11. Invasion - regulation of cell migration Flashcards
What percentage of human tumours are derived from epithelial tissues?
80-90% (tight junctions and polarised, based on top of BM)
What happens to polarity in tumours?
Loss of polarisation
How do cells pass through the BM?
- Secrete proteases to clip the BM
* Then make protrusions bu cleaving ECM proteins and invade the surrounding tissue
What are the 2 types of cell motility?
- Individual (single cell)
* Collective (group of cells)
What do both types of motility require?
- Integrins
* Proteases
Name 4 methods of migration for different tumour types, and give examples of these tumours
- Amoeboid - lymphoma, leukaemia
- Mesenchymal - fibrosarcoma, glioblastoma
- Cluster/cohorts - epithelial, melanoma
- Multicellular - epithelial, vascular
Which cell migration types are the following found in: • Integrins • Proteases (clears ECM for tracks) • Cadherins • Gap junctions
- Integrins - all
- Proteases - all
- Cadherins - collective
- Gap junctions - collective
Describe which physiological event tumour cell invasion mimics
• Morphogenetic events
• Normally if a confluent monolayer is scraped, cells sense the space and migrate together to close the gap (healing)
- collective migration
• Tumour cells demonstrate this but it is not organised
• Contact inhibition is ineffective in tumour cells, and growth is faster
How does the cytoskeleton of a cell change morphology when it is stimulated to migrate?
- Actin-based cytoskeleton moves towards apex
* Microtubule cytoskeleton moves basally
How are cells attached to the ECM?
- Focal adhesions
- Cytoskeleton is engaged
- Hooking onto ECM by dimer integrin receptors - transmembrane proteins with a short cytoplasmic tail (no enzymatic activity)
- Integrins have docking places for cytoskeletal proteins - form a plaque/complex of proteins which mediates the interaction with actin fibres
What are filopodia?
- Finger-like protrusions rich in actin filaments
- Sense the environment, telling the cell where they should attach
- Coordinate movement
What is vinculin?
- Actin-binding protein
* Bundling protein
What are lamellipodia?
- Sheet-like protrusions rich in actin filaments
- Project to the front of the cell in the same direction as the movement of the cell
- The sheets then ruffle back so the cell can move
What is hapoptatic motility?
Directional motility or outgrowth of cells with no purpose
What is chemotactic motility?
Movement in response to a chemical stimulus
What are the steps of cell movement?
- Extension - lamellipod
- Adhesion - focal adhesion
- Translocation - back of cell contracts to move the cell forward
- De-adhesion - old adhesions left behind
Cell moves one step at a time
Are actin monomers polarised?
Yes - different structures on each end ( + and - )
There is a complex regulation between what states of actin?
Monomer (small soluble subunits) and large filamentous polymer
What happens to the general actin arrangement when a signal reaches the cell (causing a change in polarity)?
- Rapid disassembly of the filaments
- Rapid diffusion of the actin monomers
- Reassembly at the side of the cell that is going to the source
- Repolarisation of cell
What form and arrangement is actin in, in the filopodium?
- Filamentous form
* Parallel arrangement
How are stress fibres organised?
- Anti-parallel organisation of the filaments
* Necessary for contraction
At what structures do stress fibres end?
Focal adhesions
How are the fibres arranged in the lamellipodia?
- No direct fibres
- Branched and cross-linked fibres, like a net
- Provide support to the big sheet of membrane
What is the rate-limiting step in the organisation of the cytoskeleton?
- Nucleation
- Requires a lot of energy
- Arp (actin-related proteins) help monomers to form a trimer, to initiate polymerisation
- The arp-2,3 complex binds to the minus end of the actin filament to form a trimer
- Once this step happens, filaments can form
Describe the step in the organisation of the cytoskeleton following nucleation?
Elongation
• Different classes of proteins assist:
• e.g. profilin binds to G-actin (monomeric) and drags it to the actin filament
• e.g. thymosin binds to actin and inhibits the polymerisation process (acts like a brake)
Describe the step in the organisation of the cytoskeleton following elongation?
Capping
• Capping proteins regulate the elongation process
• Binds to the end of the filament and prevents monomers from being added
• Once adding is blocked, a disassembly process results in the shortening of the filament
Name the capping proteins
+ end
• Cap Z
• Gelsolin
• Fragmin
- end
• Tropomodulin
• Arp complex
What is ‘severing’ in the organisation of the cytoskeleton?
