The importance of aquaporin water channel protein structures

PIPs from Fragaria vesca: A structural analysis of native and mutated protein

Aquaporins are an ancient family of membrane channel proteins present in all eukaryotes and most prokaryotes, and apart from water, allow the transport of neutral solutes and organic compounds through the pore. These proteins are essential role differentially expressed during ripening in Fragaria vesca fruits. Fv PIP2-1a is intensively expressed in fruits, inclusive several other proteins member are differentially expressed in fruit but also in other plant tissues. Phylogenetic analysis shows that Fv PIP2-1a grouped with other Fragaria proteins and far apart from other F. vesca PIP proteins. A structural model for Fv PIP2-1a protein was built by comparative modelling methodology, which was validated and refined by molecular dynamics simulation. Fv PIP2-1a structure consists of 6 transmembrane regions and two NPA domains. The mobilization of water was analyzed by molecular docking simulations in wildtype and two mutants. Interestingly, the mutant FvPIP2-1a_H214G allowed the prediction of an increment in the flux of water molecules. On contrary, structural analysis predicted that H214E mutation blocked passage of water associated to constriction of the pore.

Membrane-mediated protein interactions drive membrane protein organization

The plasma membrane's main constituents, i.e., phospholipids and membrane proteins, are known to be organized in lipid-protein functional domains and supercomplexes. No active membrane-intrinsic process is known to establish membrane organization. Thus, the interplay of thermal fluctuations and the biophysical determinants of membrane-mediated protein interactions must be considered to understand membrane protein organization. Here, we used high-speed atomic force microscopy and kinetic and membrane elastic theory to investigate the behavior of a model membrane protein in oligomerization and assembly in controlled lipid environments. We find that membrane hydrophobic mismatch modulates oligomerization and assembly energetics, and 2D organization. Our experimental and theoretical frameworks reveal how membrane organization can emerge from Brownian diffusion and a minimal set of physical properties of the membrane constituents.

Vasopressin-Dependent Disorders: What Is New in Children?

Arginine vasopressin (AVP)-mediated osmoregulatory disorders, such as diabetes insipidus (DI) and syndrome of inappropriate secretion of antidiuretic hormone (SIADH) are common in the differential diagnosis for children with hypo- and hypernatremia and require timely recognition and treatment. DI is caused by a failure to concentrate urine secondary to impaired production of or response to AVP, resulting in hypernatremia. Newer methods of diagnosing DI include measuring copeptin levels; copeptin is AVP's chaperone protein and serves as a surrogate biomarker of AVP secretion. Intraoperative copeptin levels may also help predict the risk for developing DI after neurosurgical procedures. Copeptin levels hold diagnostic promise in other pediatric conditions, too. Recently, expanded genotype and phenotype correlations in inherited DI disorders have been described and may better predict the clinical course in affected children and infants. Similarly, newer formulations of synthetic AVP may improve pediatric DI treatment. In contrast to DI, SIADH, characterized by inappropriate AVP secretion, commonly leads to severe hyponatremia. Contemporary methods aid clinicians in distinguishing SIADH from other hyponatremic conditions, particularly cerebral salt wasting. Further research on the efficacy of therapies for pediatric SIADH is needed, although some adult treatments hold promise for pediatrics. Lastly, expansion of home point-of-care sodium testing may transform management of SIADH and DI in children. In this article, we review recent developments in the understanding of pathophysiology, diagnostic workup, and treatment of better outcomes and quality of life for children with these challenging disorders.

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.

Aquaporins during pregnancy

Aquaporins (AQPs) are water channels proteins that facilitate water flux across cell membranes in response to osmotic gradients. Despite of the differences in the mammalian placentas, the conserved combination of AQPs expressed in placental and fetal membranes throughout gestation suggests that these proteins may be important in the regulation of fetal water homeostasis. Thus, AQPs may regulate the amniotic fluid volume and participate in the trans-placental transfer of water. Apart from their classical roles, recent studies have revealed that placental AQPs may also cooperate in cellular processes such as the migration and the apoptosis of the trophoblasts. Aquaglyceroporins can also participate in the energy metabolism and in the urea elimination across the placenta. Many factors including oxygen, hormones, acid-basis homeostasis, maternal dietary status, interaction with other transport proteins and osmotic stress are proposed to regulate their expression and function during gestation and alterations result in pathological pregnancies.

