combinations of α and β heterodimers can adjust the binding affinity and binding specificity of intercellular interaction
    • Binding site is inbetween α and β heterodimers
      • α subunits bind to components of extracellular matrix
      • β subunits binds to only leukocytes

  • Requires activation in order to bind to ICAM
  • Phosphorylation of integrin causes integrins to dissociate from actin cortex and disperse
    • Acts in mitosis to stop integrin from binding fibronectin allowing cells to round up and undergo mitosis
    • Thought to play a role in cancer metastasis

Glanzmann’s disease

  • Normally, integrin needs to be activated to bind fibrinogen and participate in clotting
    • Must bind to collagen of thrombin first to activate integrin
    • Lack of β3 integrin causes causes excessive bleeding due to lack of clotting

Diapedesis

  • Process of migration through extracellular matrix to infection site
  • Second phase of cell motility
    • Occurs within connective tissue
  • Two types:
    • Slow Moving
      • Occurs in fibroblasts and growth cones of neurons
    • Fast Moving
      • occurs in leukocytes and macrophages
      • Can see cytoplasmic streaming
  • Four steps:
    • Extension of lamellipodia (podosomes)
    • Adhesion via attachments by focal contacts to adhesion molecules and collagen
      • Attaches cell to substrate
      • Gives cell point to pull against
    • Cytoplasmic flow forward
      • Cytoskeleton (actin) shifts forward
      • Cytoplasm goes from a gel to a sol state
    • Retraction with footprint
      • Trailing edge thins
      • Forms retraction fiber
        • Fibers snaps to leave a “footprint”

Possible Mechanisms

  • 1st Possibility: Leading Edge
    • Actin filaments attached to cell membrane by myosin I
    • Profilin adds actin monomers to leading edge of actin, extending cytoplasm forward
  • 2nd Possibility: Actin Filament Elongation
    • No profiling, actin monomers added to leading edge, extending cytoplasm
  • 3rd Possibility: Mysoin Movement
    • Actin filaments added without profiling
    • Myosin moves within fluid mosaic, pushing actin filaments forward
  • Intrinsic to all three mechanisms is the concept that the nucleus and all other organelles are attached to the cytoskeleton and are moved forward with the framework

Cell Navigation


  • Chemotactic Factors
  • Calcium concentration gradient

Extracellular Matrix Order


  • Integrin is central to matrix organization
    • Orientation of cytoskeleton in cell orients the assembly of secreted extracellular matrix molecules in the vicinity
    • The oriented extracellular matrix reaches other cells and orients the cytoskeleton of those cells
    • Newly oriented cells now secrete an oriented matrix in their vicinity
      • Ordering of cytoskeletons now propagated by newly oriented cells

Organ Formation


  • Basal lamina and mesenchyme presence essential for salivary gland formation
    • Removal of basal lamina, mesenchyme results in non-differentiation
  • Endoderm, mesoderm, and basal lamina must interact to form functional mature gland

Reconstruction of Myoneural Junctions


  • Reconstruction of regenerating myoneural junctions dependent on basal lamina

  • Basal lamina surrounds muscle cell
  • Synaptic regions contain special forms of collagen IV and laminin
  • In injury, both skeletal muscle and nerve are destroyed, leaving just the basal lamina
    • Blocking formation of either skeletal muscle doesn’t prevent the formation of nerve or vice versa
    • Basal lamina contains information necessary for regeneration of skeletal muscle and nerve

Lamina


  • Lamina regulates neuronal outgrowth
    • Growth cones select specific molecules to migrate on

Fibronectin


  • Fibronectin promotes migration
    • Phosphorylation inactivates binding of Integrin
    • Promotes migration in mesenchyme
  • Incorporated in blood clots
    • Promotes migration of epithelial cells across wound
    • Promotes infiltration of fibroblasts to migrate into clot from surrounding connective tissue to repair gap and form scar tissue