The aquaporin family of water channel proteins in clinical medicine

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

AQP0 is a novel surface marker for deciphering abnormal erythropoiesis

Background

Hematopoiesis occurs in the bone marrow, producing a complete spectrum of blood cells to maintain homeostasis. In addition to light microscopy, chromosome analysis, and polymerase chain reaction, flow cytometry is a feasible and fast method for quantitatively analyzing hematological diseases. However, because sufficient specific cell markers are scarce, dyserythropoietic diseases are challenging to identify through flow cytometry.

Methods

Bone marrow samples from C57BL/B6 mice and one healthy donor were analyzed using traditional two-marker (CD71 and glycophorin A) flow cytometry analysis. After cell sorting, the gene expressions of membrane proteins in early and late erythropoiesis precursors and in nonerythroid cells were characterized using microarray analysis.

Results

Among characterized gene candidates, aquaporin 0 (AQP0) expressed as a surface protein in early- and late-stage erythropoiesis precursors and was not expressed on nonerythroid cells. With the help of AQP0 staining, we could define up to five stages of erythropoiesis in both mouse and human bone marrow using flow cytometry. In addition, because patients with dyserythropoiesis generally exhibited a reduced population of APQ0^high cells relative to healthy participants, the analysis results also suggested that the levels of APQ0^high cells in early erythropoiesis serve as a novel biomarker that distinguishes normal from dysregulated erythropoiesis.

Conclusions

AQP0 was successfully demonstrated to be a marker of erythroid differentiation. The expression levels of AQP0 are downregulated in patients with dyserythropoiesis, indicating a critical role of AQP0 in erythropoiesis. Accordingly, the level of AQP0^high in early erythroid precursor cells may serve as a reference parameter for diagnosing diseases associated with dyserythropoiesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02343-4.

Targeting Aquaporins in Novel Therapies for Male and Female Breast and Reproductive Cancers

Aquaporins are membrane channels in the broad family of major intrinsic proteins (MIPs), with 13 classes showing tissue-specific distributions in humans. As key physiological modulators of water and solute homeostasis, mutations, and dysfunctions involving aquaporins have been associated with pathologies in all major organs. Increases in aquaporin expression are associated with greater severity of many cancers, particularly in augmenting motility and invasiveness for example in colon cancers and glioblastoma. However, potential roles of altered aquaporin (AQP) function in reproductive cancers have been understudied to date. Published work reviewed here shows distinct classes aquaporin have differential roles in mediating cancer metastasis, angiogenesis, and resistance to apoptosis. Known mechanisms of action of AQPs in other tissues are proving relevant to understanding reproductive cancers. Emerging patterns show AQPs 1, 3, and 5 in particular are highly expressed in breast, endometrial, and ovarian cancers, consistent with their gene regulation by estrogen response elements, and AQPs 3 and 9 in particular are linked with prostate cancer. Continuing work is defining avenues for pharmacological targeting of aquaporins as potential therapies to reduce female and male reproductive cancer cell growth and invasiveness.

An overview of carbonic anhydrases and membrane channels of synoviocytes in inflamed joints

The highly aggressive fibroblast-like synoviocytes (FLSs) are inflammatory mediators involved in synovial joint destruction. Membrane channels and transporters are essential components of the cell migration apparatus and are involved in various cellular functions. Although evidence is emerging that cell migration is a physiological/pathological process, the mechanism of highly dynamic synoviocytes linked to the membrane channels and carbonic anhydrases (CAs) in inflamed joints is only partially understood. In this review, topics covered will give a brief overview of CAs and the membrane channels of synoviocytes. We have also systematically focused on the role of FLS channels and transporters under various conditions, including rheumatoid arthritis (RA), to understand the pathophysiology of the migration of synoviocytes as inflammatory mediators in joints.

Aquaporins in Urinary System

Several aquaporin (AQP )-type water channels are expressed in kidney: AQP1 in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2 -6 in the collecting duct; AQP7 in the proximal tubule; AQP8 in the proximal tubule and collecting duct; and AQP11 in the endoplasmic reticulum of proximal tubule cells. AQP2 is the vasopressin-regulated water channel that is important in hereditary and acquired diseases affecting urine-concentrating ability. The roles of AQPs in renal physiology and transepithelial water transport have been determined using AQP knockout mouse models. This chapter describes renal physiologic insights revealed by phenotypic analysis of AQP knockout mice and the prospects for further basic and clinical studies.

Characterizing the normal proteome of human ciliary body

BACKGROUND

The ciliary body is the circumferential muscular tissue located just behind the iris in the anterior chamber of the eye. It plays a pivotal role in the production of aqueous humor, maintenance of the lens zonules and accommodation by changing the shape of the crystalline lens. The ciliary body is the major target of drugs against glaucoma as its inhibition leads to a drop in intraocular pressure. A molecular study of the ciliary body could provide a better understanding about the pathophysiological processes that occur in glaucoma. Thus far, no large-scale proteomic investigation has been reported for the human ciliary body.

RESULTS

In this study, we have carried out an in-depth LC-MS/MS-based proteomic analysis of normal human ciliary body and have identified 2,815 proteins. We identified a number of proteins that were previously not described in the ciliary body including importin 5 (IPO5), atlastin-2 (ATL2), B-cell receptor associated protein 29 (BCAP29), basigin (BSG), calpain-1 (CAPN1), copine 6 (CPNE6), fibulin 1 (FBLN1) and galectin 1 (LGALS1). We compared the plasma proteome with the ciliary body proteome and found that the large majority of proteins in the ciliary body were also detectable in the plasma while 896 proteins were unique to the ciliary body. We also classified proteins using pathway enrichment analysis and found most of proteins associated with ubiquitin pathway, EIF2 signaling, glycolysis and gluconeogenesis.

CONCLUSIONS

More than 95% of the identified proteins have not been previously described in the ciliary body proteome. This is the largest catalogue of proteins reported thus far in the ciliary body that should provide new insights into our understanding of the factors involved in maintaining the secretion of aqueous humor. The identification of these proteins will aid in understanding various eye diseases of the anterior segment such as glaucoma and presbyopia.

Roles of AQP5/AQP5-G103D in carbamylcholine-induced volume decrease and in reduction of the activation energy for water transport by rat parotid acinar cells

In order to assess the contribution of the water channel aquaporin-5 (AQP5) to water transport by salivary gland acinar cells, we measured the cell volume and activation energy (E (a)) of diffusive water permeability in isolated parotid acinar cells obtained from AQP5-G103D mutant and their wild-type rats. Immunohistochemistry showed that there was no change induced by carbamylcholine (CCh; 1 μM) in the AQP5 detected in the acinar cells in the wild-type rat. Acinar cells from mutant rats, producing low levels of AQP5 in the apical membrane, showed a minimal increase in the AQP5 due to the CCh. In the wild-type rat, CCh caused a transient swelling of the acinus, followed by a rapid agonist-induced cell shrinkage, reaching a plateau at 30 s. In the mutant rat, the acinus did not swell by CCh challenge, and the agonist-induced cell shrinkage was delayed by 8 s, reaching a transient minimum at around 1 min, and recovered spontaneously even though CCh was persistently present. In the unstimulated wild-type acinar cells, E (a) was 3.4 ± 0.6 kcal mol(-1) and showed no detectable change after CCh stimulation. In the unstimulated mutant acinar cells, high E (a) value (5.9 ± 0.1 kcal mol(-1)) was detected and showed a minimal decrease after CCh stimulation (5.0 ± 0.3 kcal mol(-1)). These results suggested that AQP5 was the main pathway for water transport in the acinar cells and that it was responsible for the rapid agonist-induced acinar cell shrinkage and also necessary to keep the acinar cell volume reduced during the steady secretion in the wild-type rat.

