Mutations in aquaporin-1 in phenotypically normal humans without functional CHIP water channels

Effect of hydrogen peroxide on lens transparency, intracellular pH, gap junction coupling, hydrostatic pressure and membrane water permeability

Clouding of the eye lens or cataract is an age-related anomaly that affects middle-aged humans. Exploration of the etiology points to a great extent to oxidative stress due to different forms of reactive oxygen species/metabolites such as Hydrogen peroxide (H2O2) that are generated due to intracellular metabolism and environmental factors like radiation. If accumulated and left unchecked, the imbalance between the production and degradation of H2O2 in the lens could lead to cataracts. Our objective was to explore ex vivo the effects of H2O2 on lens physiology. We investigated transparency, intracellular pH (pHi), intercellular gap junction coupling (GJC), hydrostatic pressure (HP) and membrane water permeability after subjecting two-month-old C57 wild-type (WT) mouse lenses for 3 h or 8 h in lens saline containing 50 μM H2O2; the results were compared with control lenses incubated in the saline without H2O2. There was a significant decrease in lens transparency in H2O2-treated lenses. In control lenses, pHi decreases from ∼7.34 in the surface fiber cells to 6.64 in the center. Experimental lenses exposed to H2O2 for 8 h showed a significant decrease in surface pH (from 7.34 to 6.86) and central pH (from 6.64 to 6.56), compared to the controls. There was a significant increase in GJC resistance in the differentiating (12-fold) and mature (1.4-fold) fiber cells compared to the control. Experimental lenses also showed a significant increase in HP which was ∼2-fold higher at the junction between the differentiating and mature fiber cells and ∼1.5-fold higher at the center compared to these locations in control lenses; HP at the surface was 0 mm Hg in either type lens. Fiber cell membrane water permeability significantly increased in H2O2-exposed lenses compared to controls. Our data demonstrate that elevated levels of lens intracellular H2O2 caused a decrease in intracellular pH and led to acidosis which most likely uncoupled GJs, and increased AQP0-dependent membrane water permeability causing a consequent rise in HP. We infer that an abnormal increase in intracellular H2O2 could induce acidosis, cause oxidative stress, alter lens microcirculation, and lead to the development of accelerated lens opacity and age-related cataracts.

Dual function of the choroid plexus: Cerebrospinal fluid production and control of brain ion homeostasis

The choroid plexus is a small monolayered epithelium located in the brain ventricles and serves to secrete the cerebrospinal fluid (CSF) that envelops the brain and fills the central ventricles. The CSF secretion is sustained with a concerted effort of a range of membrane transporters located in a polarized fashion in this tissue. Prominent amongst these are the Na+/K+-ATPase, the Na+,K+,2Cl- cotransporter (NKCC1), and several HCO3- transporters, which together support the net transepithelial transport of the major electrolytes, Na+ and Cl-, and thus drive the CSF secretion. The choroid plexus, in addition, serves an important role in keeping the CSF K+ concentration at a level compatible with normal brain function. The choroid plexus Na+/K+-ATPase represents a key factor in the barrier-mediated control of the CSF K+ homeostasis, as it increases its K+ uptake activity when faced with elevated extracellular K+ ([K+]o). In certain developmental or pathological conditions, the NKCC1 may revert its net transport direction to contribute to CSF K+ homeostasis. The choroid plexus ion transport machinery thus serves dual, yet interconnected, functions with its contribution to electrolyte and fluid secretion in combination with its control of brain K+ levels.

Aquaporin water channels: roles beyond renal water handling

Aquaporin (AQP) water channels are pivotal to renal water handling and therefore in the regulation of body water homeostasis. However, beyond the kidney, AQPs facilitate water reabsorption and secretion in other cells and tissues, including sweat and salivary glands and the gastrointestinal tract. A growing body of evidence has also revealed that AQPs not only facilitate the transport of water but also the transport of several small molecules and gases such as glycerol, H2O2, ions and CO2. Moreover, AQPs are increasingly understood to contribute to various cellular processes, including cellular migration, adhesion and polarity, and to act upstream of several intracellular and intercellular signalling pathways to regulate processes such as cell proliferation, apoptosis and cell invasiveness. Of note, several AQPs are highly expressed in multiple cancers, where their expression can correlate with the spread of cancerous cells to lymph nodes and alter the response of cancers to conventional chemotherapeutics. These data suggest that AQPs have diverse roles in various homeostatic and physiological systems and may be exploited for prognostics and therapeutic interventions.

Lack of the human choline transporter-like protein SLC44A2 causes hearing impairment and a rare red blood phenotype

Blood phenotypes are defined by the presence or absence of specific blood group antigens at the red blood cell (RBC) surface, due to genetic polymorphisms among individuals. The recent development of genomic and proteomic approaches enabled the characterization of several enigmatic antigens. The choline transporter-like protein CTL2 encoded by the SLC44A2 gene plays an important role in platelet aggregation and neutrophil activation. By investigating alloantibodies to a high-prevalence antigen of unknown specificity, found in patients with a rare blood type, we showed that SLC44A2 is also expressed in RBCs and carries a new blood group system. Furthermore, we identified three siblings homozygous for a large deletion in SLC44A2, resulting in complete SLC44A2 deficiency. Interestingly, the first-ever reported SLC44A2-deficient individuals suffer from progressive hearing impairment, recurrent arterial aneurysms, and epilepsy. Furthermore, SLC44A2_null individuals showed no significant platelet aggregation changes and do not suffer from any apparent hematological disorders. Overall, our findings confirm the function of SLC44A2 in hearing preservation and provide new insights into the possible role of this protein in maintaining cerebrovascular homeostasis.

