Cell Motility

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CELLULAR MOTILITY Lecture 5 January 20th, 2006
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Cell Motility

• Migration/Movement of cells requires
• 1- Anchorage
• 2- Active cytosketelon

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Intercellular Motility (movements
between cell sheets) Cell movements are
important for the morphogenesis of the neural tube

• During the formation of the neural tube, a flat
  layer of cells must roll into a tube (neural
  tube). This is possible because of contractile
  forces generated by actin filaments at the apical
  ends of the cell.

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Dynamics of actin polymerization
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Treadmilling
T
D
T
D
D
(-)
()
ADP-actin
ATP-actin
Steady state concentration
Css Filament turnover Pure
actin 0.1 µM 3µm / 90 min
Lamellipodium ?
3 µm / 1 min
2 µM
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Actin Depolymerizing Factor (ADF) / Cofilin

• Ubiquitous, conserved, stimulus-responsive
  regulators of actin dynamics in motile processes
• Embryonic development
• Yeast endocytosis
• Cytokinesis
• Listeria propulsion
• Localized in motile regions of cells (at the
  leading edge)

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ADF increases the treadmilling of F-actin
ADF binds F-ADP-Actin better than F-ATP-Actin
T
D
D
D
D
T
Increase in Css gt faster end growth
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Profilin increase the rate of actin-based
motility
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Synergy between Profilin and ADF
2. Profilin transports actin-ATP to leading edge.
1. Profilin binding increases rate of actin-ADP
to actin-ATP.
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MOLECULAR BASIS OF CELL MOTILITY
Contraction
Contraction
Leading edge
Net movement
FOCAL ADHESION
Actin network Focal adhesions Focal
complexes Microtubules Stress fibers Forces on
the substrate Forces on the cell cortex
Relaxation
Others
Rho kinase
Rho
Myosin phosphatase
MLCK
Ca /Calmodulin
PAK
Some regulators of Focal adhesion
turnover FAK PAK Src Microtubules Calpain Rac/R
ho
Contraction
Rac
Cdc 42
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Actin Filaments in Migrating Cells
The organization of actin fibers varies
significantly in different areas of the cell.
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Filament branching array in lamellipodia
Branching of actin cytoskeleton at the leading
edge requires Arp 2/3
Branching
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The Arp2/3 complex downstream target of multiple
signaling pathways leading to actin assembly
ARP Actin Related Protein. Has similar
structure to actin.

• Seven conserved subunits including Arp2 and
  Arp3.
• Is activated at the surface of Listeria by the
  ActA protein to stimulate actin polymerization
  and bacterial propulsion.
• Localizes at motile regions of cell.
• Generates new actin filaments in a
  stimulus-responsive and spatially controlled
  fashion.

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WASP family proteins activators of Arp2/3
complex

(signaling molecules)
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WASP family proteins activators of Arp2/3
complex

Arp2/3 G-actin
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Dynamics of actin polymerization at the leading
edge
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Reconstitution of actin-based movement from pure
proteins (Loisel et al., Nature 1999)

• Proteins required for movement
• 1) N-WASP (resp. ActA)-activated Arp2/3
  site-directed generation of barbed ends
• 2) Actin, ADF/cofilin,Capping protein chemostat
  maintaining a high steady-state concentration of
  ATP-G-actin
• Not required, but improve movement
• VASP, Profilin, a-actinin.

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Role of capping proteins in motility
funnelling the treadmilling

• Capping proteins are known to be required for
  efficient motility in vivo.
• Capping proteins block most of the barbed ends,
  hence they increase the concentration of
  monomeric ATP-actin.

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Making many types of actin fibersRho family
GTPases
Rho Myosin Actin stress fibers Contractility Micro
tubule stability gene expression Cell cycle
Rac Cell migration Actin polymerization Cell
spreading Membr. trafficking Gene expression Cell
cycle apoptosis
Cdc42 Actin polymerization Cell polarity
Migration Gene expression Cell cycle apoptosis
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Signal-transduction pathways involved in
Rho-induced stress-fiber assembly
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Signal-transduction pathways induced by Rac and
Cdc42 (shown in red), which are thought to
contribute to the formation of actin-containing
lamellipodia and filopodia respectively