An impaired routing of wild-type aquaporin-2 after tetramerization with an aquaporin-2 mutant explains dominant nephrogenic diabetes insipidus.

Autosomal recessive and dominant nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin, are caused by mutations in the aquaporin-2 (AQP2) gene. Missense AQP2 proteins in recessive NDI have been shown to be retarded in the endoplasmic reticulum, whereas AQP2-E258K, an AQP2 mutant in dominant NDI, was retained in the Golgi complex. In this study, we identified the molecular mechanisms underlying recessive and dominant NDI. Sucrose gradient centrifugation of rat and human kidney proteins and subsequent immunoblotting revealed that AQP2 forms homotetramers. When expressed in oocytes, wild-type AQP2 and AQP2-E258K also formed homotetramers, whereas AQP2-R187C, a mutant in recessive NDI, was expressed as a monomer. Upon co-injection, AQP2-E258K, but not AQP2-R187C, was able to heterotetramerize with wild-type AQP2. Since an AQP monomer is the functional unit and AQP2-E258K is a functional but misrouted water channel, heterotetramerization of AQP2-E258K with wild-type AQP2 and inhibition of further routing of this complex to the plasma membrane is the cause of dominant NDI. This case of NDI is the first example of a dominant disease in which the 'loss-of-function' phenotype is caused by an impaired routing rather than impaired function of the wild-type protein.

An aquaporin-2 water channel mutant which causes autosomal dominant nephrogenic diabetes insipidus is retained in the Golgi complex.

Mutations in the aquaporin-2 (AQP2) water channel gene cause autosomal recessive nephrogenic diabetes insipidus (NDI). Here we report the first patient with an autosomal dominant form of NDI, which is caused by a G866A transition in the AQP2 gene of one allele, resulting in a E258K substitution in the C-tail of AQP2. To define the molecular cause of NDI in this patient, AQP2-E258K was studied in Xenopus oocytes. In contrast to wild-type AQP2, AQP2-E258K conferred a small increase in water permeability, caused by a reduced expression at the plasma membrane. Coexpression of wild-type AQP2 with AQP2-E258K, but not with an AQP2 mutant in recessive NDI (AQP2-R187C), revealed a dominant-negative effect on the water permeability conferred by wild-type AQP2. The physiologically important phosphorylation of S256 by protein kinase A was not affected by the E258K mutation. Immunoblot and microscopic analyses revealed that AQP2-E258K was, in contrast to AQP2 mutants in recessive NDI, not retarded in the endoplasmic reticulum, but retained in the Golgi compartment. Since AQPs are thought to tetramerize, the retention of AQP2-E258K together with wild-type AQP2 in mixed tetramers in the Golgi compartment is a likely explanation for the dominant inheritance of NDI in this patient.

Importance of the mercury-sensitive cysteine on function and routing of AQP1 and AQP2 in oocytes.

To discriminate between water transport of of aquaporin-2 (AQP2) mutants in nephrogenic diabetes insipidus and that of an AQP2 molecule used to drag them to the oolemma, we investigated the mercury sensitivity of wild-type and AQP2 C181S proteins in oocytes. Incubation with HgCl2 inhibited the osmotic water permeability (Pf) of human (h) AQP2 by 40%, whereas inhibition of hAQP1 was 75%. Oocytes expressing hAQP1 C189S revealed a Pf comparable to wild-type hAQP1, but mercury sensitivity was lost. In contrast, no increase in Pf was obtained when hAQP2 C181S was expressed. Also, expression of rat AQP2 C181A and C181S mutants did not increase the Pf, which contrasts with published observations. Immunocytochemistry and immunoblotting revealed that only AQP1, AQP1 C189S, and AQP2 were targeted to the plasma membrane and that AQP2 mutant proteins are retarded in the endoplasmic reticulum. In conclusion, water transport through AQP2 is less sensitive to mercury inhibition than through AQP1. Furthermore, substitution of the mercury-sensitive cysteine for a serine results in an impaired routing of human and rat AQP2. Similar mutations have no effect on AQP1 function, which is indicative of structural differences between AQP1 and AQP2.

Aquaporin-2 transfection of Madin-Darby canine kidney cells reconstitutes vasopressin-regulated transcellular osmotic water transport.

