A new water channel (aquaporin-8, gene symbol AQP8) was isolated from rat pancreas and liver by homology cloning. Ribonuclease protection assay showed intense expression of the gene in pancreas and liver, less intense in colon and salivary gland, and negligible in other organs. The full-length cDNA was obtained by ligation of ϳ1.4-kilobase (kb) cDNA isolated from the rat liver cDNA library to ϳ0.5 kb of the 5-end fragment obtained by the rapid amplification of cDNA ends method. A major transcript of ϳ1.45 kb was demonstrated in liver and colon by Northern blot analysis. Expression of the cRNA in Xenopus oocytes markedly enhanced osmotic water permeability in a mercury-sensitive manner, indicating a water channel function of this molecule. The open reading frame encoded a 263-amino acid protein with a predicted molecular size of 28 kDa. Hydropathy analysis represented six membranespanning domains and five connecting loops containing two sites of NPA motif as preserved in other aquaporins. Unlike other mammalian aquaporins, AQP8 has an unusual structure with a long N terminus and a short C terminus, which are found in plant aquaporin, ␥-tonoplast intrinsic protein. By in situ hybridization, AQP8 mRNA expression was assumed in hepatocytes, acinal cells of pancreas and salivary gland, and absorptive colonic epithelial cells. The physiological role(s) of AQP8 remain to be elucidated.The aquaporins are water-selective membrane channels found in many species of animals and plants as the family of major intrinsic protein (MIP) 1 (1-3). Aquaporin-1 (AQP1) is the first protein recognized as a channel-forming integral membrane protein of 28 kDa (CHIP-28) expressed in mammalian red blood cells (1, 4) and then as a water channel expressed in renal proximal tubules (5-7) and other water-permeable epithelia (8, 9). Thereafter, four other aquaporins have been cloned in mammals. AQP2 is the vasopressin-regulated water channel, exclusively present in apical membranes of principal cells of collecting ducts in the kidney (10 -13), whereas AQP3 is a water channel locating basolateral membranes of collecting duct cells and transporting water and small solutes such as glycerol and urea (14). AQP4 and AQP5 were cloned from brain and salivary gland, respectively, and were presumed to be implicated in the sensation of osmotic change in hypothalamus and secretion in exocrine glands (15,16). In plants, tonoplast intrinsic protein (TIP) is an integral membrane protein and belongs to the MIP family (17, 18). ␥-TIP was found in the vegetative organs of plant and not in seeds, although ␣-TIP and ␤-TIP were seed-specific (19).The previous studies showed the presence of unique aquaporins in various exocrine glands such as salivary gland and lacrimal gland (16). Since pancreas is a secretory organ, secreting various digestive enzymes such as lipase, amylase, and protease in a volume of about 2,000 ml/day in humans (20), it may be reasonable to speculate the presence of an aquaporin family in pancreas. In the present study, we attempted to isolate...

A family of water-selective channels, aquaporins (AQP), has been demonstrated in various organs and tissues. However, the localization and expression of the AQP family members in the gastrointestinal tract have not been entirely elucidated. This study aimed to demonstrate the expression and distribution of several types of the AQP family and to speculate on their role in water transport in the rat gastrointestinal tract. By RNase protection assay, expression of AQP1–5 and AQP8 was examined in various portions through the gastrointestinal tract. AQP1 and AQP3 mRNAs were diffusely expressed from esophagus to colon, and their expression was relatively intense in the small intestine and colon. In contrast, AQP4 mRNA was selectively expressed in the stomach and small intestine and AQP8 mRNA in the jejunum and colon. Immunohistochemistry and in situ hybridization demonstrated cellular localization of these AQP in these portions. AQP1 was localized on endothelial cells of lymphatic vessels in the submucosa and lamina propria throughout the gastrointestinal tract. AQP3 was detected on the circumferential plasma membranes of stratified squamous epithelial cells in the esophagus and basolateral membranes of cardiac gland epithelia in the lower stomach and of surface columnar epithelia in the colon. However, AQP3 was not apparently detected in the small intestine. AQP4 was present on the basolateral membrane of the parietal cells in the lower stomach and selectively in the basolateral membranes of deep intestinal gland cells in the small intestine. AQP8 mRNA expression was demonstrated in the absorptive columnar epithelial cells of the jejunum and colon by in situ hybridization. These findings may indicate that water crosses the epithelial layer through these water channels, suggesting a possible role of the transcellular route for water intake or outlet in the gastrointestinal tract.

Aquaporin (AQP) 5 gene was recently isolated from salivary gland and identified as a member of the AQP family. The mRNA expression and localization have been examined in several organs. The present study was focused on elucidation of AQP5 expression and localization in the eye, salivary gland, and lung in rat. RNase protection assay confirmed intense expression of AQP5 mRNA in these organs but negligible expression in other organs. To examine the mRNA expression sites in the eye, several portions were microdissected for total RNA isolation. AQP5 mRNA was enriched in cornea but not in other portions (retina, lens, iris/ciliary body, conjunctiva, or sclera). AQP5 was selectively localized on the surface of corneal epithelium in the eye by immunohistochemistry and immunoelectron microscopy using an affinity-purified anti-AQP5 antibody. AQP5 was also localized on apical membranes of acinar cells in the lacrimal gland and on the microvilli protruding into intracellular secretory canaliculi of the serous salivary gland. In the lung, apical membranes of type I pulmonary epithelial cells were also immunostained with the antibody. These findings suggest a role of AQP5 in water transport to prevent dehydration or to secrete watery products in these tissues.

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