Disease relevance of Aqp2

• Here, we report an inducible mouse model of AQP2 gene deletion with severe polyuria in adult mice [1].
• Diabetes insipidus in mice with a mutation in aquaporin-2 [2].
• In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy and SIADH both AQP2 expression levels and apical plasma membrane targetting is increased suggesting a role for AQP2 in the development of water retention [3].
• Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting is seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure [3].
• Frame-shift mutations within the C terminus of aquaporin 2 (AQP2) cause autosomal-dominant nephrogenic diabetes insipidus (AD-NDI) [4].

High impact information on Aqp2

• Western blot analysis and immunoelectron microscopic studies showed that the loss of functional V2Rs had no significant effect on the basal expression levels of aquaporin-2 and the bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1) [5].
• The C-T change led to the substitution of a Ser (S256) by a Leu in the cytoplasmic tail of the Aqp2 protein, preventing its phosphorylation at S256 and the subsequent accumulation of Aqp2 on the apical membrane of the collecting duct principal cells [6].
• Here, we describe the identification of a single base pair change in aquaporin-2 (Aqp2) in cph mutants through genetic linkage mapping [6].
• The interference with normal trafficking of Aqp2 by this mutation resulted in a severe urine concentration defect. cph homozygotes demonstrated polydipsia and produced a copious amount of hypotonic urine [6].
• Immunofluorescence of collecting duct cells in the AD-NDI mouse revealed that the mutant AQP2 was missorted to the basolateral instead of apical plasma membrane [4].

Chemical compound and disease context of Aqp2

• In vitro studies in transfected cells have indicated that chemical chaperones including glycerol (0.5-1.2 M) and trimethylamine oxide (TMAO, 50-100 mM) can correct defective trafficking of some proteins, including deltaF508 CFTR in cystic fibrosis and AQP2 mutants in nephrogenic diabetes insipidus [7].

Biological context of Aqp2

• Sequence comparison of the Aqp2 cDNA with a 5.5-kb mouse genomic DNA indicated three introns (lengths 2.4, 0.9, and 0.6 kb) separating four exons with boundaries at amino acids 120, 175, and 202 [8].
• This study has revealed the genetic basis for the classical cph mutation and has provided direct genetic evidence that S256 in Aqp2 is indispensable for the apical accumulation, but not the general glycosylation or membrane association, of Aqp2 [6].
• AQP2 is the vasopressin-regulated water channel that is important in hereditary and acquired diseases affecting urine-concentrating ability [9].
• During normal water intake, urinary osmolality (Uosm), plasma Na concentration, urine volume, and renal aquaporin-2 (AQP2) levels were unchanged, but plasma AVP concentration was reduced in CD ET-1 KO animals [10].
• METHODS: Various fragments of the 5'-flanking region of the murine AQP-2 gene up to -9.5 kb were cloned into a luciferase (Luc) reporter plasmid, and they were transiently transfected into Madin-Darby canine kidney cells [11].

Anatomical context of Aqp2

• Expression in Xenopus oocytes indicated that Aqp2 encoded a mercurial-sensitive, water-selective channel [8].
• STUDY DESIGN: A systematic immunohistochemical study of Aqp2 protein expression was performed on embryonic mouse inner ears ranging from embryonic day 10 (otocyst stage) to embryonic day 18 (just before birth) [12].
• During early cochlear duct formation (embryonic days 12 and 13), expression of Aqp2 is homogeneous; later, it becomes restricted to specific regions of the endolymphatic compartment (embryonic days 15 and 18) [12].
• Endolymphatic duct and sac and stria vascularis expression of Aqp2 was noted to occur fairly late during development but demonstrated a distinct pattern of immunolabeling [12].
• Previous in vitro experiments using renal cell lines suggest recessive Aqp2 mutations result in improper trafficking of the mutant water pore [2].

Associations of Aqp2 with chemical compounds

• Tamoxifen injections over 10 days resulted in AQP2 gene excision, with undetectable full-length AQP2 transcript in kidney and a >95% reduction in immunoreactive AQP2 protein [1].
• Although cycloheximide administration had no effect on transient alterations of AQP2 protein content, these effects were attenuated by administration of chloroquine, a lysosomal inhibitor, or lactacystin, a proteasomal inhibitor [13].
• Dual influence of aldosterone on AQP2 expression in cultured renal collecting duct principal cells [14].
• Finally, both aldosterone and actinomycin D delayed AQP2 protein decay following AVP washout, but in a non-cumulative manner [14].
• Fine regulation of water reabsorption by the antidiuretic hormone [8-arginine]vasopressin (AVP) occurs in principal cells of the collecting duct and is largely dependent on regulation of the aquaporin-2 (AQP2) water channel [15].

Other interactions of Aqp2

• Mouse model of inducible nephrogenic diabetes insipidus produced by floxed aquaporin-2 gene deletion [1].
• Interestingly, AQP3 protein expression in the collecting duct was increased by about fivefold after AQP2 gene excision [1].
• Hypertonicity regulates the aquaporin-2 promoter independently of arginine vasopressin [11].
• AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2 [3].
• The transcriptional effects of TAp73 on AQP-2 and B2R were independent of p53 [16].

Analytical, diagnostic and therapeutic context of Aqp2

• Northern blot analysis showed a single 1.7-kb Aqp2 transcript expressed only in kidney (medulla > cortex); transcript expression was increased approximately 20-fold in 48-h water-deprived mice [8].
• Genomic Southern blot analysis revealed a single-copy Aqp2 gene [8].
• Along the collecting duct, the induction and subapical to plasma membrane translocation of the aquaporin-2 water channel in WD EP(1)(-/-) mice appeared equivalent to that of WD WT mice as determined by quantitative RT-PCR and immunohistochemistry [17].
• Combined RNase protection assay, Western blot, and immunofluorescence analyses revealed that physiological concentrations of AVP added to the basal side, but not to the apical side, of cells grown on filters induced both AQP2 mRNA and apical protein expression [15].
• Chromatin immunoprecipitation assays demonstrated that the endogenous AQP-2 promoter was occupied by TAp73 in a developmentally regulated manner [16].

References