Direct Cell-Cell Signalling

Question 1

What are Gap Junctions?

Answer 1

Direct cell-cell contact via formation of a cytoplasmic bridge

Question 2

What is the gap distance separating two adjacent cells at Gap Junctions?

Answer 2

2-5 nm

Question 3

What is the shape of the Gap Junction channel?

Answer 3

Hexagonal

Question 4

What is the name of the proteins that make up Gap Junction channels?

Answer 4

Connexins

Question 5

What do Gap Junctions allow the direct movement of between the cytoplasms of adjacent cells?

Answer 5

Inorganic ions and small water-soluble molecules

Question 6

What is the structure of Connexin?

Answer 6

Four-Pass Transmembrane Proteins with N- and C-termini at the cytoplasmic side

Question 7

How many Connexin molecules of one plasma membrane form the hexagonal cylinder of a Gap Junction?

Answer 7

6

Question 8

What is the diameter of a Gap Junction?

Answer 8

1-2 nm

Question 9

What is the hexagonal cylinder of a Gap Junction known as?

Answer 9

Connexon

Question 10

How many Connexin isoforms are present in vertebrates?

Answer 10

Over 20

Question 11

What are Homotypic Channels?

Answer 11

Single isoform type of Connexin

Question 12

TRUE OR FALSE:
Most cells express at least two connexins

Answer 12

True.
Assemble into Hetero-Oligomeric Connexons creating Heterotypic Channels

Question 13

What does the diversity in Connexon channel composition lead to?

Answer 13

Differences in channel permeability to various molecules

Question 14

What are THREE examples of diseases that mutations in the connexin gene cause?

Answer 14

1. Neurosensory deafness
2. Cataract or heart malformations
3. X-linked Charcot-Marie-Tooth

Question 15

Do Gap Junctions undergo reversible or irreversible conformational changes in response to changes in the cell?

Answer 15

Reversible.
E.g. large influx of Ca2+ from extracellular matrix into damaged cell causes gap junction channels to immediately close

Question 16

What are THREE functions of Gap Junctions?

Answer 16

1. Movement of ions
2. Movement of metabolites
3. Movement of secondary messengers

Question 17

How does the movement of ions across gap junctions affect an Electrical Synapse?

Answer 17

Permit extremely rapid and reliable transmission of signals between neurons

Question 18

How does the movement of ions across gap junctions affect a Muscle Contraction?

Answer 18

Important in propagation of depolarisation from the pacemaker of the heart to cardiac muscle cells that causes a heart contraction

Question 19

What is the reason for diffusion of metabolites through gap junctions?

Answer 19

Ensure metabolites are shared between coupled cells

Question 20

How was movement of metabolites through gap junctions demonstrated?

Answer 20

Co-culture of normal cell-line with mutant one lacking the enzyme thymidine kinase, and therefore unable to incorporate radioactive thymidine (tracer) into their DNA

Question 21

What is the reason for movement of secondary messengers through gap junctions?

Answer 21

Permits propagation of signals through coupled cells

Question 22

What is an example of the movement of secondary metabolites through gap junctions?

Answer 22

• Beating of cilia of respiratory epithelial cells spreads as synchronised waves in a cell monolayer
• Coupling is mediated by passage of IP3 through gap junctions

Question 23

What is an example of experimental evidence for the permeability of Gap Junctions?

Answer 23

Antibodies against connexins can block intercellular communication when micro-injected into coupled cells