Aquaporins in Urinary System

There are at least eight aquaporins (AQPs) expressed in the kidney. Including AQP1 expressed in proximal tubules, thin descending limb of Henle and vasa recta; AQP2, AQP3, AQP4, AQP5, and AQP6 expressed in collecting ducts; AQP7 expressed in proximal tubules; AQP8 expressed in proximal tubules and collecting ducts; and AQP11 expressed in the endoplasmic reticulum of proximal tubular epithelial cells. Over years, researchers have constructed different AQP knockout mice and explored the effect of AQP knockout on kidney function. Thus, the roles of AQPs in renal physiology are revealed, providing very useful information for addressing fundamental questions about transepithelial water transport and the mechanism of near isoosmolar fluid reabsorption. This chapter introduces the localization and function of AQPs in the kidney and their roles in different kidney diseases to reveal the prospects of AQPs in further basic and clinical studies.

Genetic analysis of the aquaporin water channels AQP12A and AQP12B in patients with chronic pancreatitis

BACKGROUND

Alterations in genes specifically expressed in the pancreas have been associated with chronic pancreatitis (CP). A significant percentage of patients with non-alcoholic CP, however, do not have mutations in known risk genes, suggesting the existence of further susceptibility genes. Four aquaporins are expressed in the exocrine pancreas: AQP1, AQP5, AQP8 and AQP12, the latter being found exclusively in this organ. Therefore, we investigated the two AQP12 genes, AQP12A and AQP12B, in CP patients.

METHODS

We analyzed all exons and adjacent intronic regions of AQP12A and AQP12B in 292 German patients with non-alcoholic CP and 143 control subjects by direct DNA sequencing.

RESULTS

In total, we discovered 41 non-synonymous changes, three of which were nonsense variants. Genotype and allele frequencies of these variants did not differ significantly between patients and controls (all p-values >0.05). Remarkably, we found a common nonsense variant in AQP12B, p.S152Tfs∗24, with an allele frequency of 15.7% in controls, including 2.8% homozygous subjects. This finding suggests that AQP12B is physiologically dispensable for normal pancreatic function.

CONCLUSIONS

Our results suggest that genetic alterations in AQP12A and AQP12B do not predispose to the development of non-alcoholic CP.

Cerebrospinal fluid production by the choroid plexus: a century of barrier research revisited

Cerebrospinal fluid (CSF) envelops the brain and fills the central ventricles. This fluid is continuously replenished by net fluid extraction from the vasculature by the secretory action of the choroid plexus epithelium residing in each of the four ventricles. We have known about these processes for more than a century, and yet the molecular mechanisms supporting this fluid secretion remain unresolved. The choroid plexus epithelium secretes its fluid in the absence of a trans-epithelial osmotic gradient, and, in addition, has an inherent ability to secrete CSF against an osmotic gradient. This paradoxical feature is shared with other 'leaky' epithelia. The assumptions underlying the classical standing gradient hypothesis await experimental support and appear to not suffice as an explanation of CSF secretion. Here, we suggest that the elusive local hyperosmotic compartment resides within the membrane transport proteins themselves. In this manner, the battery of plasma membrane transporters expressed in choroid plexus are proposed to sustain the choroidal CSF secretion independently of the prevailing bulk osmotic gradient.

The permeability of human red blood cell membranes to hydrogen peroxide is independent of aquaporins

Hydrogen peroxide (H₂O₂) not only is an oxidant but also is an important signaling molecule in vascular biology, mediating several physiological functions. Red blood cells (RBCs) have been proposed to be the primary sink of H₂O₂ in the vasculature because they are the main cellular component of blood with a robust antioxidant defense and a high membrane permeability. However, the exact permeability of human RBC to H₂O₂ is neither known nor is it known if the mechanism of permeation involves the lipid fraction or protein channels. To gain insight into the permeability process, we measured the partition constant of H₂O₂ between water and octanol or hexadecane using a novel double-partition method. Our results indicated that there is a large thermodynamic barrier to H₂O₂ permeation. The permeability coefficient of H₂O₂ through phospholipid membranes containing cholesterol with saturated or unsaturated acyl chains was determined to be 4 × 10⁻⁴ and 5 × 10⁻³ cm s⁻¹, respectively, at 37 °C. The permeability coefficient of human RBC membranes to H₂O₂ at 37 °C, on the other hand, was 1.6 × 10⁻³ cm s⁻¹. Different aquaporin-1 and aquaporin-3 inhibitors proved to have no effect on the permeation of H₂O₂. Moreover, human RBCs devoid of either aquaporin-1 or aquaporin-3 were equally permeable to H₂O₂ as normal human RBCs. Therefore, these results indicate that H₂O₂ does not diffuse into RBCs through aquaporins but rather through the lipid fraction or a still unidentified membrane protein.

Molecular mechanisms of brain water transport

Our brains consist of 80% water, which is continuously shifted between different compartments and cell types during physiological and pathophysiological processes. Disturbances in brain water homeostasis occur with pathologies such as brain oedema and hydrocephalus, in which fluid accumulation leads to elevated intracranial pressure. Targeted pharmacological treatments do not exist for these conditions owing to our incomplete understanding of the molecular mechanisms governing brain water transport. Historically, the transmembrane movement of brain water was assumed to occur as passive movement of water along the osmotic gradient, greatly accelerated by water channels termed aquaporins. Although aquaporins govern the majority of fluid handling in the kidney, they do not suffice to explain the overall brain water movement: either they are not present in the membranes across which water flows or they appear not to be required for the observed flow of water. Notably, brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow may not describe all transmembrane fluid transport in the brain. The cotransport of water is an unconventional molecular mechanism that is introduced in this Review as a missing link to bridge the gap in our understanding of cellular and barrier brain water transport.

A Functional Aqp1 Gene Product Localizes on The Contractile Vacuole Complex in Paramecium multimicronucleatum

In a ciliate Paramecium, the presence of water channels on the membrane of contractile vacuole has long been predicted by both morphological and physiological data, however, to date either the biochemical or the molecular biological data have not been provided. In the present study, to examine the presence of aquaporin in Paramecium, we carried out RT-PCR with degenerated primers designed based on the ParameciumDB, and an aquaporin cDNA (aquaporin 1, aqp1) with a full-length ORF encoding 251 amino acids was obtained from Paramecium multimicronucleatum by using RACE. The deduced amino acid sequence of AQP1 had NPA-NPG motifs, and the prediction of protein secondary structure by CNR5000 and hydropathy plot showed the presence of six putative transmembrane domains and five connecting loops. Phylogenetic analysis results showed that the amino acid sequence of AQP1 was close to that of the Super-aquaporin group. The AQP1-GFP fusion protein clearly demonstrated the subcellular localization of AQP1 on the contractile vacuole complex, except for the decorated spongiome membrane. The functional analyses of aqp1 were done by RNA interference-based gene silencing, using an established feeding method. The aqp1 was found to be crucial for the total fluid output of the cell, the function of contractile vacuole membranes.

