Ser-256 phosphorylation dynamics of aquaporin 2 during maturation from the endoplasmic reticulum to the vesicular compartment in renal cells

Aquaporin 2 (AQP2) phosphorylation at Ser-256 by protein kinase A (PKA) is a key signal for vasopressin-stimulated AQP2 insertion into the plasma membrane in renal cells. This study underscores the possible role of phosphorylation at Ser-256 in regulating AQP2 maturation. AQP2-transfected renal CD8 cells were incubated with brefeldin A (BFA) to accumulate newly synthesized AQP2 in the endoplasmic reticulum ( ER), and AQP2 flow from ER to the vesicular compartment was analyzed after BFA washout. We found that a) in the ER, AQP2 is weakly phosphorylated; b) the amount of phosphorylated AQP2 (p-AQP2) at Ser-256 increased significantly during transit in the Golgi, even in the presence of the PKA inhibitor H89; and c) AQP2 transport from the Golgi to the vasopressin-regulated vesicular compartment occurred with a concomitant decrease in p-AQP2 at Ser-256. These results support the hypothesis that AQP2 transition in the Golgi apparatus is associated with a PKA-independent increase in AQP2 phosphorylation at Ser-256. Conversely, impaired constitutive phosphorylation in a Golgi-associated compartment occurring in cells expressing mutated S256A-AQP2 or E258K-AQP2 causes phosphorylation-defective AQP2 routing to lysosomes. This result might explain the molecular basis of the dominant form of nephrogenic diabetes insipidus caused by the mutation E258K-AQP2, in which the phenotype is caused by an impaired routing of AQP2.