2004 Global Researcher Conference Proceeding

April 09 - 11, 2004

Conference: 2004 Global Researcher Conference
Title: Wild-type aquaporin-2 rescues a novel aquaporin-2 mutant in recessive Nephrogenic Diabetes Insipidus to the apical plasma membrane
Authors: Kamsteeg, Erik-Jan; Savelkoul, Paul J.M.; de Mattia, Fabrizio; Bichet, Daniel G.; Robertson, Gary; Deen, Peter M.T.
Institutions: University of Nijmegen, NHGRI, Hopital du Sacre-Coeur de Montreal, Northwestern University Medical School

The aquaporin-2 (AQP2) water channel forms the apical entry pathway for water in the renal collecting ducts. Mutations in the AQP2 gene cause autosomal nephrogenic diabetes Insipidus (NDI), which can be inherited as a recessive or dominant trait. AQP2 mutants in recessive NDI are retained in the endoplasmic reticulum and do not oligomerize with wild type (wt) AQP2. Mutants in dominant NDI, on the other hand, are missorted to other organelles and oligomerize with wt-AQP2, thereby missorting wt-AQP2.

Analysis of two new families with a recessive form of NDI revealed compound heterozygous haplotypes with missense mutations in both alleles, encoding AQP2-P262L/AQP2-A190T and AQP2-P262L/AQP2-R187C, respectively. Expression in oocytes confirmed that AQP2-A190T and AQP2-R187C are retained in the ER, whereas AQP2-P262L is located in the plasma membrane and facilitates water transport similar to that of wt-AQP2. When expressed in renal epithelial cells, AQP2-P262L is present in an intracellular compartment and the basolateral plasma membrane. Interestingly, co-expression with wt-AQP2 resulted in the formation of hetero-oligomers like mutants in dominant NDI. However, in contrast to mutants in dominant NDI, wt-AQP2 was able to redirect AQP2-P262L to the apical plasma membrane. The rescue of the missorting of AQP2-P262L by wt-AQP2 comprises a novel mechanism in recessive NDI.

Mutations in the AQP2 gene result in autosomal NDI and can be inherited either as a recessive or dominant trait. (In the recessive form, both parents must carry the defective gene; in the dominant form, only one parent need carry the defective gene.) Mutant AQP2s in recessive NDI can’t get to the cell membrane to perform their functions because they are retained in the cell interior in a quality control structure called the endoplasmic reticulum (ER). These mutants do not interact with normal AQP2s that are present in the carriers. Mutant AQP2s in dominant NDI are directed to other cell structures, and also cannot reach the cell membrane, but they can interact with normal AQP2s and, in so doing, they prevent them from reaching the cell membrane as well. Kamsteeg, et al., discovered a new AQP2 mutation that resulted in a recessive form of NDI. Two families were identified with this mutation, which produces the mutant AQP2-P262L. This mutant has a leucine amino acid residue (L) where a proline residue (P) should have been. The mutation in the other allele encodes AQP2-A190T and AQP2-R187C for the first and second family, respectively. AQP2-A190T has a threonine residue (T) where an alanine residue (A) should have been. AQP2-R187C has a cysteine (C) where an arginine (R) should have been.

The research team found that AQP2-A190T and AQP2-R187C are retained in the ER. AQP2-P262L, however, is found in an intracellular compartment distinct from the ER. Furthermore, P262L interacts with normal AQP2s and, moves with them to the cell membrane. Thus, unlike mutants in dominant NDI, the P262L is rescued by normal AQP2s when it interacts with them.