Cloning and functional expression of human aquaporin8 cDNA and analysis of its gene

Aquaporins and Ion Channels as Dual Targets in the Design of Novel Glioblastoma Therapeutics to Limit Invasiveness

Current therapies for Glioblastoma multiforme (GBM) focus on eradicating primary tumors using radiotherapy, chemotherapy and surgical resection, but have limited success in controlling the invasive spread of glioma cells into a healthy brain, the major factor driving short survival times for patients post-diagnosis. Transcriptomic analyses of GBM biopsies reveal clusters of membrane signaling proteins that in combination serve as robust prognostic indicators, including aquaporins and ion channels, which are upregulated in GBM and implicated in enhanced glioblastoma motility. Accumulating evidence supports our proposal that the concurrent pharmacological targeting of selected subclasses of aquaporins and ion channels could impede glioblastoma invasiveness by impairing key cellular motility pathways. Optimal sets of channels to be selected as targets for combined therapies could be tailored to the GBM cancer subtype, taking advantage of differences in patterns of expression between channels that are characteristic of GBM subtypes, as well as distinguishing them from non-cancerous brain cells such as neurons and glia. Focusing agents on a unique channel fingerprint in GBM would further allow combined agents to be administered at near threshold doses, potentially reducing off-target toxicity. Adjunct therapies which confine GBM tumors to their primary sites during clinical treatments would offer profound advantages for treatment efficacy.

Classification and Gene Structure of Aquaporins

Aquaporins (AQPs) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, 13 AQPs, distributed widely in specific cell types in various organs and tissues, have been characterized in humans. A pair of NPA boxes forming a pore is highly conserved among all aquaporins and is also key residues for the classification of AQP superfamily into four groups according to primary sequences. AQPs may also be classified based on their transport properties. So far, chromosome localization and gene structure of 13 human AQPs have been identified, which is definitely helpful for studying phenotypes and potential targets in naturally occurring and synthetic mutations in human or cells.

Critical Role of Aquaporins in Cancer: Focus on Hematological Malignancies

Simple Summary

Aquaporins are proteins able to regulate the transfer of water and other small substances such as ions, glycerol, urea, and hydrogen peroxide across cellular membranes. AQPs provide for a huge variety of physiological phenomena; their alteration provokes several types of pathologies including cancer and hematological malignancies. Our review presents data revealing the possibility of employing aquaporins as biomarkers in patients with hematological malignancies and evaluates the possibility that interfering with the expression of aquaporins could represent an effective treatment for hematological malignancies.

Abstract

Aquaporins are transmembrane molecules regulating the transfer of water and other compounds such as ions, glycerol, urea, and hydrogen peroxide. Their alteration has been reported in several conditions such as cancer. Tumor progression might be enhanced by aquaporins in modifying tumor angiogenesis, cell volume adaptation, proteases activity, cell–matrix adhesions, actin cytoskeleton, epithelial–mesenchymal transitions, and acting on several signaling pathways facilitating cancer progression. Close connections have also been identified between the aquaporins and hematological malignancies. However, it is difficult to identify a unique action exerted by aquaporins in different hemopathies, and each aquaporin has specific effects that vary according to the class of aquaporin examined and to the different neoplastic cells. However, the expression of aquaporins is altered in cell cultures and in patients with acute and chronic myeloid leukemia, in lymphoproliferative diseases and in multiple myeloma, and the different expression of aquaporins seems to be able to influence the efficacy of treatment and could have a prognostic significance, as greater expression of aquaporins is correlated to improved overall survival in leukemia patients. Finally, we assessed the possibility that modifying the aquaporin expression using aquaporin-targeting regulators, specific monoclonal antibodies, and even aquaporin gene transfer could represent an effective therapy of hematological malignancies.

Differences in the localization of AQP1 and expression patterns of AQP isoforms in rat and mouse sciatic nerve and changes in rat AQPs expression after nerve crush injury

In the peripheral nervous system aquaporins (AQPs) have been reported in both peripheral neurons and glial cells. Previously we described the precise localization of AQP1 in the rat sciatic nerve, which is present in both Remak and myelin Schwann cells, and is enriched in the Schmidt-Lanterman incisures. In this work, we found that AQP1 in mouse is only present in Remak cells, showing a different localization between these species. However, after nerve crush injury the level of AQP1 mRNA expression remains constant at all times studied in rat and mouse. We then performed RT-PCR of nine AQP (AQP1-9) isoforms from rat and mouse sciatic nerve, we found that in rat only five AQPs are present (AQP1, AQP4, AQP5, AQP7 and AQP9), whereas in mouse all AQPs except AQP8 are expressed. Then, we studied the expression by RT-PCR of AQPs in rat after nerve crush injury, showing that AQP1, AQP4 and AQP7 expression remain constant at all times studied, while AQP2, AQP5 and AQP9 are upregulated after injury. Therefore, these two closely related rodents show different AQP1 localization and have different AQPs expression patterns in the sciatic nerve, possibly due to a difference in the regulation of these AQPs. The expression of AQP1 in Remak cells supports the involvement of AQP1 in pain perception. Also, in rat the upregulation of AQP2, AQP5 and AQP7 after nerve injury suggests a possible role for these AQPs in promoting regeneration following injury.

Boric acid transport activity of human aquaporins expressed in Xenopus oocytes

Boric acid is a vital micronutrient that is toxic at high concentrations in animals. However, the mechanisms underlying boric acid transport in animal cells remain unclear. To identify the plasma membrane boric acid channels in animals, we analyzed the function of human aquaporins (AQPs), which are homologous to the nodulin-like intrinsic protein family of plant boric acid channels. When human AQPs were expressed in Xenopus laevis oocytes, the results of the swelling assay showed that boric acid permeability significantly increased in oocytes expressing AQP3, 7, 8, 9, and 10, but not in those expressing AQP1, 2, 4, and 5. The boric acid influxes of these oocytes were also confirmed by elemental quantification. Electrophysiological analysis using a pH microelectrode showed that these AQPs transported boric acid (B(OH)3 ) but not borate ions (B(OH)4- ). These results indicate that AQP3, 7, 8, 9, and 10 act as boric acid transport systems, likely as channels in humans.

