Isolation of Human aquaporin-CD Gene

Title: Isolation of Human aquaporin-CD Gene
Authors: Uchida, Shinichi; Sasaki, Sei; Fushimi, MD, Kiyohide; Marumo, MD, Fumiaki
Publisher: Journal of Biological Chemistry
Date Published: September 23, 1994
Reference Number: 294
The human gene encoding aquaporin-CD (AQP-CD) was isolated, and its structural organization was characterized. The gene appeared to exist as a single copy in the human genome and comprises four exons distributing over 5 kilobases. The size range of exons is 81-761 base pairs, and that for introns is approximately 3000 to approximately 250 base pairs. The exon-intron boundaries of human AQP-CD gene are identified at identical positions in other related genes, the human AQP-CHIP gene and the human major intrinsic protein gene. The major transcription initiation sites were identified to positions 93 and 94 base pairs upstream of the ATG initiation codon by primer extension and ribonuclease protection assay. The 5'-flanking region of the hAQP-CD gene was characterized by a TATA box, two GATA consensus sequences, an AP-1 site, an AP-2 site, three E-boxes, and a cyclic AMP-responsive element. These structural features will lead to a better understanding of the mechanisms of tissue-specific expression and the regulation by dehydration in AQP-CD gene and will also be of help in search for possible genetic disorders in human AQP-CD gene.
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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.
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Aquaporins (AQPs) are a family of proteins that act as channels through which water can flow across cell membranes. Different AQPs are located in different parts of the body. Some are regulated by specific hormones and some operate independently of hormones. One such AQP, AQP-CD (now referred to as AQP2), is located in the principal cells of the kidney collecting duct (CD). When signaled by the antidiuretic hormone, arginine vasopressin (AVP), AQP2s are inserted into the apical membrane of the CD principal cells. Once inserted, much more water is allowed to flow through the apical membrane than usual. Thus, the kidney can reabsorb the body water flowing through the CD, a process essential to maintain body water balance.

Uchida, et al., were able to isolate the AQP-CD gene and determine its structure. This gene contains four coding sequences (exons) and three non-coding sequences (introns) distributed over 5000 nucleotides. (Genes are comprised of unique sequences of four nucleotide bases - adenine, thymine, guanine, cytosine - linked together by sugar and phosphate side chains.) The exon-intron boundaries are located in the same three points within the coding sequence as two other related genes: the aquaporin CHIP (AQP1) gene and the major intrinsic protein gene (AQP0).

To understand how the AQP-CD gene expresses itself, the authors examined the promoter region of the gene: that segment which controls the expression of the gene. They identified the positions in the gene where the process of gene transcription begins. That is, the process that begins the synthesis of the AQP-CD protein.

Rat AQP-CD increases when rats are dehydrated. At present, it is not clear whether this increase is the result of an increase in the rate of AQP-CD gene transcription or a decrease in the rate of degradation of AQP-CD mRNA. Their analysis, and further research based on it, will provide a greater understanding of how a AQP-CD gene expresses itself and how it is regulated through dehydration. It will also be of help in search of possible genetic disorders in human AQP-CD gene.