Lecture 4 - Cell Adhesion and Communication

Question 1

There are 5 types of lateral connections. Name them.

Answer 1

1. Tight junctions.
2. Adherens juntions.
3. Desmosomes.
4. Gap junctions.
5. ‘Non-junctional’ adhesions (proteins such as cadherins, selectins and integrins).

Question 2

Connections with the basement membrane?

Answer 2

Non-junctional (integrins) and attachment junctions (hemidesmosomes and focal adhesions).

Question 3

Define the 3 distinct classes of cell adhesion junctions.

Answer 3

1. Occluding (tight) junctions - form a barrier between epithelial layers that prevents the movement of molecules from passing through the intracellular space between cells.
2. Attachment junctions - links two cells using their cytoskeleton, or links a cell to the basement membrane.
3. Communication junctions - links two adjacent cells using their cytoplasm.

Question 4

Where are occluding (tight) junctions found?

Answer 4

Just below the apical surface.

Question 5

What do tight junctions consist of?

Answer 5

Made of bands of interconnected strands of integral membrane proteins that wrap around the cell. Transmembrane proteins in these include claudins, junctional adhesion molecules (JAMs) and occludins.

Question 6

How many claudins are there?

Answer 6

24

Question 7

The claudins are the core proteins of tight junctions. What structure do they form?

Answer 7

Paracellular pores.

Question 8

Define paracellular transport.

Answer 8

Transfer of substances through the intracellular space inbetween the cells.

Question 9

Why is transcellular transport beneficial?

Answer 9

Allows the transport of substances against their concentration gradient (active transport).

Question 10

Briefly explain the active transport mechanism that is used to absorb glucose.

Answer 10

Sodium-potassium ATPase pumps Na+ out of the cell, exchanged for K+. The concentration of Na+ is now low in the cell = diffusion = shuttles glucose as it does. Na+ glucose symporter used. The concentration of glucose is now high in the intestinal epithelial cell, enters the blood via GLUT2.

Question 11

Two types of attachment junctions?

Answer 11

Adherens junctions and desmosomes (hemidesmoses)

Question 12

Main protein that makes up an adheren junction?

Answer 12

Cadherin (dimer, one from each cell).

Question 13

Adheren junctions link what type of filament from each cell? Which protein (besides cadherin) is involved?

Answer 13

Actin filaments, and catenin.

Question 14

Desmosomes link what type of filament from each cell?

Answer 14

Intermediate filaments.

Question 15

Name the 4 types of proteins involved in desmosomes.

Answer 15

Cadherins (desmocollins, desmogleins) and the adaptor proteins plagoglobulins and desmoplakin (form the cytoplasmic plaque).

Question 16

Hemidesmosomes are attachment junctions that link the extracellular matrix to the intermediate filaments. What transmembrane proteins are involved in this structure?

Answer 16

Integrins.

Question 17

Hemidesmosomes also form cytoplasmic plaques. Which adaptor protein do they use?

Answer 17

Plectin.

Question 18

Describe the structure of integrin.

Answer 18

Heterodimer, one alpha and one beta subunit.

Question 19

Why can integrins bind to different components of the ECM?

Answer 19

There are 18 different alpha subunits and 8 different beta subunits.

Question 20

How are integrins regulated?

Answer 20

By signalling cascades that indicate whether a cell needs to adhere more tightly to the ECM.

Question 21

‘Inside-out signalling’

Answer 21

When signalling inside the cell affects the role of integrins on the outside of the cell.

Question 22

‘Outside-in signalling’

Answer 22

Cell binding to ECM can cluster integrins, forming docking sites for signalling molecules.

Question 23

How are integrins recycled for focal adhesions?

Answer 23

Integrin is endocytosed, breaking the attachment to the substrate. The internalised integrin is then recycled to establish a new focal adhesion in the direction of movement.

Question 24

What do gap junctions form?

Answer 24

Channels between cells that allow the transport of small molecules.

Question 25

Describe the structure of a gap junction.

Answer 25

They consist of two halves (connexons). Each connexon consists of 6 subunits, called connexins. Each connexin consists of 4 TM alpha helices.

Question 26

How are gap junctions regulated?

Answer 26

They can alternate between open and closed states, regulated by the phosphorylation of connexins. This is in response to changes in intracellular pH and/or calcium concentration. Can also be protective, e.g. if adjacent cells undergo apoptosis.