Epithelial Transport

Epithelial Transport

Multiple Choice

1. Membrane transporters play important roles in the flux of molecules between cells and lumen. Which one of the following is true with regards to epithelial membrane transport?

• A. The transport of molecules across the apical membrane may be uniport or symport but not antiport.

• B. The basolaterally located membrane transporters are usually symporter or antiporter.

• C. In indirect active transport, the concentration gradient is higher in the lumen than the cells.

• D. Membrane channels function independently from each other.

Answer Explanation

• Only statement B is correct...A is incorrect--transport across membrane may be uniport, symport or even antiport. C is also incorrect--Indirect active transport is also known as secondary active transport, for example Na/glucose cotransporter. Concentration maybe higher in cell than in lumen, but energy derived from Na/K ATPase creates electrochemical gradient for Na/glucose to be absorbed from lumen into cell. D is obviously incorrect, as different transporters act together, for example the Na/glucose cotransporter.

Multiple Choice

2. In the pulmonary epithelial and mucosal cell

• A. the Na+/K+-ATPase actively pumps Na+ from the mucus lining pulmonary lumen into the cell.

• B. the opening of CFTR leads to the removal of Na+ from the mucus lining the pulmonary lumen by epithelial sodium channel (ENaC).

• C. ENaC allows diffusion of Na+ from the cell into blood plasma.

• D. ENaC allows diffusion of Na into the lumen (not blood)

Feedback for incorrect answers

• A. incorrect, as Na is pumped out of the cell (not in)

  B. incorrect, as opening of CFTR inhibits ENaC in the airways

  C. correct

  D. incorrect, ENaC allows diffusion of Na into the lumen (not blood)

Multiple Choice

3. Activation of CFTR in airway epithelial cells:

• A. causes the concentration of Na+ and Cl- to increase in the blood.

• B. requires both binding of ATP and phosphorylation of the regulatory domain by protein kinase A (PKA)

• C. occurs in ciliated cells which helps to clear the lungs of inhaled particulates.

• D. actively pumps Cl- from the blood into the cell.

Feedback for incorrect answers

• A. incorrect, as it causes inhibition of ENaC (causing less Na to be reabsorbed from the mucus)

  B. correct

  C. incorrect, as clearance of inhaled particles occurs due to ciliary movements (with well hydrated mucus) and not directly due to CFTR activation

  D. incorrect, as Cl moves from the cell into the airways

Multiple Choice

4. Cystic Fibrosis (CF) is commonly associated with mucus obstruction in the airways but mucus plugging is not manifested in the sweat glands although the latter is also implicated in this disease. Which one of the flowing is the most likely reason for this observation?

• A. Airways contain higher expression of Cystic Fibrosis transmembrane conductance regulator (CFTR) than the sweat glands.

• B. Secretion of chloride by the sweat glands does not require activation by cAMP, unlike the airways.

• C. Airway epithelia contain more heterogenous cell populations than the sweat glands

• D. Airways contain secretory epithelium but sweat glands do not.

Feedback for incorrect answers

• C is correct, because the problem with mucus obstruction has to do with abnormal mucus production, rather than a direct effect of abnormal cftr. Skin does not express mucus producing cells as the airways (and intestines--they are secreted by Goblet cells) do, and so does not exhibit mucus obstruction. The skin cells are mainly secretory cells, whereas airways contain ciliated, Goblet, secretory and basal cells.

Multiple Choice

5. CFTR is also known to transport HCO3- in addition to Cl-. Which of the following statements is true with regards to CFTR involvement in HCO3- secretion?

• A. CFTR only mediates efflux of HCO3- and is not known to participate in influx of HCO3- into cells.

• B. CFTR may work in concordance with basolateral Na+/HCO3- co-transporter to enhance luminal HCO3- secretion.

• C. Defective CFTR expression in the female reproductive tract may not affect the sperm.

• D. CFTR transports HCO3- into the lumen more efficiently than Cl- , therefore it does not require co-existence of Cl-/HCO3- exchanger

Feedback for incorrect answers

• A is incorrect--it can go bothways (influx or efflux). Example of CFTR in influx is for sperm capacitation (https://academic.oup.com/view-large/figure/60640599/bire-80-01-01-f10.jpeg). Example of efflux is in HCO3 secreion in pancreas (https://openi.nlm.nih.gov/detailedresult?img=PMC4921137_MI2016-7596531.001&req=4). Answer is B, as it work with basolateral Na/HCO3 co-transporter to enhance luminal HCO3 secretion. C is incorrect because CFTR transports Cl- primarily and only about 25% of Cl- is transporting HCO3.