Bird aquaporins: Molecular machinery for urine concentration

Water conservation is one of the most challenging processes for terrestrial vertebrates and is necessary for their survival. Birds are the only vertebrate animals other than mammals that have the ability to concentrate their urine. Previously, we identified and characterized aquaporins (AQP)1-4 responsible for urine concentration in Japanese quail kidneys. Today, a total of 13 orthologs for these genes have been reported in birds. Bird AQPs can be classified into four subfamilies: 1) Classical AQPs (AQP0-5 and novel member, AQP4-like) that conserve the selectivity filter; 2) aquaglyceroporins (AQP3, 7, 9 and 10) that retain an aspartic acid residue in the second NPA box and expand the pore to accept larger molecules; 3) unorthodox AQPs (AQP11-12) which structurally resemble their mammalian counterparts; 4) AQP8-type, a subfamily that differs from mammalian AQP8. Interestingly, over the course of time, birds lost their mammalian counterpart AQP6 but obtained a novel AQP4-like aquaporin member. In quail and/or chicken kidneys, at least six AQPs are expressed. Quail AQP1 (qAQP1) is expressed in both cortical and medullary proximal tubules but is absent in the descending limb (DL) and the thick ascending limb (TAL), supporting our previous finding that the DL and TAL are water impermeable. AQP2, an arginine vasotocin (AVT)-sensitive water channel, is exclusively expressed in the principal cells of the collecting duct (CD). AQP4 is unlikely to participate in free water resorption from the collecting duct (CD), and only AQP3 may represent an exit pathway for water reabsorbed apically via AQP2. While AQP9 is not expressed in mammalian kidneys, AQP9 was recently found in chicken kidneys. This review summarizes the current knowledge of the structure, function and expression of bird AQPs.

Aquaporins in insulin resistance and diabetes: More than channels!

Aquaporins (AQPs) are part of the family of the integral membrane proteins. Their function is dedicated to the transport of water, glycerol, ammonia, urea, H₂O₂, and other small molecules across the biological membranes. Although for many years they were scarcely considered, AQPs have a relevant role in the development of many diseases. Recent discoveries suggest, that AQPs may play an important role in the process of fat accumulation and regulation of oxidative stress, two crucial aspects of insulin resistance and type-2 diabetes (T2D).

Insulin resistance (IR) and T2D are multi-faceted systemic diseases with multiple connections to obesity and other comorbidities such as hypertension, dyslipidemia and metabolic syndrome. Both IR and T2D transcends different tissues and organs, creating the maze of mutual relationships between adipose fat depots, skeletal muscle, liver and other insulin-sensitive organs. AQPs with their heterogenous properties, distinctive tissue distribution and documented involvement in both the lipid metabolism and regulation of the oxidative stress appear to be feasible candidates in the search for the explanation to this third-millennium plague. A lot of research has been assigned to adipose tissue AQP7 and liver tissue AQP9, clarifying their relationship and coordinated work in the induction of hepatic insulin resistance. Novel research points also to other aquaporins, such as AQP11 which may be associated with the induction of insulin resistance and T2D through its involvement in hydrogen peroxide transport.

In this review we collected recent discoveries in the field of AQP's involvement in the insulin resistance and T2D. Novel paths which connect AQPs with metabolic disorders can give new fuel to the research on obesity, insulin resistance and T2D - one of the most worrying problems of the modern society.

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Presentation of the current knowledge on the involvement of aquaporins in disease state.

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Overview on latest research regarding insulin resistance and AQPs.

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Conceptualisation of an AQPs network involved regulation of lipid-related metabolism.

Aquaporins in health and disease

Aquaporins (AQPs) are transmembrane channel proteins that mainly facilitate the water translocation through the plasma cell membrane. For several years these proteins have been extensively examined for their biologic role in health and their potential implication in different diseases. Technological improvements associated with the methods employed to evaluate the functions of the AQPs have provided us with significant new knowledge. In this chapter, we will examine the role of AQPs in health and disease based on the latest currently available evidence.

Fluid resuscitation via the rectum ameliorates hemodynamic disorders through adjusting aquaporin expression in an experimental severe acute pancreatitis model

Acute pancreatitis is an acute abdominal disease, with 10-20% of the cases deteriorating rapidly, accompanied by persistent organ failure and further development into severe acute pancreatitis (SAP). The aim of the present study was to investigate the mechanism of fluid resuscitation via the rectum in the early stages of SAP and the role of aquaporins (AQPs). An SAP model was constructed by injection of 5% sterile sodium taurocholate into the biliopancreatic duct of Sprague Dawley rats, and the mean arterial pressure (MAP) was continuously monitored via femoral artery catheterization. At 30 min after the construction of the SAP model, the rats in the fluid resuscitation groups were resuscitated with normal saline at a rate of 4 ml/kg/h through the venous or the rectal route. The AQP and Na⁺-K⁺-ATPase levels, and the correlation of the MAP and colon AQPs at the early stages of SAP were analyzed. The results demonstrated that the mRNA level of AQP-3 and AQP-4 in the distal colon decreased significantly in the group subjected to fluid resuscitation via the rectum, while no significant differences were identified in the Na⁺-K⁺-ATPase levels of the colon in that group. Furthermore, a negative correlation was identified between the expression of AQPs and the MAP (P<0.01). Thus, fluid resuscitation via the rectum appears to ameliorate hemodynamic disorders through adjusting the expression of AQP-3 and AQP-4 in the distal colon in an experimental SAP model.

Aquaporins in the lung

The lung is the interface between air and blood where the exchange of oxygen and carbon dioxide occurs. The surface liquid that is directly exposed to the gaseous compartment covers both conducting airways and respiratory zone and forms the air-liquid interface. The barrier that separates this lining fluid of the airways and alveoli from the extracellular compartment is the pulmonary epithelium. The volume of the lining fluid must be kept in a range that guarantees an appropriate gas exchange and other functions, such as mucociliary clearance. It is generally accepted that this is maintained by balancing resorptive and secretory fluid transport across the pulmonary epithelium. Whereas osmosis is considered as the exclusive principle of fluid transport in the airways, filtration may contribute to alveolar fluid accumulation under pathologic conditions. Aquaporins (AQP) facilitate water flux across cell membranes, and as such, they provide a transcellular route for water transport across epithelia. However, their contribution to near-isosmolar fluid conditions in the lung still remains elusive. Herein, we discuss the role of AQPs in the lung with regard to fluid homeostasis across the respiratory epithelium.

