Lecture 23

Question 1

What are gap junctions also known as?

Answer 1

Connexons

Question 2

What are gap junctions?

Answer 2

Gap junctions are a plaque containing groupings (from a few to thousands) of intercellular connexon channels

Question 3

What do gap junctions exclude?

Answer 3

They exclude other transmembrane proteins

Question 4

What is the function of gap junctions?

Answer 4

Provide conduits for the movement of molecules up to 1 kD

Question 5

How do connexon channels arise?

Answer 5

Connexon channels arise from the joining of two connexon hemichannels from adjacent cells

Question 6

How many connexin proteins form the connexin hemichannel?

Answer 6

6 connexin proteins form a connexon hemichannel

Question 7

How does the hemichannel work?

Answer 7

The 2 hemichannels bind tightly to not allow molecules to leak between the cells, extracellularly

Question 8

How is one connexon formed?

Answer 8

1/2 connexon forms with 1/2 connexon on other side to make 1 connexon

Question 9

What are connexins?

Answer 9

4 transmembrane alpha helices
N and C termini are in the cytoplasmic side of the membrane
NOT A MARVEL structure protein

Question 10

How do the extracellular loops interact in connexins?

Answer 10

2 extracellular loops interact with 2 extracellular loops from the neighbouring connexin

Question 11

How often do connexins turnover?

Answer 11

Connexins turnover over severeal hours

Question 12

How are connexins added and removed?

Answer 12

New connexins (RED) enter at the periphery of the gap junctions, old connexins (GREEN) are removed ferom the middle of the plaque

Question 13

What is the connexon aqueous pore?

Answer 13

Hemichannels are cone shaped 10nm long, narrowing to 1.2 wide

Question 14

What does the passing of hydrophilic molecules up to 1 kD include?

Answer 14

The passing of hydrophilic molecules up to 1 kD includes the passage of:
1) Ions (for electrical coupling)
2) Peptides
3) Second messengers (signalling molecules)
4) Metabolites (shape nutrients)
5) ATP
6) RNA

Question 15

Who proposed the term Connexin?

Answer 15

Daniel Goodenough in JCB
He the continued the pioneering work by himself, then got some grad students
Discovered Cx43, Cx32

Question 16

Which connexin protein is most ubiquitously expressed?

Answer 16

Cx43

Question 17

How many connexin proteins are there?

Answer 17

There are at least 24 connexin proteins (humans have 21 connexin genes)

Question 18

What is the range of sizes of connexin proteins?

Answer 18

Range in size from 26-60 kD
(Usually named based on their molecular weights.
Eg. Cx43, Cx32, Cx26)

Question 19

What do different connexins make?

Answer 19

Different connexins make different channels (different permeability and charge selectivity)

Question 20

Can connexons have mixed connexins?

Answer 20

Yes
They usually pair with identical connexins in the neighbouring cell, but not always

Question 21

What can this give rise to?

Answer 21

This can give rise to novel connexon properties:
1) May pass molecules better from one cell to the other instead of passing the molecules equally well in both directions (each hemichannel would habe to be open to pass the molecules)
2) Very useful for directional transfer of material or electric impulses
3) In some cases connexins can be replaced with different connexins to rescue KO animals

Question 22

What do insects use?

Answer 22

Insects use innexins (invertebrate connexins)

Question 23

What are innexins?

Answer 23

They are structurally similar to connexins, but don’t have any sequence similarities
Hemichannels are made of 8 proteins, not 6

Question 24

Can connexons move material between cells?

Answer 24

Connexons can open and close to allow material to move from cell to cell using:
1) Voltage-dependent gating based on the difference in voltage between neighbouring cells
2) Protein and calcium levels
3) Protein kinases (enzymes that add a phosphate to the connexins)

Question 25

Are all connexons open?

Answer 25

No, all connexons are not always open at the gap junction plaque
~20% are open in the ehart at a time
1% in a nerve cell

Question 26

How long does it take to open and close the connexon?

Answer 26

It usually takes seconds to open and close the connexon, which is pretty slow for voltage-gated channels

Question 27

How are gap junctions closed?

Answer 27

Oleamide (fatty acid made in brain) blocks gap junctions and makes animals sleep
High concentrations of calcium can also clsoe connexons

Question 28

What is Halothane?

Answer 28

Closes gap junctions, closes electrical current (general anesthesic)

Question 29

What else are gap junctions useful for?

Answer 29

1) Synchronizing cilia beating
2) Provide holes for osteocytes (deep in bone) to get nutrients from the far away blood vessels
3) Action potentials betwen cardiac and smooth muscle

Question 30

What is Oculodentodigital dysplasia (ODDD)?

Answer 30

Mutations in Cx43
Over 60 different mutations documented
14 mutations have been investigted in the lab for functional effects
Most show normal localization
BUT,
All have reduced or no ability to form functional gap junctions
Many of the mutant forms of the protein have dominant negative effects on the wild-type Cx43

Question 31

What are the cardiovascular connexins?

Answer 31

Cx37, Cx40, Cx43

Question 32

Is there a cardiovascular phenotype?

Answer 32

Usually there is no cardiovascular phenotype… even ODDD patients rarely have cardiovascular problems

Question 33

Why is there no cardiovascular phenotype?

Answer 33

Likely due to redundancies and replacement by one connexin for another

Question 34

What are some rare problems of cardiovascular connexin mutation?

Answer 34

Idiopathic atrial fibrillation
Out of 15 patients, 4 had mutations in Cx40, all in the transmembrane domain
A genetic mutation in Cx37 is strongly associated with atherosclerosis and heart attacks

Question 35

What do mutations do Cx46 and Cx50 do?

Answer 35

Mutations to 2 connexins, Cx46 and Cx50, that are in the lens have been associated with cataracts

Question 36

What is the Charcot-Marie-Tooth disease?

Answer 36

A disease of progressive peripheral nerve degeneration. Cx32 contributes to it

Question 37

What is Cx32?

Answer 37

Cx32 is in the mylein surrounding the nerves making gap junctions between the fold of the Schwann cell plasma membrane
Over 200 different mutations in Cx32 are associated with this disease

Question 38

What do most mutations in Cx32 cause?

Answer 38

Most mutations cause a loss of function as gap junction plaques fail to form
Other mutations cause the connexins to gate (open/close) improperly

Question 39

What is recessive non-syndromic sensorineural hearing loss?

Answer 39

Recessive non-syndromic sensorineural hearing loss
10-50% of patients have a mutations in Cx26
Most have a truncation of the protein or missense mutations in the last 2/3 of the protein