Current status of diagnosis and Mesenchymal stem cells therapy for acute pancreatitis

Acute pancreatitis (AP) is an acute gastrointestinal disorder that is the most common and requiring emergency hospitalization. Its incidence is increasing worldwide, thus increasing the burden of medical services. Approximately 20% of the patients develop moderate to severe necrotizing pancreatitis associated with pancreatic or peri-pancreatic tissue necrosis and multiple organ failure. There are many reports about the anti-inflammatory effect of mesenchymal stem cells (MSCs) on pancreatitis and the repair of tissue damage. MSCs cells come from a wide range of sources, autologous MSCs come from bone marrow and allogeneic MSCs such as umbilical cord blood MSCs, placenta-derived MSCs, etc. The wide source is not only an advantage of MSCs but also a disadvantage of MSCs. Because of different cell sources and different methods of collection and preparation, it is impossible to establish a unified standard method for evaluation of efficacy. The biggest advantage of iMSCs is that it can be prepared by a standardized process, and can be prepared on a large scale, which makes it easier to commercialize. This paper reviews the present status of diagnosis and progress of MSCs therapy for AP.

Plant Aquaporins: Diversity, Evolution and Biotechnological Applications

The plasma membrane forms a permeable barrier that separates the cytoplasm from the external environment, defining the physical and chemical limits in each cell in all organisms. The movement of molecules and ions into and out of cells is controlled by the plasma membrane as a critical process for cell stability and survival, maintaining essential differences between the composition of the extracellular fluid and the cytosol. In this process aquaporins (AQPs) figure as important actors, comprising highly conserved membrane proteins that carry water, glycerol and other hydrophilic molecules through biomembranes, including the cell wall and membranes of cytoplasmic organelles. While mammals have 15 types of AQPs described so far (displaying 18 paralogs), a single plant species can present more than 120 isoforms, providing transport of different types of solutes. Such aquaporins may be present in the whole plant or can be associated with different tissues or situations, including biotic and especially abiotic stresses, such as drought, salinity or tolerance to soils rich in heavy metals, for instance. The present review addresses several aspects of plant aquaporins, from their structure, classification, and function, to in silico methodologies for their analysis and identification in transcriptomes and genomes. Aspects of evolution and diversification of AQPs (with a focus on plants) are approached for the first time with the aid of the LCA (Last Common Ancestor) analysis. Finally, the main practical applications involving the use of AQPs are discussed, including patents and future perspectives involving this important protein family.

Binding of a small molecule water channel inhibitor to aquaporin Z examined by solid-state MAS NMR

Aquaporins are integral membrane proteins that facilitate water flow across biological membranes. Their involvement in multiple physiological functions and disease states has prompted intense research to discover water channel activity modulators. However, inhibitors found so far are weak and/or lack specificity. For organic compounds, which lack of high electron-dense atoms, the identification of binding sites is even more difficult. Nuclear magnetic resonance spectroscopy (NMR) requires large amounts of the protein, and expression and purification of mammalian aquaporins in large quantities is a difficult task. However, since aquaporin Z (AqpZ) can be purified and expressed in good quantities and has a high similarity to human AQP1 (~ 40% identity), it can be used as a model for studying the structure and function of human aquaporins. In the present study, we have used solid-state MAS NMR to investigate the binding of a lead compound [1-(4-methylphenyl)1H-pyrrole-2,5-dione] to AqpZ, through mapping of chemical shift perturbations in the presence of the compound.

Aquaporin-graphene interface: relevance to point-of-care device for renal cell carcinoma and desalination

The aquaporin superfamily of hydrophobic integral membrane proteins constitutes water channels essential to the movement of water across the cell membrane, maintaining homeostatic equilibrium. During the passage of water between the extracellular and intracellular sides of the cell, aquaporins act as ultra-sensitive filters. Owing to their hydrophobic nature, aquaporins self-assemble in phospholipids. If a proper choice of lipids is made then the aquaporin biomimetic membrane can be used in the design of an artificial kidney. In combination with graphene, the aquaporin biomimetic membrane finds practical application in desalination and water recycling using mostly Escherichia coli AqpZ. Recently, human aquaporin 1 has emerged as an important biomarker in renal cell carcinoma. At present, the ultra-sensitive sensing of renal cell carcinoma is cumbersome. Hence, we discuss the use of epitopes from monoclonal antibodies as a probe for a point-of-care device for sensing renal cell carcinoma. This device works by immobilizing the antibody on the surface of a single-layer graphene, that is, as a microfluidic device for sensing renal cell carcinoma.

