L4 - CF and Small Molecules I Flashcards
What is the CF gene product?
A Cl channel in the apical membrane
What is the structure of the CF Cl channel?
12 transmembrane spanning domains - 2 groups of 6 - Between each group is the regulatory domain Two nucleotide binding domains - NBD1 and NBD2 - Important in the gating of the channel
What is CFTR phosphorylated by?
PKA
In the regulatory domain
Plays an important role in the opening and closing of CFTR
Where are lots of CFTR mutations found?
NBD1 and NBD2
What is the most common CFTR mutation?
Delta F508
Where is the Delta F508 CFTR mutation found?
In NBD1
in 70-90% of individuals
What are the 6 CFTR mutation classes?
1 - Null production 2 - Trafficking 3 - Regulation 4 - Conduction 5 - Partial reduction mRN~a 6 - High turnover CFTR Mutations can belong to multiple classes depending on its impact on CFTR
Class I mutation - null production
mRNA from gene product is unstable
No CFTR made
Class II mutation - trafficking
CFTR protein is made but is not trafficked effectively to the membrane
E.g. F508
- Made but misfolded - meaning cell targets the protein for degradation rather than trafficking it to membrane
Class III mutation - regulation
CFTR is made and trafficked to membrane but is not regulated appropriately
Normally activation by PKA mediated phosphorylation - with mutation this has no effect
Or this phosphorylation does not occur
- Get differences in channel open probability
Class IV mutation - conduction
CFTR is made and trafficked to membrane but it does not open as it should do
The way the protein responds to regulation is not correct
Open probability of channel is lower than it should be
Class V mutation - partial reduction mRNA
mRNA is made but the amount is reduced
Reduced amount of CFTR
Class VI mutation - high turnover CFTR
Amount of time the CFTR spends at the apical membrane is reduced
High turnover of CFTR from cell membrane
E.g. F50D
- Doesn’t traffic to membrane properly and when it gets there it is removed quickly
What is the diagnostic threshold for sweat Cl concentration?
If a patient has severe CF their sweat Cl is typically above 60mmol/L Cl
In clinic one of the first tests is take a sweat sample and measure Cl
If your sweat chloride is 100 mmol what class mutation are you likely to have?
Classes 1-3
Very low CFTR function
Pancreases doesn’t work
If your sweat chloride is 80 mmol what class mutation are you likely to have?
Classes 4-5
10% CFTR function
Tend to have pancreatic sufficiency
- Don’t secrete digestive enzymes from pancreases because of CFTRs role in bicarbonate secretion
What is Congenital bilateral aplasia of the Vas Deferens ?
2 CF mutations
- Less severe mutations
Issue – inconclusive sweat test as sweat Cl below 60mmol
- Therefore may not get access to CF treatments
What is the next step if you get an inconclusive sweat test?
Genetic tests - but a lot of work
Nasal potential difference experiments
Biopsy of gut and looked at response to Ach
- Lost even in mild CF patients
What % normal CFTR protein do you need to get normal CFTR function?
50%
Some studies have even suggested 15%
How is Cl handled in the upper airway?
NKCC1 on basolateral membrane
- Brings Cl ions in using the Na gradient
CFTR opens in apical membrane
- Loss of Cl from the cell
- This drives water movement between the cells
- Sets height of ASL
How is Cl handling altered in the upper airway in CF patients?
Non-functioning CFTR Less Cl and water secretion Height of periciliary layer drops Cilia bend Thick mucus not cleared Airways problems/infection
How do ENaC and CFTR interact in the upper airway?
When CFTR is active, ENaC is inhibited and vice versa
Cell type specific
How does ENaC and CFTR interaction affect Na and Cl handling in the upper airways in a CF patient?
In CF CFTR in non-functional –> extra ENaC function
Reduced Cl secretion
Enhanced Na absorption
Exacerbates the impact on the ASL - height drops even further
How is Cl handled in the alveolar?
Movement of Cl reversed compared to upper airways
K/Cl cotransporter on basolateral membrane
- Removes Cl using K gradient
- Intracellular Cl is low
If you open CFTR
- Driving force is for Cl enter into cell
- Net absorption of Cl from ASL
