Lecture 16. Cell Motility and Chemotaxis

Question 1

What does Dictyostellium discoideum (amoeba) do in response to cAMP?

Answer 1

Aggregates

Question 2

What is chemotaxis?

Answer 2

The ability to sense and move towards or away from a directional signal

Question 3

What functions of the body rely on chemotaxis?

Answer 3

Directed cell movements during development (e.g. gastrulation, neural crest migration and primordial germ cell migration)
Immune surveillance (phagocytosis of pathogens)
The inflammatory response to injury (lymphocyte migration)
Wound healing (fibroblast migration)

Question 4

How can a neutrophil successfully chase down and phagocytose a bacterium as a part of immune surveillance?

Answer 4

The bacteria secretes a tri-peptide which is sensed by receptors on the neutrophil plasma membrane

Question 5

How can lymphocytes rapidly migrate in response to injury?

Answer 5

Piercing a zebrafish larvae can visualise the process (larva is optically transparent)
Lymphocytes can be seen migrating to the site of injury. At later times, Fibroblasts which initially appear static will migrate to the wound site to repair the wounded area

Question 6

How is the lamellipod extended by actin filament assembly and disassembly at the leading edge?

Answer 6

Requires actin (particularly branching actin) polymerisation regulated by the Arp2/3 complex which sits on the side of a filament in order to nucleate new actin filament formation
As this actin is polymerised, it pushes the plasma membrane forward int eh direction of the chemotaxic agent after receiving a signal from the extracellular surface through a transmembrane receptor

Question 7

What actin structures make up the lamellipodia?

Answer 7

Branched networks that help push the cell forward

Question 7

How does a particular cell move forward during chemotaxis?

Answer 7

1. Locomotion begins with the extension of one or more Lamellipodia from the leading edge of the cell. This involves branched actin polymerisation which pushes the membrane forward
   2) New focal adhesions (feet) are formed at specialised sites on the plasma-membrane. These contain Integrins which link actin to the extracellular matrix (ECM)
   3) The bulk of the cytoplasm is pushed forward by contraction of Actin-Myosin II bundles (stress fibers) at the rear of the cell in a process called translocation
   4) The trailing edge of the cell detaches from the ECM and retracts into the cell body. During this process the endocytic machinery internalises integrins and transports them to the front of the cell to be used again (endocytic recycling)

Question 8

What actin structures make up the filopodium?

Answer 8

Tight parallel bundles

Question 9

What actin structures make up the stress fibres near the back end of the cell?

Answer 9

Contractile bundles that have to be far enough apart to allow myosin motor proteins to enter, allowing for contraction

Question 10

What regulates the actin assembly?

Answer 10

The Rho superfamily of GTPases
Microinjection of dominant activated forms of the Cdc42, Rac and Rho (members of the Rho superfamily of GTPases) have different effects on actin filament organisation in cells

Question 11

What role does Rho have in the formation of stress fibres?

Answer 11

Rho activates formin which makes straight actin filament cables which are then bundles together to create contractile acting bundles
Rho kinase stimulates the movement of a myosin motor protein

Question 12

What role does Rac have in the formation of the lamellipodia?

Answer 12

Rac activates WASp which activates ARP2/3 complex which activates actin polymerisation, activating lamellipodia formation

Question 13

What role does Cdc42 have in the formation of the filopodia?

Answer 13

Cdc42 activates the formin which causes actin polymerisation that creates the filopodia (no myosin activity required)

Question 14

How is the intracellular distribution of each class of GTPase controlled furing chemotaxis?

Answer 14

Each class of GTPase is controlled so that a gradient is formed within the cell. At the leading (growing) edge Cdc42 activation promotes formin and Arp2/Arp3 dependent actin assembly to promote filopodia and lamellapodia growth.
This in turn stimulates Rac GTPase which further promotes branched actin network assembly behind the leading edge. Rac activation leads to Rho GTPase activation at the lagging edge of the cell which promotes stress fibre formation and myosin II activation which powers forward movement of the bulk of the cell contents. This systems is highly dynamic as the cell can rapidly change direction in response to changes in concentration and direction of chemotactic signals.

