Vasopressin Type-2 Receptor and Aquaporin-2 Water Channel Mutants in Nephrogenic Diabetes Insipidus

Title: Vasopressin Type-2 Receptor and Aquaporin-2 Water Channel Mutants in Nephrogenic Diabetes Insipidus
Authors: Deen, Peter M.T.; Knoers, Nine
Publisher: American Journal of Medicine and Science
Date Published: November 01, 1998
Reference Number: 201
The regulation of water excretion by the kidney is one of the few physiologic processes that are prominent in everyday life. This process predominantly occurs in renal collecting duct cells, where transcellular water reabsorption is induced after binding of the pituitary hormone arginine-vasopressin to its vasopressin type-2 receptor and the subsequent insertion of aquaporin-2 (AQP2) water channels in the apical membrane of these cells. Removal of the hormone triggers endocytosis of AQP2 and restores the water-impermeable state of the collecting duct cells. Nephrogenic diabetes insipidus is characterized by the inability of the kidney to concentrate urine in response to vasopressin; the vasopressin type-2 receptor and the AQP2 water channel have both been shown to be involved in this disease. This article focuses on mutations in the vasopressin V2 receptor and aquaporin-2 water channel identified in nephrogenic diabetes insipidus patients, and on the effects of these mutations on the transport and function of these proteins upon expression in cell systems.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

The kidney is the main organ involved in balancing body water. The nephrons are the main working units of the kidneys. There are about one million nephrons in each kidney, and each nephron consists of a filter, called a glomerulus, and a tube called a tubule. The section of the tubule nearest the glomerulus is called the proximal tubule, and the section furthest from the glomerulus is called the distal tubule. There is a section in between the proximal and distal tubules called the Loop of Henle.

The average adult passes about 180 liters of body water through the glomeruli every day, only 1 or 2 liters of which is expelled as urine. So the kidney has to reabsorb most of the body water that passes through the glomeruli. About 90% of the water reabsorption occurs in proximal tubules and the descending limbs of the Loop of Henle. The remaining 10% is reabsorbed in the kidney collecting ducts (CDs).

This is the molecular sequence that allows body water to be reabsorbed through the CDs:

  1. The antidiuretic hormone, arginine vasopressin (AVP), responding to sensors that monitor the concentration solutes in serum and the volume of plasma circulating in the body, binds with the vasopressin-2 receptor (V2R). V2Rs are located in the basolateral membranes of the principal cells of the kidney CDs. (If you think of the principle cell as an upright rectangle and its membrane as the perimeter, the bottom and sides of the rectangle are the basolateral membranes and the top is the apical membrane.)
  2. When AVP binds with V2R it stimulates the activity of the enzyme, adenylyl cyclase (AdC) via a Gs protein to which the V2R is coupled.
  3. In turn, the AdC increases the level of the activity of cyclic adenosine monophosphate (cAMP).
  4. cAMP activates protein kinase A, which induces little sacs called water channel vesicles (WCVs) that contain aquaporin-2 (AQP2) water channels to travel from beneath the apical membrane to the apical membrane.
  5. When the WCVs reach the apical membrane they fuse with it and the AQP2s are inserted into it. Normally, the apical membrane does not let water permeate through it, but when the AQP2s are inserted into the membrane it increases the membrane's water permeability up to 100% because the AQP2s act as channels through which water can flow.

This is how the kidney can reabsorb the body water flowing through the kidney CDs. The water that is not reabsorbed is concentrated urine, which is later voided.

When AVP absents itself from the principal cell, the AQP2s are retrieved from the apical membrane and brought back inside the cell. This returns the apical membrane to its relatively low degree of water permeability and the kidney no longer is able to reabsorb the water. Thus, the action of AQP2s, regulated by AVP, allow the kidney to reabsorb water and concentrate urine.

Nephrogenic diabetes insipidus (NDI) is a disorder characterized by the kidneys' inability to respond to AVP. As a result, the NDI patient is unable to reabsorb the body water or concentrate urine. The two primary symptoms of NDI are polyuria (the chronic passage of large volumes of urine) and polydipsia (chronic, excessive thirst). NDI may either be acquired or inherited. The acquired form is generally less severe and more common. NDI can be acquired through long-term use of lithium and some other prescription drugs. It can also occur as a result of a low-protein diet or as a result of an abnormally low amount of potassium in the blood, obstruction of ureters, or degenerative kidney disease.

