Invasion – Regulation of Cell Motility Flashcards
What are the changes that occur in the cells that occur during tumour progression?
- Genetic alterations lead to…
- Hyperproliferation
- Disassembly of cell-cell contacts
- Loss of polarity
- Increased motility
- Cleavage of ECM proteins
What are the different types of tumour cell migration?
- Single cell migration (ameboid)
- Mesenchymal single cells
- Mesenchymal chains
- Clusters/cohorts
- Multicellular strands/sheets
What does collective migration require?
- Modulation of cell-cell contacts
- Communication between cells e.g gap junctions
What physiological phenomena does tumour migration mimic?
- Morphogenesis e.g. angiogenesis when forming breast tissue in breast-feeding mothers
- This is where there must be collective migration of cells for example in vascular sprouting
What did a comparison of the expression profile of invasive cells vs primary tumours show to be upregulated in invasive cells?
- Cytoskeleton regulation
- Motility machinery
1) What makes normal migrating cells stop moving?
2) How are tumour cells different in this aspect?
1) Contact inhibition of locomotion
2) They lose contact inhibition of locomotion so they can multilayer
What are the 2 types of cell motility - in terms of what they are motile in response to?
- Hapoptatic - random movement
- Chemotactic - movement along a chemical gradient
What is another term for ECM proteins?
- Substratum
In cell motility, what structures allow attachment to the substratum, briefly describe these structures (much detail not needed) and what happens?
- Focal adhesions
- These are integrins which extend extracellularly, have a transcellular domain and an intracellular domain
- The intracellular domain binds actin cytoskeleton filaments at the ends of the cell
- They hook onto the ECM matrix and provide points of attachment
- Traction forces are generated about this attachment
1) What is the name of the monomers of actin filaments?
2) What is the name of the actin filaments?
1)
- G-actin
2)
- F-actin
What are filopodia?
- Finger-like protrusions that are rich in actin filaments
- They sense the local environment
What are lamellipodia?
- Sheet-like protrusions that are rich in actin filaments
- In order for the cell to migrate in a direction, the sheets of membrane project to the front of the cell
- The sheets then ruffle back, so the cell can move
What are the four main stages of cell movement and briefly describe what is happening?
- Extension - the focal adhesions act like feet, hooking onto substratum, cell protrusions (lamellipod) protrude and extend
- Adhesion - the focal adhesion sticks onto the substratum (picture suction cups-like thing going on)
- Translocation - the back of the cell must contract in order to propel the rest of the cell forward, there are various cytoskeletal changes and so forth
- De-adhesion - the focal adhesions at the back are left behind (picture a suction cup coming off)
What are the attachments between the cell and the surface that it is moving along called?
- Focal adhesions
What are the monomers of actin filaments?
- G-actin
Describe the polarity of actin filaments
- They have a plus end and a minus end
- The monomers preferentially get added on at the plus end
6 processes in actin filament formation and arrangement?
- Nucleation
- Elongation
- Capping
- Severing
- Cross-linking / bundling
- Branching
Describe what happens in nucleation - in regards actin
- Arp-2,3 binds the minus end of the actin filament to form the initial trimer, and extend the filament
Describe what happens in the elongation step in regards actin
- After Arp-2,3 binds to the actin filament and forms a trimer in nucleation, elongation occurs
- Profilin binds G-actin and drags it over actin filament
- Thymosin also competes with profilin to inhibit the polymerisation process
What do Beta-4 thymosin and ADF / cofilin do?
- It competes with profilin in order to prevent it from elongating the actin filament
- They sequester the G-actin to prevent it being used in elongation
Describe what happens in the capping process in regards actin filaments
- After nucleation and elongation, capping of the actin filament ends occurs
- This is in order to prevent more monomers from being added on
- CapZ, Gelsolin and Fragmin / severin cap off the +ve end and tropomodulin and the Arp complex cap off the -ve end
Describe what happens in the severing process in regards the actin filament and why this is useful
- The actin filaments once formed can be severed into smaller branches which themselves can be polymerised, therefore it is a method to speed up the actin polymerisation process, severed ends may then be re-annealed or just form separate fibres
- The filaments are severed by the following proteins:
- Gelsolin
- ADF / cofilin
- Fragmin / severin
Describe what happens in the branching process in regards actin filaments and the importance of this
- Arp-2 causes branching appearance of actin filaments, arranging them at 70 degrees (because Arp-2 complex can initiate polymerisation forming new filaments at angles on already existing filaments to form branches)
- This facilitates nucleation and elongation
- It facilitates lamellar (lamellipod) protrusion also
What protein complex is important in initiating polymerisation?