- Regulation of filament size
- The unsevered actin filament grows and shrinks
- Severing proteins chop the filament up
- This counter-intuitively generates more ends so the filaments grow rapidly
Name the severing proteins
- Gelsolin
- ADF/cofilin
- Fragmin/severin
What does fascin do?
Bind filaments together at a particular distance
What does fimbrin do?
Bind filaments at a long distance from one another, together
What is alpha-actinin?
A dimer, which binds filaments
What does spectrin, filamin and dystrophin do?
- Cross-link the filaments in particular angles
* Dystrophin helps attach filaments to the plasma membrane
Which cross-linking protein is mutated in muscular diseases?
Dystrophin
For motor proteins coming in, what does this depend on?
- Way in which the proteins are bundling
* Distance of the filaments (if too close, motor proteins can’t come in)
Wha happens if there are 2 shorter separate filaments connecting to the same longer filament, and they move towards each other?
Buckling
What is the precise angle of cross-linking proteins in the lamellar?
70º
Which protein is responsible for the branching appearance of filaments as the cells move forward in the lamellar?
Arp-2 complex
What does the Arp-2 complex do once it binds to the sides of the filament at 70º?
Nucleates (forming a trimer of G-actin) and elongates (branches)
What happens to the rigid cell cortex when the cell needs to move and project?
- Must be broken - allows the cell membrane to flow forward
- Called ‘gel-sol’ transition
- Gel is a rigid structure of the actin cytoskeleton - cross-linking proteins hold the filaments as a mesh here
- Gel mesh is broken down if the membrane pushes through - severing
- Actin cross-linking proteins are still present - just no mesh
- Allows a sol that can flow - cytoplasm can move to another area
What is Wiskott-Aldrich Syndrome (WAS)?
- Immune cells impaired as they cannot remodel cytoskeleton
- Cannot phagocytose
- Immunodeficiency, eczema, autoimmunity
What is Bullous Pemphigoid?
- Autoimmune skin disease
- Breakdown between skin cells and their BM
- Due to antibody damage
How do the filaments organise themselves in:
• Lamellipodium extension
• Focal adhesion formation
Extension
• Lot of actin polymerisation
Focal adhesion formation • Assembling • Nucleation • Elongation • Capping • Severing • Branching • Bundling
What happens if the cells don’t contract at the back and detach to move it forward?
Cell will rip apart
Where in the filopodia does elongation occur?
Only at the tip - keeps chain localised and thin
What are the 4 signalling mechanisms that regulate the actin cytoskeleton?
- Ion flux changes
- Phosphoinositide signalling (phospholipid binding)
- Kinases/phosphatases - phosphorylation of cytoskeleton proteins
- Signalling cascades via small GTPases - master regulators
Which super-family do the Rho subfamily of small GTPases belong to?
Ras super-family
Describe the control of the actin cytoskeleton by small G proteins?
- Activation - GDP => GTP
- Small G proteins bind to effectors
- Effectors are the messengers that carry out actions
- Proteins inactivated by hydrolysis of GTP => GDP
What are the best know proteins in the Rho family?
- Cdc42
- Rac
- Rho
What happens when CDC42, RAC and RHO are activated?
- Cdc42 - induces filopodia
- Rac - induces lamellipodia, expansion and flattening of the cell
- Rho - induces stress fibres
What does Rac protein activate and induce?
- Activates WAVE and Arp-2/3
* Induces polymerisation
What does activated Cdc42 activate?
- WASP
- This regulates Arp-2/3
- Branching out to activate many proteins
- Mutated in Wiskott-Aldrich Syndrome
Which Rho family proteins does focal adhesion assembly involve?
Rac and Rho
Which Rho family proteins does contraction to retract the cell involve?
Mostly Rho
What happens to the cell in contraction if Rho is blocked?
Cell may rip apart as it’s involved in detatchment
Which processes of filopodia does Cdc42 control?
- Exploratory processes
* Driving polarised motility and actin polymerisation
Does lamellipodia or filopodia occur first?
Filopodia, as it tests where the cell is going