Early stages of mesothelioma, screening and biomarkers

The early diagnosis of mesothelioma is notoriously difficult, both from a clinical and pathological perspective. Patients often undergo several medical investigations without definitive diagnosis. The discovery of biomarkers that can be assessed in pleural effusions, histological samples, and serum may assist with the difficult early diagnosis of mesothelioma. In this chapter we focus on those markers that have been examined in the setting of either early diagnosis of mesothelioma in symptomatic individuals or that have been proposed as suitable for screening of asbestos-exposed individuals, with an emphasis on cytology and histology.

Aquaporin 1, a potential therapeutic target for migraine with aura

The pathophysiology of migraine remains largely unknown. However, evidence regarding the molecules participating in the pathophysiology of migraine has been accumulating. Water channel proteins, known as aquaporins (AQPs), notably AQP-1 and AQP-4, appears to be involved in the pathophysiology of several neurological diseases. This review outlines newly emerging evidence indicating that AQP-1 plays an important role in pain signal transduction and migraine and could therefore serve as a potential therapeutic target for these diseases.

Relationship between aquaporin-4 expression and brain edema in cirrhotic rats with hepatic encephalopathy

AIM: To investigate the mechanism of brain edema in cirrhotic rats with hepatic encephalopathy, and to provide the theoretical basis for management of brain edema.

METHODS: Rats were divided into 4 groups randomly: normal group (n = 10), normal ammonia burden group (n = 10), cirrhosis group (n = 20), cirrhosis ammonia burden group (n = 20). After models were generated successfully, the arterial plasma ammonia was measured by dry biochemical method, and the evans blue (EB) content was examined by colorimetric method; the brain water content (BWC) was measured by means of dry-wet method and the aquaporin-4 (APQ-4) in the brain tissue was assessed by immunohistochemistry.

RESULTS: In comparison with that in the normal group and nomal ammonia burden group, the arterial plasma ammonia was significantly increased in the cirrhosis group and cirrhosis ammonia burden group (420.18 ± 75.91, 97.20 ± 29.66 μmol/L vs 42.62 ± 10.11, 59.33 ± 15.06 μmol/L; P < 0.05 or P < 0.01). The EB content (1.96 ± 0.55 μg/g) and brain water content (75.14 ± 5.68) in the cirrhosis ammonia burden group were higher than those in the nomal group, nomal ammonia burden group, and cirrhosis group (1.96 ± 0.55 μg/g vs 1.05 ± 0.18, 1.19 ± 0.38, 1.41 ± 0.46 μg/g; 75.14 ± 5.68 vs 62.14 ± 2.29, 66.27 ± 4.57 65.58 ± 4.14; all P < 0.05). The location of APQ-4 expression in cirrhotic rats was consistent with that in normal rats, but both the intensity and area of APQ-4 expression were increased as compared with those in normal controls (P < 0.05).

CONCLUSION: APQ-4 expression is up-regulated in cirrhotic rats with hepatic encephalopathy, suggesting that APQ-4 may play a role in the occurrence of hepatic encephalopathy.

The difference of aquaporin 5 distribution in acinar and ductal cells in lacrimal and parotid glands

PURPOSE

This study was designed to clarify the physiological function and tissue distribution of aquaporin 5 (AQP5) in the lacrimal and parotid glands.

METHODS

Saliva and tear volumes were compared in AQP5 knockout (AQP5-/-) mice and wild-type mice. Immunohistochemistry and immunoblot analysis were performed in wild-type and AQP5-/- mice.

RESULTS

Immunofluorescence of AQP5 staining showed that AQP5 was localized mainly in the ductal cells rather than in the acinar cells of the lacrimal gland. In contrast, in the parotid gland, AQP5 was observed abundantly in acinar cells with undetectable staining in ductal cells. Tear secretion was not changed in AQP5-/- mouse, although saliva secretion was significantly reduced.

CONCLUSIONS

AQP5 distribution in acinar cells and ductal cells was completely opposite in the lacrimal and parotid glands. The physiological role of AQP5 might be dependent on the characteristic tissue distribution of the protein in the lacrimal and parotid glands.

Expression of water channel proteins (aquaporins) in the rat Eustachian tube and middle ear mucosa

CONCLUSION

Diverse expression of the different subtypes of aquaporins in different parts of the Eustachian tube and middle ear suggests region-specific functions of the aquaporins in the normal physiology of the tubotympanum and also suggests that they may play roles in the pathophysiology of otitis media.

OBJECTIVES

The epithelial cells of the middle ear and Eustachian tube must maintain adequate water balance for normal function of the mucociliary system. Since aquaporins (AQPs) are known to play critical roles in water homeostasis, we investigated their expression in the tubotympanum of the rat.

METHODS

The expression of AQP subtypes 1, 2, 4, 5, and 7 were examined in the rat Eustachian tube and middle ear using RT-PCR, Western blotting, and immunohistochemistry.

RESULTS

Transcripts for AQP 1, 4, and 5 were detected in the Eustachian tube and middle ear. Expression of these molecules at the protein level was confirmed by Western blot analysis. Immunohistochemical analysis demonstrated that AQP 4 was localized to the basolateral membranes of ciliated epithelial cells while AQP 5 was localized to the apical surface of serous gland cells, but not goblet cells, in the rat Eustachian tube. AQP 1 was found to be expressed by the subepithelial fibroblasts.

Novel role of AQP-1 in NO-dependent vasorelaxation

Nitric oxide (NO) produced by endothelial cells diffuses to vascular smooth muscle cells to cause dilatation of the renal vasculature and other vessels. Although it is generally assumed that NO moves from cell to cell by free diffusion, we recently showed that aquaporin-1 (AQP-1) transports NO across cell membranes. AQP-1 is expressed in endothelial and vascular smooth muscle cells. We hypothesized that diffusion of NO into vascular smooth muscle cells and out of endothelial cells is facilitated by AQP-1, and transport of NO by AQP-1 is involved in endothelium-dependent relaxation. In intact aortic rings from AQP-1 −/− mice, vasorelaxation induced by acetylcholine (which increases endogenous NO) was reduced ( P < 0.0001 vs. control). No differences were found in the relaxation caused by intracellular delivery of NO or intracellular cGMP between strains. In endothelium-denuded aortic rings from AQP-1 −/− mice, the vasorelaxant capability of NO released in the extracellular space was reduced ( P < 0.0001 vs. control). Influx of NO (5 μM) into vascular smooth muscle cells was 0.17 ± 0.02 f.u./s for control and 0.07 ± 0.01 f.u./s for AQP-1 −/− mice, 62% lower ( P < 0.002). NO released by endothelial cells in response to 1 μM acetylcholine was 96.2 ± 17.7 pmol NO/mg for control and 41.9 ± 13.4 pmol NO/mg for AQP-1 −/− mice, 56% reduction ( P < 0.04). NOS3 expression was 1.33 ± 0.29 O.D. units for control and 3.84 ± 0.76 O.D. units for AQP-1 −/− mice, 188% increase ( P < 0.01). We conclude that 1) AQP-1 facilitates NO influx into vascular smooth muscle cells, 2) AQP-1 facilitates NO diffusion out of endothelial cells, and 3) transport of NO by AQP-1 is required for full expression of endothelium-dependent relaxation.