High-fat diet effects on amniotic fluid volume and amnion aquaporin expression in non-human primates

Western style, high-fat diet (HFD) and associated high lipid levels have deleterious effects on fetal and placental development independent of maternal obesity and/or diabetes. Our objectives were to determine whether HFD without development of obesity would alter amniotic fluid volume (AFV) and amnion aquaporin (AQP) expression in a non-human primate model. Japanese macaques were fed either a control diet or HFD before and during pregnancy. The four quadrant amniotic fluid index (AFI) was used as an ultrasonic estimate of AFV at 120 days gestation. Amnion samples were collected at 130 days gestation by cesarean section and AQP mRNA levels were determined by quantitative RT-PCR. Similar to that in human, AQP1, AQP3, AQP8, AQP9, and AQP11 were expressed in the macaque amnion with significant differences in levels among AQPs. In macaque, neither individual AQPs nor expression profiles of the five AQPs differed between control and non-obese HFD animals. There were regional differences in AQP expression in that, AQP1 mRNA levels were highest and AQP8 lowest in reflected amnion while AQP3, AQP9, and AQP11 were not different among amnion regions. When subdivided into control and HFD groups, AQP1 mRNA levels remain highest in the reflected amnion of both groups. The HFD did not significantly affect the AFI, but AFI was positively correlated with AQP11 mRNA levels independent of diet. Collectively, these data suggest that HFD in pregnant non-obese individuals may have at most modest effects on AFV as the AFI and amnion AQP expression are not substantially altered.

Structural Basis for Mutations of Human Aquaporins Associated to Genetic Diseases

Aquaporins (AQPs) are among the best structural-characterized membrane proteins, fulfilling the role of allowing water flux across cellular membranes. Thus far, 34 single amino acid polymorphisms have been reported in HUMSAVAR for human aquaporins as disease-related. They affect AQP2, AQP5 and AQP8, where they are associated with nephrogenic diabetes insipidus, keratoderma and colorectal cancer, respectively. For half of these mutations, although they are mostly experimentally characterized in their dysfunctional phenotypes, a structural characterization at a molecular level is still missing. In this work, we focus on such mutations and discuss what the structural defects are that they appear to cause. To achieve this aim, we built a 3D molecular model for each mutant and explored the effect of the mutation on all of their structural features. Based on these analyses, we could collect the structural defects of all the pathogenic mutations (here or previously analysed) under few main categories, that we found to nicely correlate with the experimental phenotypes reported for several of the analysed mutants. Some of the structural analyses we present here provide a rationale for previously experimentally observed phenotypes. Furthermore, our comprehensive overview can be used as a reference frame for the interpretation, on a structural basis, of defective phenotypes of other aquaporin pathogenic mutants.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

Characterization of AQPs in Mouse, Rat, and Human Colon and Their Selective Regulation by Bile Acids

In normal individuals, the epithelium of the colon absorbs 1.5–2 l of water a day to generate dehydrated feces. However, in the condition of bile acid malabsorption (BAM), an excess of bile acids in the colon results in diarrhea. Several studies have attempted to address the mechanisms contributing to BAM induced by various bile acids. However, none have addressed a potential dysregulation of aquaporin (AQP) water channels, which are responsible for the majority of transcellular water transport in epithelial cells, as a contributing factor to the onset of diarrhea and the pathogenesis of BAM. In this study, we aimed to systematically analyze the expression of AQPs in colonic epithelia from rat, mouse, and human and determine whether their expression is altered in a rat model of BAM. Mass spectrometry-based proteomics, RT-PCR, and western blotting identified various AQPs in isolated colonic epithelial cells from rats (AQP1, 3, 4, 7, 8) and mice (AQP1, 4, 8). Several AQPs were also detected in human colon (AQP1, 3, 4, 7–9). Immunohistochemistry localized AQP1 to the apical plasma membrane of epithelial cells in the bottom of the crypts, whereas AQP3 (rat, human) and AQP4 (mice, human) were localized predominantly in the basolateral plasma membrane. AQP8 was localized intracellularly and at the apical plasma membrane of epithelial cells. Rats fed sodium cholate for 72 h had significantly increased fecal water content, suggesting development of BAM-associated diarrhea. Colonic epithelial cells isolated from this model had significantly altered levels of AQP3, 7, and 8, suggesting that these AQPs may be involved in the pathogenesis of bile acid-induced diarrhea.

Aquaporins in the Colon as a New Therapeutic Target in Diarrhea and Constipation

Aquaporins (AQPs) play important roles in the water transport system in the human body. There are currently 13 types of AQP, AQP0 through AQP12, which are expressed in various organs. Many members of the AQP family are expressed in the intestinal tract. AQP3 is predominantly expressed in the colon, ultimately controlling the water transport. Recently, it was clarified that several laxatives exhibit a laxative effect by changing the AQP3 expression level in the colon. In addition, it was revealed that morphine causes severe constipation by increasing the AQP3 expression level in the colon. These findings have shown that AQP3 is one of the most important functional molecules in water transport in the colon. This review will focus on the physiological and pathological roles of AQP3 in the colon, and discuss clinical applications of colon AQP3.

Hepatocyte and Sertoli Cell Aquaporins, Recent Advances and Research Trends

Aquaporins (AQPs) are proteinaceous channels widespread in nature where they allow facilitated permeation of water and uncharged through cellular membranes. AQPs play a number of important roles in both health and disease. This review focuses on the most recent advances and research trends regarding the expression and modulation, as well as physiological and pathophysiological functions of AQPs in hepatocytes and Sertoli cells (SCs). Besides their involvement in bile formation, hepatocyte AQPs are involved in maintaining energy balance acting in hepatic gluconeogenesis and lipid metabolism, and in critical processes such as ammonia detoxification and mitochondrial output of hydrogen peroxide. Roles are played in clinical disorders including fatty liver disease, diabetes, obesity, cholestasis, hepatic cirrhosis and hepatocarcinoma. In the seminiferous tubules, particularly in SCs, AQPs are also widely expressed and seem to be implicated in the various stages of spermatogenesis. Like in hepatocytes, AQPs may be involved in maintaining energy homeostasis in these cells and have a major role in the metabolic cooperation established in the testicular tissue. Altogether, this information represents the mainstay of current and future investigation in an expanding field.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

Knockdown of aquaporin-8 induces mitochondrial dysfunction in 3T3-L1 cells

Background

Aquaporin-8 (AQP8), a member of the aquaporin water channel family, is expressed in various tissue and cells, including liver, testis, and pancreas. AQP8 appears to have functions on the plasma membrane and/or on the mitochondrial inner membrane. Mitochondrial AQP8 with permeability for water, H₂O₂ and NH₃ has been expected to have important role in various cells, but its information is limited to a few tissues and cells including liver and kidney. In the present study, we found that AQP8 was expressed in the mitochondria in mouse adipose tissues and 3T3-L1 preadipocytes, and investigated its role by suppressing its gene expression.

