The NPC motif of aquaporin-11, unlike the NPA motif of known aquaporins, is essential for full expression of molecular function.

The recently identified molecule aquaporin-11 (AQP11) has a unique amino acid sequence pattern that includes an Asn-Pro-Cys (NPC) motif, corresponding to the N-terminal Asn-Pro-Ala (NPA) signature motif of conventional AQPs. In this study, we examined the effect of the mutation of the NPC motif on the subcellular localization, oligomerization, and water permeability of AQP11 in transfected mammalian cells. Furthermore, the effect was also assessed using zebrafish. Site-directed mutation at the NPC motif did not affect the subcellular localization of AQP11 but reduced its oligomerization. A cell swelling assay revealed that cells expressing AQP11 with a mutated NPC motif had significantly lower osmotic water permeability than cells expressing wild-type AQP11. Zebrafish deficient in endogenous AQP11 showed a deformity in the tail region at an early stage of development. This phenotype was dramatically rescued by injection of human wild-type AQP11 mRNA, whereas the effect of mRNA for AQP11 with a mutated NPC motif was less marked. Although the NPA motif is known to be important for formation of water-permeable pores by conventional AQPs, our observations suggest that the corresponding NPC motif of AQP11 is essential for full expression of molecular function.

Disruption of aquaporin-11 produces polycystic kidneys following vacuolization of the proximal tubule.

Aquaporin-11 (AQP11) has been identified with unusual pore-forming NPA (asparagine-proline-alanine) boxes, but its function is unknown. We investigated its potential contribution to the kidney. Immunohistochemistry revealed that AQP11 was localized intracellularly in the proximal tubule. When AQP11 was transfected in CHO-K1 cells, it was localized in intracellular organelles. AQP11-null mice were generated; these mice exhibited vacuolization and cyst formation of the proximal tubule. AQP11-null mice were born normally but died before weaning due to advanced renal failure with polycystic kidneys, in which cysts occupied the whole cortex. Remarkably, cyst epithelia contained vacuoles. These vacuoles were present in the proximal tubules of newborn mice. In 3-week-old mice, these tubules contained multiple cysts. Primary cultured cells of the proximal tubule revealed an endosomal acidification defect in AQP11-null mice. These data demonstrate that AQP11 is essential for the proximal tubular function. AQP11-null mice are a novel model for polycystic kidney diseases and will provide a new mechanism for cystogenesis.