Structure of CFTR
- 12 TM domains
- 2 groups of 6 – domain 1 and 2
- Regulatory / R domain – where its phosphorylated by PKA – important role in opening and closing
- Nucleotide binding domain – NBD1 and NBD2 – important for opening and closing
How many CFTR mutations is there?
What mutation is the most common?
around 1200
delta F508 is the most common
What is a Class I Null production mutation ?
unstable mRNA, meaning no CFTR protein being made
what is a Class II Trafficking mutation?
o Cftr protein made but not trafficked correctly
o Delta F508 mutation – misfolded – targeted for degradation instead of being trafficked to the membrane
what is a Class III Regulation mutation?
o Protein made, goes to membrane, but not regulated effectively – not activated correctly. Different ways this can happen, e.g. phosphorylation cannot occur, change in open probability
what is a Class IV Conduction mutation?
o Gating mutations
o Protein made, moved to membrane, cannot open. The way the protein responds to regulation
o Open probability lower
what is a Class V Partial reduction mRNA mutation?
o mRNA made, but its reduced, amount of protein made is reduced, so amount of protein at membrane reduced.
What is the diagnostic threshold for sweat chloride?
normal is 20mmol-L-1
Diagnostic threshold 60mmol-L-1
above 60 = diagnosed with cystic fibrosis
What are the features of the upper airway Na+ and Cl- handling?
gradient
• When CFTR opens in apical membrane – loss of Cl from cell, drives water movement between cells
• Changing height of the PCL – which cilia project into to beat
o When CFTR is active, ENaC is inhibited
what happens if CFTR is non-functional in upper airways?
o Less Cl secretion o Less water secretion o Height of liquid layer drops o Cilia bend over o Clearance of mucus is reduced o Thick mucus with bacteria and viruses trapped in lungs = increased risk of infection
what are the features of alveolar Na and Cl handling?
• K Cl co-transport protein on basolateral membrane – takes Cl out using K gradient
o Meaning intracellular Chloride in alveolar cells is quite low
o CFTR – driving force is for Cl to move into the cell
o Net absorption of Cl from the airway surface liquid layer
• Think CFTR actually activates ENaC – showing interactions are v tissue specific
• Reabs of Na and Cl drives water reabsorption
• Height of liquid layer in alveolar is optimum for gas exchange
Why is cystic fibrosis associated with alveolar oedema?
o Increased fluid in alveoli – impacts ability to get oxygen into their bodies
o Not getting Cl reabsorption
What are the features of Na and Cl handling in the distal sweat glands?
CFTR reabsorbs Cl from lumen of sweat gland
• Driving force is for Cl to enter the cell via first CFTR in apical membrane, then a second driving force to the leave the cell through the basolateral CFTR channel
• Usually CFTR independent secretion of chloride into lumen of sweat gland
• Whatever is in lumen is then lost via sweat at surface of the skin
• Distal sweat gland absorbs Na and Cl from sweat that’s being produced
What is the difference to the sweat gland during cystic fibrosis?
CFTR is non-functional
o No absorption of Cl
o No activation of ENaC
o High levels of NaCl in sweat
What are the current treatments for cystic fibrosis?
• Nebulised antibiotics e.g. tobramycin – fight infection
• Inhaled bronchodilators – open airways
• Mycolytics e.g. pulmozyme – breakdown mucus
• Nebulised hypertonic (highly concentrated NaCl solution) saline - Hydrates ASL, improves mucociliary clearance
o Osmotic gradient being created for water to move between the cells into airway surface liquid layer
• Oral antibiotics – fight infection
o Antibiotic resistance is a massive problem – patient can acc die from bacterial infection
• Pancreatic enzymes – breakdown food
• Fat soluble vitamins - To help absorb sufficient vitamin levels
• Steroids - To help absorb sufficient vitamin levels
• Exercise - Helps clear mucous
• Physiotherapy - Helps clear mucous
• High energy supplements - Helps with sufficient nutrient absorption
What is the problem with the current treatments for CF?
All treat symptoms rather than the cause
what are the 2 types of drugs that could be used to treat CF?
- Potentiator = increases open probability of the channel -Particularly useful for gating mutations
- Correctors = traffic mutant protein to the membrane
What are the actions of Ivacaftor on the G551D mutation?
increases short circuit current of CFTR increases channel opening enhanced Cl secretion - ASL height closer to normal increase ciliary beat frequency sweat chloride drops below threshold