Methods

AQP8-knocked down (shAQP8) cells were established using a vector expressing short hairpin RNA. Cellular localization of AQP8 was examined by western blotting and immunocytochemistry. Mitochondrial function was assessed by measuring mitochondrial membrane potential, oxygen consumption and ATP level measurements.

Results

In 3T3-L1 cells, AQP8 was expressed in the mitochondria. In shAQP8 cells, mRNA and protein levels of AQP8 were decreased by about 75%. The shAQP8 showed reduced activities of complex IV and ATP synthase; it is probable that the impaired mitochondrial water handling in shAQP8 caused suppression of the electron transport and ADP phosphorylation through inhibition of the two steps which yield water. The reduced activities of the last two steps of oxidative phosphorylation in shAQP8 cause low routine and maximum capacity of respiration and mitochondrial hyperpolarization.

Conclusion

Mitochondrial AQP8 contributes to mitochondrial respiratory function probably through maintenance of water homeostasis.

General significance

The AQP8-knocked down cells we established provides a model system for the studies on the relationships between water homeostasis and mitochondrial function.

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AQP8 was expressed in mouse adipose tissues and 3T3–L1 preadipocytes.

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AQP8 was localized in mitochondria in 3T3–L1 cells.

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Knockdown of AQP8 lowered routine and maximum capacity of mitochondrial respiration.

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Knockdown of AQP8 reduced activities of complex IV and ATP synthase in mitochondria.

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AQP8 contributes to maintain mitochondrial respirationprobably via water excretion.

The expression of the multiple splice variants of AQP8 in porcine testes at different developmental stages

Aquaporin 8 (AQP8) may be involved in cytoplasmic condensation and fluid formation in the vas deferens during the process of spermiogenesis. In the present study, using reverse transcription polymerase chain reaction (RT-PCR) combined with DNA sequencing technology, the splice isoforms of AQP8 post-transcriptionally formed by alternative splicing in porcine testis at various developmental stages were analysed, and the differences in the amino acid sequences encoded by the splice isoforms were compared. The results showed that a total of three splice isoforms of AQP8 mRNA, including full-length mRNA, were produced via alternative splicing in porcine testes at the four developmental stages. Because the number of bases removed during splicing was not an integer multiple of 3, the amino acid sequence and protein structure of AQP8 were significantly altered after alternative splicing.

[Water regulation in the cochlea : Do molecular water channels facilitate potassium-dependent sound transduction?]

BACKGROUND

Sound transduction in the cochlea critically depends on the circulation of potassium ions (K(+)) along so-called "K(+) recycling routes" between the endolymph and perilymph. These K(+) currents generate high ionic and osmotic gradients, which potentially impair the excitability of sensory hair cells and threaten cell survival in the entire cochlear duct. Molecular water channels-aquaporins (AQP)-are expressed in all cochlear supporting cells along the K(+) recycling routes; however, their significance for osmotic equilibration in cochlear duct cells is unknown.

METHODS

The diffusive and osmotic water permeabilies of Reissner's membrane, the organ of Corti and the entire cochlear duct epithelium were determined. Expression of the potassium channel Kir4.1 and the water channel AQP4 in the cochlear duct was investigated by immunohistochemistry.

RESULTS

The calculated water permeability values indicate the extent of AQP-facilitated water flux across the cochlear duct epithelium. Immunohistochemically, Kir4.1 and AQP4 were found to colocalize in distinct membrane domains of supporting cells along the K(+)-recycling routes.

CONCLUSION

These observations suggest the presence of a rapid AQP-mediated water exchange between the endolymph, the cells of the cochlear duct and the perilymph. The subcellular colocalization of Kir4.1 and AQP4 in epithelial supporting cells indicates functional coupling of potassium and water flow in the cochlea. Finally, this offers an explanation for the hearing impairment observed in individuals with mutations in the AQP4 gene.

Subcellular localization of selectively permeable aquaporins in the male germ line of a marine teleost reveals spatial redistribution in activated spermatozoa

In oviparous vertebrates such as the marine teleost gilthead seabream, water and fluid homeostasis associated with testicular physiology and the external activation of spermatozoa is potentially mediated by multiple aquaporins. To test this hypothesis, we isolated five novel members of the aquaporin superfamily from gilthead seabream and developed paralog-specific antibodies to localize the cellular sites of protein expression in the male reproductive tract. Together with phylogenetic classification, functional characterization of four of the newly isolated paralogs, Aqp0a, -7, -8b, and -9b, demonstrated that they were water permeable, while Aqp8b was also permeable to urea, and Aqp7 and -9b were permeable to glycerol and urea. Immunolocalization experiments indicated that up to seven paralogous aquaporins are differentially expressed in the seabream testis: Aqp0a and -9b in Sertoli and Leydig cells, respectively; Aqp1ab, -7, and -10b from spermatogonia to spermatozoa; and Aqp1aa and -8b in spermatids and sperm. In the efferent duct, only Aqp10b was found in the luminal epithelium. Ejaculated spermatozoa showed a segregated spatial distribution of five aquaporins: Aqp1aa and -7 in the entire flagellum or the head, respectively, and Aqp1ab, -8b, and -10b both in the head and the anterior tail. The combination of immunofluorescence microscopy and biochemical fractionation of spermatozoa indicated that Aqp10b and phosphorylated Aqp1ab are rapidly translocated to the head plasma membrane upon activation, whereas Aqp8b accumulates in the mitochondrion of the spermatozoa. In contrast, Aqp1aa and -7 remained unchanged. These data reveal that aquaporin expression in the teleost testis shares conserved features of the mammalian system, and they suggest that the piscine channels may play different roles in water and solute transport during spermatogenesis, sperm maturation and nutrition, and the initiation and maintenance of sperm motility.

