The aquaporin family of water channel proteins in clinical medicine

Identification of aquaporin 5 (AQP5) within the cochlea: cDNA cloning and in situ localization

Dysfunction of fluid and electrolyte homeostasis is considered to cause variety of inner ear disorders. One group of candidate proteins that may play a critical role in the inner ear fluid homeostasis is the aquaporins, a family of proteins whose members have well defined roles in fluid transport in variety of organs. This study reports the identification of AQP5, a member of the aquaporin family, within the rat inner ear and its in situ localization. AQP5 was initially identified within rat cochlear RNA via RT-PCR and sequence analysis of the amplified fragments. Immunoblot of cochlear homogenate yielded a predominant AQP5-immunoreactive band of M(r) 35 kDa. The anti-AQP5 immunoreactivity, indicating expression of the AQP5 polypeptide, was localized within the cochlea in situ to the cell types that form the lateral wall of the cochlear duct-the external sulcus (ES) cells and the cells of the spiral prominence. Expression of AQP5 was observed in the apical turn but not the basal turn of the cochlea; nor was it observed in the vestibular neuroepithelia or its supporting cells. The restricted expression of AQP5 to the apical turns of the cochlea suggests its potential role in low frequency hearing.

Brain adaptation to water loading in rabbits as assessed by NMR relaxometry

The present study was undertaken to investigate the cerebral adaptation to hypoosmolar stress in adult Pannon white rabbits by applying proton nuclear magnetic resonance relaxometry. Progressive hyponatremia was induced by combined administration of hypotonic dextrose in water and 8-deamino-arginine vasopressin over a hydration period of 3, 24, and 48 h. Each group comprised five animals. After completing the hydration protocols, blood was taken to determine plasma osmolality (freezing point depression) and sodium concentration (ion-selective electrode) and, at about the same time, T2-weighted images were made. After the in vivo measurements, the animals were killed and brain tissue samples were obtained to measure water content (desiccation method) and T1 and T2 relaxation times (proton nuclear magnetic resonance method). Free and bound water fractions were calculated by using multicomponent fits of the T2 relaxation curves. It was shown that brain water content and T1 relaxation time remained unchanged despite the progressing hyponatremia. By contrast, T2 relaxation time increased steadily from the control value of 100.2 +/- 7.7 ms to attain its maximum of 107.5 +/- 8.5 ms (p < 0.05) after 48 h of hydration. Using biexponential analysis, fast and slow components of the T2 relaxation curve could be distinguished that corresponded to the bound (T21) and free (T22) water fractions. In response to hyponatremia, the bound water fraction was markedly depressed from 6.5 +/- 3.0% to 3.6 +/- 0.9% (3 h, p < 0.05) and 3.9 +/- 0.8% (24 h, p < 0.05); then it approached the initial value of 5.3 +/- 2.5% by the end of the hydration period of 48 h. It is concluded that restructuring of brain water is a contributory factor to the successful adaptation to hypotonic environment.

Cloning, structural organization and chromosomal localization of the mouse aquaporin-8 water channel gene (Aqp8)

The gene encoding the mouse Aquaporin-8 water channel protein (Aqp8) was cloned and its genomic structure was defined. Aqp8 consists of six exons with boundaries at amino acids 1-4, 5-87, 88-129, 130-201, 202-246 and 247-261 which partially correspond to those of other known aquaporin genes. All splice sites conform to the GT-AG rule except the first one which is GT-GG. Primer extension and RNase protection analyses using mouse liver RNA demonstrated three initiation transcription sites located 385, 156 and 146 bp upstream from the translational start codon. No defined TATA box was found in the 5'-flanking region where numerous CAAT motifs and one GATA box were identified. Fluorescence in situ hybridization localized the Aqp8 locus to mouse chromosome 7F3. The 7F region is syntenic with human chromosomes 11, 16 and 10. These results (i) reveal marked structural distinction between the Aqp8 gene and the other known mammalian aquaporin genes, (ii) may now permit the molecular characterization of Aqp8 expression and (iii) represent a fundamental step for the construction of a target vector to generate transgenic Aqp8 knockout mice.