The study of isotopic enrichment of water in human plasma and erythrocyte

Life does not sustain without water. For water, there is a natural abundance of stable isotope hydrogen and oxygen. Water molecules get across cell membranes through a plasma membrane protein, named aquaporin. Moreover, the kidney is the main organ to maintain water homeostasis. Here, we study the stable isotopic ratios of hydrogen and oxygen in human blood plasma and erythrocyte corresponding to kidney functions. We extract waters from human plasma and erythrocyte, collected from 110 participants, including 51 clinically stable outpatients with end-stage renal disease (ESRD) and 59 subjects with normal renal function (NRF). We observed that (i) both extracellular (blood plasma) and intracellular (erythrocyte) biology waters are isotopic differences between the ESRD and NRF participants, (ii) the natural abundance of isotopic waters of ESRD is hypo-isotopic, and (iii) the isotopic enrichment of water between erythrocyte and blood plasma are distinct. In addition, we introduce an empirical formula using entropy transformation to describe isotopic water enrichment for biology. Accordingly, the natural abundance of stable isotope water of blood plasma and erythrocyte may be possibly put in practice a new sign for assessments of kidney dysfunctions.

Urinary extracellular vesicles: Origin, role as intercellular messengers and biomarkers; efficient sorting and potential treatment options

Urinary extracellular vesicles (uEVs) are a heterogenous group of vesicles consisting mainly of microvesicles and exosomes that originate predominantly (99.96%) from kidney, the urinary tract epithelium and the male reproductive tract. Secreted EVs contain molecular cargo from parental cells and provide an attractive source for biomarkers, a potential readout of physiological and pathophysiological mechanisms, and events associated with the urinary system. uEVs are readily enriched and isolated from urine samples and we review 6 standard methods that allow for downstream analysis of the uEV cargo. Although the use of uEVs as a surrogate readout for physiological changes in tissue protein levels is widespread, the protein abundance in uEVs is affected significantly by mechanisms that regulate protein sorting and secretion in uEVs. Data suggest that baseline kidney tissue and uEV levels of apical membrane-associated electrolyte transport proteins are not directly related in human patients. Recent evidence indicates that EVs may contribute to physiological and pathophysiological intercellular signalling and EVs confer protection against renal ischemia-reperfusion injury. The therapeutic use of EVs as information carriers has mainly been explored in vitro and a major hurdle lies in the translation of the in vitro findings into an in vivo setting. Thus, the EV research field is moving from a technical focus to a more physiological focus, allowing for a deeper understanding of human physiology, development of diagnostic tools and potential treatment strategies for precision medicine.

Physiological functions of urea transporter B

Urea transporters (UTs) are membrane proteins in the urea transporter protein A (UT-A) and urea transporter protein B (UT-B) families. UT-B is mainly expressed in endothelial cell membrane of the renal medulla and in other tissues, including the brain, heart, pancreas, colon, bladder, bone marrow, and cochlea. UT-B is responsible for the maintenance of urea concentration, male reproductive function, blood pressure, bone metabolism, and brain astrocyte and cardiac functions. Its deficiency and dysfunction contribute to the pathogenesis of many diseases. Actually, UT-B deficiency increases the sensitivity of bladder epithelial cells to apoptosis triggers in mice and UT-B-null mice develop II-III atrioventricular block and depression. The expression of UT-B in the rumen of cow and sheep may participate in digestive function. However, there is no systemic review to discuss the UT-B functions. Here, we update research approaches to understanding the functions of UT-B.

Aquaporin water channels: New perspectives on the potential role in inflammation

Aquaporins (AQPs) are a family of membrane water channel proteins that osmotically modulate water fluid homeostasis in several tissues; some of them also transport small solutes such as glycerol. At the cellular level, the AQPs regulate not only cell migration and transepithelial fluid transport across membranes, but also common events that are crucial for the inflammatory response. Emerging data reveal a new function of AQPs in the inflammatory process, as demonstrated by their dysregulation in a wide range of inflammatory diseases including edematous states, cancer, obesity, wound healing and several autoimmune diseases. This chapter summarizes the discoveries made so far about the structure and functions of the AQPs and provides updated information on the underlying mechanisms of AQPs in several human inflammatory diseases. The discovery of new functions for AQPs opens new vistas offering promise for the discovery of mechanisms and therapeutic opportunities in inflammatory disorders.

Exploring the Potential Roles of Band 3 and Aquaporin-1 in Blood CO2 Transport-Inspired by Comparative Studies of Glycophorin B-A-B Hybrid Protein GP.Mur

