Cell+Motility


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

toc

=**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
 * 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