Cystic Fibrosis

Question 1

Trachea

Answer 1

Cartilaginous and fibromuscular tube
Tracheal wall has 4 different layers - mucosa, submucosa, cartilage/muscle, adventitia
Main cell type = ciliated, goblet

Question 2

Cilia

Answer 2

Tiny “hair-like” structures on the surface of the cell that sweep mucus/dust/bacteria up to the back of the throat for swallowing

Question 3

Goblet cells

Answer 3

Secret mucus to protect to protect the mucous membrane that lines the respiratory tract

Question 4

Bronchi

Answer 4

Division of the trachea
Relatively large lumen
Surrounded by cartilage
Contain mucous and serous cells

Question 5

Serous cells

Answer 5

Secrete serous fluid - a pale yellow/transparent bodily fluid, benign in nature

Question 6

Bronchioles

Answer 6

Branches of the bronchi
No longer contain cartilage or glands in their submucosa
Generally < 1 mm diameter
Contain ciliated, goblet and club cells

Question 7

Club cells

Answer 7

Bronchiolar exocrine cell
Main function is to protect the bronchiolar epithelium through secretion of substances and detoxification of harmful substances that are inhaled into the lungs
Can also act as stem cells - multiply and differentiate into ciliated cells to regenerate bronchiolar epithelium

Question 8

Alveoli

Answer 8

Site of gas exchange with the blood
Each alveolus is wrapped in capillaries
Typical pair of human lungs contains 700 mil alveoli

Question 9

Alveolar cells =

Answer 9

= pneumocytes

Question 10

Type I alveolar cells

Answer 10

Squamous, cover 90-95 % of alveolar surface
Involved in gas exchange
Cannot replicate, susceptible to toxic insults

Question 11

Type II alveolar cells

Answer 11

60 % of alveolar cells but cover small fraction of alveolar surface area
Involved in surfactant production and ion secretion
Precursor of type I cells - can differentiate into type I cells in the event of damage

Question 12

What is the main driving force for fluid movement across the alveolus?

Answer 12

Sodium movement

Question 13

Sodium enters alveolar epithelial cells through…

Answer 13

…the apical membrane via epithelial sodium channels (ENaC)

Question 14

Sodium is pumped out of alveolar epithelial cells through…

Answer 14

…the basolateral membrane via Na/K-ATPase

Question 15

How does water move out of alveolar epithelial cells?

Answer 15

Passively down its osmotic gradient either paracellularly or through aquaporins (type I)

Question 16

Adaptations to lung fluid at birth

Answer 16

Foetal lungs are fluid-filled to provide a growth environment. ‘Leaky’ epithelium and low expression of ENaC
At birth, the lungs need to be cleared of fluid. Hormone surge from the mother leads to increased catecholamines (activate Na/K-ATPase) and increased corticosteroids (increase ENaC expression)
This leads to rapid movement of Na from alveolar lumen into tissues, bringing water with it down the osmotic gradient

Question 17

ARDS

Answer 17

Acute Respiratory Distress Syndrome
Caused by 'direct' and 'indirect' damage/clinical insults
Direct = lung infection, aspiration
Indirect = sepsis, shock, trauma
Leads to ALI (Acute Lung Injury)

Question 18

Acute Lung Injury

Answer 18

Injury of the alveolar-capillary membrane
Inflammation
Increased permeability leading to pulmonary oedema
Impaired gas exchange

Question 19

How does lung infection lead to pulmonary oedema?

Answer 19

Infection leads to the release of interferons that downregulate Na/K-ATPase
ENaC activity is consequently reduced and there is a decreased ionic drive for water resorption - the diminished Na transport decreases alveolar fluid clearance

Question 20

Therapies for pulmonary oedema

Answer 20

Beta agonists e.g. i.v. salbutamol
Salbutamol increases Na resorption
But actually increased mortality - thought to be due to systemic effects of salbutamol

Question 21

Hypoxic Pulmonary Oedema

Answer 21

Acute altitude sickness leads to regional pulmonary vaso/venoconstriction
This leads to increased perfusion pressure and an increased hydrostatic drive of fluid into the lungs

Question 22

Cystic Fibrosis

Answer 22

The most common lethal genetic disorder
Caused by a defect in Cl ion transport
1 in 25 carriers in Western populations, with 1 in 2500 live births affected
Lungs infection and dysfunction are the most common causes of morbidity