The molecular basis of the inherited forms of NDI is a mutation of one of two genes: either the V2R gene or the AQP2 gene. NDI resulting from V2R gene mutations are inherited in an X-linked recessive manner. This is because the V2R gene is located on the X chromosome. A female can carry the disease and not express its symptoms severely or at all, but if her son inherits the X-chromosome that carries a mutated V2R gene, then he will display the symptoms of NDI. X-linked NDI is by far the most common form of inherited NDI, affecting approximately four males per million. Inherited NDI has been found in many different ethnic groups worldwide.

NDI that results from mutations of the AQP2 gene may affect both males and females. This is because the AQP2 gene is carried on chromosome 12, an autosomal chromosome that is inherited by males and females equally, one from each parent. In most cases, AQP2 gene mutations are inherited in an autosomal recessive manner, which means both the chromosome inherited from the mother and father must bear a mutated AQP2 gene. However, there have been a few instances where a child inherited an AQP2 mutation from only one parent and still developed NDI. This inheritance pattern is called autosomal dominant.

Researchers have been able to clone both the AQP2 gene and the V2R gene. This has enabled them to determine the structure of both the normally formed proteins and their mutant counterparts. By studying both the normal and the mutant forms of these AQP2 proteins, researchers are able to understand the relationship between the gene's form and function. At present, more than 80 different mutations of the V2R gene and 14 different mutations of the AQP2 gene have been identified.

Mutations have been identified on every part of the V2R gene except its tail end. Different mutations result in different structural alterations that cause different functional defects. Some of the mutations result in a severely truncated V2R, others result in the amino acid sequence that comprises the V2R (the V2R is comprised of 371 amino acids) being altered by a single amino acid substitution (missense mutation). Both extremes have functional consequences.

Not all of the more than 80 mutations associated with X-linked NDI have been studied to determine the functional results of their altered structure. Of those that have, most of the mutations have resulted in V2Rs that cannot leave endoplasmic reticulum (ER). Normally, V2Rs are folded, synthesized, then through binding to other molecules, they are further modified in the ER. If a mutation prevents the V2Rs from being properly folded, linked or further assembled, they cannot leave the ER and are degraded, never being able to express themselves on the cell surface where they need to be in order to bind with AVP.

A number of the V2R gene mutations allow their V2Rs to leave the ER, but when they arrive at the cell surface, they are unable to bind with AVP because they have a low affinity for it. A few of the mutations allow their V2Rs to bind with AVP, but at a lower affinity than normal. One allows its V2Rs to bind with AVP, but the bond is unable to generate cAMP. By studying the structure and resultant functional defect, researchers are becoming more clear on what function each segment (and sometimes each amino acid of each segment) of the V2R is supposed to perform. This knowledge may some day be applied to genetic therapy for NDI.

That each mutation should undergo a functional analysis is made clear by the finding that three of the V2R mutations identified with different NDI patients were found to result in no dysfunction, and thus were not the cause of the patients' NDI. (The patients' NDI were caused by other mutations in their V2R genes.)

Of the 14 AQP2 gene mutations, one involves a deletion of a nucleotide and one signals the protein development to end early. Both of these result in a protein that stops short of normal development. The rest of the mutations result in one amino acid substituting for another, thus producing an altered amino acid sequence. Like the V2R mutations, the majority of the AQP2 gene mutations result in misfolded AQP2s that do not pass the quality control of the ER, are held there and degraded. Thus they are unable to get to the apical membrane where they must be to do their job.

An interesting finding is that all AQP2 gene mutations that result in defects in the part of AQP2 in between the transmembrane domains 1 and 6 are nonfunctional, whereas AQP2s with defects in loop A or C (AQP2s have 5 loops: A, B, C, D, E) are functional. Still, all result in NDI because the functional AQP2s are unable to make the apical membranes as water permeable as the normal AQP2s.

Another interesting finding is that the AQP2 gene that resulted in the autosomal dominant inheritance pattern (the majority of AQP2 gene-caused NDI is inherited in an autosomal recessive pattern) resulted in AQP2s with defects in its tail end (C-terminus). This is in contrast to the AQP2 mutations of the recessive pattern that result in defects in the part of the AQP2 called the transmembrane domain. This suggests there may be a relationship between the inheritance pattern (dominant or recessive) and the site of the mutation-caused defect.