Mitochondrial aquaporin-8 in renal proximal tubule cells: evidence for a role in the response to metabolic acidosis

Mitochondrial ammonia synthesis in proximal tubules and its urinary excretion are key components of the renal response to maintain acid-base balance during metabolic acidosis. Since aquaporin-8 (AQP8) facilitates transport of ammonia and is localized in inner mitochondrial membrane (IMM) of renal proximal cells, we hypothesized that AQP8-facilitated mitochondrial ammonia transport in these cells plays a role in the response to acidosis. We evaluated whether mitochondrial AQP8 (mtAQP8) knockdown by RNA interference is able to impair ammonia excretion in the human renal proximal tubule cell line, HK-2. By RT-PCR and immunoblotting, we found that AQP8 is expressed in these cells and is localized in IMM. HK-2 cells were transfected with short-interfering RNA targeting human AQP8. After 48 h, the levels of mtAQP8 protein decreased by 53% (P < 0.05). mtAQP8 knockdown decreased the rate of ammonia released into culture medium in cells grown at pH 7.4 (-31%, P < 0.05) as well as in cells exposed to acid (-90%, P < 0.05). We also evaluated mtAQP8 protein expression in HK-2 cells exposed to acidic medium. After 48 h, upregulation of mtAQP8 (+74%, P < 0.05) was observed, together with higher ammonia excretion rate (+73%, P < 0.05). In vivo studies in NH(4)Cl-loaded rats showed that mtAQP8 protein expression was also upregulated after 7 days of acidosis in renal cortex (+51%, P < 0.05). These data suggest that mtAQP8 plays an important role in the adaptive response of proximal tubule to acidosis possibly facilitating mitochondrial ammonia transport.

Water channel proteins in the gastrointestinal tract

Water transport through the human digestive system is physiologically crucial for maintaining body water homeostasis and ensure digestive and absorptive functions. Within the gastrointestinal tract, water recirculates, being secreted with the digestive juices and then almost entirely absorbed by the small and large intestine. The importance of aquaporins (AQPs), transmembrane water channel proteins, in the rapid passage of water across plasma membranes in the gastrointestinal tract appears immediately evident. Several AQP isoforms are found in gastrointestinal epithelia, with AQP1, 3, 7, 10 and 11 being the most abundantly expressed in the whole gut. On the other hand, AQP4 and 8 are located selectively in the stomach and colon, respectively. Here we review AQP expression and localization at the tissue, cellular and subcellular level in gastrointestinal epithelia, and their modification in various gut diseases.

Expression and subcellular localization of the AQP8 and AQP1 water channels in the mouse gall-bladder epithelium

BACKGROUND INFORMATION

Transepithelial transport of water is one of the most distinctive functions by which the gall-bladder rearranges its bile content. Water is reabsorbed from the gall-bladder lumen during fasting, whereas it is secreted into the lumen following meal ingestion. Nevertheless, the molecular mechanism by which water is transported across the gall-bladder epithelium remains mostly unclear.

RESULTS

In the present study, we investigate the presence and subcellular localization of AQP (aquaporin) water channels in the mouse gall-bladder epithelium. Considerable AQP8 mRNA was detected in the gall-bladder epithelium of mouse, calf, rabbit, guinea pig and man. Studies of subcellular localization were then addressed to the mouse gall-bladder where the transcript of a second AQP, AQP1, was also detected. Immunoblotting experiments confirmed the presence of AQP8 and AQP1 at a protein level. Immunohistochemistry showed intense expression of AQP8 and AQP1 in the gall-bladder epithelial cells where AQP8 was localized in the apical membrane, whereas AQP1 was seen both in the apical and basolateral membranes, and in vesicles located in the subapical cytoplasm.

CONCLUSIONS

The pattern of subcellular distribution of AQP8 and AQP1 strongly corroborates the hypothesis of a transcellular route for the movement of water across the gall-bladder epithelium. Osmotic water would cross the apical membrane through AQP8 and AQP1, although AQP1 would be the facilitated pathway for the movement of water across the basolateral membrane. The presence of two distinct AQPs in the apical membrane is an unusual finding and may relate to the membrane's ability both to absorb and secrete fluid. It is tempting to hypothesize that AQP1 is hormonally translocated to the gall-bladder apical membrane to secrete water as in the bile duct epithelium, a functional homologue of the gall-bladder epithelium, whereas apical AQP8 may account for the absorption of water from gall-bladder bile.

Reconstitution of water channel function and 2D-crystallization of human aquaporin 8

Among the thirteen human aquaporins (AQP0-12), the primary structure of AQP8 is unique. By sequence alignment it is evident that mammalian AQP8s form a separate subfamily distinct from the other mammalian aquaporins. The constriction region of the pore determining the solute specificity deviates in AQP8 making it permeable to both ammonia and H(2)O(2) in addition to water. To better understand the selectivity and gating mechanism of aquaporins, high-resolution structures are necessary. So far, the structure of three human aquaporins (HsAQP1, HsAQP4, and HsAQP5) have been solved at atomic resolution. For mammalian aquaporins in general, high-resolution structures are only available for those belonging to the water-specific subfamily (including HsAQP1, HsAQP4 and HsAQP5). Thus, it is of interest to solve structures of other aquaporin subfamily members with different solute specificities. To achieve this the aquaporins need to be overexpressed heterologously and purified. Here we use the methylotrophic yeast Pichia pastoris as a host for the overexpression. A wide screen of different detergents and detergent-lipid combinations resulted in the solubilization of functional human AQP8 protein and in well-ordered 2D crystals. It also became evident that removal of amino acids constituting affinity tags was crucial to achieve highly ordered 2D crystals diffracting to 3Å.

Pregnant phenotype in aquaporin 8-deficient mice

AIM

Aquaporin 8 (AQP8) is expressed within the female reproductive system but its physiological function reminds to be elucidated. This study investigates the role of AQP8 during pregnancy using AQP8-knockout (AQP8-KO) mice.

METHODS

Homozygous AQP8-KO mice were mated, and the conception rate was recorded. AQP8-KO pregnant mice or their offspring were divided into 5 subgroups according to fetal gestational day (7, 13, 16, 18 GD) and newborn. Wild type C57 pregnant mice served as the control group. The number of pregnant mice, total embryos and atrophic embryos, as well as fetal weight, placental weight and placental area were recorded for each subgroup. The amount of amniotic fluid in each sac at 13, 16, and 18 GD was calculated. Statistical significance was determined by analysis of variance of factorial design and chi-square tests.

RESULTS

Conception rates did not differ significantly between AQP8-KO and wild type mice. AQP8-KO pregnant mice had a significantly higher number of embryos compared to wild type controls. Fetal/neonatal weight was also significantly greater in the AQP8-KO group compared to age-matched wild type controls. The amount of amniotic fluid was greater in AQP8-KO pregnant mice than wild type controls, although the FM/AFA (fetal weight/amniotic fluid amount) did not differ. While AQP8-KO placental weight was significantly larger than wild type controls, there was no evidence of placental pathology in either group.

