8. Invasion and Regulation of Cell Migration

Question 1

Outline the steps of tumour progression

Answer 1

Steps of tumour progression (benign-> malignant) :

1. Homeostasis.
2. Genetic alterations.
3. Hyper-proliferation.
4. De-differentiation:
   
   • Disassembly of cell-cell contacts.
   • Loss of cell polarity.
5. Invasion:
   
   • Increased motility.
   • Cleavage of ECM proteins
   • Epithelial tumour -> Basement membrane -> stroma

Question 2

Describe types of tumour cell migration

Answer 2

Types of Migration

• Either migrate as individual cells/ solo or as a cluster of cells
• Migratory strategy:
  
  • Integrins, proteases are used for individual and collective
  • Cadherins and gap junctions (for coordination) are only used for collective
• Individual cell migration:
  
  • Amoeboid – e.g. lymphomas.
  • Mesenchymal (single) – e.g. Fibrosarcoma.
• Collective cell migration:
  
  • Mesenchymal (chains) – e.g. Fibrosarcoma, glioblastoma, anaplastict.
  • Cluster/cohorts – e.g. Epithelial cancers.
  • Multicellular strands/sheets – e.g. Epithelial cancers.
    
    • Collective cell migration requires more coordination to metastasise and so still has some cell-cell junctions.

Tumour cell metastasis/migration MIMICS PHYSIOLOGICAL morphogenic events.

E.G. Branching morphogenesis in the mammary glands.

E.G. Migration of primary glial cells to repair a scratch wound (the cells stop migrating when the contact is made) – conversely, tumour cells will have no clear migration front and no sense of direction.

E.G. vascular sprouting

Administration of EGF can result in upregulation of the genes involved in:

• Cytoskeleton regulation.
• Motility machinery.

Question 3

Name reasons for cell movement

Answer 3

Stimuli to move a cell:

• organogenesis and morphogenesis
• wounding
• growth factors/chemoattractants
• dedifferentiation (tumours)

Question 4

Outline the process of cell motility

Answer 4

Motility

• The cells attach to the ECM via integrins.
  
  • Attachment to substratum – epithelium cells already have their attachements
  • Focal adhesions – close to ECM
  • Filamentous actin – ropes that organize like bundles within cell – attachement of substractum and the Integrins (monomers: a and b)
  • tail organized a plaque of cytoskeletal proteins
  • Main function: signalling port and connection to cytoskeleton

For motility, the cell uses:

1. Filopodia
   
   • finger-like projections rich in actin filaments.
   • A bundle of parallel filaments.
2. Lamellipodia
   
   • sheet-like protrusions rich in actin filaments.
   • Branched and crosslinked filaments.

Control is needed in motility for/to:

• Coordinate happenings inside the cell itself.
• Regulate adhesion/release, de-adhesion of cell-ECM.
• To respond to external influences (checking for growth factor and nutrients - sensors and directionality)

Motility: hapoptatic vs chemoptatic

Mechanism:

1. Extension
2. Adhesion
3. Translocation
4. Deadhesion

• Contraction of Filopodia and Lamellipodia can break old adhesions, allowing the cell to maintain a motion.
• A signal to move could be a nutrient source and the filaments can rapidly disassemble and then reassemble at a new site to move the cell.
• Actin filaments have a polarity – there is a plus and minus end on which different proteins can bind.
• Depending on the proteins that bind, the actin filaments can carry out different functions.

Question 5

Discuss actin filament polarity

Answer 5

G - actins -> small soluble subunits

F - actins -> large filamentous polymer

Question 6

Outline the steps of actin filament remodelling

Answer 6

1. Nucleation:

• Attachment of the actin to the cell inner membrane.
• ARP proteins form a complex and bind to actin monomers to create a nucleated actin filament (ARPs bind to the minus end).
• This is the limiting step in actin dynamics.

2. Elongation:

• Profilin facilitates actin monomer binding to the actin filament.
• Thymosin reduces actin monomer binding by sequestering the free monomers so they are not available to bind to the actin filament.

3. Capping & Severing:

• Capping – addition of a capping molecule (to + or – end) to limit elongation.
  
  • Plus-end caps – Cap Z, Gelsolin, Fragmin/Severin.
  • Minus-end caps – Tropomodulin, Arp complex.
• Severing – breaking up actin filaments:
  
  • Unsevered actin filaments grow/shrink slowly.
  • Severed populations grow/shrink more rapidly.
    
    • Proteins – gelsolin, ADF/cofilin, fragmin/severin.

4. Crosslinking and Bundling, Branching:

Crosslinking & bundling:

• This produces differing arrangements of actin filaments.
• Proteins involved include:

1. a-actinin.
2. Fimbrin.
3. Filamin.
4. Spectrin.
5. Villin.
6. Vinculin.

5. Branching:

• This protein enables branches of actin to come off at 70degree angles.
• Protein – Arp complex.

Question 7

Discuss gel-sol transition by actin filament severing

Answer 7

Gel-Sol Transition by Actin Severing

• Gels are rigid and have NOT been severed.
• Sols are not rigid (i.e. can flow) and HAVE been severed.

If it can flow: you have cross ligaments but are not connecte to teachother

Question 8

Discuss cell movement signalling pathways

Answer 8

Regulation via Signalling Pathways on the Cytoskeleton

1. Ion-flux changes – i.e. intracellular calcium.
2. Phosphoinositide signalling – i.e. phospholipid binding.
3. Kinases/phosphatases – i.e. phosphorylation of cytoskeletal proteins.
4. Signalling cascades via small GTPases.

Regulation via Signalling Pathways on the Cytoskeleton – Small GTPases:

1. Rho, Rac, Cdc42 sub-families belongs to the Ras super-family.
2. Cdc42 -> Filopodia production.
3. Rac -> Lamellipodia production.
4. Rho -> stress fibre production.
5. Over-leaf picture is one of how small GTPases contribute to movement.