Lens fiber cell elongation and differentiation is associated with a robust increase in myosin light chain phosphorylation in the developing mouse

Myosin II, a molecular motor, plays a critical role in cell migration, cell shape changes, cell adhesion, and cytokinesis. To understand the role of myosin II in lens fiber cell elongation and differentiation, we determined the distribution pattern of nonmuscle myosin IIA, IIB, and phosphorylated regulatory myosin light chain-2 (phospho-MLC) in frozen sections of the developing mouse lens by immunofluorescence analysis. While myosin IIA was distributed uniformly throughout the differentiating lens, including the epithelium and fibers, myosin IIB was localized predominantly to the epithelium and the posterior tips of the lens fibers. In contrast, immunostaining with a di-phospho-MLC antibody localized intensely and precisely to the elongating and differentiating primary and secondary lens fibers, co-localizing with actin filaments. An in situ analysis of Rho GTPase activation revealed that Rho-GTP was distributed uniformly throughout the embryonic lens, including epithelium and fibers. Inhibition of myosin light chain kinase (MLCK) activity by ML-7 in organ cultured mouse lenses led to development of nuclear lens opacity in association with abnormal fiber cell organization. Taken together, these data reveal a distinct spatial distribution pattern of myosin II isoforms in the developing lens and a robust activation of MLC phosphorylation in the differentiating lens fibers. Moreover, the regulation of MLC phosphorylation by MLCK appears to be critical for crystallin organization and for maintenance of lens transparency and lens membrane function.

Expression of aquaporin water channels in rat taste buds

In order to gain insight into the molecular mechanisms that allow taste cells to respond to changes in their osmotic environment, we have used primarily immunocytochemical and molecular approaches to look for evidence of the presence of aquaporin-like water channels in taste cells. Labeling of isolated taste buds from the fungiform, foliate, and vallate papillae in rat tongue with antibodies against several of the aquaporins (AQPs) revealed the presence of AQP1, AQP2, and AQP5 in taste cells from these areas. AQP3 antibodies failed to label isolated taste buds from any of the papillae. There was an apparent difference in the regional localization of AQP labeling within the taste bud. Antibodies against AQP1 and AQP2 labeled predominantly the basolateral membrane, whereas the AQP5 label was clearly evident on both the apical and basolateral membranes of cells within the taste bud. Double labeling revealed that AQP1 and AQP2 labeled many, but not all, of the same taste cells. Similar double-labeling experiments with anti-AQP2 and anti-AQP5 clearly showed that AQP5 was expressed on or near the apical membranes whereas AQP2 was absent from this area. The presence of these 3 types of AQPs in taste buds but not in non-taste bud-containing epithelia was confirmed using reverse transcription-polymerase chain reaction. Experiments using patch clamp recording showed that the AQP inhibitor, tetraethylammonium, significantly reduced hypoosmotic-induced currents in rat taste cells. We hypothesize that the AQPs may play roles both in the water movement underlying compensatory mechanisms for changes in extracellular osmolarity and, in the case of AQP5 in particular, in the gustatory response to water.

A naturally occurring point mutation in the rat aquaporin 5 gene, influencing its protein production by and secretion of water from salivary glands

A greater than twofold diversity in the expression level of aquaporin 5 (AQP5) has been observed in the membrane fraction of the submandibular gland (SMG) in Sprague-Dawley rats (Murdiastuti K, Miki O, Yao C, Parvin MN, Kosugi-Tanaka C, Akamatsu T, Kanamori N, and Hosoi K. Pflügers Arch 445: 405-412, 2002). In the present study, breeding between brother and sister rats was repeated within high AQP5 producers and low ones to obtain inbred offspring. High- and low-producer rats from 3rd to 18th generations were used for experiments. By Western blotting, levels of AQP5 proteins in the parotid and lacrimal glands, and lungs were all low in low producers, whereas they were all high in high producers, implying genetic variations of the gene for this water channel. Despite this implication, AQP5 mRNA levels were almost the same between the two groups by Northern blotting, suggesting the irrelevance of transcriptional regulation for this diversity. AQP5 cDNAs from the SMGs of the two groups were sequenced. The nucleotide sequence of AQP5 cDNA from low producers indicated the existence of a point mutation at nt 308 (G308A), leading to a replacement of (103)Gly with (103)Asp in the third transmembrane domain, but no alteration was detected in the Kozak area. The existence of such a mutation was confirmed by the assessment of genomic DNA also. This mutation may have resulted in an abnormal membrane insertion or ineffective trafficking of AQP5, since the rats having this mutation showed extremely low membrane expression of AQP5 in the SMG acinar cells and decreased water secretion from their salivary glands.

Expression and localization of aquaporin-5 in the epithelial ovarian tumors

OBJECTIVE

To investigate the expression and localization of aquaporin-5 (AQP5) in epithelial ovarian tumors and its clinic significance.

METHODS

The expression of AQP5 protein and mRNA in 65 cases epithelial ovarian tumors and 13 cases normal tissue were measured by immunohistochemical technique, Western blotting and RT-PCR, respectively.

RESULTS

AQP5 is mainly localized in the basolateral membranes of benign tumor cells, the apical and basolateral membrane of borderline cells and scattered in the membrane of malignant cells and almost no or weak staining in normal ovarian epithelium. The AQP5 expression in ovarian malignant and borderline tumors was significantly higher than that of benign tumors (P < 0.05) and normal tissue (P < 0.05). Of all the epithelial ovarian malignant tumors, the AQP5 expression in cases with ascites volume more than 1000 ml was higher than that of ascites volume less than 500 ml (P < 0.05). Increased AQP5 protein level was associated with lymph node metastasis (P < 0.05). There is a positive correlation between ascites amount and the expression of AQP5 protein and mRNA (P < 0.05), as well as lymph node metastasis and the expression of AQP5 protein and mRNA (P < 0.05). The AQP5 expression was not related with FIGO stage, grade and histological type (P > 0.05).

CONCLUSION

The data suggest that overexpression of AQP5 play an important role in tumorigenesis of epithelial ovarian tumors, which may be related to the ascites formation of ovarian carcinoma.