Question 23

Structural changes associated with cystic fibrosis

Answer 23

Thickening of the bronchial wall
Bronchiectasis (permanent enlargement/dilation of the bronchial tree)
Pneumothorax (abnormal collection of are in the pleural space between the lungs and chest wall)

Question 24

Neutrophilic inflammation

Answer 24

= the massive infiltration of neutrophils into the airways as a consequence of epithelial secretion of pro inflammatory mediators

Question 25

Consequences of neutrophilic inflammation

Answer 25

Frustrated phagocytosis
Failure to clear bacteria/dying neutrophils
Epithelial damage
Decline in lung function

Question 26

CFTR

Answer 26

Cystic Fibrosis Transmembrane Conductance Regulator
Cl- channel
Cystic fibrosis is caused by no/mutated expression of CFTR

Question 27

Structure of CFTR

Answer 27

2 transmembrane domains, each connected to a nucleotide-binding domain in the cytoplasm
The NBDs are connected via a regulatory domain

Question 28

Gating of CFTR

Answer 28

“ATP-gated ion channel”
CFTR opens when the R domain is phosphorylated by PKA and ATP is bound at the NBDs
Cl- flows down its electrochemical gradient (generally export from the cell in airways), which means sodium will follow, which means water will follow

Question 29

Normal CFTR

Answer 29

There is a balance between ion flow and water entry/export

This leads to a thin layer (approx. 7 micrometres) of fluid on the apical surface of the epithelial cells

Question 30

Defective CFTR

Answer 30

There is no Cl- flow out of the cell so there is no gradient for water to move out of the cell
Leading to a thin ‘Air-Surface Liquid’ (ASL) layer (approx. 1 micrometre)
This leads to sticky/viscous mucus that is difficult for the cilia to clear, providing a growth environment for bacteria

Question 31

Established therapies for cystic fibrosis

Answer 31

Neonatal screening for early recognition - proper nutrition and enzyme supplements
Antibiotics e.g. inhaled tobramycin against pathogens
Beta agonists to activate residual CFTR activity
Hypertonic saline
Mucolysis
Gene therapy trials ongoing

Question 32

CFTR gene

Answer 32

Nearly 300 mutations in the CFTR gene have been descibed
These mutations have many molecular consequences for the synthesis of CFTR and its transport through the cell to the cell surface

Question 33

Defects in CFTR

Answer 33

Class I = defective/reduced protein production (10 %), e.g. nonsense mutations/premature termination codon, e.g. G542X
Class II = defective processing (88 %), i.e. no transport from the ER to the Golgi. Resulting from in-frame deletion e.g. DF508
Class III = defective regulation (2-3 %), i.e. no transport from the Golgi to the cell surface. Resulting from substitution e.g. G551D (and DF508)
Class IV = defective conduction (< 2 %), where CFTR reaches the cell membrane and some of the protein is functional, but Cl- transport is reduced due to channel narrowing
Class V = decreased surface expression (least common). Normal CFTR but less of it is expressed
Class VI = decreased surface stability (mutations in PDZ binding domain)

Question 34

CFTR activators and potentiators

Answer 34

Lumacaftor = ‘chaperone’ during protein folding, increases number of CFTR trafficked to cell surface
Ivacaftor = ‘potentiator’ of CFTR already at the cell surface, increases the probability of a defective channel being open for Cl- to pass through
These 2 drugs have ‘synergistic’ effects

Question 35

Ivacaftor

Answer 35

For treating patients with G551D mutation (class III)

Question 36

Lumacaftor/ivacaftor combination

Answer 36

For treating patients with DF508 mutation (class II)

Question 37

Other therapeutic strategies for cystic fibrosis

Answer 37

1. Activation of alternative Cl- channels

2. Blocking sodium resorption

Question 38

Activation of alternative Cl- channels as a therapy for cystic fibrosis

Answer 38

e.g. activators of calcium-activated chloride channels (CACC)
e.g. Denufusol = P2Y2 agonist
Second phase III trial showed compound to be ineffective at maintaining lung function - possibly because Cl- channels in goblet cells were also activated, which would increase mucus production

Question 39

Blocking sodium resorption as a therapy for cystic fibrosis

Answer 39

e.g. through blocking ENaC (but does present risk of hyperkalaemia)
e.g. Amiloride = kidney diuretic, tested in CF but had a very short half life in the airways
New compounds may be more effective - some Na+ channel blockers are in the pipeline e.g. QBW276, SPX-101, AZD5634