Glycosylation is not essential for vasopressin-dependent routing of aquaporin-2 in transfected Madin-Darby canine kidney cells.

Abstract

Glycosylation has been shown to be important for proper routing and membrane insertion of a number of proteins. 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. 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. 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. The 1-desamino-8-D-arginine vasopressin-induced transcellular osmotic water permeability (Pf) of WT10 cells by a factor of 2.6 +/- 0.2 was reduced to 1.5 +/- 0.1 after pretreatment with the glycosylation inhibitor tunicamycin. However, when WT10 cells were incubated with 8-br-cAMP, the Pf increased by a factor 2.8 +/- 0.2 and by 2.9 +/- 0.2 after prior incubation with tunicamycin. Immunoblot analyses revealed that in WT10 cells, 34% of AQP2 is glycosylated, which was reduced to 2% after tunicamycin treatment. 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. However, in tunicamycin-treated WT10 cells, all AQP2 in the apical membrane was unglycosylated, whereas in untreated cells 30% of AQP2 in the apical membrane was glycosylated. These results prove that glycosylation has no function in the routing of AQP2 in Madin-Darby canine kidney cells.