Water permeability in human airway epithelium

Osmotic water permeability (P(f)) was studied in spheroid-shaped human airway epithelia explants derived from nasal polyps by the use of a new improved tissue collection and isolation procedure. The fluid-filled spheroids were lined with a single cell layer with the ciliated apical cell membrane facing the outside. They were capable of surviving hours of experiment involving continuous superfusion of the bathing medium and changes of osmolarity. A new image analysis technique was developed for measuring the spheroid diameters, giving high time and measurement resolutions. The transepithelial P(f), determined by the changes of the apical solution osmolarity, was not influenced by the presence of glucose, Na(+), or Na(+)/glucose-cotransport inhibitors in the bath, but was sensitive to the aquaporin (AQP) inhibitor HgCl(2). The measured P(f) levels and the values of activation energy were in the range of those seen in AQP-associated water transport. Together, these results indicate the presence of an AQP in the apical membrane of the spheroids. Notably, identical values for P(f) were found in CF and non-CF airway preparations, as was the case also for the calculated spontaneous fluid absorption rates.

Microarray analysis of rat brain gene expression after chronic administration of sodium valproate

Valproic acid has been used to treat mania and bipolar disorder, but its mechanism of action is not agreed on. We used rat genome U34A Affymetrix oligonucleotide microarrays, containing 8799 known probesets, to determine the effect of 30-day daily intraperitoneal administration of valproate (200mg/kg) on rat brain gene expression. We found 87 down-regulated genes and 34 up-regulated genes of at least a 1.4-fold change in valproate-treated compared to control rats. The experiments were done on five independent samples for each group, each in duplicate. The genes affected are known to be involved in a variety of pathways, including synaptic transmission, ion channels and transport, G-protein signaling, lipid, glucose and amino-acid metabolism, transcriptional and translational regulation, phosphoinositol cycle, protein kinases and phosphatases, and apoptosis. Our results suggest that the therapeutic effect of valproate may involve the modulation of multiple signaling pathways.

Subcellular redistribution of AQP5 by vasoactive intestinal polypeptide in the Brunner's gland of the rat duodenum

Aquaporin (AQP)5, an exocrine-type water channel, was detected in the rat duodenum by Western blot analysis, and was localized by immunohistochemistry in the secretory granule membranes as well as in the apical and lateral aspects of the plasma membrane of Brunner's gland cells. Incubation of duodenal slices with vasoactive intestinal polypeptide (VIP) in vitro significantly increased the amount of AQP5 in the apical membrane fraction in a dose- and time-dependent manner with the amount reaching a plateau at 100 nM VIP and becoming near maximal after a 30-s incubation. Protein kinase inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7, 50 muM), and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89; PKA-specific, 1 muM) blocked this increase, but PKC-specific inhibitor calphostin C did not, implying the involvement of PKA but not PKC in this cellular event. Intravenous injection with VIP (40 mug/kg body wt) provoked dilation of the lumen of the Brunner's gland at 2 and 7 min and increased the staining intensity of AQP5 in the apical and lateral membranes. AQP1 (both nonglycosylated and glycosylated forms) was also found to localize in the apical and basolateral membranes of cells of Brunner's gland. VIP, however, did not provoke any significant change in the AQP1 level in the apical membrane, as judged from the results of the above in vitro and in vivo experiments. These results suggest that VIP induced the exocytosis of granule contents and simultaneously caused translocation of AQP5 but not of AQP1 to the apical membrane in Brunner's gland cells.

Developmental expression of aquaporin-3 in zebrafish embryos (Danio rerio)

Fish embryos have never been successfully cryopreserved because of the low permeability of cryoprotectants into the yolk. Recently, we used aquaporin-3 fused with a green fluorescent protein (AQP3GFP) to modify the zebrafish embryo, and demonstrated that the pores functioned physiologically. This increased the water and cryoprotectant permeability of the membranes. We have continued our work on AQP3-modified embryos and here we report their developmental expression of AQP3, the success of various culture media on their survival and development, and their reproductive success. The AQP3GFP expression begins within 30 m after the mRNA AQP3GFP injection into the yolk of the 1- to 4-cell embryo. This expression is distributed in the membranes throughout the blastoderm and the yolk syncytial layer within 24 h. It diminishes after 96 h. We found no difference in the survival or normal development of embryos from AQP3GFP or wild-type adults. Additionally, zebrafish embryos did not require special culture medium to survive after AQP3GFP modification. In fact, they survived best in embryo medium (ca. 40 mOsm). Embryos reared entirely in embryo medium had a higher percent survival and a higher percent normal development than those exposed to a high osmolality sucrose culture medium (ca. 330 mOsm). The mechanism whereby these embryos can maintain their internal osmolality in a hypoosmotic solution with water channels in their membranes is unknown.

Molecular Mechanisms and Drug Development in Aquaporin Water Channel Diseases: The Translocation of Aquaporin-5 From Lipid Rafts to the Apical Plasma Membranes of Parotid Glands of Normal Rats and the Impairment of It in Diabetic or Aged Rats

Salivary secretion from rat salivary glands occurs in response to stimulation by acetylcholine and norepinephrine released from nerve endings. Aquaporin-5 (AQP5) localizes in lipid rafts under control conditions and is induced to traffic to the apical plasma membrane in interlobular ducts of rat parotid glands by the activation of M3 muscarinic acetylcholine receptors or alpha1-adrenoceptors. This review will focus on the mechanisms of the translocation of AQP5 from lipid rafts to the apical plasma membrane in the interlobular duct cells of parotid glands of normal rats and the impairment of its translocation in diabetic or senescent rats.

Red cell antigens as functional molecules and obstacles to transfusion

Blood group antigens (BGAs) can act as functional molecules but also can evoke autoantibodies and alloantibodies, causing autoimmune hemolytic anemia, hemolytic disease of the newborn and hemolytic transfusion reactions. In Section I, Dr. Marilyn Telen discusses physiologic and pathologic functions of RBC BGA-bearing molecules. She reviews some associations of BGAs with RBC membrane integrity and hemolytic anemia; association of BGAs with enzymatic and transport functions; and adhesion molecules expressed by RBCs, especially with reference to their pathophysiological role in sickle cell disease. In Section II, Dr. Lawrence Petz discusses the problems of providing blood for patients who have RBC autoantibodies. He provides an algorithm for excluding the presence of "hidden" alloantibodies, when all units appear to be incompatible due to the autoantibody. He emphasizes that clinicians should be aware of these approaches and not accept "the least incompatible unit." In Section III, Dr. George Garratty describes two processes, in development, that produce RBCs that result in RBCs that can be described as "universal" donor or "stealth" RBCs. The first process involves changing group A, B, or AB RBCs into group O RBCs by removing the immunospecific sugars responsible for A and B specificity by using specific enzymes. The second process involves covering all BGAs on the RBC surface using polyethylene glycol (PEG). Results of in vitro and in vivo studies on these modified RBCs are discussed.