Multiple Choice

6. Various pharmacological approaches have been developed to treat complications of CF. Which of the following is NOT one of the principles in treating complications related to this disease?

• A. Airway complications can be overcome by giving wide spectrum antibiotics and mucolytics.

• B. Gut complications can be partially overcome by giving probiotics.

• C. Gut and airway complications can be overcome by activation of non-CFTR chloride secreting mechanisms.

• D. Airway complications due to mucus obstruction can be overcome by activation of epithelial sodium channel (ENaC) activity.

Feedback for incorrect answers

•  Options A & B are established. Drugs that work by activating other chloride channels are undergoing clinical trials -- Denufosol (Ph3) and Duramycin. Both act by activating Calcium activated chloride channel (CaCC). Option D is incorrect because activation of ENaC will make it worse (refer slides 21-23)

Multiple Choice

7. CF has been known to be associated with male and female infertility. Which one of the statements below correctly describes the role of CFTR in reproductive functions?

• A. CFTR is essential for fluid and Cl- secretions in the testes.

• B. CFTR expression and functional activity in the female reproductive tract are decreased by estrogen.

• C. CFTR is essential for normal development and function of the vas deferens

• D. During embryo implantation, decreased epithelial sodium channel (ENaC) activity is due to increase in CFTR function.

Feedback for incorrect answers 

•  CFTR mutations are detected in men with congenital bilateral absence of vas deferens (CBAVD) and, with a lower frequency, in men with congenitalunilateral absence of vas deferens (CUAVD). Exactly what is the role of CFTR during embryogenesis is not known, but the association is apparent. Just before implantation, there is maximal expression of ENaC and CFTR to ensure maximal fluid reabsorption, so that the blastocyst is immobile during implantation.

Multiple Choice

8. CFTR mutation can result in several pathophysiological changes in the airways. Which of the following is NOT true with regards to CFTR functions in the airways?

• A. Defective CFTR can indirectly induce cellular inflammation by increasing innate immunity.

• B. Defective CFTR alters the pH of airway surface liquid that impairs viscosity of the mucus, resulting in mucus plugging.

• C. Defective CFTR directly diminishes activity of epithelial antimicrobial peptide, resulting in chronic lung infection.

• D. Defective CFTR impairs mucociliary clearance

Feedback for incorrect answers 

•  *Apologies, I must have overlooked the fact that this question was put in the negative. Here is the explanation, and the correct answer is A. Please feel free to ask if you are still not clear*

  Although the cells in innate immunity are increased in cellular inflammation, the innate immunity is defective rather than increased, as eventually it causes more infections in people with CF. Other answers are correct.

  
  

Multiple Choice

9. Mr and Mrs Brown have 5 children, Jenny, Bill, Peter, Nigel and Celia. Bill and Celia suffer from cystic fibrosis. The other children show no symptoms of the disease, but genetic testing revealed that they were all carriers.Which of the following is NOT a probable genotype of the parents for the mutation in the CF gene involved?

• A. Mother heterozygous, father heterozygous

• B. Mother homozygous, father heterozygous

• C. Mother homozygous, father homozygous

• D. Father homozygous, mother heterozygous

Feedback for incorrect answers 

•  A, Probable, the family has carrier and affected children.

  B, Another probable scenario, as 2 of the children are affected and three others are just carriers.

  C. Not probable. If both parents are homozygous CF, all children will be affected.

  D. Another probable scenario, as 2 of the children are affected and three others are just carriers.

Multiple Choice

10. The following is the fate of mutant delF508 CFTR protein

• A. It is secreted from the cell

• B. It is trafficked into storage vesicles

• C. It will not be fully glycosylated

• D. It is protected from degradation by chaperones

Feedback for incorrect answers

•  A and B. are incorrect. The ΔF508 mutant protein will not be trafficked to storage vesicles, nor will the normal/mutant protein be released from the cell for secretion.