Interferon-alpha upregulates gene expression of aquaporin-5 in human parotid glands

Aquaporins are a family of homologous membrane proteins that function as highly selective water channels. Aquaporin-5 (AQP5) is uniquely present in lacrimal and salivary glands, where it accounts for normal tear and saliva production. We tested the hypothesis that orally administered human interferon-alpha (HuIFN-alpha) benefits persons with xerostomia by augmenting the production of AQP5 protein by parotid gland epithelium. Cells from three human parotid glands were cultured with and without human lymphoblastoid IFN-alpha, and assayed for AQP5 mRNA levels by reverse transcriptase polymerase chain reaction (RT-PCR), and AQP5 protein levels by Western blot. Intracellular localization of AQP5 protein was done using confocal microscopy. The functional integrity of the glandular tissue was confirmed by RT-PCR analysis of alpha-amylase 1 and basic proline-rich protein transcripts. AQP5 was constitutively expressed in human parotid gland tissue, with AQP5 protein restricted to the plasma membranes and cytoplasmic vesicles of acinar cells. IFN-alpha augmented AQP5 transcription and protein production in a concentration-dependent manner, and increased the size of intensity of staining of AQP5-containing cytoplasmic vesicles in acinar cells. We conclude that IFN-alpha upregulates AQP5 gene expression in human parotid acinar cells in vitro. To our knowledge, this is the first demonstration that IFN-alpha regulates the gene expression of an aquaporin.

Neonatal and adult rabbit renal brush border membrane vesicle solute reflection coefficients

The interaction between solute and water in epithelial transport is represented by the solute reflection coefficient. Because the osmotic water transport process changes in the rabbit proximal tubule during maturation, there is a potential for the solute reflection coefficients to also undergo maturational changes. In the present study, we directly examined solute reflection coefficients in neonatal and adult brush border membrane vesicles (BBMV) using the stop-flow light-scattering technique. Reflection coefficients for NaCl, KCl, NaHCO3 and urea were found to be identical in the neonatal and adult BBMV and were not different from 1. Thus, although the water transport pathway undergoes changes in the proximal tubule during maturation, there is no evidence for changes in solute and water interaction. Because the reflection coefficients are not different from 1, there is no evidence for solvent drag in the proximal tubule apical membrane in either the neonatal or adult tubule.

Water transport across yeast vacuolar and plasma membrane-targeted secretory vesicles occurs by passive diffusion

To determine whether solute transport across yeast membranes was facilitated, we measured the water and solute permeations of vacuole-derived and late secretory vesicles in Saccharomyces cerevisiae; all permeations were consistent with passive diffusive flow. We also overexpressed Fps1p, the putative glycerol facilitator in S. cerevisiae, in secretory vesicles but observed no effect on water, glycerol, formamide, or urea permeations. However, spheroplasts prepared from the strain overexpressing Fps1p showed enhanced glycerol uptake, suggesting that Fps1p becomes active only upon insertion in the plasma membrane.

Expression pattern of aquaporin water channels in the inner ear of the rat. The molecular basis for a water regulation system in the endolymphatic sac

Mammalian aquaporins constitute a family of so far 10 related water channel proteins which mediate osmotically driven water fluxes across the plasma membrane. Because regulation of the ionic composition and osmolality of inner ear fluids is of great functional significance, we investigated the expression patterns of aquaporins in five defined areas of the rat inner ear by RT-PCR. The tissues used were stria vascularis, endolymphatic sac, Reissner's membrane, vestibulum and organ of Corti. Aquaporin 1 transcripts were detected in all tissues and are probably constitutive. Aquaporin 5 was only expressed in the organ of Corti and in Reissner's membrane. We show that aquaporin 2, so far considered to be specific to the principal cells of the renal collecting duct, is expressed in the endolymphatic sac. Aquaporin 2 expression was not detected in any other inner ear region. The postnatal appearance of aquaporin 2 transcripts in the endolymphatic sac resembled that in the kidney, i.e. it increased postnatally until day 4. The full-length DNA for aquaporin 2 was cloned from cDNA of the endolymphatic sac. It had an irrelevant Ile54Thr mutation because it could be functionally expressed in Xenopus oocytes. Also exclusively in the endolymphatic sac of the inner ear, we detected transcripts for aquaporin isoforms 3 and 4 which are known to be expressed in the renal principal cells. In the kidney, aquaporin 2 regulation involves vasopressin-stimulated, cAMP-dependent phosphorylation of Ser256 of aquaporin 2 which is stored in cytosolic vesicles. These storage vesicles also contain a serpentine calcium/polycation-sensing receptor. Vesicle shuffling to the plasma membrane involves proteins such as vesicle-associated membrane protein VAMP2, syntaxin-4 and the small GTPase Rab3a. Using RT-PCR we were able to demonstrate the expression of all of these components. By analogy the data suggest that in the endolymphatic sac of the inner ear a system for cellular water permeability is in place which may share many similarities with that characterized in the principal cells of the renal collecting duct. These findings may have a number of interesting pharmacological implications which need to be addressed in future studies.