Distúrbios na secreção e ação do hormônio antidiurético

A manutenção da constância da osmolalidade plasmática e do equilíbrio hidroeletrolítico deve-se à regulação do volume extracelular e da natremia, através da integração entre as ações do hormônio antidiurético (ADH), o sistema renina-angiotensina-aldosterona (SRAA) e o mecanismo da sede. Distúrbios na síntese, secreção ou ação do ADH podem resultar em síndromes poliúricas, em que ocorre excreção aumentada de urina hipotônica, resultante da ingestão excessiva de água, secreção ou ação inadequadas do ADH ou alterações nos canais de água da aquaporina-2 (AQP2). A hiponatremia pode ocorrer por depleção de sal, mecanismos dilucionais ou metabólicos, além de ser uma freqüente ocorrência após a cirurgia hipofisária, tendo sido descrita em 9 a 35% de pacientes operados. A causa pode ser devida a uma lesão transitória ou definitiva da hipófise posterior, a síndrome de secreção inapropriada do hormônio antidiurético (SIADH) ou, mais freqüentemente, a síndrome cerebral perdedora de sal (SCPS). A apresentação clínica de ambas as síndromes é similar e o diagnóstico diferencial pode apresentar dificuldades. A determinação do estado volêmico é essencial para o diagnóstico, já que os pacientes portadores de SIADH caracterizam-se por serem euvolêmicos ou hipervolêmicos, enquanto aqueles com a SCPS são hipovolêmicos. Os critérios para o diagnóstico incluem parâmetros clínicos, determinação da osmolalidade plasmática e urinária e os testes de restrição hídrica, sobrecarga hipertônica e o da furosemida, importante na discriminação entre SIADH e SCPS. O tratamento das síndromes poliúricas depende da etiologia e inclui o uso de análogos do ADH, diuréticos, ou outras drogas tais como clorpropramida, clofibrato, corticóides e carbamazepina. O tratamento da SIADH necessita de restrição de líqüidos e/ou furosemida para diminuir o volume de água extracelular. O tratamento da SCPS, ao contrário, implica em reposição de volume com fornecimento de suplementação de sódio e líqüidos, sendo que a fludrocortisona pode ser uma boa alternativa terapêutica.

Down-regulation of aquaporin 3 in bronchiectatic airways in vivo

Bronchiectasis is characterized pathologically by permanent abnormal bronchial dilation, and clinically by chronic sputum production. Aquaporin 3 (AQP3), a recently described water channel that is also found in large airway cell membrane, could play a role in the pathogenesis and particularly that of bronchorrhea in bronchiectasis. However, little is known of its in vivo distribution and physiological role in human airways. We have, therefore, performed this quantitative immunohistochemistry study on endobronchial biopsies to evaluate the expression and clinical relevance of AQP3 in patients with idiopathic bronchiectasis (n = 25, 15 F, 64.3 +/- 11.5 years) and control subjects (n = 14, 5 F, 57.5 +/- 12.0 years). Quantitative image analysis was performed to evaluate the expression of AQP3 in the bronchial epithelial cells. Our results show that AQP3 was predominantly expressed in the basal cells of the epithelial layer in both groups. Expression of AQP3 was significantly reduced in the basal, but not columnar, epithelial cells in bronchiectasis compared with control airways (p = 0.02, 0.35). Only bronchiectatic patients with regular sputum production, but not their counterparts, had significant downregulation of epithelial AQP3 expression compared with control airways (p = 0.004, 0.24). Our findings suggest that AQP3 could have an important role in the pathogenesis of increased mucus production in bronchiectasis.

Altering fish embryos with aquaporin-3: an essential step toward successful cryopreservation

Fish populations are globally threatened by overharvesting and habitat degradation. The ability to bank fish embryos by cryopreservation could be crucial for preserving species diversity, for aquaculture (allowing circannual fish farming), and for managing fish models used in human biomedical research. However, no nonmammalian embryo has ever been successfully cryopreserved. For fish, low membrane permeability prevents cryoprotectants from entering the yolk to prevent cryodamage. Here, we present evidence of a membrane mechanism hindering cryopreservation of fish and propose a novel solution to this obstacle. Zebrafish (Danio rerio) embryos have rectifying membranes that allow water to leave but not to reenter readily. This feature may be an evolutionary trait that allows freshwater embryos to grow in hypoosmotic environments without osmoregulatory organs. However, this trait may also prevent successful fish embryo cryopreservation because both water and cryoprotectants must move into and out of cells. As a solution, we injected zebrafish embryos with mRNA for the aquaporin-3 water channel protein and demonstrated increased membrane permeability to water and to a cryoprotectant. Modeling indicates that sufficient cryoprotectant enters aquaporin-3-expressing zebrafish embryos to allow cryopreservation.

Water permeability of an ovarian antral follicle is predominantly transcellular and mediated by aquaporins

Ovarian folliculogenesis is characterized, in part, by the formation and expansion of the fluid-filled antrum. Development of this cavity requires water influx, which may occur by transcellular or pericellular transport mechanisms. To assess the contribution of these mechanisms to the water permeability of an antral follicle, the rate of (3)H(2)O and (14)C-inulin (a complex sugar restricted to the extracellular compartment) uptake into isolated follicles was determined. The rate of H(2)O movement was 3.5-fold greater than that of inulin, suggesting that water enters a follicle primarily by transcellular pathways. Preincubation of the follicles with 50 microM HgCl(2) [a nonspecific aquaporin (Aqp) inhibitor] decreased H(2)O movement to levels seen with inulin, indicating that transcellular water movement is mediated through Aqp. To demonstrate the functional presence of Aqp in granulosa cells, we show that swelling in response to a hypotonic insult is attenuated by preincubation with 50 microM HgCl(2). Flow cytometry demonstrated the presence of Aqps-7, -8, and -9, thus identifying candidate Aqp potentially mediating water movement into antral follicles. These results suggest that water permeability of antral follicles occurs primarily through transcellular mechanisms, which may be mediated by Aqps -7, -8, and/or -9 in granulosa cells.

Positive and negative regulation of water channel aquaporins in human small intestine by cholera toxin

Analysis of osmotic water permeability of aquaporin (AQP) 1, AQP3 and AQP4, which are expressed in human small intestine, in the presence or absence of cholera toxin (CT) was performed using a Xenopus oocyte expression system. When treated with CT, water permeability of AQP4 was facilitated while that of AQP3 was suppressed. AQP1 did not show any significant change of water permeability when treated with CT. An adenylyl cyclase accelerator forskolin showed similar effects as CT did, suggesting that changes of the water permeability of AQP4 and AQP3 were due to an increase of intracellular cAMP concentration. A possibility that these AQPs are responsible molecules for causing acute secretory diarrhoea as in cholera is considered.

Aquaporins in brain: distribution, physiology, and pathophysiology

Water homeostasis in the brain is of central physiologic and clinical importance. Neuronal activity and ion water homeostasis are inextricably coupled. For example, the clearance of K+ from areas of high neuronal activity is associated with a concomitant water flux. Furthermore, cerebral edema, a final common pathway of numerous neurologic diseases, including stroke, may rapidly become life threatening because of the rigid encasement of the brain. A water channel family, the aquaporins, facilitates water flux through the plasma membrane of many cell types. In rodent brain, several recent studies have demonstrated the presence of different types of aquaporins. Aquaporin 1 (AQP1) was detected on epithelial cells in the choroid plexus whereas AQP4, AQP5 and AQP9 were localized on astrocytes and ependymal cells. In rodent brain, AQP4 is present on astrocytic end-feet in contact with brain vessels, and AQP9 is found on astrocytic processes and cell bodies. In basal physiologic conditions, AQP4 and AQP9 appear to be implicated in brain homeostasis and in central plasma osmolarity regulation. Aquaporin 4 may also play a role in pathophysiologic conditions, as shown by the reduced edema formation observed after water intoxication and focal cerebral ischemia in AQP4-knockout mice. Furthermore, pathophysiologic conditions may modulate AQP4 and AQP9 expression. For example, AQP4 and AQP9 were shown to be upregulated after ischemia or after traumatic injuries. Taken together, these recent reports suggest that water homeostasis in the brain is maintained by regulatory processes that, by control of aquaporin expression and distribution, induce and organize water movements. Facilitation of these movements may contribute to the development of edema formation after acute cerebral insults such as ischemia or traumatic injury.