  C. The correct answer. CFTR is glycosylated twice during its maturation, in the ER and Golgi apparatus. ΔF508 mutation induced CF misfolding, causing its retention in endoplasmic reticulum (ER) and thus, the second glycosylation in Golgi will not occur. The mutant ΔF508 is eventually degraded by proteasome. Proteasome is a protein complex which destroys damaged or misfolded proteins in the cell by proteolysis.

  D. Incorrect. Chaperones help F508 proper folding, thereby preventing degradation, but it cannot correct ΔF508 mutant protein misfolding and its eventual degradation.

Multiple Choice

11. Which of the following statements about CF is NOT correct?

• A. More than 2,000 different mutations in the CFTR gene have been identified

• B. The most common mutation in Caucasians results in the loss of a phenylalanine residue

• C. CF occurs more frequently in women than men

• D. A CF patient homozygous for R117H mutant allele will have a milder CF disease

Feedback for incorrect answers

•  A, Correct. More than 1700 different mutations CFTR mutations have been identified

  B. Correct. Deletion of Phe508 is the most common mutation

  ΔF508 protein is It is retained in ER., all children will be affected.

  C. Incorrect. CF is not a sex-linked genetic disorder

  D. The R117H mutant allele is widely known as a mild disease conferring mutant allele.

Multiple Choice

12. Small molecule therapy has currently been used for the treatment of CF. Which of the following is NOT true with regards to this therapy?

• A. It helps to restore CFTR trafficking to the apical membrane.

• B. It increases the stability of CFTR mutant protein

• C. It restores CFTR gating function

• D. It enhances transcription of CFTR gene

Feedback for incorrect answers

•  A. Lumacaftor acts as chaperone during protein folding and increases the number of CFTR proteins that are trafficked to the cell surface, for Class II mutations.

• B. When lumacaftor is used, mutant CFTR can be protected from being degraded and more CFTR is available on the membrane, and these increases stability of mutant protein.

• C. Gating mutation such as G551D can be corrected using ivacaftor.

• D. There exist no pharmacotherapy agent for CF which can enhance CFTR transcription (so then this is the incorrect statement and thus, the answer...)

Multiple Choice

13. SGLT2 inhibitors block how many percent of glucose reabsorption in the proximal renal tubule?

• A. 20%

• B. 40%

• C. 50%

• D. 90%

SGLT2

• is a SLC carrier or ABC?

• SGLT2 inhibitors are used in the treatment of diabetes mellitus

Multiple Choice

14. ABC transport proteins binds ATP to this site to cause hydrolysis and power the conformational change required for the substrate to be transported out of the cell.

• A. Transmembrane domain (TMD)

• B. Nucleotide binding domain (NBD)

• C. Substrate binding protein

• D. Protein binding pocket

The ATP binds to the NBD which is usually inside the cytoplasm, after the substrate binds inside the binding pocket.

Multiple Choice

15. A 44-year-old man is being treated for neurosyphilis and is prescribed daily IM Procaine Penicillin with oral probenecid 4 times a day. What is the purpose of co-administering these two drugs?

• A. To reduce adverse effects of IM penicillin

• B. To enhance clinical effects of penicillin

• C. To prolong penicillin half life

• D. To reduce bacterial resistance to penicillin

• Probenecid competitively inhibits SLC22A6 (OAT1) and SLC22A8 (OAT3) (and other drug transporters), increases the half-life of antibiotics

Multiple Choice

16. You are a doctor at a very sophisticated medical facility. You ordered an SLC-genotyping for your patient, Mr Cheng, prior to commencing on statin treatment to reduce Mr Cheng’s cholesterol level. His results came back as SLCO1B1 – poor function. How will this affect your decision to treat Mr Cheng?

• A. Avoid statin and choose another drug

• B. Use higher statin dose for better efficacy

• C. Reduce statin dose for less adverse effect

• D. Combine statin with ezetimibe to reduce adverse effect

Feedback for incorrect answers

• This result signifies that the patient has two copies of a decreased function allele. Based on the genotype result, this patient is predicted to have low SLCO1B1 function. This patient may be at a high risk for an adverse response to medications that are affected by SLCO1B1. To avoid an untoward drug response, dose adjustments or alternative therapeutic agents may be necessary for medications affected by SLCO1B1. If simvastatin is prescribed to a patient with low SLCO1B1 function, there is a high risk of developing simvastatin-associated myopathy; such patients may need an alternative statin agent, and creatine kinase levels may need to be monitored routinely.