A Fully Active Nonglycosylated V2 Vasopressin Receptor

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Title: A Fully Active Nonglycosylated V2 Vasopressin Receptor
Authors: Birnbaumer, Mariel; Innamorati, Giulio; Sadeghi, Hamid
Publisher: Molecular Pharmacology
Date Published: September 01, 1996
Reference Number: 147
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The human V2 vasopressin receptor belongs to the superfamily of G protein-coupled receptors believed to be anchored to the plasma membrane by seven transmembrane regions. The extracellular portion of the human V2 vasopressin receptor contains one site susceptible to N-linked glycosylation. Metabolic labeling and immunoprecipitation of the receptor expressed in transfected cells were applied to examine whether the protein was indeed glycosylated. The V2 vasopressin receptor expressed transiently was glycosylated, but glycosidase treatment to test the complexity of the sugar moiety linked to asparagine revealed that the majority of the receptor protein lacked complex carbohydrates, an indication of an improperly processed protein. This immature protein displayed a tendency to form aggregates. In contrast with these data, testing of the sugar complexity of the receptor protein synthesized in stably transfected cells identified the predominant form as an appropriately processed receptor protein. Mutagenesis of asparagine 22 to glutamine produced on expression in transfected cells a nonglycosylated receptor with ligand binding affinity and coupling characteristics almost identical to those of the wild-type form. After exposure to elevated concentrations of AVP (100 nM), the nonglycosylated form desensitized to the same extent as the wild-type receptor.

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 vasopressin-2 receptor (V2R) is a member of the family of G protein-coupled receptors (GPCRs). V2R resides in the principal cells of the kidney collecting duct, and its function is to bind with the antidiuretic hormone, arginine vasopressin (AVP). When V2R binds with AVP, a process is set into motion that results in the kidneys reabsorbing water and concentrating urine. The sequence goes like this:

  1. AVP binds with V2R.
  2. This binding activates the Gs/andenylyl cyclase system, which increases the intracellular level of an important metabolic regulator called cAMP.
  3. Increased cAMP levels activate protein kinase A, which signals the water transporting proteins known as aquaporin-2 (AQP2) to travel to the top portion of the kidney collecting duct cell membrane.
  4. AQP2 insertion into the kidney collecting duct cell membranes makes the membranes much more water permeable than usual.
  5. It is this increased permeability of the cell membranes that allows water to be reabsorbed, urine to be concentrated, and body water balance to be maintained.

The cell membrane is the thin, permeable band that encircles a cell, separating it from the environment. The majority of the V2R sits inside the cell membrane in seven folded clumps called transmembrane helices. Part of the V2R snakes outside the cell to form three curves called extracellular loops 1, 2 and 3; part of the V2R snakes inside the cell forming three curves called intracellular loops 1, 2 and 3. The end of the V2R called the amino terminus sits outside the cell with the extracellular loops; the other end, called the carboxy-terminal, sits inside the cell with the intracellular loops. (You can look at a diagram of a V2R for a clearer understanding.)

Scientists predict that the extracellular amino terminus is a spot where a process called N-linked glycosylation can happen. Glycosylation is a process where glycosyl groups (groups of atoms called radicals that form a fundamental part of molecules) are linked to other molecules, such as V2R. Research on pigs led researchers to conclude that glycosylation plays a major role in the transport and function of V2Rs. Innamorati, et al., examined the impact of glycosylation on the synthesis and function of the human V2R to see if the hypothesis regarding glycosylation and the human V2R was true.

They expressed normal V2Rs and mutant V2Rs in laboratory cell cultures. The mutant V2Rs lacked the asparagine required for glycosylation to occur, so the mutants would be nonglycosylated V2Rs. The authors then compared the mutant and normal V2R's ability to function and found that the functional properties of the nonglycosylated human V2R were unaltered.

The nonglycosylated V2R could get to the cell surface and perform all its receptor functions. It was able to bind with AVP; it could be desensitized to AVP (a necessary step to ensure the AVP initiated urine concentration does not last longer than it needs to); and it could be internalized (brought into the cell to be less available to AVP at the appropriate time). Thus, normal V2R, even though it is glycosylated, seems to belong to a class of receptors for which glycosylation does not play a role in function. However, glycosylation does seem to extend the half-life of the V2R when compared to the half-life of nonglycosylated V2R.