Expression patterns of aquaporins in the inner ear: evidence for concerted actions of multiple types of aquaporins to facilitate water transport in the cochlea

Water transport between the perilymph and endolymph is important in regulations of volume and osmotic pressure of the inner ear labyrinth. It is now known that expression of water channels (aquaporins or AQPs) in the cell membrane dramatically increases the ability of water to cross epithelial cells. The aims of the current study were to investigate the cellular localization of AQPs by immunolabeling, and to study the developmental expression and relative abundance of various subtypes of AQPs. We report here that AQP3, AQP7 and AQP9 were expressed in the inner ear. Specific subtypes of AQPs were found in discrete regions expressed by both epithelial cells and fibrocytes in cochlear and vestibular organs. Semi-quantitative measurements showed that AQP4 and AQP1 were the two most abundantly expressed AQP subtypes in the inner ear, and their expressions were dramatically upregulated during development. These data showed a highly localized and largely non-overlapping distribution pattern for different subtypes of AQPs in the inner ear, suggesting the existence of regional subtype-specific water transport pathways, and global regulation of water transport in the inner ear may require concerted actions of multiple types of AQPs.

Experimental study on brain oxygenation in relation to tissue water redistribution and brain oedema

The aim of the present experimental research was to study brain oxygenation parameters in relation to tissue water movement and brain cortex oedema caused by focal brain ischemia. It has been demonstrated that local osmotic dehydration of the parietal brain cortex, mercury compounds (aquaporin inhibitors) and brain cortex oedema resulting from focal brain ischemia all influence extra-capillary oxygen transport lowering tissue respiration rates and oxygen transfer coefficients. The changes of brain oxygenation parameters in case of cortex ischemic oedema are reflected in the gas composition (oxygen and carbon dioxide partial pressure) of the blood in v. jugularis. A short course of hyperbaric treatment results in normalization of water content in the brain. The results are interpreted in terms of the functioning of tissue microcirculation that might participate, at the extra-capillary level, in brain oxygenation.

Salivary acinar cells from aquaporin 5-deficient mice have decreased membrane water permeability and altered cell volume regulation

Aquaporins (AQPs) are channel proteins that regulate the movement of water through the plasma membrane of secretory and absorptive cells in response to osmotic gradients. In the salivary gland, AQP5 is the major aquaporin expressed on the apical membrane of acinar cells. Previous studies have shown that the volume of saliva secreted by AQP5-deficient mice is decreased, indicating a role for AQP5 in saliva secretion; however, the mechanism by which AQP5 regulates water transport in salivary acinar cells remains to be determined. Here we show that the decreased salivary flow rate and increased tonicity of the saliva secreted by Aqp5(-)/- mice in response to pilocarpine stimulation are not caused by changes in whole body fluid homeostasis, indicated by similar blood gas and electrolyte concentrations in urine and blood in wild-type and AQP5-deficient mice. In contrast, the water permeability in parotid and sublingual acinar cells isolated from Aqp5(-)/- mice is decreased significantly. Water permeability decreased by 65% in parotid and 77% in sublingual acinar cells from Aqp5(-)/- mice in response to hypertonicity-induced cell shrinkage and hypotonicity-induced cell swelling. These data show that AQP5 is the major pathway for regulating the water permeability in acinar cells, a critical property of the plasma membrane which determines the flow rate and ionic composition of secreted saliva.

Nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus, which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone arginine vasopressin. Polyuria, with hyposthenuria, and polydipsia are the cardinal clinical manifestations of the disease. About 90% of patients with congenital nephrogenic diabetes insipidus are males with the X-linked recessive form of the disease (OMIM 304800) who have mutations in the arginine vasopressin receptor 2 gene (AVPR2), which codes for the vasopressin V2 receptor. The gene is located in chromosomal region Xq28. In <10% of the families studied, congenital nephrogenic diabetes insipidus has an autosomal-recessive or autosomal-dominant (OMIM 222000 and 125800, respectively) mode of inheritance. Mutations have been identified in the aquaporin-2 gene (AQP2), which is located in chromosome region 12q13 and codes for the vasopressin-sensitive water channel. When studied in vitro, most AVPR2 mutations result in receptors that are trapped intracellularly and are unable to reach the plasma membrane. A few mutant receptors reach the cell surface but are unable to bind arginine vasopressin or to properly trigger an intracellular cyclic AMP signal. Similarly, aquaporin-2 mutant proteins are misrouted and cannot be expressed at the luminal membrane. Chemical or pharmacological chaperones have been found to reverse the intracellular retention of aquaporin-2 and arginine vasopressin receptor 2 mutant proteins. Because many hereditary diseases stem from the intracellular retention of otherwise functional proteins, this mechanism may offer a new therapeutic approach to the treatment of those diseases that result from errors in protein kinesis.

A 76-bp deletion in the Mip gene causes autosomal dominant cataract in Hfi mice

Hfi is a dominant cataract mutation where heterozygotes show hydropic lens fibers and homozygotes show total lens opacity. The Hfi locus was mapped to the distal part of mouse chromosome 10 close to the major intrinsic protein (Mip), which is expressed only in cell membranes of lens fibers. Molecular analysis of Mip revealed a 76-bp deletion that resulted in exon 2 skipping in Mip mRNA. In Hfi/Hfi this deletion resulted in a complete absence of the wildtype Mip. In contrast, Hfi/+ animals had the same amount of wildtype Mip as +/+. Results from pulse-chase expression studies excluded hetero-oligomerization of wildtype and mutant Mip as a possible mechanism for cataract formation in the Hfi/+. We propose that the cataract phenotype in the Hfi heterozygote mutant is due to a detrimental gain of function by the mutant Mip resulting in either cytotoxicity or disruption in processing of other proteins important for the lens. Cataract formation in the Hfi/Hfi mouse is probably a combined result of both the complete loss of wildtype Mip and a gain of function of the mutant Mip.

mRNAs encoding aquaporins are present during murine preimplantation development

The present study was conducted to investigate the mechanisms underlying fluid movement across the trophectoderm during blastocyst formation by determining whether aquaporins (AQPs) are expressed during early mammalian development. AQPs belong to a family of major intrinsic membrane proteins and function as molecular water channels that allow water to flow rapidly across plasma membranes in the direction of osmotic gradients. Ten different AQPs have been identified to date. Murine preimplantation stage embryos were flushed from the oviducts and uteri of superovulated CD1 mice. Reverse transcription-polymerase chain reaction (RT-PCR) methods employing primer sets designed to amplify conserved sequences of AQPs (1-9) were applied to murine embryo cDNA samples. PCR reactions were conducted for up to 40 cycles involving denaturation of DNA hybrids at 95 degrees C, primer annealing at 52-60 degrees C and extension at 72 degrees C. PCR products were separated on 2% agarose gels and were stained with ethidium bromide. AQP PCR product identity was confirmed by sequence analysis. mRNAs encoding AQPs 1, 3, 5, 6, 7, and 9 were detected in murine embryos from the one-cell stage up to the blastocyst stage. AQP 8 mRNAs were not detected in early cleavage stages but were present in morula and blastocyst stage embryos. The results were confirmed in experimental replicates applied to separate embryo pools of each embryo stage. These results demonstrate that transcripts encoding seven AQP gene products are detectable during murine preimplantation development. These findings predict that AQPs may function as conduits for trophectoderm fluid transport during blastocyst formation.