Endocrinologic and metabolic complications in the intensive care unit

Critical illness provides major stresses on all body systems, including those serving important regulatory functions. Endocrinologic and metabolic abnormalities are common on presentation and during hospitalization in the intensive care unit. Some of these abnormalities are the focus of this article. The authors review abnormalities of the adrenal and thyroid glands and in the metabolism of glucose, and include a brief review of abnormalities of sodium and calcium metabolism.

Molecular insights into the physiology of the 'thin film' of airway surface liquid

The epithelia that line the airways of the lung exhibit two general functions: (1) airway epithelia in all regions 'defend' the lung against infectious and noxious agents; and (2) airway epithelia in the proximal regions replenish water lost from airway surfaces, i.e. the 'insensible water loss', consequent to conditioning inspired air. How airway epithelia perform both functions, and co-ordinate them in health and disease, is the subject of this review.

Lessons on renal physiology from transgenic mice lacking aquaporin water channels

Several aquaporin-type water channels are expressed in kidney: AQP1 in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2, AQP3, and AQP4 in the collecting duct; AQP6 in the papilla; and AQP7 in the proximal tubule. AQP2 is the vasopressin-regulated water channel that is important in hereditary and acquired diseases affecting urine-concentrating ability. It has been difficult to establish the roles of the other aquaporins in renal physiology because suitable aquaporin inhibitors are not available. One approach to the problem has been to generate and analyze transgenic knockout mice in which individual aquaporins have been selectively deleted by targeted gene disruption. Phenotype analysis of kidney and extrarenal function in knockout mice has been very informative in defining the role of aquaporins in organ physiology and addressing basic questions regarding the route of transepithelial water transport and the mechanism of near iso-osmolar fluid reabsorption. This article describes new renal physiologic insights revealed by phenotype analysis of aquaporin-knockout mice and the prospects for further basic and clinical developments.

Membrane permeability modeling: Kedem-Katchalsky vs a two-parameter formalism

The analysis of experiments for the purpose of determining cell membrane permeability parameters is often done using the Kedem-Katchalsky (KK) formalism (1958). In this formalism, three parameters, the hydraulic conductivity (Lp), the solute permeability (Ps), and a reflection coefficient (final sigma), are used to characterize the membrane. Sigma was introduced to characterize flux interactions when water and solute (cryoprotectant) cross the membrane through a common channel. However, the recent discovery and characterization of water channels (aquaporins) in biological membranes reveals that aquaporins are highly selective for water and do not typically cotransport cryoprotectants. In this circumstance, sigma is a superfluous parameter, as pointed out by Kedem and Katchalsky. When sigma is unneeded, a two-parameter model (2P) utilizing only Lp and Ps is sufficient, simpler to implement, and less prone to spurious results. In this paper we demonstrate that the 2P and KK formalism yield essentially the same result (Lp and Ps) when cotransporting channels are absent. This demonstration is accomplished using simulation techniques to compare the transport response of a model cell using a KK or 2P formalism. Sigma is often misunderstood, even when its use is appropriate. It is discussed extensively here and several simulations are used to illustrate and clarify its meaning. We also discuss the phenomenological nature of transport parameters in many experiments, especially when both bilayer and channel transport are present.

Nephrogenic diabetes insipidus

In nephrogenic diabetes insipidus, the kidney is unable to concentrate urine despite normal or elevated concentrations of the antidiuretic hormone arginine vasopressin (AVP). In congenital nephrogenic diabetes insipidus (NDI), the obvious clinical manifestations of the disease, that is polyuria and polydipsia, are present at birth and need to be immediately recognized to avoid severe episodes of dehydration. Most (>90%) congenital NDI patients have mutations in the AVPR2 gene, the Xq28 gene coding for the vasopressin V2 (antidiuretic) receptor. In <10% of the families studied, congenital NDI has an autosomal recessive inheritance and mutations of the aquaporin-2 gene (AQP2), ie, the vasopressin-sensitive water channel, have been identified. When studied in vitro, most AVPR2 mutations lead to receptors that are trapped intracellularly and are unable to reach the plasma membrane. A minority of the mutant receptors reach the cell surface but are unable to bind AVP or to trigger an intracellular cyclic adenosine-monophosphate (cAMP) signal. Similarly AQP2 mutant proteins are trapped intracellularly and cannot be expressed at the luminal membrane. The acquired form of NDI is much more common than the congenital form, is almost always less severe, and is associated with downregulation of AQP2. The advances described here are examples of "bedside physiology" and provide diagnostic tools for physicians caring for these patients.

