Disruption of Aquaporin-11 Produces Polycystic Kidneys following Vacuolization of the Proximal Tubule

Abstract

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.Aquaporins (AQPs) are a family of membrane proteins that facilitate the transport of water and small solutes (8,15,21). They are distributed widely in nature from bacteria to animals. Eleven aquaporins (AQP0 to AQP10) have been identified and functionally characterized in humans. We reported the most recent AQP, AQP10 (11, 13). Their physiological importance is documented by the targeted disruption in mice (knockout mice) and by the discovery of humans and mice with nonfunctioning mutations. Of nine AQPs disrupted in mice and humans (AQP0 to AQP7 and AQP10), only AQP2-null mice die due to massive polyuria from nephrogenic diabetes insipidus (23). The milder phenotypes in AQP disruptions in general are surprising, since water is vital for organisms. Therefore, AQPs seem to be not critically essential for the survival of mammals but seem to be involved in the quality of their lives.The completion of human genome projects has revealed two more aquaporin-like genes, which we have deposited in GenBank under the names of AQPX1 and AQPX2 (9). They are renamed AQP11 and AQP12 with the approval of the Human Gene Nomenclature Committee. Rat AQP11 (AQPX1) is highly expressed in the testis and moderately expressed in the kidney, liver, and brain. On the other hand, rat AQP12 (AQPX2) is selectively expressed in the pancreas. They share similar genome structures with three exons, which are distinct from other AQPs: AQP0, AQP1, AQP2, AQP4, AQP5, and AQP6 have four exons; AQP3, AQP7, AQP8, AQP9, and AQP10 have six exons. In humans, AQP11 is mapped to chromosome 11q14 and AQP12 to chromosome 2q34-37, to which no diseases have been mapped. Moreover, we were not able to express them functionally in Xenopus oocytes. Therefore, their functions and physiological significance remain to be clarified.Previous AQPs have two highly conserved, short sequences named NPA (asparagine-proline-alanine) boxes. Each one of the NPA boxes has ...

Aquaporins in spermatozoa and testicular germ cells: identification and potential role

“…npg [68], rat [67]). Most recently, localization of the protein has been restricted to the late stages of spermiogenesis, first appearing in step 16 of spermatid elongation in the caudal cytoplasm and gradually becoming concentrated in vesicular organelles [69].…”

“…Although the water permeability of AQP11 was in doubt when it was initially expressed in plasma membranes [67], a high water conductance has since been shown when it is expressed in liposomes [81]. In Aqp11 knockout mice [68], cells in the proximal renal tubule show vacuolation, which results in the formation of polycystic kidney. Unfortunately, as the animals die of renal failure before puberty, this transgenic model sheds no light on the role of AQP11 in the testis, in which the protein is first expressed when the first wave of spermatogenesis is near completion [69].…”

NPA motifs play a key role in plasma membrane targeting of aquaporin‐4

“…However, the functions of NPA motifs in AQP biogenesis remain largely unknown. Few AQP members with variations in NPA motifs such as AQP11 and AQP12 do not express in the plasma membrane (11,12), suggesting an important role of NPA motifs in AQP plasma membrane targeting.…”

Dissecting the Roles of Aquaporins in Renal Pathophysiology Using Transgenic Mice

“…The mechanisms responsible for the association of AQP11 deficiency with renal cystogenesis remain unknown, though their elucidation might offer new insights on how and why renal cysts form in polycystic kidney disease. We found that primary cultures of proximal tubule cells from neonatal kidneys of AQP11 null mice manifest a mild defect in endosomal acidification (34). However, it is unclear whether defective endosomal acidification is involved in cyst pathogenesis or constitutes an epiphenomenon.…”

“…Whether AQP11 functions as a water channel has been debated, though a recent report showed a small increase in water permeability in liposomes reconstituted with recombinant AQP11 (33). In kidney, AQP11 appears to be expressed in intracellular vesicles in proximal tubule (34), though available antibodies are limited in their specificity. AQP11 null mice manifest an interesting phenotype of vacuolization and cyst formation in proximal tubule, similar in some respects to polycystic kidney disease.…”