Congenital progressive polymorphic cataract caused by a mutation in the major intrinsic protein of the lens, MIP (AQP0)

BACKGROUND

Congenital cataract, when inherited as an isolated abnormality, is phenotypically and genetically heterogeneous. Although there is no agreed nomenclature for the patterns of cataract observed, a recent study identified eight readily identifiable phenotypes.

METHODS

The Moorfields Eye Hospital genetic eye clinic database was used to identify a four generation family with isolated autosomal dominant congenital cataracts. All individuals (affected and unaffected) underwent a full ophthalmic assessment.

RESULTS

The results of the molecular linkage study identifying a missense mutation in the gene encoding the major intrinsic protein of the lens (MIP) have been published elsewhere. Affected individuals had bilateral discrete progressive punctate lens opacities limited to mid and peripheral lamellae with additional asymmetric polar opacification. One young female had predominantly cortical cataract and another had serpiginous nuclear opacities.

CONCLUSIONS

This phenotype has not been recorded in human families before and has been termed polymorphic. The pattern of opacification appears to reflect the distribution of MIP in the lens. Furthermore, this is the first clear evidence of allelic heterogeneity in this condition following the identification of a family with lamellar cataracts who have a different mutation within the MIP gene.

Surface tongue-and-groove contours on lens MIP facilitate cell-to-cell adherence

The lens major intrinsic protein (MIP, AQP0) is known to function as a water and solute channel. However, MIP has also been reported to occur in close membrane contacts between lens fiber cells, indicating that it has adhesive properties in addition to its channel function. Using atomic force and cryo-electron microscopy we document that crystalline sheets reconstituted from purified ovine lens MIP mostly consisted of two layers. MIP lattices in the apposing membranes were in precise register, and determination of the membrane sidedness demonstrated that MIP molecules bound to each other via their extracellular surfaces. The surface structure of the latter was resolved to 0.61 nm and revealed two protruding domains providing a tight "tongue-and-groove" fit between apposing MIP molecules. Cryo-electron crystallography produced a projection map at 0.69 nm resolution with a mirror symmetry axis at 45 degrees to the lattice which was consistent with the double-layered nature of the reconstituted sheets. These data strongly suggest an adhesive function of MIP, and strengthen the view that MIP serves dual roles in the lens.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

Developmental expression of urine concentration-associated genes and their altered expression in murine infantile-type polycystic kidney disease

Currently, there is little understanding of what factors regulate the development of urine concentrating capability in normal or polycystic kidney. The present study examined the developmental expression of genes associated with urine concentration in developing mice, including C57BL/6J-cpk/cpk mice with autosomal recessive-infantile (AR) polycystic kidney disease (PKD). Concentration of urine requires: 1) medullary collecting ducts (CD) located within a hypertonic interstitium, 2) CD cell expression of functional arginine vasopressin V2 receptors (AVP-V2R), and 3) the presence of appropriate CD water channels (aquaporins, AQP 2 and 3). An increase in urine osmolarity, normally seen between 1 and 3 weeks of age, was absent in cpk cystic mice. Aldose reductase mRNA expression (a gene upregulated by medullary hyperosmolarity) increased in normal mice, but remained low in the cystic kidney, suggesting the absence of a hypertonic medullary interstitium. AVP-V2R, AQP2, and AQP3 mRNA expression normally increase between 7 and 14 days. However, all were dramatically overexpressed even at 7 days of age in the cpk kidney in vivo, but decreased in vitro. Activation of the AVP-V2 receptor stimulates the production of cAMP, a substance known to promote cyst enlargement. To determine if CD cAMP, generated from increased AVP-V2Rs, was accelerating the PKD, cystic mice and their normal littermates were treated with OPC31260, a relatively specific AVP-V2R antagonist. OPC31260 treatment of cystic mice led to an amelioration of the cystic enlargement and azotemia. Treatment also decreased renal AQP2 mRNA but increased AVP-V2R and AQP3 mRNA expression in vivo. AVP upregulates the expression of AVP-V2R, AQP2, and AQP3 mRNAs in vitro. Renal EGF, known to inhibit AVP-V2R activity, downregulates AVP-V2R mRNA in vitro. Brief in vivo EGF treatment, known to decrease PKD in cpk mice, led to increased expression of AVP-V2R, AQP2, and AQP3 mRNAs at 2 weeks in both normal and cystic mice but no change was evident at 3 weeks of age. In conclusion, the development of urinary concentration ability correlates with the development of an increased medullary osmotic gradient which is diminished in murine ARPKD. However, CD genes associated with this process are overexpressed in vivo but underexpressed in vitro in the cystic kidney. The overexpression and/or overactivity of the AVP-V2R appears to contribute to the progression of PKD since an AVP-V2R antagonist inhibits cystic renal enlargement in the cpk mouse.

Aquaporin water channel in salivary glands

Water secretion from salivary glands, which are innervated by parasympathetic and sympathetic nerves, occurs in response to the stimulation by neurotransmitters. In general, parasympathetic or sympathetic stimulation produces a high flow of saliva as a result of the activation of M3 muscarinic or alpha1-adrenergic receptors, respectively. The secretory mechanisms of fluid secretion were osmotically regulated in response to a transepithelial ion gradient generated by ion transport systems that were located in the apical or basolateral membranes of the acinar cells. Recently, the identification of water-specific channels, or aquaporins (AQPs), in many mammalian tissue and cell types has provided insight into the molecular basis of water movement across biological membranes. It has been reported that several AQPs are expressed in salivary glands and especially AQP5 plays an important role in fluid secretion. This review will focus on the role of AQP5 in the movement of water across the apical plasma membrane in relation to the physiology and pathophysiology of salivary glands.