The Cl^—/HCO₃^— exchanger band 3 is functionally relevant to blood CO₂ transport. Band 3 is the most abundant membrane protein in human red blood cells (RBCs). Our understanding of its physiological functions mainly came from clinical cases associated with band 3 mutations. Severe reduction in band 3 expression affects blood HCO₃^—/CO₂ metabolism. What could happen physiologically if band 3 expression is elevated instead? In some areas of Southeast Asia, about 1–10% of the populations express GP.Mur, a glycophorin B-A-B hybrid membrane protein important in the field of transfusion medicine. GP.Mur functions to promote band 3 expression, and GP.Mur red cells can be deemed as a naturally occurred model for higher band 3 expression. This review first compares the functional consequences of band 3 at different levels, and suggests a critical role of band 3 in postnatal CO₂ respiration. The second part of the review explores the transport of water, which is the other substrate for intra-erythrocytic CO₂/HCO₃^— conversion (an essential step in blood CO₂ transport). Despite that water is considered unlimited physiologically, it is unclear whether water channel aquaporin-1 (AQP1) abundantly expressed in RBCs is functionally involved in CO₂ transport. Research in this area is complicated by the fact that the H₂O/CO₂-transporting function of AQP1 is replaceable by other erythrocyte channels/transporters (e.g., UT-B/GLUT1 for H₂O; RhAG for CO₂). Recently, using carbonic anhydrase II (CAII)-filled erythrocyte vesicles, AQP1 has been demonstrated to transport water for the CAII-mediated reaction, CO_2(g) + H₂O ⇌ HCO₃^—_(aq) + H⁺_(aq). AQP1 is structurally associated with some population of band 3 complexes on the erythrocyte membrane in an osmotically responsive fashion. The current findings reveal transient interaction among components within the band 3-central, CO₂-transport metabolon (AQP1, band 3, CAII and deoxygenated hemoglobin). Their dynamic interaction is envisioned to facilitate blood CO₂ respiration, in the presence of constantly changing osmotic and hemodynamic stresses during circulation.

Water transport and homeostasis as a major function of erythrocytes

Erythrocytes have long been known to change volumes and shapes in response to different salt concentrations. Aquaporin-1 (AQP1) was discovered in their membranes more than 20 yr ago. The physiological roles of volume changes and AQP1 expression, however, have remained unclear. We propose that rapid water exchange through AQP1 coupled with large capacity for volume change may allow erythrocytes to play an important role in water regulation. In this study, we showed that erythrocytes in situ gradually reduced their volumes by 39% in response to the hyperosmotic corticomedullary gradient within mouse kidneys. AQP1 knockout (KO) erythrocytes, however, displayed only minimal reduction. Constructing a microfluidic device resembling capillary flow with an extracellular fluorescent reporter demonstrated that water exchanges between erythrocytes and their hypotonic or hypertonic surroundings in vitro reached steady state in ~60 ms. AQP1 KO erythrocytes, however, did not show significant change. To simulate the water transport in circulation, we built basic units consisting of three compartments (i.e., erythrocyte, plasma, and interstitial fluid) using Kedem-Katchalsky equations for membrane transport, and connected multiple units to account for the blood flow. These simulations agreed with experimental results. Importantly, volume-changing erythrocytes in capillaries always "increase" the osmotic gradient between plasma and interstitial fluid, making them function as "micropumps" to speed up the regulation of local osmolarity. Trillions of these micropumps, mobile throughout the body, may further contribute to water homeostasis. These insights suggest that the enhanced exchange of water, in addition to O₂ and CO₂, may well be the third major function of erythrocytes. NEW & NOTEWORTHY Physiological roles of erythrocyte volume change and aquaporin-1 were proposed and investigated here. We conclude that fast water transport by aquaporin-1 coupled with large volume-change capacity allows erythrocytes to enhance water exchange with local tissues. Furthermore, their huge number and mobility allow them to contribute to body water homeostasis.

Water permeability is a measure of severity in acute appendicitis

Acute appendicitis is the most common indication for pediatric abdominal emergency surgery. Determination of the severity of appendicitis on clinical grounds is challenging. Complicated appendicitis presenting with perforation, abscess or diffuse peritonitis is not uncommon. The question remains why and when acute appendicitis progresses to perforation. The aim of this study was to assess the impact of water permeability on the severity of appendicitis. We show that AQP1 expression and water permeability in appendicitis correlate with the stage of inflammation and systemic infection parameters, leading eventually to perforation of the appendix. AQP1 is also expressed within the ganglia of the enteric nervous system and ganglia count increases with inflammation. Severity of appendicitis can be correlated with water permeability measured by AQP1 protein expression and increase of ganglia count in a progressive manner. This introduces the question if regulation of water permeability can present novel curative or ameliorating therapeutic options.

Phenotypic homogeneity with minor deviance in osmotic fragility of Sahel goat erythrocytes in non-ionic sucrose media during various physiologic states

BACKGROUND

Erythrocyte swelling in non-ionic sucrose media and the subsequent osmotic lysis are influenced by mechanisms of regulatory volume adjustment and osmotic water permeability. Kinetics of transmembrane water and ion fluxes in varied physiologic states may determine the phenotype of erythrocyte osmotic fragility (EOF) and affect estimates of EOF.

METHODS

Effects of sex, age, late pregnancy (third trimester) and lactation on the haemolysis of Sahel goat erythrocytes incubated in a series of hyposmotic non-ionic sucrose media were investigated.

RESULTS

The fragiligram was sigmoidal in 72 (97%) out of 74 goats. Two male (3%) goats with low and high extreme median erythrocyte fragilities (MEF), had non-sigmoidal curves. The mean fragilities at osmolarities of 30-300 mosmol/L of sucrose and the mean osmolarities responsible for 10%-90% haemolysis (CH10-CH90) were not significantly different between males and non-pregnant dry (NPD) females, amongst the age groups and between pregnant or lactating and NPD female goats. The MEF (CH50) of the goats were at osmolarities of 126-252 mosmol/L (median of data: 171 mosmol/L) with a mean of 175.24±16.20 mosmol/L. Therefore, phenotypic homogeneity of EOF occurred with minor deviance, since EOF variables were not differentiated by sex, age, late pregnancy or lactation.

CONCLUSIONS

Physiologic states of the goat did not affect EOF phenotype in non-ionic sucrose media. Sigmoidal fragility phenotype seemed to be homogeneously conserved by osmoregulatory mechanisms not partitioned by sex, age, late pregnancy or lactation, but a minor non-sigmoidal curve might have occurred due to altered erythrocyte osmotic behaviour that would require further investigation.

