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 ...
Aquaporin-5 (AQP5) is a water channel protein and is considered to play an important role in water movement across the plasma membrane. We raised anti-AQP5 antibody and examined the localization of AQP5 protein in rat salivary and lacrimal glands by immunofluorescence microscopy. AQP5 was found in secretory acinar cells of submandibular, parotid, and sublingual glands, where it was restricted to apical membranes including intercellular secretory canaliculi. In the submandibular gland, abundant AQP5 was also found additionally at the apical membrane of intercalated duct cells. Upon stimulation by isoproterenol, apical staining for AQP5 in parotid acinar cells tended to appear as clusters of dots. These results suggest that AQP5 is one of the candidate molecules responsible for the water movement in the salivary glands.
SUMMARY Aquaporins (AQPs) are membrane water channel proteins expressed in various tissues in the body. We surveyed the immunolocalization of AQP3, an isoform of the AQP family, in rat epithelial tissues. AQP3 was localized to many epithelial cells in the urinary, digestive, and respiratory tracts and in the skin. In the urinary tract, AQP3 was present at transitional epithelia. In the digestive tract, abundant AQP3 was found in the stratified epithelia in the upper part, from the oral cavity to the forestomach, and in the simple and stratified epithelia in the lower part, from the distal colon to the anal canal. In the respiratory tract, AQP3 was present in the pseudostratified ciliated epithelia from the nasal cavity to the intrapulmonary bronchi. In the skin, AQP3 was present in the epidermis. Interestingly, AQP3 was present at the basal aspects of the epithelia: in the basolateral membranes in the simple epithelia and in the multilayered epithelia at plasma membranes of the basal to intermediate cells. During development of the skin, AQP3 expression commenced late in fetal life. Because these AQP3-positive epithelia have a common feature, i.e., they are exposed to an environment of possible water loss, we propose that AQP3 could serve as a water channel to provide these epithelial cells with water from the subepithelial side to protect them against dehydration. Rapid water movement across the plasma membranes of cells is mediated by the membrane water channel proteins, aquaporins (AQPs). Ten isoforms of the AQP family have been thus far identified in mammals (AQP0-AQP9: for review see Brown et
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