Osmotic water permeabilities of cultured, well-differentiated normal and cystic fibrosis airway epithelia

Current hypotheses describing the function of normal airway surface liquid (ASL) in lung defense are divergent. One theory predicts that normal airways regulate ASL volume by modulating the flow of isosmotic fluid across the epithelium, whereas an alternative theory predicts that ASL is normally hyposmotic. These hypotheses predict different values for the osmotic water permeability (P(f)) of airway epithelia. We measured P(f) of cultures of normal and cystic fibrosis (CF) airway epithelia that, like the native tissue, contain columnar cells facing the lumen and basal cells that face a basement membrane. Xz laser scanning confocal microscopy recorded changes in epithelial height and transepithelial volume flow in response to anisosmotic challenges. With luminal hyperosmotic challenges, transepithelial and apical membrane P(f) are relatively high for both normal and CF airway epithelia, consistent with an isosmotic ASL. Simultaneous measurements of epithelial cell volume and transepithelial water flow revealed that airway columnar epithelial cells behave as osmometers whose volume is controlled by luminal osmolality. Basal cell volume did not change in these experiments. When the serosal side of the epithelium was challenged with hyperosmotic solutions, the basal cells shrank, whereas the lumen-facing columnar cells did not. We conclude that (a) normal and CF airway epithelia have relatively high water permeabilities, consistent with the isosmotic ASL theory, and the capacity to restore water on airway surfaces lost by evaporation, and (b) the columnar cell basolateral membrane and tight junctions limit transepithelial water flow in this tissue.

Missense mutations in MIP underlie autosomal dominant 'polymorphic' and lamellar cataracts linked to 12q

Human inherited cataract is both clinically diverse and genetically heterogeneous. Here we report the identification of the first mutations affecting the major intrinsic protein of the lens, MIP, encoded by the gene MIP on 12q14. MIP is a member of the aquaporin family of membrane-bound water channels. The mutations identified are predicted to disturb water flux across the lens cell membrane.

The choroid plexuses and the barriers between the blood and the cerebrospinal fluid

1. The fluid homeostasis of the brain depends both on the endothelial blood-brain barrier and on the epithelial blood-cerebrospinal fluid (CSF) barrier located at the choroid plexuses and the outer arachnoid membrane. 2. The brain has two fluid environments: the brain interstitial fluid, which surrounds the neurons and glia, and the CSF, which fills the ventricles and external surfaces of the central nervous system. 3. CSF acts as a fluid cushion for the brain and as a drainage route for the waste products of cerebral metabolism. 4. Recent findings suggest that CSF may also act as a "third circulation" conveying substances secreted into the CSF rapidly to many brain regions.

Altered gravity downregulates aquaporin-1 protein expression in choroid plexus

Aquaporin-1 (AQP1) is a water channel expressed abundantly at the apical pole of choroidal epithelial cells. The protein expression was quantified by immunocytochemistry and confocal microscopy in adult rats adapted to altered gravity. AQP1 expression was decreased by 64% at the apical pole of choroidal cells in rats dissected 5.5-8 h after a 14-day spaceflight. AQP1 was significantly overexpressed in rats readapted for 2 days to Earth's gravity after an 11-day flight (48% overshoot, when compared with the value measured in control rats). In a ground-based model that simulates some effects of weightlessness and alters choroidal structures and functions, apical AQP1 expression was reduced by 44% in choroid plexus from rats suspended head down for 14 days and by 69% in rats suspended for 28 days. Apical AQP1 was rapidly enhanced in choroid plexus of rats dissected 6 h after a 14-day suspension (57% overshoot, in comparison with control rats) and restored to the control level when rats were dissected 2 days after the end of a 14-day suspension. Decreases in the apical expression of choroidal AQP1 were also noted in rats adapted to hypergravity in the NASA 24-ft centrifuge: AQP1 expression was reduced by 47% and 85% in rats adapted for 14 days to 2 G and 3 G, respectively. AQP1 is downregulated in the apical membrane of choroidal cells in response to altered gravity and is rapidly restored after readaptation to normal gravity. This suggests that water transport, which is partly involved in the choroidal production of cerebrospinal fluid, might be decreased during spaceflight and after chronic hypergravity.

cAMP regulated membrane diffusion of a green fluorescent protein-aquaporin 2 chimera

To study the membrane mobility of aquaporin water channels, clones of stably transfected LLC-PK1 cells were isolated with plasma membrane expression of GFP-AQP1 and GFP-AQP2, in which the green fluorescent protein (GFP) was fused upstream and in-frame to each aquaporin (AQP). The GFP fusion did not affect AQP tetrameric association or water transport function. GFP-AQP lateral mobility was measured by irreversibly bleaching a spot (diameter 0.8 microm) on the membrane with an Argon laser beam (488 nm) and following the fluorescence recovery into the bleached area resulting from GFP translational diffusion. In cells expressing GFP-AQP1, fluorescence recovered to >96% of its initial level with t(1/2) of 38 +/- 2 s (23 degrees C) and 21 +/- 1 s (37 degrees C), giving diffusion coefficients (D) of 5.3 and 9.3 x 10(-11) cm(2)/s. GFP-AQP1 diffusion was abolished by paraformaldehyde fixation, slowed >50-fold by the cholesterol-binding agent filipin, but not affected by cAMP agonists. In cells expressing GFP-AQP2, fluorescence recovered to >98% with D of 5.7 and 9.0 x 10(-11) cm(2)/s at 23 degrees C and 37 degrees C. In contrast to results for GFP-AQP1, the cAMP agonist forskolin slowed GFP-AQP2 mobility by up to tenfold. The cAMP slowing was blocked by actin filament disruption with cytochalasin D, by K(+)-depletion in combination with hypotonic shock, and by mutation of the protein kinase A phosphorylation consensus site (S256A) at the AQP2 C-terminus. These results indicate unregulated diffusion of AQP1 in membranes, but regulated AQP2 diffusion that was dependent on phosphorylation at serine 256, and an intact actin cytoskeleton and clathrin coated pit. The cAMP-induced immobilization of phosphorylated AQP2 provides evidence for AQP2-protein interactions that may be important for retention of AQP2 in specialized membrane domains for efficient membrane recycling.

Decreased expression of aquaporin (AQP)1 and AQP5 in mouse lung after acute viral infection

Intratracheal infection of mice with adenovirus is associated with subsequent pulmonary inflammation and edema. Water movement through the air space-capillary barrier in the distal lung is facilitated by aquaporins (AQPs). To investigate the possibility that distal lung AQPs undergo altered regulation under conditions of aberrant fluid handling in the lung, we analyzed messenger RNA (mRNA) and protein expression of AQPs 1 and 5 in the lungs of mice 7 and 14 d after infection with adenovirus. Here, we demonstrate that AQP1 and AQP5 show decreased expression following adenoviral infection. Northern blot analysis showed significantly decreased mRNA levels of AQP1, which is expressed in the capillary endothelium, and AQP5, which is expressed in alveolar epithelium, in the lungs of mice both 7 and 14 d after infection. Immunoblotting studies demonstrated significantly reduced levels of AQP1 and AQP5 protein after infection as well. In addition, mRNA expression of the alpha subunit of the epithelial sodium channel was reduced in the lungs of mice 7 and 14 d after adenoviral infection. In contrast, mRNA expression of the alpha1 subunit of the Na,K-adenosine triphosphatase in the lung was unaltered. Immunohistochemical analysis demonstrated that the decreases in AQP1 and AQP5 expression were not localized to regions of overt inflammation but were found throughout the lung. Thus, this study provides the first report of AQP gene regulation in an in vivo model of pulmonary inflammation and edema. Decreased AQP1 and AQP5 levels during adenoviral infection suggest a role for AQP1 and AQP5 in the abnormal fluid fluxes detected during pulmonary inflammation.