Disease relevance of AQP8

• Differential expression of aquaporin 8 in human colonic epithelial cells and colorectal tumors [1].
• Expression of aquaporin 8 in colonic epithelium with diarrhoea-predominant irritable bowel syndrome [2].

High impact information on AQP8

• Expression of human AQP8 and plant Arabidopsis TIP1;1 and TIP1;2 in yeast decreased growth and survival in the presence of H(2)O(2) [3].
• Aquaporin (AQP)8-facilitated transport of NH(3) has been suggested recently by increased NH(3) permeability in Xenopus oocytes and yeast expressing human or rat AQP8 [4].
• We tested the proposed roles of AQP8-facilitated NH(3) transport in mammalian physiology by comparative phenotype studies in wild-type vs. AQP8-null mice [4].
• Mammalian AQP8 sharing the aquaammoniaporin signature also complemented NH4+ transport deficiency in yeast [5].
• Aquaporin-7 appears to play a role in the cryopreservation of the sperm whereas aquaporin-8 is responsible for the secretion of pancreatic juice [6].

Biological context of AQP8

• The cDNA and the gene of human aquaporin8 (AQP8) were cloned from human testis cDNA and a genomic library, respectively [7].
• The upregulation of mitochondrial AQP8 in the hypothyroid liver was confirmed by immunogold electron microscopy [8].
• Overall, our data indicate that the T3 modulation of the AQP8 gene is a rapid downregulation of transcription [8].
• Assignment of the aquaporin-8 water channel gene (AQP8) to human chromosome 16p12 [9].
• Our results suggest that AQP8 may transport ammonium in vivo and physiologically contribute to the acid-base equilibrium [10].

Anatomical context of AQP8

• The expression of human AQP8 cRNA in Xenopus oocytes increased osmotic water permeability by 10-fold [7].
• Modulation of hepatic AQP8 expression may be relevant to the regulation of mitochondrial metabolism by thyroid hormones [8].
• Since thyroid hormone triiodothyronine (T3) is known to modulate both the shape and the metabolic activities of liver mitochondria, it was interesting to investigate the expression and distribution of AQP8 as well as the osmotic water permeability of the IMM in liver mitochondria from rats in different thyroid states [8].
• Considerable AQP8 mRNA was detected in the gall-bladder epithelium of mouse, calf, rabbit, guinea pig and man [11].
• CONCLUSION: Our result suggests that the expression of AQP8 is a marker of normal proliferating colonic epithelial cells and suggest these cells to be involved in fluid transport in the colon [1].

Associations of AQP8 with chemical compounds

• Previous studies in frog oocytes suggested that AQP8 was permeable to water, urea and ammonium, but no direct characterization had yet been reported [10].
• Incubation of WISH cells with monobutyryl cyclic adenosine monophosphate resulted in a 2-fold increase in aquaporin 8 messenger RNA level (P <.001) [12].
• In situ hybridization with biotin-labeled aquaporin-8 riboprobe was undertaken to localize the cellular expression of aquaporin-8 gene in these human fetal membranes [13].
• Moreover, hAQP8 is permeable to ammonium analogues (formamide and methylammonium) [10].
• All three AQP8 proteins transport water. rAQP8 and hAQP8 are not permeable to urea or glycerol [10].

Other interactions of AQP8

• The roles of AQP-8 and AQP-3 in water absorption remain to be established [14].
• While the localization of AQP5 in this gland has been verified in several laboratories, there have been differing reports regarding AQP8 localization [15].
• In the rat PG, AQP8 was invested near intercalated ducts and adjacent acini, whereas NKCC1 localized in the basolateral membranes of acinar cells [15].
• Thus, in agreement with an earlier report by other investigators (Elkjaer et al., Am J Physiol Renal Physiol 281:F1047-F1057, 2001), we report that AQP8 is expressed in the myoepithelial cells, but not in the acinar cells, of both the rat SMG and PG [15].

Analytical, diagnostic and therapeutic context of AQP8

• AQP8-facilitated NH(3) transport was confirmed in mammalian cell cultures expressing rat or mouse AQP8, in which the fluorescence of a pH-sensing yellow fluorescent protein was measured in response to ammonia (NH(3)/NH(4)(+)) gradients [4].
• In situ hybridization demonstrated that AQP8 was expressed in the cells facing the lumen in the normal colonic epithelium [1].
• METHODS: Specimens were taken from the proximal ascending colon or distal descending colon of D-IBS patients (n = 26) and healthy volunteers (n = 30), and AQP8 mRNA expression of each specimen was determined by fluorescent quantitative reverse transcription polymerase chain reaction (FQ-RT-PCR) [2].
• RESULTS: Reverse transcriptase-polymerase chain reaction detected aquaporin 8 expression in WISH cells [12].

References