Cancer 11 - Invasion - regulation of cell migration Flashcards
How do tumours progress (stages)
- Homeostasis
- Genetic alterations
- Hyper-proliferaion
- De-differentiation
- Invasion
In the de-differentiation stage of tumour progression, what things happen
- Cell-cell contacts disassemble
2. Loss of polarity of the cell
In the invasion stage of tumour progression, what things happen
- Increased motility
2. Cleavage of ECM proteins (e.g. via MMPs)
What 4 molecular mechanisms regulate motility
- Microfilaments
- Regulation of actin dynamics
- Cytoskeletal proteins
- Signalling proteins
What are the 2 types of tumour migration/motility
- Single cell migration - amoeboid (circular) or mesenchymal - requires integrins and proteases
- Collective cell migration - requires cadherins and gap junctions - groups of cells detach and forms clusters/cohorts or multicellular sheets - cells follow leader cell
Tumour cell metastasis mimics morphogenetic events. Give an example of this
e.g. collective cell migration in vascular sprouting
Give 2 differences between invasive cells and primary tumours
Invasive cells have an up regulation of cytoskeleton regulation and motility machinery
What gives invasive cells stimuli to move
- Organogenesis and morphogenesis
- Wounding
- GFs/chemoattractants
- Dedifferentiation
What controls the direction of invasive cells to go?
Polarity
What signals invasive cells to stop moving
Contact-inhibition motility
How do invasive cells move?
Through specialised structures (focal adhesion, lamellae, filopodium)
What hooks cells onto substratum whilst they move?
Focal adhesions (on the terminal end of actin filaments)
Integrin does most of the hooking
What are filopodia and how are they linked into bundles
Finger like protrusions containing many actin filaments that are cross linked into bundles by actin binding proteins (e.g. fascin and fimbrin, vinculin)
What is a lamellopodia
Sheet like membrane rich with actin filaments - they project to the front then ruffle back when the cell moves to allow movement
Control of cell movement occurs within the cell to coordinate what is happening in different parts; but also outside the cell. Why is this control needed?
Within cell - To regulate adhesion/release of cell/ECM receptors.
Outside cell - to respond to external influences (requires sensors)
Describe the 2 types of motility
cell movement involves cell changing shape
- Haptotaxis - no purpose
2. Chemotaxis - purpose (e.g. responding to GF)
Describe the 4 phases involved in cell motility
- Extension - of cell body in direction of movement
- Adhesion - led by lamellipodium, then filopodia hook onto ECM - forming new focal adhesion
- Translocation - contraction of cell body - bring back of cell forwards - needs energy
- De-adhesion of previous focal adhesions
Explain actin filament polarity
G-actin (small soluble units) and F-actin (large, twisted, filamentous polymer)
- Signal reaches cell (e.g. nutrient source)
- Filaments rapidly disassembled and monomers rapidly diffuse across cell –> reform at the end of the cell towards the site of the signal
- Cell repolarises moving towards the nutrient signal
What filament organisation do stress fibres have?
Antiparallel filament organisation - affect whole cell body when contract to produce force
Stress fibres have focal adhesions on their ends
What filament organisation do filopodia have?
Parallel filament organisation
What filament organisation do lamellipodium have?
Branched and cross linked filaments –> provides support to big membrane sheet
What is the key process in the remodelling of actin filaments
Nucleation
What is the limiting step in the actin remodelling dynamic
Nucleation - lots of energy needed
What happens in nucleation
- Arp 2/3 protein complexes (Actin like proteins) cause actin monomers to form trimers —> eventually form filaments
- Arp proteins usually located at the minus end (so that the plus end is free to add monomers)
What step happens after nucleation. What happens in this step
Elongation
- Profilin binds G-actin and brings it to filament (promotes assembly)
- Thymosin binds G-actin but inhibits polymerisation - doesn’t bring G actin to filament
Profilin and thymosin are competing
What is sequestered in the elongation step
B4-thymosin
ADF/cofilin (these do not inhibit polymerisation)
What steps follow elongation
Capping, then severing
Capping = capping proteins regulate elongation of actin filament (stop elongation when necessary)
Capping proteins at + end: CapZ, gelsolin, fragmin/severin
Capping proteins at - end: Tropomodulin, Arp complex
Severing = regulating filament size - severing filaments promotes growth as more ends are made
Severing proteins: gelsolin, ADF/cofilin, fragmin/severin
Name a protein that is both a capping protein and a severing protein - its function being determined by regulation
Gelsolin
Which end does profilin-(G) actin bind for elongation
Plus end
When the filament is broken down, what are the 2 possible outcomes
- Cell glues filament pieces back together (annealing)
2. Short filament grows a new fibre
What occurs after severing
Cross-linking (+ bundling of newly formed filaments)
- Fascin binds filaments at particular distances
- Fibrin binds filaments at longer distances
- Spectrin, filamin, dystrophin cross link multiple filaments at particular angles
Bundling occurs due to Vinculin - affects motor protein binding - if distance between filaments wide enough (enough distance), then motor protein enters (Eg myosin) - allows contraction
What occurs after cross linking?
What is particular about this step
Branching
Occurs in the lamella at 70 degrees.
Arp2 complex causes branching appearance of filaments in lamella as cells move forward
Which 2 things does the Arp2 complex do?
Nucleation
Filament elongation/angling
Name a disease that is not caused by deregulation of the actin cytoskeleton
Alzheimers
How can a gel-sol transition be achieved with the cytoskeleton
Cross linking proteins hold cytoskeleton - creates a scaffold/mesh
Severing actin filaments allows cytoplasm flow in parts of the cell
Describe different uses of actin organisation in cell movement
- Polymerisation in lamellipodium extension
- Attachment to ECM at new adhesion
- Gel/sol transition on cortex at membrane going forward in translocation
- Detachment during de-adhesion
Explain the actin organisation processes occurring in lamellar protrusion
Polymerisation, disassembly, branching, capping
Severing at back of lamellae, releases monomers which goes to front of cell –> F actin filament assembled at front and generates pushing force of cells to move forward
Explain the actin organisation processes in filopodia
Polymerisation, bundling, cross-linking
Form bundles, then quickly elongate —> pushes membrane out in localised position
Once filopodia sense no more room to go forwards or no stimulus –> collapse of filament by attracting capping proteins —> retraction by eroding base
What other cell types use bundling (a form of actin reorganisation)
Cilia, filopodia, microvilli, stereocilia
Branching - a different for of actin reorganisation - used in lamellipodia
4 signalling mechanisms that regulate the actin cytoskeleton
- Ion flux changes (i.e. intracellular Ca)
- Phosphoinositide signalling (phospholipid binding)
- Kinases/phosphatases (phosphorylation cytoskeletal proteins)
- Signalling cascades via small GTPases
Which superfamily does the Rho subfamily of small GTPases belong to
Ras super family
Name some family members of the Ras superfamily
Rac - expansion + flattening of cell, lamellipodia
Rho - stress fibre formation
Cdc42 - promotes filopodia growth
Rac & Cdc42 also activates many other cytoskeletal proteins (involved in polymerisation and organisation)
What does the Rho subfamily of small GTPases do
Activated by receptor tyrosine kinase, adhesion receptors and signal transduction pathways
Their expression is unregulated in different tumours
They participate in various cytoskeletal processes
When is the Rho GTPase inactivated and how
Inactive when bound to GDP - inactivated by hydrolysis
Which GTPase causes de-adhesion
Rho
Lamellipodium is controlled by which GTPase
Rac
Focal adhesion is controlled by which GTPases
Rac and Rho
Contraction and de-adhesion is controlled by which GTPase
Rho