CONCLUSION

The results suggest that AQP8 deficiency plays an important role in pregnancy outcome.

Piscine aquaporins: an overview of recent advances

Aquaporins are a superfamily of integral membrane proteins that facilitate the rapid and yet highly selective flux of water and other small solutes across biological membranes. Since their discovery, they have been documented throughout the living biota, with the majority of research focusing on mammals and plants. Here, we review available data for piscine aquaporins, including Agnatha (jawless fish), Chondrichthyes (chimaeras, sharks, and rays), Dipnoi (lungfishes), and Teleostei (ray-finned bony fishes). Recent evidence suggests that the aquaporin superfamily has specifically expanded in the chordate lineage consequent to serial rounds of whole genome duplication, with teleost genomes harboring the largest number of paralogs. The selective retention and dichotomous clustering of most duplicated paralogs in Teleostei, with differential tissue expression profiles, implies that novel or specialized physiological functions may have evolved in this clade. The recently proposed new nomenclature of the piscine aquaporin superfamily is discussed in relation to the phylogenetic signal and genomic synteny, with the teleost aquaporin-8 paralogs used as a case study to illustrate disparities between the underlying codons, molecular phylogeny, and physical locus. Structural data indicate that piscine aquaporins display similar channel restriction residues found in the tetrapod counterparts, and hence their functional properties seem to be conserved. However, emerging evidence suggests that regulation of aquaporin function in teleosts may have diverged in some cases. Cell localization and experimental studies imply that the physiological roles of piscine aquaporins extend at least to osmoregulation, reproduction, and early development, although in most cases their specific functions remain to be elucidated.

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

Mammalian spermatozoa have relatively high water permeability and swell readily, as in the hypo-osmotic swelling test used in the andrology clinic. Physiologically, spermatozoa experience changes in the osmolality of the surrounding fluids in both the male and the female tracts on their journey from the testis to the ovum. Sperm volume regulation in response to such osmotic challenges is important to maintain a stable cell size for the normal shape and function of the sperm tail. Alongside ion channels for the fluxes of osmolytes, water channels would be crucial for sperm volume regulation. In contrast to the deep knowledge and numerous studies on somatic cell aquaporins (AQPs), the understanding of sperm AQPs is limited. Among the 13 AQPs, convincing evidence for their presence in spermatozoa has been confined to AQP7, AQP8 and AQP11. Overall, current findings indicate a major role of AQP8 in water influx and efflux for sperm volume regulation, which is required for natural fertilization. The preliminary data suggestive of a role for AQP7 in sperm glycerol metabolism needs further substantiation. The association of AQP11 with the residual cytoplasm of elongated spermatids and the distal tail of spermatozoa supports the hypothesis of more than just a role in conferring water permeability and also in the turnover and recycling of surplus cellular components made redundant during spermiogenesis and spermiation. This would be crucial for the maintenance of a germinal epithelium functioning efficiently in the production of spermatozoa.

Solute transporters and aquaporins are impaired in celiac disease

BACKGROUND INFORMATION

Celiac disease is a chronic inflammatory disorder of the small bowel induced in genetically susceptible subjects by gluten ingestion. Diarrhoea, weight loss and malabsorption represent the major clinical presentation of the disease. Here we examined the possible alteration in the expression and localization of water channels [AQPs (aquaporins)] and some solute transporters in duodenal mucosa of celiac disease patients. Duodenal biopsies from untreated celiacs, treated celiacs, healthy controls and disease controls were considered in the present study. The expressions of some AQPs and transporter mRNAs in human duodenal biopsies were determined by semi-quantitative RT-PCR (reverse transcription PCR) and real-time RT-PCR. The localization of AQPs 3, 7 and 10 and of SGLT1 (Na+/glucose co-transporter 1), PEPT1 (H+/oligopeptide transporter 1) and NHE3 (Na+/H+ exchanger 3) was evaluated by immunohistochemistry.

RESULTS

AQPs 3, 7, 10 and 11, SGLT, PEPT and NHE, CFTR (cystic fibrosis transmembrane conductance regulator) and NKCC (Na-K-2Cl co-transporter) mRNAs were expressed in duodenal biopsies of healthy controls, treated celiac patients and disease controls. The expression of transcripts was virtually absent in duodenal biopsies of untreated celiac disease patients except for CFTR and NKCC. In healthy controls, immunohistochemistry revealed a labelling in the apical membrane of surface epithelial cells of the duodenum. The immunolabelling was heavily reduced or absent in untreated celiac patients, while it was normal in patients consuming a gluten-free diet for at least 12 months.

CONCLUSIONS

Our results indicate that the main routes for water and solute absorption are deficient in celiac disease and may play a role in the onset of malabsorption symptoms.

Immunolocalization of aquaporin-1, -5, and -7 in the avian testis and vas deferens

Thirteen mammalian aquaporin (AQP) isoforms have been identified, and they have a unique tissue-specific pattern of expression. AQPs have been found in the reproductive system of both male and female humans, rats, and mice. However, tissue expression and cellular and subcellular localization of AQPs have been poorly investigated in the male reproductive system of birds. The localization of AQP subtypes (AQP1, 2, 3, 4, 5, 7, 8, 9, and 11) in the goose testis and vas deferens has been studied through immunohistochemistry and immunobloting. Interestingly, the testicular and deferential tissues were positive for AQP1, -5, and -7 but not the others. AQP1 immunoreactivity was detected in the capillary endothelial cells of testis and vas deferens. AQP5 was localized in the interstitial tissue of the testis, including Leydig cells, as well as in the basal cells of vas deferens. Double-labeling confocal microscopy revealed coexpression of AQP5 with capillary AQP1 in the testis. AQP7 was expressed in elongated spermatid and spermatozoa tails in the testis, as well as spermatozoa tails in the vas deferens. These results suggest that several subtypes of AQPs are involved in the regulation of water homeostasis in the goose male reproductive system.

Aquaporin 2 mutations in nephrogenic diabetes insipidus

Water reabsorption in the renal collecting duct is regulated by the antidiuretic hormone vasopressin (AVP). When the vasopressin V2 receptor, present on the basolateral site of the renal principal cell, becomes activated by AVP, aquaporin-2 (AQP2) water channels will be inserted in the apical membrane, and in this fashion, water can be reabsorbed from the pro-urine into the interstitium. The essential role of the vasopressin V2 receptor and AQP2 in the maintenance of body water homeostasis became clear when it was shown that mutations in their genes cause nephrogenic diabetes insipidus, a disorder in which the kidney is unable to concentrate urine in response to AVP. This review describes the current knowledge on AQP2 mutations in nephrogenic diabetes insipidus.

