Lecture 5 Flashcards
What is the structure of Cav channels
24 TMDs (four lots of six), 4 pore regions, 4 voltage sensitive regions
What regulatory subunits are present on the Cav channels
Variety of regulatory subunits
Beta1-4, alpha 2 and delta subunits interact with the alpha 1 subunit and regulate its activity, sensitivity to voltage and to calcium itself.
Which Cav channel types are regulated by high voltages
L type, N type and P/Q type
Which Cav channels are regulated by intermediate voltages
R type
Which Cav channels are regulated by low voltages
T type
What type of channel is Cav1.2
L type
What is the gene encoding Cav1.2
CACNA1C
Where is Cav1.2 found
Heart brain and lungs
how many alternative splicing loci does the CACNA1C gene have
12 - leading to 42 splice variants
Where are the Cav1.2 channels found in the heart
On the T tubules close to the sarcoplasmic reticulum
Which two types of inactivation does the Cav1.2 channel exhibit and what type of activation?
Voltage dependent activation
Voltage dependent inactivation
Ca dependent inactivation - calcium that enters feeds back on the channel causing it to close. (beta subunit plays important role in regulating inactivation)
What are some symptoms of Timothy syndrome
Webbed digits, heart arrhythmia, immune deficient, increased risk of autism etc
In the context of LTQS how does Timothy syndrome effect the patient
Patiens show a 2:1 atrioventricular block - 2P waves to each QRS. Meaning there is a mismatch in timing between the atria and the ventricles, as a consequence there is an increased risk of arrhythmia and sudden cardiac death.
What changes in the T wave signify a problem with repolarisation in cardiac myocytes.
Alternating T wave polarity - T wave goes from one direction to the other. Sign of a significant repolarisation defect in ventricular muscles.
What is ventricular tachycardia
Fast repetitive contractions of the ventricles in a completely uncoordinated manner with relation to the atria.
What were the findings of a study into 17 children with Timothy syndrome
10 of the children died during the study at a mean age of 2.5
12/17 experienced life threatening arrhythmias and the 7 survivors had motor and cognitive impairment
Why is motor and cognitive impairment common in patients with Timothy Syndrome
Because Cav1.2 is also found in the brain with impacts on development.
What mutation of the Cav1.2 channel do patients of Timothy syndrome suffer from.
Not a mutation - instead it is an unusual splice form of Cav1.2
Gain of function seen at position G406R (at the end of the first TMD)
What is different to most other Cav channels in the splice variant of Cav1.2
Glycine at position 406 becomes an argenine - glycine seems important due to its conservation between Cav channels.
What is the result of the G406R splice variant on IV on inactivation of Cav1.2
Slowing of inactivation - activation is normal.
Less inactivation over the same time period as the WT channel. Effect is on voltage dependent inactivation - with only a small impact on Ca dependent inactivation
How does the voltage dependent inactivation differ between the WT Cav1.2 and G406R splice variant at 0mV
In the WT at 0Mv there’s only about 20% of the Ca current left. In the G406R splice variant there’s about 80% left - this means the Cav1.2 channels are staying open longer and prolonging the plateau phase, also prolonging the QT
What causes the uplift in both WT and G406R Cav1.2 channels at the lower end of relative Ca current
Relief from the Ca dependent inactivation of the channel.
How is Ca dependent inactivation of the Cav1.2 channel prevented experimentally
Using Ba instead of Ca. Ba doesn’t feedback and block the channel, therefore the only inactivation is dependent on the voltage changing. (Cav channels will also allow other divalent cations through its pore)
What does the use of Ba show when repeating IV analysis of both WT and G406R Cav1.2 channels
Splice variant shows almost complete loss of voltage dependent inactivation, almost all the inactivation of G406 Cav1.2 channels is therefore mediated by Ca feedback.
What percentage of channels would be expected to be the G406R variant if the patient was heterozygous
predicted 11.5% of channels are the G406R variant however this is capable of extending action potentials by 17%
Which SQTS types are due to changes in Cav1.2 function
SQTS4 and SQTS5
4 = CACNA1C effecting the alpha subunit of the Cav1/2 channel (loss of function)
5= CACNB2B effecting the beta subunit (loss of function)
What changes are seen on a patients ECG suffering from SQTS4/5
Accentuated J-wave (just after the QRS wave
ST segment is also elevated.
What is the mutation responsible for beta subunit inactivity
S481L
Which two mutations have effects on the alpha subunit
G490R - Between the first loop between TMD 6 and 1
A39V - N terminal mutation
What effect does the A39V N terminus mutation have on Ca current
Smaller Ca current, about 25% smaller means the plateau phase will be shorter and repolarisation will occur earlier
What effect does the G409R mutation have on Ca current
Only produces a tiny current, they are very slow calcium currents through this specific calcium channel
Is the impact of the Cav1.2 mutants likely to be due to gating or trafficking
Gating - location studies showed that Cav1.2 expression was similar in the WT and mutant forms. Only the A39V showed a potential trafficking problem