- Arp2/3
- This forms a trimer with actin and is good at initiating polymerisation
Describe what happens in cross-linking and bundling of actin filaments, mentioning some proteins involved and the roles they play
- Fascin - bind filaments
- Fimbrin - bind filaments at a long distance
- Alpha-actinin - dimer that brings together filaments
- Spectrin, filamin and dystrophin all cross-link actin filaments
Why is it important that actin filaments are certain distances apart?
- For motor proteins to be able to slot in
- E.g. Myosin
What is the limiting step in actin dynamics?
- Formation of Arp2/3-actin trimers to initiate polymerisation
State two proteins that bind to free G-actin (excluding Arp-2,3) and describe how they affect elongation
- Promote elongation – profilin (these deliver the G-actin to the growing filament)
- Sequesters G-actin - beta–4 thymosin, ADF / cofilin
Name some + end capping proteins
- CapZ
- Gelsolin
- Fragmin/severin
Name some – end capping proteins
- Tropomodulin
- Arp2/3
Name some severing proteins
- Gelsolin
- Fragmin / severin
- ADF / Cofilin
Why might it be desirable to sever actin filaments?
- Actin filaments can grow and shrink more rapidly when severed - more bits for polymerisation to act on
What can happen to single filaments of actin to improve their structural integrity?
- They can be bundled or cross-linked
Name some proteins involved in the processes of bundling and cross-linking actin filaments
- Alpha-actinin
- Fimbrin
- Filamin
- Spectrin
- Villin
- Vinculin
Which protein allows branching of the actin filaments?
- Arp2/3
At what angle do actin filaments branch in the lamellar?
- 70 degrees
Summarise the actions of Arp2/3
- They initiate nucleation
- They cap filaments
- They cause branching
What is the gel-sol transition (including what the gel and sol states are) and how is it carried out? Also why is this useful?
- The actin filaments can be severed to break the cross-linking proteins holding the mesh-like gel structure and makes the cell more fluid and able to flow as a sol structure
- This, alongside actin polymerisation, is important in rearranging cell polarity for directional motility
Describe the actin processes that take place during the protrusion of lamellipodia
- There is disassembly of actin filaments at the back end of the cell from F-actin (filamentous) to G-actin (monomeric actin)
- There is then migration of the G-actin to the end where lamellipod protrusion will occur (leading edge) and assembly of the actin filament
- Branching and capping also occurs
- There is net filament assembly at the leading edge
Describe the actin processes that take place during the formation of filopodia
- Actin polymerisation
- Bundling and cross-linking (NO branching because there is only parallel arrangement of actin fibres at the lamellipod)
- Elongation for the protrusion in a localised position
- As soon as the finger wants to retract (e.g. due to removal of a stimulus), capping occurs to stop elongation and the filopod will collapse at the base
State four signalling mechanisms that regulate the actin cytoskeleton
- Ion flux changes (e.g. calcium levels)
- Phosphoinositide signalling
- Kinases/phosphatases
- Small GTPases
What are the three most important small GTPases in terms of the actin cytoskeleton and what does activation of each cause?
- Cdc42 – filopodia - binds WASP, which in turn activates Arp-2,3 to control polarised motility and actin mobilisation in filopodia
- Rac – lamellipodia - activates WAVE and Arp-2,3 to induce polymerisation
- Rho – stress fibres - induces stress fibre contraction to aid mobility
- NOTE: these are all part of the Ras super-family of GTPases
Explain how Rac causes actin polymerisation/organisation
- Rac binds to and activates WAVE
- WAVE then activates Arp2/3, which is important in actin organisation
Explain how Cdc42 causes actin polymerisation/organisation
- Cdc42 binds to WASP
- WASP also activates Arp2/3
Which small GTPases are involved in lamellipodia protrusion?
- Rac
Which small GTPases are involved in focal adhesion assembly?
- Rac and Rho
Which small GTPases are involved in contraction?
- Rho (stress fibres are important for contraction)