Comparative temporal and dose-dependent morphological and transcriptional uterine effects elicited by tamoxifen and ethynylestradiol in immature, ovariectomized mice

BACKGROUND

Uterine temporal and dose-dependent histopathologic, morphometric and gene expression responses to the selective estrogen receptor modulator tamoxifen (TAM) were comprehensively examined to further elucidate its estrogen receptor-mediated effects. These results were systematically compared to the effects elicited by the potent estrogen receptor ligand 17alpha-ethynylestradiol (EE) to identify pathways similarly and uniquely modified by each compound.

RESULTS

Three daily doses of 100 microg/kg TAM elicited a dose-dependent increase in uterine wet weight (UWW) in immature, ovariectomized C57BL/6 mice at 72 hrs with concurrent increases in luminal epithelial cell height (LECH), luminal circumference and glandular epithelial tubule number. Significant UWW and LECH increases were detected at 24 hrs after a single dose of 100 microg/kg TAM. cDNA microarray analysis identified 2235 differentially expressed genes following a single dose of 100 microg/kg TAM at 2, 4, 8, 12, 18 and 24 hrs, and at 72 hrs after three daily doses (3 x 24 hrs). Functional annotation of differentially expressed genes was associated with cell growth and proliferation, cytoskeletal organization, extracellular matrix modification, nucleotide synthesis, DNA replication, protein synthesis and turnover, lipid metabolism, glycolysis and immunological responses as is expected from the uterotrophic response. Comparative analysis of TAM and EE treatments identified 1209 common, differentially expressed genes, the majority of which exhibited similar profiles despite a temporal delay in TAM elicited responses. However, several conserved and treatment specific responses were identified that are consistent with proliferation (Fos, Cdkn1a, Anapc1), and water imbibition (Slc30a3, Slc30a5) responses elicited by EE.

CONCLUSION

Overall, TAM and EE share similar gene expression profiles. However, TAM responses exhibit lower efficacy, while responses unique to EE are consistent with the physiological differences elicited between compounds.

Regulation of amniotic fluid volume

Water arrives in the mammalian gestation from the maternal circulation across the placenta. It then circulates between the fetal water compartments, including the fetal body compartments, the placenta and the amniotic fluid. Amniotic fluid is created by the flow of fluid from the fetal lung and bladder. A major pathway for amniotic fluid resorption is fetal swallowing; however, in many cases the amounts of fluid produced and absorbed do not balance. A second resorption pathway, the intramembranous pathway (across the amnion to the fetal circulation), has been proposed to explain the maintenance of normal amniotic fluid volume. Amniotic fluid volume is thus a function both of the amount of water transferred to the gestation across the placental membrane, and the flux of water across the amnion. Water flux across biologic membranes may be driven by osmotic or hydrostatic forces; existing data suggest that intramembranous flow in humans is driven by the osmotic difference between the amniotic fluid and the fetal serum. The driving force for placental flow is more controversial, and both forces may be in effect. The mechanism(s) responsible for regulating water flow to and from the amniotic fluid is unknown. In other parts of the body, notably the kidney, water flux is regulated by the expression of aquaporin water channels on the cell membrane. We hypothesize that aquaporins have a role in regulating water flux across both the amnion and the placenta, and present evidence in support of this theory. Current knowledge of gestational water flow is sufficient to allow prediction of fetal outcome when water flow is abnormal, as in twin-twin transfusion syndrome. Further insight into these mechanisms may allow novel treatments for amniotic fluid volume abnormalities with resultant improvement in clinical outcome.

Proteomic knowledge of human aquaporins

Aquaporins (AQPs) are an ubiquitous family of proteins characterized by sequence similarity and the presence of two NPA (Asp-Pro-Ala) motifs. At present, 13 human AQPs are known and they are divided into two subgroups according to their ability to transport only water molecules (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8), or also glycerol and other small solutes (AQP3, AQP7, AQP9, AQP10, AQP12). The genomic, structural, and functional aspects of this family are briefly described. In particular, proteomic approaches to identify and characterize the most studied AQPs, mainly through SDS-PAGE followed by MS analysis, are discussed. Moreover, the clinical importance of the best studied aquaporin (AQP1) in human diseases is also provided.

Myocardial water handling and the role of aquaporins

Cardiac surgery is performed in approximately 770,000 adults and 30,000 children in the United States of America annually. In this review we outline the mechanistic links between post-operative myocardial stunning and the development of myocardial edema. These interrelated processes cause a decline in myocardial performance that account for significant morbidity and mortality after cardiac surgery. Factors leading to myocardial edema include hemodilution, ischemia and reperfusion as well as osmotic gradients arising from pathological change. Several members of the aquaporin family of water transport proteins have been described in the myocardium although their role in the pathogenesis and resolution of cardiac edema is not established. This review examines evidence for the involvement of aquaporins in myocardial water handling during normal and pathological conditions.

Cardiac aquaporin expression in humans, rats, and mice

Water accumulation in the heart is important in ischemia-reperfusion injury and operations performed by using cardiopulmonary bypass, with cardiac dysfunction associated with myocardial edema being the principal determinant of clinical outcome. As an initial step in determining the role of aquaporin (AQP) water channels in myocardial edema, we have assessed the myocardial expression of AQPs in humans, rats, and mice. RT-PCR revealed expression of AQP-1, -4, -6, -7, -8, and -11 transcripts in the mouse heart. AQP-1, -6, -7, and -11 mRNAs were found in the rat heart as well as low levels of AQP-4 and -9. Human hearts contained AQP-1, -3, -4, -5, -7, -9, -10, and -11 mRNAs. AQP-1 protein expression was confirmed by Western blot analysis in all three species. AQP-4 protein was detected in the mouse heart but not in the rat or human heart. To determine the potential functional consequences of myocardial AQP expression, water permeability was measured in plasma membrane vesicles from myocardial cells of wild-type versus various AQP knockout mice. Water permeability was reduced by AQP-1 knockout but not by AQP-4 or AQP-8 knockout. With the use of a model of isolated rat heart perfusion, it was found that osmotic and ischemic stresses are not associated with changes in AQP-1 or AQP-4 expression. These studies support a possible functional role of AQP-1 in myocardium but indicate that early adaptations to osmotic and ischemic stress do not involve transcriptional or posttranslational AQP-1 regulation.