The distribution and function of aquaporins in the kidney: resolved and unresolved questions

The membrane water channel aquaporin (AQP) family is composed of 13 isoforms in mammals, eight of which are reportedly expressed in the kidney: AQP1, 2, 3, 4, 6, 7, 8, and 11. These isoforms are differentially expressed along the renal tubules and collecting ducts. AQP1 and 7 are distributed in the proximal tubules, whereas AQP2, 3, and 4 occur in the collecting duct system. They play important roles in the reabsorption of water and some solutes across the plasma membrane. In contrast to other aquaporins found in the kidney, AQP6, 8, and 11 are localized to the cytoplasm rather than to the apical or basolateral membranes. It is therefore doubtful that these isoforms are directly involved in water or solute reabsorption. AQP6 is localized in acid-secreting type A intercalated cells of the collecting duct. AQP8 has been found in the proximal tubule but its cellular location has not yet been defined by immunohistochemistry. AQP11 seems to be localized in the endoplasmic reticulum (ER) of proximal tubule cells. Interestingly, polycystic kidneys develop in AQP11-null mice. Many vacuole-like structures are seen in proximal tubule cells in kidneys of newborn AQP11-null mice. Subsequently, cysts are generated, and most of the mice die within a month due to severe renal failure. Although ER stress and impairment of polycystin-1, the product of the gene mutated in autosomal-dominant polycystic kidney disease, are possible causes of cystogenesis in AQP11-null mice, the exact mechanism of pathogenesis and the physiological function of AQP11 are yet to be resolved.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

AQP1 expression in human gingiva and its correlation with periodontal and peri-implant tissue alterations

Aquaporins (AQPs) are a family of hydrophobic integral membrane proteins that function as transmembrane channels and play an important role in tissue homeostasis. Aquaporin-1 (AQP1), in particular, has been reported to be involved in several biological processes including inflammation, angiogenesis, wound healing and others. Periodontitis and peri-implantitis can be defined as inflammatory processes that affect the tissues surrounding a tooth or an osseointegrated implant, respectively. To date, there are limited data about the involvement of AQPs in these diseases. The aim of this study was to evaluate the possible link between the histomorphological alterations and the expression of AQP1 in healthy, pathological and healed periodontal and peri-implant gingival tissues. The results obtained showed that changes in organization of collagen fibers were observed in periodontitis and peri-implantitis, together with an increase in the percentage of area occupied by inflammatory cell infiltration and an increase of AQP1 immunostaining, which was located in the endothelial cells of the vessels within the lamina propria. Moreover, in healed periodontal and peri-implant mucosa a restoration of histomorphological alterations was observed together with a concomitant decrease of AQP1 immunostaining. These data suggested a possible link between the degree of inflammatory state and the presence of AQP1, where the latter could be involved in the chain of inflammatory reactions triggered at periodontal and peri-implant levels.

Energetic and molecular water permeation mechanisms of the human red blood cell urea transporter B

Urea transporter B (UT-B) is a passive membrane channel that facilitates highly efficient permeation of urea. In red blood cells (RBC), while the major function of UT-B is to transport urea, it is assumed that this protein is able to conduct water. Here, we have revisited this last issue by studying RBCs and ghosts from human variants with defects of aquaporin 1 (AQP1) or UT-B. We found that UT-B's osmotic water unit permeability (pfunit) is similar to that of AQP1. The determination of diffusional permeability coefficient (Pd) allowed the calculation of the Pf/Pd ratio, which is consistent with a single-file water transport. Molecular dynamic simulations of water conduction through human UT-B confirmed the experimental finding. From these results, we propose an atomistic description of water-protein interactions involved in this permeation. Inside the UT-B pore, five water molecules were found to form a single-file and move rapidly along a channel by hydrogen bond exchange involving two critical threonines. We further show that the energy barrier for water located in the central region coincides with a water dipole reorientation, which can be related to the proton exclusion observed experimentally. In conclusion, our results indicate that UT-B should be considered as a new member of the water channel family.

The AQP1 mutation c.601delG causes the Co-negative phenotype in four patients belonging to the Romani (Gypsy) ethnic group

BACKGROUND

The Colton blood group antigens Co(a), Co(b) and Co3 are encoded by the AQP1 gene which produces a water channel forming integral protein. The extremely rare Co-deficiency enables immunisation against the Co3 isoantigen.

MATERIALS AND METHODS

Four patients from different regions of Europe who belong to the ethnic minority of Romani (Gypsy) presented with irregular antibodies against a high frequency red blood cell antigen. Positive cross-matches with all red blood cells tested were reported. An Anti-Co3 antibody was identified as the cause of incompatibility in the four cases. The genetic background was determined by polymerase chain reaction typing with sequence-specific primers and by DNA sequencing.

RESULTS

The Co(a-b-) phenotype was confirmed in the four patients despite the fact that genotyping revealed the CO*01 allele of the AQP1 gene. A homozygous AQP1 c.601delG mutation, leading to a frame shift and producing a premature stop in the next codon, was responsible for the Co-negative phenotype in all four cases. While one patient was successfully transfused with blood from his sibling with the identical mutation, another case, a baby affected by haemolytic disease of the newborn, recovered without transfusion.

DISCUSSION

Despite the difficulties in undertaking a population study to determine the prevalence of this AQP1 c.601delG allele in the ethnic minority of Romani, the observations described in this report clearly suggest an accumulation of this mutation, which causes the Co(a-b-) phenotype, in Romani (Gypsy) patients. Further studies are necessary to prove such an accumulation.

Aquaporins in the eye: expression, function, and roles in ocular disease

BACKGROUND

All thirteen known mammalian aquaporins have been detected in the eye. Moreover, aquaporins have been identified as playing essential roles in ocular functions ranging from maintenance of lens and corneal transparency to production of aqueous humor to maintenance of cellular homeostasis and regulation of signal transduction in the retina.

SCOPE OF REVIEW

This review summarizes the expression and known functions of ocular aquaporins and discusses their known and potential roles in ocular diseases.

MAJOR CONCLUSIONS

Aquaporins play essential roles in all ocular tissues. Remarkably, not all aquaporin function as a water permeable channel and the functions of many aquaporins in ocular tissues remain unknown. Given their vital roles in maintaining ocular function and their roles in disease, aquaporins represent potential targets for future therapeutic development.

