Lecture 7 - Cell Polarity

Question 1

ILOs

Answer 1

1. Describe in some details the molecular mechanisms that orchestrate cell polarity, in particular GTPases
2. Understand how tools in biology have led to the discovery of these mechanisms
3. Understand that subcellular localisation is important for cell function (“where”)
4. Bridge signalling pathways/ sub‐cellular structures/ consequences for the cell as a whole
5. Interpret scientific data

Question 2

What is polarity?

Answer 2

• Polarity: Having one end morphologically and/or functionally distinct from the other
• F‐actin and MTs are polarised
• Cells are polarised
• Organisms are polarised

Question 3

Why is polarity necessary?

Answer 3

1. Cellular morphogenesis
2. Tissue morphogenesis
3. Organism morphogenesis: Single-celled, Multicellular

Although the final structures are different, the mechanisms used to organise and regulate the machinery are common to many eukaryotes

Question 4

Polarity is achieved by cytoskeletal remodelling

Answer 4

1. Requires a cell to sense and respond to its environment
2. Many signalling pathways converge on the actin and microtubule cytoskeletons
3. The cytoskeleton is a highly plastic organelle
4. Remodelling can occur in a matter of a few minutes (mitotic spindle, turnover of actin in lamellipodia )
5. Cross‐talk exists
6. Polarity of actin filaments and microtubules is critical for function

Question 5

The environment - Taxis

Answer 5

• “a motion or orientation of a cell, organism, or part in response to an external stimulus”. E.g. chemotaxis, phototaxis

Question 6

Attraction and repulsion guidance cues

Answer 6

‐ Adhesion (aka contact) type (e.g. ECM, other cell): short range

‐ Chemical type (e.g. secreted molecule): long range

Question 7

Cooperation between guidance cues

Answer 7

Question 8

Commissural neurons use guidance cues to project onto the other side of the CNS

Answer 8

Question 9

Signalling pathways

How do cells respond to their environment?

Answer 9

‐ Role of any cell surface receptor is to transduce an extracellular signal to the cell, so the cell responds to this extracellular signal

‐ Recruitment and/or activation of intermediates

‐ Downstream effectors (e.g. the cytoskeleton)

Extra‐cellular polarity cues activate intra‐cellular GTPases

Question 10

GTPases (aka G‐proteins)

Answer 10

• Large heterotrimeric
• Initiation and elongation factors
• SRP/SR Family
• Tubulins and Cytoskeletal Motor
• Small monomeric GTPases (aka small-G or Ras superfamily)

Question 11

Substrate level phosphorylation

Answer 11

Question 12

Once activated, small GTPases activate targets, directly or via more molecular switches

Answer 12

Question 13

Cdc42: a central regulator of cell polarity

Answer 13

is a small GTPase

Question 14

Discovery of PAR proteins in the worm C. elegans

Answer 14

• PARtitioning defective
• PAR proteins responsible for asymmetric positioning of the mitotic spindle + of proteins and RNAs involved in cell fate distinctions
• Now 6 PAR proteins

Question 15

Data handling practice

Answer 15

Answer: A, C, E

we are observing the localisation of PAR2 (tagged with GFP) in embryos that are either unmodified or lack par‐6 or par‐5. Here we have no information about PAR6 and PAR5 proteins (localisation, interaction).

Question 16

Thus, work from C. elegans showed that PAR proteins…

Answer 16

Thus, work from C. elegans showed that PAR proteins…

1. Relocalise over time
2. Depend on each other: anterior PAR proteins required to prevent posterior PAR proteins from localizing anteriorly, and vice versa
3. Polarise the cell

Question 17

PAR proteins also drive polarity in higher eukaryotes (aka Par3‐Par6‐aPKC complex)

Answer 17

Mutuated PAR proteins cause polyps

Question 18

Epithelial apical-basal polarity

Answer 18

Gives the cell a top and a bottom

- Organelles + proteins differentially distributed

• Distinct membrane lipid composition

Apical surface

- Permeable to water ‐soluble molecules

• Faces external milieu
• Surface area can be increased

Basolateral surface:

• Impermeable
• Faces internal milieu + blood supply

Cell ‐cell junctions

Question 19

PAR proteins organise the polarity of cells in relation to their neighbours and basal lamina

Answer 19

• With 2 other complexes: Scribble and Crumbs

1. Influence each other’s activity (by phosphorylation) and localisation at cell‐cell junctions
2. Organise cell‐cell junction
3. Bind and position small GTPases (and conversely…)

Question 20

PAR proteins also organise the polarity of individual cells

Answer 20

• Lkb1 is a human Par protein
• Mutations in humans= abnormal growth of lining of gut (e.g. polyps)
• Human colon cells isolated from gut polarise depending on level Lkb1

Question 21

What do PAR proteins do anyway?

Answer 21

1. They are Kinases: switch other proteins on or off
2. Scaffolding factors: interact and recruit other proteins

Question 22

Data handling practice

Remodelling of the cytoskeleton

Answer 22

Answer: B,D,E

Cdc42 (and its GPCR!) will bind the non‐hydrolysable form of GTP and will not be able to be turned off. Thus, more actin polymerization and more mating.

Question 23

Rho GTP
Rac GTP

Cdc42 GTP

Answer 23

Question 24

Other cellular functions are regulated by Cdc42

Answer 24

1. Migrating fibroblast
2. Cytotoxic t cell
3. Epithelial cell
4. Polarized dividing cell