Steve Ward Flashcards
Cell migration
A fundamental and critical function that requires the coordination of several cellular processes that operate in a cycle
A highly integrated, multi-step process
Roles of cell migration
Wound healing (innate and adaptive immune response)
Regeneration
Development
Tumour metastases and angiogenesis
Chemokinesis
Migration driven by soluble chemokines, without a cue gradient to provide a directional bias
Haptokinesis
Migration along a surface, using immobilised ligands e.g. chemokines/integrins, without a cue gradient to provide a directional bias
Chemotaxis
Migration driven by a gradient of soluble chemokines, occurring when there is an asymmetry in chemoattractant (i.e. in its rate/density) and local cue gradients can be followed
Haptotaxis
Migration along a surface, guided by a gradient of immobilised chemoattractants/adhesion receptor ligands, providing a directional bias
Roles of chemotaxis
Immune cell development, immunosurveillance, activation
Sourcing of nutrients (by prokaryotes)
Metastatic cancer migration to growth factors
Embryonic cell development
Formation of multi-cellular structures in protozoa
Amoeboid chemotaxis
Consists of 3 key components, each with overlapping but distinct properties
- Pseudopodia formation
- Polarisation
- Directional sensing
Pseudopodia formation in amoeboid chemotaxis
Regulated by the chemoattractant
G-protein independent
Enhanced by PIP3
Polarisation in amoeboid chemotaxis
Elongation of the cell shape
Cells ‘turn’ towards the gradient because the anterior is more sensitive than the posterior
G-protein dependent
Enhanced by PIP3
Directional sensing in amoeboid chemotaxis
Occurs even in immobilised cells
Proteins containing PH domains accumulate at the leading edge of the membrane
G-protein dependent
PIP3 independent
Mediators involved in establishing and maintaining cell polarity in response to extracellular stimuli
RhoGTPases PI3Ks Integrins Microtubules Vesicular transport
Cdc42
Rho G protein
“Master regulator” of cell polarity
How does Cdc42 influence polarity?
Restricts where lamellipodia form
Localises Golgi and MTOC in front of the nucleus, oriented towards the leading edge of the cell
How does Cdc42 exert its effects on MTOC?
Through PAR6 and aPKC, leading to activation of dyne motors that “pull” the MTOC into its new position
What does the polarisation of PIP3/PIP2 in response to cellular exposure chemoattractants involve?
The localised accumulation of PI3Ks and PTEN, which generate/remove PIP3/PIP2
PI3Ks rapidly accumulate at the leading edge of cells in response to chemoattractant, whereas PTEN is restricted to the sides/rear, restricting protrusions to the front of the cell
Initial experiments
Cells with altered PI3K/PTEN activity can usually still migrate but exhibit significantly reduced directionality along a chemoattractant gradient
More recent experiments
PI3K/PTEN activity are dispensable for chemotaxis
Hoeller and Kay, 2007
Multiple KO strain lacking all 5 Dictyostelium class I pi3k genes as well as PTEN was still able to undergo chemotaxis in strong chemoattractant gradients, but showed reduced speed Therefore showing that the polarisation of membrane PIP3 is not essential for directed chemotaxis but ensures rapid movement
How is activation of the actin polymerisation machinery achieved?
Through activation of Rac GEFs
Actin polymerisation is required for protrusions
Rac positive feedback loops for maintaining directional protrusions
Stimulates recruitment/activation of PI3K
Microtubule polymerisation activates Rac and Rac, in turn, stabilises microtubules
WASP/WAVE proteins
Targets of Rac and Cdc42 - associate with WASP/WAVE proteins, that can then bind to Arp2/3 complexes via their WCA domains, promoting Arp2/3 activation
Arp2/3 complex
Binds to pre-existing actin filaments at 70 degrees, leading to the nucleation of new actin filaments that branch off in all directions
This results in the formation of a dense web/branched network of actin filaments
Actin polymerisation is regulated by…
…accessory proteins that control filament length and stability
Profilin
Facilitates growth of the actin filament through (+) end elongation
Profilin binds to the (+) end of actin monomers, leading the (-) end free to bind to the (+) end of the growing actin polymer
Controlling the local concentration of profiling can lead to explosive polymer growth in the required areas of the cell
Cofilin
Destabilises the actin filament helix through binding and inducing torsional strain, leading to depolymerisation
Selectively binds to ADP-actin (most new actin in growing polymers is ATP-actin), therefore selectively destabilising older polymers
CapZ
Capping protein
Binds to ("caps") the (+) end of actin filaments, favouring their disassemblyTropomodulin
Capping protein
Binds to ("caps") the (-) end of actin filaments, favouring growthHow are protrusions stabilised?
Through the formation of adhesions (Rac- and Cdc42-dependent)
Adhesions stabilise lamellipodia through mediating attachment to the ECM
