Renal Diseases

Question 1

What are the symptoms of Liddle’s syndrome?

Answer 1

Na+ retention, fluid retention, hypertension, hypokalaemia, metabolic alkalosis, low renin and aldosterone levels

Question 2

What causes Liddle’s syndrome?

Answer 2

An autosomal dominant, gain of function mutation of ENaC in principal cells

Question 3

What is the structure of the ENaC protein?

Answer 3

Alpha, beta and gamma subunits - all needed for normal function

Question 4

Where is the Liddle’s mutation in the ENaC protein?

Answer 4

In the COOH tail of beta or gamma subunits

Deletion of the proline rich motif

Question 5

What is the normal function of the proline rich motif in ENaC?

Answer 5

It allows the ubiquitination of ENaC which is important to allow the endocytosis of ENaC from the membrane

Question 6

What happens as a result of this loss of the proline rich motif in Liddle’s syndrome?

Answer 6

Endocytosis can’t occur at the same rate and therefore Na+ reabsorption is increased

Question 7

How do you experimentally show the effect of Liddle’s syndrome?

Answer 7

Over express in xenopus oocytes, traces will show there are roughly double the amount of Na channels in mutants compared with wild-type

Question 8

How do the symptoms seen in patients with Liddle’s syndrome arise?

Answer 8

Excessive Na+ movement into the cell also leads to excessive H2O - this causes hypertension
More Na+ influx means more K+ secretion - hypokalaemia
-ve membrane potential drives H+ secretion - metabolic alkalosis

Question 9

How do Liddle’s patients and normal individuals respond differently to an increase in blood pressure?

Answer 9

Normal individual:
Increased BP = decreased renin and aldosterone = loss of ENaC from apical membrane of principal cells = decreased Na+
Liddle’s individual:
Hypertension = decreased renin and aldosterone = ENaC cannot be removed = no reduction in Na+ reabsorption =hypertension persists

Question 10

How is Liddle’s syndrome treated?

Answer 10

Amiloride
Spironolactone (MR receptor agonist) shows hypertension is nothing to do with aldosterone and it is to do with ENaC

Question 11

What are the symptoms of diabetes insipidus?

Answer 11

Polyuria with compensatory polydipsia

Severe dehydration

Question 12

What are the four types of diabetes insipidus?

Answer 12

Primary polydipsia (suppressed AVP production from excessive H2O)
Gestational (decreased AVP levels, metabolised by placental enzymes)
Central
Nephrogenic

Question 13

What causes central diabetes insipidus?

Answer 13

Acquired - infection, head trauma, surgery

Congenital - 67 mutations in the AVP gene, make a version of vasopression that can’t be trafficked

Question 14

What causes nephrogenic diabetes insipidus?

Answer 14

Acquired - lithium (bipolar medication), some antibiotics and antifungals, hypokalaemia (reduction in AQP2), hypercalciuria, acute and chronic renal failures
Congenital - mutations in AVPR2 (X linked) or AQP2 gene. AQP2 impacts trafficking (dominant) or function (recessive) of protein

Question 15

What are symptoms of congenital nephrogenic diabetes insipidus in infants?

Answer 15

Hypernatrimic dehydration, poor feeding, skin dryness, depressed anterior fontanelle

Question 16

What are treatments of central diabetes insipidus?

Answer 16

Desmopressin - activates vasopressin receptors and increase urine osmolality

Question 17

Why is nephrogenic diabetes insipidus difficult to treat?

Answer 17

Patients can’t respond to vasopressin

Question 18

What are some treatments of nephrogenic diabetes inspidius and how do they work?

Answer 18

Cell permeable antagonists - attach to misfolded vasopressin receptor to correct it, AQP attaches to the membrane
Cell permeable receptor agonists - stimulate misfolded vasopressin receptor and allow downstream signalling
Cell permeable agonists - binds to misfolded receptor, corrects it and stimulates it to allow downstream activation
Compounds that bypass V2R signalling - target prostaglandin receptors, cAMP, insertion of AQP2 channels

Question 19

What are the different ways you can target cells to increase the AQP in the membrane?

Answer 19

1. Prostaglandin receptor agonists induce AQP2 expression and membrane abundance
2. Stimulating cGMP levels (cGMP controls insertion) - PDE5 inhibitors prevent breakdown of cGMP
3. Stimulating cAMP levels - PDE4 inhibitors prevent breakdown of of cAMP
4. Statins - prevent internalisation of AQP2, keeps in membrane for longer
5. HSP90 inhibitors allow misfolded AQP2 to reach the membrane