Lecture 23 Flashcards
What are gap junctions also known as?
Connexons
What are gap junctions?
Gap junctions are a plaque containing groupings (from a few to thousands) of intercellular connexon channels
What do gap junctions exclude?
They exclude other transmembrane proteins
What is the function of gap junctions?
Provide conduits for the movement of molecules up to 1 kD
How do connexon channels arise?
Connexon channels arise from the joining of two connexon hemichannels from adjacent cells
How many connexin proteins form the connexin hemichannel?
6 connexin proteins form a connexon hemichannel
How does the hemichannel work?
The 2 hemichannels bind tightly to not allow molecules to leak between the cells, extracellularly
How is one connexon formed?
1/2 connexon forms with 1/2 connexon on other side to make 1 connexon
What are connexins?
4 transmembrane alpha helices
N and C termini are in the cytoplasmic side of the membrane
NOT A MARVEL structure protein
How do the extracellular loops interact in connexins?
2 extracellular loops interact with 2 extracellular loops from the neighbouring connexin
How often do connexins turnover?
Connexins turnover over severeal hours
How are connexins added and removed?
New connexins (RED) enter at the periphery of the gap junctions, old connexins (GREEN) are removed ferom the middle of the plaque
What is the connexon aqueous pore?
Hemichannels are cone shaped 10nm long, narrowing to 1.2 wide
What does the passing of hydrophilic molecules up to 1 kD include?
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
Who proposed the term Connexin?
Daniel Goodenough in JCB
He the continued the pioneering work by himself, then got some grad students
Discovered Cx43, Cx32
Which connexin protein is most ubiquitously expressed?
Cx43
How many connexin proteins are there?
There are at least 24 connexin proteins (humans have 21 connexin genes)
What is the range of sizes of connexin proteins?
Range in size from 26-60 kD
(Usually named based on their molecular weights.
Eg. Cx43, Cx32, Cx26)
What do different connexins make?
Different connexins make different channels (different permeability and charge selectivity)
Can connexons have mixed connexins?
Yes
They usually pair with identical connexins in the neighbouring cell, but not always
What can this give rise to?
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
What do insects use?
Insects use innexins (invertebrate connexins)
What are innexins?
They are structurally similar to connexins, but don’t have any sequence similarities
Hemichannels are made of 8 proteins, not 6
Can connexons move material between cells?
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)
Are all connexons open?
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
How long does it take to open and close the connexon?
It usually takes seconds to open and close the connexon, which is pretty slow for voltage-gated channels
How are gap junctions closed?
Oleamide (fatty acid made in brain) blocks gap junctions and makes animals sleep
High concentrations of calcium can also clsoe connexons
What is Halothane?
Closes gap junctions, closes electrical current (general anesthesic)
What else are gap junctions useful for?
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
What is Oculodentodigital dysplasia (ODDD)?
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
What are the cardiovascular connexins?
Cx37, Cx40, Cx43
Is there a cardiovascular phenotype?
Usually there is no cardiovascular phenotype… even ODDD patients rarely have cardiovascular problems
Why is there no cardiovascular phenotype?
Likely due to redundancies and replacement by one connexin for another
What are some rare problems of cardiovascular connexin mutation?
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
What do mutations do Cx46 and Cx50 do?
Mutations to 2 connexins, Cx46 and Cx50, that are in the lens have been associated with cataracts
What is the Charcot-Marie-Tooth disease?
A disease of progressive peripheral nerve degeneration. Cx32 contributes to it
What is Cx32?
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
What do most mutations in Cx32 cause?
Most mutations cause a loss of function as gap junction plaques fail to form
Other mutations cause the connexins to gate (open/close) improperly
What is recessive non-syndromic sensorineural hearing loss?
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