Distinct biochemical and topological properties of the 31- and 27-kilodalton plasma membrane intrinsic protein subgroups from red beet

Plasma membrane vesicles from red beet (Beta vulgaris L.) storage tissue contain two prominent major intrinsic protein species of 31 and 27 kD (X. Qi, C.Y Tai, B.P. Wasserman [1995] Plant Physiol 108: 387-392). In this study affinity-purified antibodies were used to investigate their localization and biochemical properties. Both plasma membrane intrinsic protein (PMIP) subgroups partitioned identically in sucrose gradients; however, each exhibited distinct properties when probed for multimer formation, and by limited proteolysis. The tendency of each PMIP species to form disulfide-linked aggregates was studied by inclusion of various sulfhydryl agents during tissue homogenization and vesicle isolation. In the absence of dithiothreitol and sulfhydryl reagents, PMIP27 yielded a mixture of monomeric and aggregated species. In contrast, generation of a monomeric species of PMIP31 required the addition of dithiothreitol, iodoacetic acid, or N-ethylmaleimide. Mixed disulfide-linked heterodimers between the PMIP31 and PMIP27 subgroups were not detected. Based on vectorial proteolysis of right-side-out vesicles with trypsin and hydropathy analysis of the predicted amino acid sequence derived from the gene encoding PMIP27, a topological model for a PMIP27 was established. Two exposed tryptic cleavage sites were identified from proteolysis of PMIP27, and each was distinct from the single exposed site previously identified in surface loop C of a PMIP31. Although the PMIP31 and PMIP27 species both contain integral proteins that appear to occur within a single vesicle population, these results demonstrate that each PMIP subgroup responds differently to perturbations of the membrane.

Epithelial aquaporins

The past year has brought significant advances in our understanding of the involvement of aquaporins in the regulation of water balance. Besides the identification of new mammalian aquaporins, highlights include the progress in our understanding of their cell-biological regulation and their roles in physiology and pathophysiology as deduced from natural and engineered knockout models.

High microvascular endothelial water permeability in mouse lung measured by a pleural surface fluorescence method

Transport of water between the capillary and airspace compartments in lung encounters serial barriers: the alveolar epithelium, interstitium, and capillary endothelium. We previously reported a pleural surface fluorescence method to measure net capillary-to-airspace water transport. To measure the osmotic water permeability across the microvascular endothelial barrier in intact lung, the airspace was filled with a water-immiscible fluorocarbon. The capillaries were perfused via the pulmonary artery with solutions of specified osmolalites containing a high-molecular-weight fluorescent dextran. An increase in perfusate osmolality produced a prompt decrease in surface fluorescence due to dye dilution in the capillaries, followed by a slower return to initial fluorescence as capillary and lung interstitial osmolality equilibrate. A mathematical model was developed to determine the osmotic water permeability coefficient (Pf) of lung microvessels from the time course of pleural surface fluorescence. As predicted, the magnitude of the prompt change in surface fluorescence increased with decreased pulmonary artery perfusion rate and increased osmotic gradient size. With raffinose used to induce the osmotic gradient, Pf was 0.03 cm/s at 23 degrees C and was reduced 54% by 0.5 mM HgCl2. Temperature dependence measurements gave an Arrhenius activation energy (Ea) of 5.4 kcal/mol (12-37 degrees C). The apparent Pf induced by the smaller osmolytes mannitol and glycine was 0.021 and 0.011 cm/s (23 degrees C). Immunoblot analysis showed approximately 1.4 x 10(12) aquaporin-1 water channels/cm2 of capillary surface, which accounted quantitatively for the high Pf. These results establish a novel method for measuring osmotically driven water permeability across microvessels in intact lung. The high Pf, low Ea, and mercurial inhibition indicate the involvement of molecular water channels in water transport across the lung endothelium.