GENERAL SIGNIFICANCE

Since aquaporins play key roles in ocular physiology, an understanding of these functions is important to improving ocular health and treating diseases of the eye. It is likely that future therapies for ocular diseases will rely on modulation of aquaporin expression and/or function. This article is part of a Special Issue entitled Aquaporins.

A new AQP1 null allele identified in a Gypsy woman who developed an anti-CO3 during her first pregnancy

BACKGROUND AND OBJECTIVES

The Colton blood group antigens are carried by the AQP1 water channel. AQP1(-/-) individuals, also known as Colton-null since they express no Colton antigens, do not suffer any apparent clinical consequence but may develop a clinically significant alloantibody (anti-CO3) induced by transfusion or pregnancy. Identification and transfusion support of Colton-null patients are highly challenging, not only due to the extreme rarity of this phenotype, the lack of appropriate reagents in most laboratories, as well as the possibility of confusing it with the recently described CO:-1,-2,3,-4 phenotype where AQP1 is present. This study investigated a new Colton-null case and evaluated three commercially available anti-AQP1s to identify Colton-null red blood cell samples.

METHODS

The Colton-null phenotype was investigated by standard serological techniques, AQP1 sequencing, immunoblot and flow cytometry analyses.

RESULTS

We identified and characterized the Colton-null phenotype in a Gypsy woman who developed an anti-CO3 during her first pregnancy. After developing a simple and robust method to sequence AQP1, we showed that she was apparently homozygous for a new AQP1 null allele, AQP1 601delG, whose product is not expressed in her red blood cells. We also established the Colton specificity of three commercially available anti-AQP1s in immunoblot and/or flow cytometry analyses.

CONCLUSION

This Gypsy woman represents the sixth Colton-null case characterized at the serological, genetic and biochemical levels. The validation here of new reagents and methods should facilitate the identification of Colton-null individuals.

Functional characterization of a novel aquaporin from Dictyostelium discoideum amoebae implies a unique gating mechanism

The social amoeba Dictyostelium discoideum is a widely used model organism for studying basic functions of protozoan and metazoan cells, such as osmoregulation and cell motility. There is evidence from other species that cellular water channels, aquaporins (AQP), are central to both processes. Yet, data on D. discoideum AQPs is almost absent. Despite cloning of two putative D. discoideum AQPs, WacA, and AqpA, water permeability has not been shown. Further, WacA and AqpA are expressed at the late multicellular stage and in spores but not in amoebae. We cloned a novel AQP, AqpB, from amoeboidal D. discoideum cells. Wild-type AqpB was impermeable to water, glycerol, and urea when expressed in Xenopus laevis oocytes. Neither stepwise truncation of the N terminus nor selected point mutations activated the water channel. However, mutational truncation by 12 amino acids of an extraordinary long intracellular loop induced water permeability of AqpB, hinting at a novel gating mechanism. This AqpB mutant was inhibited by mercuric chloride, confirming the presence of a cysteine residue in the selectivity filter as predicted by our structure model. We detected AqpB by Western blot analysis in a glycosylated and a non-glycosylated form throughout all developmental stages. When expressed in D. discoideum amoebae, AqpB-GFP fusion constructs localized to vacuolar structures, to the plasma membrane, and to lamellipodia-like membrane protrusions. We conclude that the localization pattern in conjunction with channel gating may be indicative of AqpB functions in osmoregulation as well as cell motility of D. discoideum.

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.

Aquaporins in sensory and pain transmission

Recent data suggest a possible involvement of Aquaporins (AQPs) in pain transmission. AQPs are small membrane channel proteins involved in osmoregulation and, to date, AQP1, AQP2, AQP3, AQP4, AQP5, AQP8 and AQP9 have been found in the nervous system. Nevertheless only AQP1, AQP2 and AQP4 seem to be involved in nociception.

In this review, direct and indirect evidences of the role of AQPs in pain processing will be reported.

Unique and analogous functions of aquaporin 0 for fiber cell architecture and ocular lens transparency

Aquaporin (AQP) 1 and AQP0 water channels are expressed in lens epithelial and fiber cells, respectively, facilitating fluid circulation for nourishing the avascular lens to maintain transparency. Even though AQP0 water permeability is 40-fold less than AQP1, AQP0 is selectively expressed in the fibers. Delimited AQP0 fiber expression is attributed to a unique structural role as an adhesion protein. To validate this notion, we determined if wild type (WT) lens ultrastructure and fiber cell adhesion are different in AQP0(-/-), and TgAQP1(+/+)/AQP0(-/-) mice that transgenically express AQP1 (TgAQP1) in fiber cells without AQP0 (AQP0(-/-)). In WT, lenses were transparent with 'Y' sutures. Fibers contained opposite end curvature, lateral interdigitations, hexagonal shape, and were arranged as concentric growth shells. AQP0(-/-) lenses were cataractous, lacked 'Y' sutures, ordered packing and well-defined lateral interdigitations. TgAQP1(+/+)/AQP0(-/-) lenses showed improvement in transparency and lateral interdigitations in the outer cortex while inner cortex and nuclear fibers were severely disintegrated. Transmission electron micrographs exhibited tightly packed fiber cells in WT whereas AQP0(-/-) and TgAQP1(+/+)/AQP0(-/-) lenses had wide extracellular spaces. Fibers were easily separable by teasing in AQP0(-/-) and TgAQP1(+/+)/AQP0(-/-) lenses compared to WT. Our data suggest that the increased water permeability through AQP1 does not compensate for loss of AQP0 expression in TgAQP1(+/+)/AQP0(-/-) mice. Fiber cell AQP0 expression is required to maintain their organization, which is a requisite for lens transparency. AQP0 appears necessary for cell-to-cell adhesion and thereby to minimize light scattering since in the AQP0(-/-) and TgAQP1(+/+)/AQP0(-/-) lenses, fiber cell disorganization was evident.

