Disease-Causing V2 Vasopressin Receptors are Retained in Different Compartments of the Early Secretory Pathway

Title: Disease-Causing V2 Vasopressin Receptors are Retained in Different Compartments of the Early Secretory Pathway
Authors: Hermosilla, Ricardo; Oueslati, Morad; Donalies, Ute; Schonenberger, Eva; Krause, Eberhard; Oksche, Alexander; Rosenthal, Walter; Schulein, Ralf
Publisher: Traffic
Date Published: December 01, 2004
Reference Number: 681
The G protein-coupled V(2) vasopressin receptor is crucially involved in water reabsorption in the renal collecting duct. Mutations in the human V(2) vasopressin receptor gene cause nephrogenic diabetes insipidus. Many of the disease-causing mutants are retained intracellularly by the quality control system of the early secretory pathway. It was previously thought that quality control system is restricted to the endoplasmic reticulum (ER). Here, we have examined the retention mechanisms of eight V(2) vasopressin receptor mutants. We show that mutants L62P, DeltaL62-R64 and S167L are trapped exclusively in the ER. In contrast, mutants R143P, Y205C, InsQ292, V226E and R337X reach the ER/Golgi intermediate compartment (ERGIC) and are rerouted to the ER. The ability of the mutant receptors to reach the ERGIC is independent of their expression levels. Instead, it is determined by their folding state. Mutant receptors in the ERGIC may be sorted into retrograde transport vesicles by an interaction of an RXR motif in the third intracellular loop with the coatomer complex I. Our data show that disease-causing mutants of a particular membrane protein may be retained in different compartments of the early secretory pathway and that the folding states of the proteins determine their retention mechanism.
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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 endoplasmic reticulum (ER) is a section of the cell that acts as part of the quality control system (QCS) of the cell. Proteins that are improperly folded (i.e., that do not achieve the normal protein shape for the type of protein they are) are retained in the ER. In short, the QCS permits only correctly folded and assembled proteins to leave it in order to travel to that section of the cell where they perform their function.

Initially, researchers believed the QCS was restricted to the ER, but more recent research indicates that the QCS extends beyond the ER to include a section of the cell called the ER/Golgi intermediate compartment (ERGIC). It was not clear as to whether all misfolded proteins got as far as the ERGIC or just a certain type of misfolded protein could escape the ER, only to end up in the ERGIC.

Hermosilla, et al., developed an elegant series of experiments to answer this question. There are over 150 different V2R mutations that result in NDI. That is, there are over 150 V2R gene mutations that carry codes that produce V2R proteins that are misfolded or have some other defect that renders them, on the whole, ineffective. 70% of them produce V2R proteins that fail to reach the cell membrane, where they must be in order to perform their function. The research team selected 8 distinct V2R mutations to use in their tests. Using a recognized method to track the movements of these mutant V2R proteins, the researchers were able to determine that 3 of the mutants were completely retained in the ER. That is, their misfoldings were such that they were not allowed past the ER. The remaining 5, however, were allowed to travel as far as the ERGIC before the cells' QCS stopped them. Hence, Hermosilla, et al., were able to demonstrate that different V2R mutations produce different V2R mutants that fall into one of two subsets:

  1. those that are trapped exclusively in the ER, and

  2. those that reach the ERGIC and are then sent back to the ER.

The researchers then looked into the differences between the 2 V2R mutant subsets to see what allowed one subset, but not the other, to get as far as the ERGIC. They discovered that it had nothing to do with the number of mutants in the cell. That is, the V2R mutants that escaped as far as the ERGIC did not do so because they expressed higher numbers in the cell than the other subset did. What did enable the V2R mutants in subset B to reach the ERGIC was, as they were synthesized in the cell, they were able to develop into a more complete shape, relatively, than subset A.

However, the V2R mutants that do get to the ERGIC are still retained as the cell?s QCS extends into this section. The researchers speculate that these mutants get sorted into tiny cellular sacs that return them to the ER by means of a particular protein residue pattern that exists on a section of the V2R called the third intracellular loop. The researchers? work shows that the QCS appears to be more complex than originally thought.