Water transport across the mammalian collecting tubule is regulated by vasopressin-dependent aquaporin-2 insertion into and retrieval from the apical cell membrane. To establish a cell line that properly expresses aquaporin-2 and its hormone-dependent shuttling, Madin-Darby canine kidney cells were stably transfected with an aquaporin-2 expression construct. Cells of a representative clone (wild-type 10 [WT-10]) were grown on semipermeable supports, and transcellular osmotic water permeability (Pf; in microm/s +/- SEM) was measured. The basal Pf of WT-10 cells, which was lowered with indomethacin, increased from 10.6 +/- 0.8 to 35.7 +/- 1.2 upon incubation with 1-desamino-8-D-arginine vasopressin (dDAVP). This increase coincided with the translocation of aquaporin-2 from an intracellular compartment to the apical membrane. The Pf of untransfected cells (6.5 +/- 0.8) was unchanged by dDAVP. Kinetic studies with WT-10 cells revealed that maximal Pf was obtained within 30 min after dDAVP addition, which remained elevated for at least 90 min. Intracellular cAMP levels peaked within 5 min after dDAVP admission and decreased to basal levels within 45 min. After preincubation with dDAVP, the Pf decreased within 15 min after dDAVP washout and returned to basal levels within 75 min. In conclusion, the WT-10 cells mimic the vasopressin-regulated transcellular water transport and aquaporin-2 translocation as found in collecting duct cells to a great extent, and therefore constitute an in vitro cell model that can be used to study the regulation of transcellular water transport in detail and provide a simplified test system for screening putative aquaporin-2 blockers.

New mutations in the AQP2 gene in nephrogenic diabetes insipidus resulting in functional but misrouted water channels.

Nephrogenic diabetes insipidus (NDI) is characterized by the inability of the kidney to concentrate urine in response to vasopressin. The autosomal recessive form of NDI is caused by mutations in the AQP2 gene, encoding the vasopressin-regulated water channel of the kidney collecting duct. This report presents three new mutations in the AQP2 gene that cause NDI, resulting in A147T-, T126M-, or N68S-substituted AQP2 proteins. Expression of the A147T and T126M mutant AQP2 proteins in Xenopus oocytes revealed a relatively small, but significant increase in water permeability, whereas the water permeability of N68S expressing oocytes was not increased. cRNA encoding missense and wild-type AQP2 were equally stable in oocytes. Immunoblots of oocyte lysates showed that only the A147T mutant protein was less stable than wild-type AQP2. The mutant AQP2 proteins showed, in addition to the wild-type 29-kd band, an endoplasmic reticulum-retarded form of AQP2 of approximately 32 kd. Immunoblotting and immunocytochemistry demonstrated only intense labeling of the plasma membranes of oocytes expressing wild-type AQP2. In summary, two mutant AQP2 proteins encoded in NDI are functional water channels. Therefore, the major cause underlying autosomal recessive NDI is the misrouting of AQP2 mutant proteins.

The human AQP4 gene: definition of the locus encoding two water channel polypeptides in brain.

The aquaporin family of membrane water transport proteins are expressed in diverse tissues, and in brain the predominant water channel protein is AQP4. Here we report the isolation and characterization of the human AQP4 cDNAs and genomic DNA. Two cDNAs were isolated corresponding to the two initiating methionines (M1 in a 323-aa polypeptide and M23 in a 301-aa polypeptide) previously identified in rat [Jung, J.S., Bhat, R.V., Preston, G.M., Guggino, W.B. & Agre, P. (1994) Proc. Natl. Acad. Sci. USA 91, 13052-13056]. Similar to other aquaporins, the AQP4 gene is composed of four exons encoding 127, 55, 27, and 92 amino acids separated by introns of 0.8, 0.3, and 5.2 kb. Unlike other aquaporins, an alternative coding initiation sequence (designated exon 0) was located 2.7 kb upstream of exon 1. When spliced together, M1 and the subsequent 10 amino acids are encoded by exon 0; the next 11 amino acids and M23 are encoded by exon 1. Transcription initiation sites have been mapped in the proximal promoters of exons 0 and 1. RNase protection revealed distinct transcripts corresponding to M1 and M23 mRNAs, and AQP4 immunoblots of cerebellum demonstrated reactive polypeptides of 31 and 34 kDa. Using a P1 and a lambda EMBL subclone, the chromosomal site of the human AQP4 gene was mapped to chromosome 18 at the junction of q11.2 and q12.1 by fluorescence in situ hybridization. These studies may now permit molecular characterization of AQP4 during human development and in clinical disorders.