Modelling the structure of the red cell membraneThis paper is one of a selection of papers published in a Special Issue entitled CSBMCB 53rd Annual Meeting — Membrane Proteins in Health and Disease, and has undergone the Journal’s usual peer review process

The red cell membrane has long been the focus of extensive study. The macromolecules embedded within the membrane carry the blood group antigens and perform many functions including the vital task of gas exchange. Links between the intramembrane macromolecules and the underlying cytoskeleton stabilize the biconcave morphology of the red cell and allow deformation during microvascular transit. Much is now known about the proteins of the red cell membrane and how they are organised. In many cases we have an understanding of which proteins are expressed, the number of each protein per cell, their oligomeric state(s), and how they are collected in large multi-protein complexes. However, our typical view of these structures is as cartoon shapes in schematic figures. In this study we have combined knowledge of the red cell membrane with a wealth of protein structure data from crystallography, NMR, and homology modelling to generate the first, tentative models of the complexes which link the membrane to the cytoskeleton. Measurement of the size of these complexes and comparison with known cytoskeletal distance parameters suggests the idea of interaction between the membrane complexes, which may have profound implications for understanding red cell function and deformation.

A dominant mutation in the gene encoding the erythroid transcription factor KLF1 causes a congenital dyserythropoietic anemia

The congenital dyserythropoietic anemias (CDAs) are inherited red blood cell disorders whose hallmarks are ineffective erythropoiesis, hemolysis, and morphological abnormalities of erythroblasts in bone marrow. We have identified a missense mutation in KLF1 of patients with a hitherto unclassified CDA. KLF1 is an erythroid transcription factor, and extensive studies in mouse models have shown that it plays a critical role in the expression of globin genes, but also in the expression of a wide spectrum of genes potentially essential for erythropoiesis. The unique features of this CDA confirm the key role of KLF1 during human erythroid differentiation. Furthermore, we show that the mutation has a dominant-negative effect on KLF1 transcriptional activity and unexpectedly abolishes the expression of the water channel AQP1 and the adhesion molecule CD44. Thus, the study of this disease-causing mutation in KLF1 provides further insights into the roles of this transcription factor during erythropoiesis in humans.

A functional AQP1 allele producing a Co(a-b-) phenotype revises and extends the Colton blood group system

BACKGROUND

The Colton blood group system currently comprises three antigens, Co(a) , Co(b) , and Co3. The latter is only absent in the extremely rare individuals of the Colton "null" phenotype, usually referred to as Co(a-b-), which lack the water channel AQP1 that carries the Colton antigens. The discovery of a Co(a-b-) individual with no AQP1 deficiency suggested another molecular basis for the Co(a-b-) phenotype.

STUDY DESIGN AND METHODS

Red blood cells were analyzed by stopped-flow light scattering and Western blotting and typed by hemagglutination and flow cytometry. Genotyping by sequencing and polymerase chain reaction-restriction fragment length polymorphism was applied. An expression system for Colton antigens was developed in mammalian cells.

RESULTS

Although Co(a-b-), the proband expressed fully functional AQP1 and had developed a novel Colton alloantibody. Sequencing of AQP1 revealed a homozygous nucleotide change (140A>G) encoding the single-amino-acid substitution Q47R. A second case was identified due to the presence of this novel Colton alloantibody. By generating an expression system for Colton antigens in K-562 cells, the Q47R substitution was shown to inhibit the expression of both Co(a) and Co(b) antigens. Other naturally occurring single-amino-acid substitutions, that is, A45T, P38L, and N192K, were also studied in this Colton antigen expression system.

CONCLUSIONS

The Co(a-b-) phenotype can be generated by a functional AQP1 allele, that is, AQP1 140G encoding AQP1 (Q47R) and allowing the development of a novel Colton alloantibody. This study also shows that the Co(b) antigen can be produced by at least two different substitutions at Amino Acid Position 45, that is, A45V and A45T.

A silenced allele in the Colton blood group system

BACKGROUND

The antigens of the Colton blood group system, Co(a) and Co(b), are encoded by a single gene that produces the aquaporin-1 (AQP1) protein, a water channel-forming protein, and are characterized by a single nucleotide polymorphism (SNP). A healthy Caucasoid blood donor originally typed as Co(a-b-) with commercial anti-Co(b) typed Co(a-b+) when retested with another anti-Co(b). Retyped with two different molecular biology methods, the sample came out Co(a)/Co(b). With the aim of understanding these discrepancies, serological, cytometric and molecular biology tests were carried out.

METHODS

Absorption/elution studies with propositus red cells and controls were performed. The region spanning exon 1 to exon 4 of the Colton gene was sequenced, and flow cytometry analyses were carried out.

RESULTS

Absorption/elution studies showed the absence of Co(a) and a weak expression of Co(b). DNA sequencing confirmed a CT heterozygosity at nucleotide position 134 (i.e. Co(a)/Co(b)), and an additional heterozygous CT was found at position 112. The presence of the Co(b) allele that encodes for the Co(b) antigen was confirmed. The new allele has the base cytosine at nucleotide 134 (Co(a)), in cis with the new nucleotide 112T. The nucleotide substitution 112C>T causes a missense mutation leading to an amino acid change from proline (CCG) to serine (TCG) at codon 38.

CONCLUSION

The substitution found at codon 38 results in a modified AQP1 protein which explains the Co(a-b+) phenotype and possibly the weak expression of Co(b).

Electrolytes in the aging

The elderly population in the United States continues to grow and is expected to double by 2050. With aging, there are degenerative changes in many organs and the kidney is no exception. After 40 years of age, there is an increase in cortical glomerulosclerosis and a decline in both glomerular filtration rate and renal plasma flow. These changes may be associated with an inability to excrete a concentrated or a dilute urine, ammonium, sodium, or potassium. Hypernatremia and hyponatremia are the most common electrolyte abnormalities found in the elderly and both are associated with a high mortality. Under normal conditions, the elderly are able to maintain water and electrolyte balance, but this may be jeopardized by an illness, a decline in cognitive ability, and with certain medications. Therefore, it is important to be aware of the potential electrolyte abnormalities in the elderly that can arise under these various conditions to prevent adverse outcomes.