Regulation of aquaporin-4 water channels by phorbol ester-dependent protein phosphorylation

The molecular mechanisms for regulating water balance in many tissues are unknown. Like the kidney, the eye contains multiple water channel proteins (aquaporins) that transport water through membranes, including two (AQP1 and AQP4) in the ciliary body, the site of aqueous humor production. However, because humans with defective AQP1 are phenotypically normal and because the ocular application of phorbol esters reduce intraocular pressure, we postulated that the water channel activity of AQP4 may be regulated by these agents. We now report that protein kinase C activators, phorbol 12,13-dibutyrate, and phorbol 12-myristate 13-acetate strongly stimulate the phosphorylation of AQP4 and inhibit its activity in a dose-dependent manner. Phorbol 12,13-dibutyrate (10 microM) and phorbol 12-myristate 13-acetate (10 nM) reduced the rate of AQP4-expressing oocyte swelling by 87 and 92%, respectively. Further, phorbol 12,13-dibutyrate significantly increased the amount of phosphorylated AQP4. These results demonstrate that protein kinase C can regulate the activity of AQP4 through a mechanism involving protein phosphorylation. Moreover, they suggest important potential roles for AQP4 in several clinical disorders involving rapid water transport such as glaucoma, brain edema, and swelling of premature infant lungs.

Cloning and functional expression of a new aquaporin (AQP9) abundantly expressed in the peripheral leukocytes permeable to water and urea, but not to glycerol

A new member (AQP9) of the aquaporin family was identified from human leukocytes by homology cloning using PCR. A full length clone was obtained by screening human liver cDNA library. AQP9 encodes a 295-amino-acid protein with the amino acid sequence identity with AQP3 (48%), AQP7 (45%), and other aquaporins (approximately 30%), suggesting that AQP3, AQP7, and AQP9 belong to a subfamily of the aquaporin family. Injection of AQP9-cRNA into Xenopus oocytes stimulated the osmotic water permeability 7-folds with a low activation energy (4.2 kcal/mol) which was inhibited by 0.3 mM mercury chloride by 48%. AQP9 also facilitated urea transport 4-folds. However, in contrast to AQP3 and AQP7, AQP9 did not stimulate the glycerol permeability, suggesting a unique permeability character. Northern blot analysis revealed the high expression of 3.5-kb messages in peripheral leukocytes >> liver > lung = spleen, but not in thymus. The possible role of AQP9 in the immunological function of leukocytes is intriguing and the identification of AQP9 with unique permeability profile may expand our understanding of water and small solute transport in the body.

Fourfold reduction of water permeability in inner medullary collecting duct of aquaporin-4 knockout mice

Aquaporin (AQP)-3 and AQP4 water channels are expressed at the basolateral membrane of mammalian collecting duct epithelium. To determine the contribution of AQP4 to water permeability in the initial inner medullary collecting duct (IMCD), osmotic water permeability (Pf) was compared in isolated perfused IMCD segments from wild-type and AQP4 knockout mice. The AQP4 knockout mice were previously found to have normal gross appearance, survival, growth, and kidney morphology and a mild urinary concentrating defect (T. Ma, B. Yang, A. Gillespie, E. J. Carlson, C. J. Epstein, and A. S. Verkman, J. Clin. Invest. 100: 957-962, 1997). Transepithelial Pf was measured in microdissected IMCDs after 18-48 h of water deprivation and in the presence of 0.1 nM arginine vasopressin (to make basolateral Pf rate limiting). Pf values (37 degrees C; means +/- SE in cm/s x 10(-3)) were 56.0 +/- 8.5 for wild-type mice (n = 5) and 13.1 +/- 3.7 for knockout mice (n = 6) (P < 0.001). Northern blot analysis of kidney showed that transcript expression of AQP1, AQP2, AQP3, and AQP6 were not affected by AQP4 deletion. Immunoblot analysis indicated no differences in protein expression of AQP1, AQP2, or AQP3, and immunoperoxidase showed no differences in staining patterns. Coexpression of AQP3 and AQP4 in Xenopus laevis oocytes showed additive water permeabilities, suggesting that AQP4 deletion does not affect AQP3 function. These results indicate that AQP4 is responsible for the majority of basolateral membrane water movement in IMCD but that its deletion is associated with a very mild defect in urinary concentrating ability.