Function of aquaporins in female and male reproductive systems

The flow of water and some other small molecules across cell membranes is important in many of the processes underlying reproduction. The fluid movement is strongly associated with the presence of aquaporins (AQPs) in the female and male reproductive systems. It has been suggested that AQPs mediate water movement into the antral follicle and play important roles in follicle development. AQPs are known to be involved in the early stage of spermatogenesis, in the secretion of tubule liquid and in the concentration and storage of spermatozoa. Fluid reabsorption in some regions of the male reproductive tract is under steroid hormone control and could be mediated by various AQPs. Also AQPs take part in the processes of fertilization, blastocyst formation (as the pathway for transtrophoectodermal water movement during cavitation) and implantation. Alterations in the expression and function or regulation of AQPs have already been demonstrated in disorders of the male reproductive system, such as abnormal sperm motility, the abnormal epididymis and infertility seen in cystic fibrosis, and varicocele. This article extensively reviews the distribution of AQPs in mammalian reproductive tissues and discusses their possible physiological and pathophysiological roles.

Purification and functional characterization of aquaporin-8

BACKGROUND INFORMATION

Aquaporins (AQPs) are a family of channels permeable to water and some small solutes. In mammals, 13 members (AQP0-AQP12) have been found. AQP8 is widely distributed in many tissues and organs. Previous studies in frog oocytes suggested that AQP8 was permeable to water, urea and ammonium, but no direct characterization had yet been reported.

RESULTS

We expressed recombinant rAQP8, hAQP8 and mAQP8 (rat, human and mouse AQP8 respectively) in yeast, purified the proteins to homogeneity and reconstituted them into proteoliposomes. Although showing high sequence similarity, AQP8 proteins from the three species had to be purified with different detergents prior to reconstitution. In stopped-flow studies, all three AQP8 proteoliposomes showed water permeability, which was inhibited by mercuric chloride and rescued by 2-mercaptoethanol. rAQP8 and hAQP8 proteoliposomes did not transport glycerol or urea but were permeable to formamide, which was also inhibited by mercuric chloride. In the oocyte transport assay, hAQP8-injected oocytes showed significantly higher [14C]methylammonium uptake than water-injected oocytes.

CONCLUSIONS

In the present study, we successfully purified rAQP8, hAQP8 and mAQP8 proteins and characterized their biochemical and biophysical properties. All three AQP8 proteins transport water. rAQP8 and hAQP8 are not permeable to urea or glycerol. Moreover, hAQP8 is permeable to ammonium analogues (formamide and methylammonium). Our results suggest that AQP8 may transport ammonium in vivo and physiologically contribute to the acid-base equilibrium.

Aquaporins are observed in the duct epithelia of the epididymal region of the large white turkey

The cellular and molecular mechanisms regulating the reuptake of the testicular fluid supporting sperm exiting the testes in the bird are not known. The presence of aquaporins, proteins involved in transmembrane water transport, was investigated. Observations were limited to the ductuli efferentes, collecting ducts, and ductus epididymis. Interestingly all of these ducts were positive for aquaporins-2, -3, and -9 but not aquaporin-7. When positive, aquaporin was observed localized over the whole cell or the apical plasma membrane of the nonciliated cells and the apical plasma membrane and cilia of the ciliated cells. This study is the first to clearly demonstrate the presence of aquaporins-2, -3, and -9 in the epididymal region of any bird. We assume the aquaporins play a role in concentrating the sperm and in the promotion of sperm maturation in the epididymal region.

Genomic organization and developmental expression of globin genes in the teleost Oryzias latipes

We isolated globin genes from a genomic DNA library of the drR strain of medaka Oryzias latipes, and walked on chromosome. The present study is the first demonstration of the full-length structure of globin gene locus in the teleosts. Two gene clusters were found. One cluster of 36 kbp consisted of nine globin genes and two pseudogenes. Based on structural and phylogenetic similarity of amino acid sequences, the cluster was named embryonic globin gene cluster (E1). The orientation of the genes was in (5')alpha0(3')-(3')beta1(5')-(5')alpha1(3')-(5')beta2(3')-(5')alpha2(3')-(3')alpha3(5')-(5')beta3(3')-(3')beta4(5')-(5')alpha4(3')-(3')psialpha(5')-(5')psibeta(3'). The other cluster of 20 kbp contained three globin genes ((3')ad.alpha1(5')-(5')ad.beta1(3')-(3')ad.alpha2(5')), and was named adult globin gene cluster (A1). Genetic linkage analysis clarified that E1 and A1 were mapped on linkage groups 8 and 19, respectively. The E1 cluster included other genes homologous to human EST clone KIAA0172, Sushi-1 retrotransposon, and protein 14 gene-like gene, while the A1 cluster linked to aquaporin-8 gene-like gene. The expression patterns of the genes were classified into four types: embryo-specific expression (alpha3, beta3, alpha4 and beta4), expression in embryo to young fish (alpha0, beta1, alpha1 and ad.alpha2), expression in young to adult fish (alpha2 and ad.alpha1) and successive expression in embryo to adult (ad.beta1).

Isolation of a novel aquaglyceroporin from a marine teleost (Sparus auratus): function and tissue distribution

The aquaporins (formerly called the major intrinsic protein family) are transmembrane channel proteins. The family includes the CHIP group, which are functionally characterised as water channels and the GLP group, which are specialised for glycerol transport. The present study reports the identification and characterisation of a novel GLP family member in a teleost fish, the sea bream Sparus auratus. A sea bream aquaporin (sbAQP)cDNA of 1047 bp and encoding a protein of 298 amino acids was isolated from a kidney cDNA library. Functional characterization of the sbAQP using a Xenopus oocyte assay revealed that the isolated cDNA stimulated osmotic water permeability in a mercury-sensitive manner and also stimulated urea and glycerol uptake. Northern blotting demonstrated that sbAQP was expressed at high levels in the posterior region of the gut, where two transcripts were identified (1.6 kb and 2 kb), and in kidney, where a single transcript was present (2 kb). In situ hybridisation studies with a sbAQP riboprobe revealed its presence in the lamina propria and smooth muscle layer of the posterior region of the gut and in epithelial cells of some kidney tubules. sbAQP was also present in putative chloride cells of the gill. Phylogenetic analysis of sbAQP, including putative GLP genes from Fugu rubripes, revealed that it did not group with any of the previously isolated vertebrate GLPs and instead formed a separate group, suggesting that it may be a novel GLP member.