Aquaporins: The renal water channels

Water is the most abundant molecule in any cell. Specialized membrane channel, proteins called aquaporins, facilitate water transport across cell membranes. At least seven aquaporins (AQP): 1, 2, 3, 4, 6, 7, and 11 are expressed in the kidneys. Aquaporins play a role in both the short-term and long-term regulation of water balance as well as in the pathophysiology of water balance disorders. Aquaporin is composed of a single peptide chain consisting of approximately 270 amino acids. Inherited central and nephrogenic diabetes insipidus are primarily due to the decreased expression of AQP2 while mutation in the AQP2 molecule is responsible for inherited central diabetes insipidus. In acquired causes of nephrogenic diabetes insipidus, there is a downregulation of AQP2 expression in the inner medulla of the kidney. Nephrotic syndrome is characterized by excessive sodium and water reabsorption, although in spite of this, patients do not develop hyponatremia. There is a marked downregulation of both AQP2 and AQP3 expression, which could be a physiologic response to extracellular water reabsorption in patients with nephrotic syndrome. There are some conditions in which aquaporin expression has been found to increase such as experimentally induced heart failure, cirrhosis, and pregnancy. Some drugs such as cisplatin and cyclosporine, also alter the expression of aquaporins. The three-pore model of peritoneal transport depicts the importance of aquaporins. Thus, the understanding of renal water channels has solved the mystery behind many water balance disorders. Further insights into the molecular structure and biology of aquaporins will help to lay a foundation for the development of future drugs.

Intact AQP0 performs cell-to-cell adhesion

Aquaporins (AQPs) constitute a major conduit for movement of water across plasma membranes. AQP0 is expressed in the fiber cells and is critical for lens transparency and homeostasis as mutations and knockout have resulted in dominant lens cataract. Several functions have been attributed for AQP0. In vitro and ex vivo experiments from several laboratories have confirmed the water permeability function of AQP0. However, this function seems paradoxical when the lens switches protein expression from AQP1 in the equatorial epithelial cells to 40 times less efficient AQP0 in the differentiating fiber cells. A possible explanation is AQP0 may perform unique function/s besides being a water pore. Indirect evidences including those from structural studies indicate a cell-to-cell adhesion role for AQP0. However, there is a lack of experimental evidence directly demonstrating the cell-to-cell adhesion capability of AQP0. We studied the adhesion property of human intact AQP0 by expressing it in adhesion-deficient mouse fibroblast L-cells using a newly devised method as well as a traditional assay. Our results reveal that AQP0 indeed can perform cell-to-cell adhesion. AQP1, two alternate splice variants of AQP4 (AQP4-M1and AQP4-M23) and E-cadherin were also tested to validate the results. Cell-to-cell adhesion and cell aggregation properties of AQP0 expressing L-cells were less than those of the positive control L-cells expressing mouse E-cadherin and greater than those of AQP4-M23. AQP1 or AQP4-M1 expressing cells did not show cell-to-cell adhesion or cell aggregation. To our knowledge, this is the first report validating the possible structural role of intact AQP0 as a cell-to-cell adhesion protein, using an in vitro expression system.

Novel therapeutic targets in Plasmodium falciparum: aquaglyceroporins

BACKGROUND

Malaria is caused by the intracellular parasite Plasmodium falciparum. The constant need for novel malaria therapies is due to the development of resistance against existing drugs.

OBJECTIVE

To summarise attempts to investigate parasitic aquaporins as drug targets in malaria.

METHODS

Starting with a summary of the history of malaria we present aquaporin structure and function relationships. Potential interactions of inhibitors with plasmodial AQP (PfAQP) are discussed. PfAQP blockage is examined in the light of recent work on knock-out parasites. Since PfAQP is able to transport other small solutes the parasites are sensitive to other compounds which are harmless to the human host.

RESULTS/CONCLUSIONS

Total blockage of PfAQP may not lead to the death of the parasite but application of PfAQP as a vehicle for toxic substances may be a further pathway for research.

Very high aquaporin-1 facilitated water permeability in mouse gallbladder

Water transport across gallbladder epithelium is driven by osmotic gradients generated from active salt absorption and secretion. Aquaporin (AQP) water channels have been proposed to facilitate transepithelial water transport in gallbladder and to modulate bile composition. We found strong AQP1 immunofluorescence at the apical membrane of mouse gallbladder epithelium. Transepithelial osmotic water permeability (Pf) was measured in freshly isolated gallbladder sacs from the kinetics of luminal calcein self-quenching in response to an osmotic gradient. Pf was very high (0.12 cm/s) in gallbladders from wild-type mice, cAMP independent, and independent of osmotic gradient size and direction. Although gallbladders from AQP1 knockout mice had similar size and morphology to those from wild-type mice, their Pf was reduced by approximately 10-fold. Apical plasma membrane water permeability was greatly reduced in AQP1-deficient gallbladders, as measured by cytoplasmic calcein quenching in perfluorocarbon-filled, inverted gallbladder sacs. However, neither bile osmolality nor bile salt concentration differed in gallbladders from wild-type vs. AQP1 knockout mice. Our data indicate constitutively high water permeability in mouse gallbladder epithelium involving transcellular water transport through AQP1. The similar bile salt concentration in gallbladders from AQP1 knockout mice argues against a physiologically important role for AQP1 in mouse gallbladder.

Overexpression of AQP5, a putative oncogene, promotes cell growth and transformation

Overexpression of several aquaporins has been reported in different types of human cancer but the role of AQPs in human carcinogenesis has not yet been clearly defined. Here, we demonstrate that ectopic expression of human AQP5 (hAQP5), a water channel expressed in lung, salivary glands, and kidney, induces many phenotypic changes characteristic of transformation both in vitro and in vivo. Furthermore, the cell proliferative ability of AQP5 appears to be dependent upon the phosphorylation of a cAMP-protein kinase (PKA) consensus site located in a cytoplasmic loop of AQP5. In addition, phosphorylation of the PKA consensus site was found to be phosphorylated preferentially in tumors. These findings altogether indicate that hAQP5 plays an important role in human carcinogenesis and, furthermore, provide an attractive therapeutic target.