Lecture 12 - Epithelial cell polarity

Question 1

What is polarity in epithelia?

Answer 1

A difference in structure, composition or function between the two poles of a cell, such as apical/basolateral in an epithelial cell.
Also refers to location of a
protein in either apical or basolateral membranes

Question 2

Why must proteins be sorted and directed to apical and basolateral membranes during epithelial cell development?

Answer 2

For the ion transport pathways that allow epithelial cells to function - Essential for epithelia to transport ions/molecules

Question 3

What is the formation of the cell-basement membrane and cell-cell interactions necessary in?

Answer 3

Establishing epithelial cell polarity

Question 4

What are associated with first cell-cell interaction/what initiates epithelia formation?

Answer 4

adherens junction

Question 5

What proteins are involved in the formation of AJs?

Answer 5

Nectin proteins
E-cadherin
Catenins

Question 6

What is the role of nectin proteins in AJ formation?

Answer 6

Nectin proteins make initial cell-cell contact. Nectins on neighbouring cells interact.
- Ca2+-independent cell adhesion.

Question 7

What is the role of E-cadherin proteins in AJ formation?

Answer 7

E-cadherin on one cell forms homodimer with E-cadherin on neighbouring cell
- requires Ca2+
Cytoplasmic tail binds to catenins

Question 8

What is the role of catenins proteins in AJ formation?

Answer 8

Link actin of e-cadherin to cytoskeleton
Catenins also link nectin and cadherin complexes to pull all the proteins together to make the AJ.

Question 9

What is the generalised function of catenins?

Answer 9

Catenins - connective/glue to link things together

Question 10

What do TJ components interact with?

Answer 10

TJ components interact with apical polarity protein complexes

Question 11

What polarity complexes are activated during the formation of TJs/epithelial polarity

Answer 11

PAR
CRB
SCRIB

Question 12

What happens when small GTP binding proteins are activated during the formation of TJs/epithelial polarity?

Answer 12

Small GTP binding protein activated - cdc42
Activates aPKC

Question 13

how are polarity complexes activated during the formation of TJs/epithelial polarity?

Answer 13

aPKC

Question 14

What are the role of polarity complexes in epithelial polarity?

Answer 14

Maintenance of polarity

Question 15

Where are the three scaffolding protein polarity complexes located?
- formation of epithelial polarity

Answer 15

1. PAR complex – apical, near TJ
2. CRB complex – apical, near TJ
3. SCRIB complex – basolateral

Question 16

What are the four domains responsible for establishing polarity?

Answer 16

PAR complex
CRB complex
SCRIB complex
aPKC

Question 17

What is aPKC?

Answer 17

aPKC is an atypical protein kinase C that is activated by small GTP binding proteins (cdc42).
aPKC is the only polarity protein that has enzymatic activity
aPKC interacts with and phosphorylates the CRB and SCRIB complexes.

Question 18

What happens when CRB and SCRIB are phosphorylated?

Answer 18

When phosphorylated, the polarity complexes are correctly locate themselves in either the apical or basolateral domain.

Question 19

What does mutual exclusion refer to?

Answer 19

How polarity complexes (or other proteins) are located in only one epithelial domain (apical or basolateral)

Question 20

Summarise the formation of TJs and epithelial polarity (7 steps)

Answer 20

1. Interactions between neighbouring cells, and between cells and basement membrane.
2. Interaction between cells forms adherens junction.
3. Small GTP proteins (cdc42) activated.
4. Cdc42 activates aPKC - a polarity complex protein.
5. Tight junctions start to form.
6. Positioning of PAR and CRB polarity complexes to apical domain, and SCRIB complex to basolateral domain.
7. Apical-basolateral polarity established.

Question 21

What happens to polarity complexes for cell division or cell removal?

Answer 21

They are pulled apart and the are reformed

Question 22

What is the main ligand for the removal of protein complexes?

Answer 22

TGF-B

Question 23

What are the two pathways of TGF-B in the removal of protein complexes?

Answer 23

Activation of SMAD proteins (transcription proteins)
Downregulation of PAR complex
- both result in Epithelial-mesenchymal transition

Question 24

Describe the TGF-B SMAD protein pathway

Answer 24

TGF-B will bind to receptor which activates SMAD2/3. SMAD2/3 is a transciption factor process which goes into nucleus to cause upregulation of the genes that becomes proteins that can disrupt (down regulate) the tight junctions via inhibition

Question 25

What can mutations in polarity complex proteins cause?

Answer 25

• Decreased formation or lack of tight junctions so barrier, gate and fence functions compromised.
• Changes in cell-cell adhesion and cell movement.
• Changes in location of apical and basolateral proteins.
• Cancer

Question 26

Normal epithelial physiology

Answer 26

High cell density – ZO1 keeps transcription factors that promote cell division localised to TJs preventing cell division. Epithelia carries out regular functions e.g. absorption/secretion.

Question 27

What is the pathophysiology for wound healing/damage to epithelia?

Answer 27

Low cell density –transcription factors that promote cell division move from TJs to nucleus to stimulate cell division (sensing a gap in the epithelia that needs to be filled with new cells to repair junctions and maintain polarity).

Question 28

What is the pathophysiology of cancer in epithelia?

Answer 28

Genetic changes in cancer cells of epithelia may promote EMT: cell division increases and loss of epithelial polarity.