Disease relevance of AQP2

• Autosomal nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin, is caused by mutations in the Aquaporin-2 (AQP2) gene [1].

High impact information on AQP2

• When co-expressed, AQP2-727deltaG, but not a mutant in recessive NDI (AQP2-R187C), formed hetero-oligomers with wild-type (wt) AQP2 and reduced the water permeability of these oocytes, because of a reduced plasma membrane expression of wt-AQP2 [1].
• Expressed in renal epithelial cells, AQP2-727deltaG predominantly localized to the basolateral membrane and late endosomes/lysosomes, whereas wt-AQP2 was expressed in the apical membrane [1].
• Upon co-expressing in these cells, wt-AQP2 and AQP2-727deltaG mainly co-localized to late endosomes/lysosomes [1].
• Instead, the N-linked glycan is important for exit from the Golgi complex and sorting of AQP2 to the plasma membrane [2].
• The mechanisms and pathways directing AQP2 from the endoplasmic reticulum to the Golgi complex and beyond have not been defined [2].

Biological context of AQP2

• These results prove that glycosylation has no function in the routing of AQP2 in Madin-Darby canine kidney cells [3].
• Surface biotinylation and subsequent semiquantitative immunoblotting revealed that stimulation by cAMP increased the level of AQP2 in the apical membrane of WT10 cells 1.5-fold. independent of the presence of tunicamycin [3].
• In mammals, the regulation of water homeostasis is mediated by the aquaporin-1 (AQP1) water channel, which localizes to the basolateral and apical membranes of the early nephron segment, and AQP2, which is translocated from intracellular vesicles to the apical membrane of collecting duct cells after vasopressin stimulation [4].
• The wild-type AQP2 is a 271 amino acid protein, whereas these mutant genes were predicted to encode 330 to 333 amino acid proteins due to the frameshift mutations leading to the creation of a new stop codon 180 nucleotides downstream [5].

Anatomical context of AQP2

• Semiquantitative immunoblotting of human kidney membranes and urine showed an AQP2 glycosylation of 35 to 45% for medulla, papilla, and urine, with low variation among individuals [3].
• The extent of glycosylation of AQP2 in human kidney and urine and the effects of deglycoylation on routing of AQP2 in an AQP2-transfected Madin-Darby canine kidney cell line (clone WT10) were investigated [3].
• This indicates that both agents lead to the functional insertion of AQP2 into the cell membrane [6].
• We first examined AQP2 mRNA expression in many cultured epithelial cells derived from kidney [7].
• In conclusion, hetero-oligomerization of AQP2-727deltaG with wt-AQP2 and consequent mistargeting of this complex to late endosomes/lysosomes results in absence of AQP2 in the apical membrane, which can explain dominant NDI in this family [1].

Associations of AQP2 with chemical compounds

• In the collecting duct, aquaporin-2 (AQP2) is inserted into the apical membrane after stimulation of vasopressin type-2 receptors and retrieved into an endosomal compartment after withdrawal of vasopressin [3].
• Immunoblot analyses revealed that in WT10 cells, 34% of AQP2 is glycosylated, which was reduced to 2% after tunicamycin treatment [3].
• These data indicate that region N220-S229 is essential for localization of AQP2 in the apical membrane and that the NH2 and COOH tail of AQP2 are essential for trafficking of AQP2 to intracellular vesicles and its shuttling to and from the apical membrane [4].

Other interactions of AQP2

• AQP1 with the entire COOH tail of AQP2 was constitutively localized in the apical membrane, whereas chimeras with shorter COOH tail segments of AQP2 were localized in the apical and basolateral membrane [4].

Analytical, diagnostic and therapeutic context of AQP2

• Reverse-transcribed polymerase chain reaction applied to primary culture of IMCD cells showed a rapid dissipation of AQP2 mRNA within 4 days after culture [7].
• Cells were grown on the membrane support to examine the localization of AQP2 proteins by immunofluorescence microscopy [5].

References