Question 15

What disrupts the movement of fibroblasts needed for wound healing?

Answer 15

If the cells express dominant negative versions of either Rac, Cdc42 or Rho GTPase
In other words all three GTPases are needed to respond to direct cell migration

Question 16

How do migratory cells such as lymphocytes, macrophages and fibroblasts attach to the extracellular matrix?

Answer 16

Via focal adhesion sites
These sites contain clustered Integrins which interact with actomyosin stress fibres on the inside of cells and fibronectin (a component of the extracellular matrix) on the outside of the cells. These dynamic linkages allow cells to migrate to where they are needed.
Microtubules are required for the efficient endocytic recycling of Integrins and other components of focal adhesions from the back for their re-use in the establishment of new focal adhesions at the front of the cell.

Question 17

What is the difference between epithelial collective migration and mesenchymal collective migration?

Answer 17

Mesenchymal collective migration happens when all the cells migrate in the same direction to the signal, but migrating independently of each other
Epithelial collective migration is movement when cells are connected to each other (such as in wound healing) - whole cell layer moving coordinately together

Question 18

What is mesenchymal cell migration a feature of?

Answer 18

Metastatic cancer cells
During tumourigenesis epithelial cells can lose adherence to their neighbours and gain the ability to migrate through extracellular matrix and invade other tissues. Entry into blood vessels aids the spread of such tumour cells around the body. In this situation deregulated cell migration has a negative impact on human health (metastasis)
Therefore it is important that the control of cell migration is critically important to make sure cells migrate only when told too

Question 19

How do nerve growth cones migrate towards each other to form new synaptic termini?

Answer 19

Using the same actin-dependent migration process that we have observed in lymphocytes and fibroblasts

Question 20

Why is neuroplasticity important?

Answer 20

Neuroplasticity is important for forming new connections in the brain, for example as a result of a new experience, to allow you to learn, form new patterns of thought, change your perception and, ultimately, change the way you experience life

Question 21

What can certain pathogenic bacteria (e.g Listeria) and viruses (Vaccinia virus) usurp?

Answer 21

The actin polymerisation machinery to form comet tails for movements

Question 22

How does Listeria usurp the polymerisation machinery to from a comet tail for movement?

Answer 22

Listeria expresses ActA on the back end of the bacteria and mimics WASp
Has the ability to artificially activate Arp2/3 complex formation and branched filament formation at the back of the bacteria which pushes the bacteria forward

Question 23

How can cilia and flagella be distinguished?

Answer 23

By their beating patterns but are nearly identical in structure. Flagella (found on sperm and many protozoa) tend to be longer than cilia

Question 24

How do cilia and flagella propel cells forward?

Answer 24

Both cilia and flagella can propel cells as they cycle rapidly beating up to 100 times a second. Co-ordinated beating of many cilia can move large cells

Question 25

How are cilia and flagella constructed?

Answer 25

From a complex array of microtubules whose organisation is different along their length. The basal body sits in the cytoplasm of the cell and tethers and nucleates the axoneme (the main body of the cilia or flagella) through a region called the transitional zone. Whereas the top of the basal body has nine pairs of triplets (much like a centriole) the axoneme has nine pairs of doublets and two central MTs which are not linked (sometimes refereed to as 9 + 2)

Question 26

What happens to the motor domain during the “power stroke” which causes movement?

Answer 26

The motor domain undergoes a conformational change that causes the microtubule-binding stalk to pivot relative to the cargo-binding tail with the result that one microtubule slides relative to the other. This sliding produces the bending movement needed for cilia to beat and propel the cell or other particles. Groups of dynein molecules responsible for movement in opposite directions are activated and inactivated in a coordinated fashion so that the cilia or flagella can bend and flex co-ordinately (sperm uses this for movement)