The Role of Aquaporins in pH-Dependent Germination of Rhizopus delemar Spores

Rhizopus delemar and associated species attack a wide range of fruit and vegetables after harvest. Host nutrients and acidic pH are required for optimal germination of R. delemar, and we studied how this process is triggered. Glucose induced spore swelling in an acidic environment, expressed by an up to 3-fold increase in spore diameter, whereas spore diameter was smaller in a neutral environment. When suspended in an acidic environment, the spores started to float, indicating a change in their density. Treatment of the spores with HgCl2, an aquaporin blocker, prevented floating and inhibited spore swelling and germ-tube emergence, indicating the importance of water uptake at the early stages of germination. Two putative candidate aquaporin-encoding genes-RdAQP1 and RdAQP2-were identified in the R. delemar genome. Both presented the conserved NPA motif and six-transmembrane domain topology. Expressing RdAQP1 and RdAQP2 in Arabidopsis protoplasts increased the cells' osmotic water permeability coefficient (Pf) compared to controls, indicating their role as water channels. A decrease in R. delemar aquaporin activity with increasing external pH suggested pH regulation of these proteins. Substitution of two histidine (His) residues, positioned on two loops facing the outer side of the cell, with alanine eliminated the pH sensing resulting in similar Pf values under acidic and basic conditions. Since hydration is critical for spore switching from the resting to activate state, we suggest that pH regulation of the aquaporins can regulate the initial phase of R. delemar spore germination, followed by germ-tube elongation and host-tissue infection.

Aquaporins: Their role in gastrointestinal malignancies

Aquaporins (AQPs) are small (~30 kDa monomers) integral membrane water transport proteins that allow water to flow through cell membranes in reaction to osmotic gradients in cells. In mammals, the family of AQPs has thirteen (AQP0-12) unique members that mediate critical biological functions. Since AQPs can impact cell proliferation, migration and angiogenesis, their role in various human cancers is well established. Recently, AQPs have been explored as potential diagnostic and therapeutic targets in gastrointestinal (GI) cancers. GI cancers encompass multiple sites including the colon, esophagus, stomach and pancreas. Research in the last three decades has revealed biological aspects and signaling pathways critical for the development of GI cancers. Since the majority of these cancers are very aggressive and rapidly metastasizes, identifying effective targets is crucial for treatment. Preclinical studies have utilized inhibitors of specific AQPs and knock down of AQP expression using siRNA. Although several studies have explored the role of AQPs in colorectal, esophageal, gastric, hepatocellular and pancreatic cancers, there is no comprehensive review compiling the available information on GI cancers as has been published for other malignancies such as ovarian cancer. Due to the similarities and association of various sites of GI cancers, it is helpful to consider these results collectively in order to better understand the role of specific AQPs in critical GI cancers. This review summarizes the current knowledge of the role of AQPs in GI malignancies with particular focus on diagnosis and therapeutic applications.

Effects of propofol and sevoflurane on aquaporin-4 and aquaporin-9 expression in patients performed gliomas resection

Post-operative cerebral edema is a threat for patients performed gliomas resection. Some studies have shown that general anesthesia drugs, such as, propofol had neuroprotective effect. Aquaporin-4 (AQP4) and Aquaporin-9 (AQP9) play an important role in maintaining brain water homeostasis under various conditions. The aim of this study was to compare the effect of propofol or sevoflurane on expression of AQP4 and AQP9 in patients performed gliomas resection. 30 patients performed gliomas resection were included in this study. The patients were randomly divided into two groups: propofol group and sevoflurane group. Fresh human gliomas specimens were obtained and hematoxylin eosin (HE) staining, immunohistochemical staining and Western blot analysis were used for observation of the expression of AQP4 and AQP9. The immunohistochemical staining of the sections showed that the percentage of AQP4 positive cells in the propofol group (14.3±4.61%) was significantly lower than that in sevoflurane group (37.3±10.01%) (n=15, P<0.05). There was no significant difference in the percentage of AQP9 positive cells in propofol group and sevoflurane group (25.8±2.67 versus 28.1±7.81%, n=15, P>0.05). Western blot analysis confirmed the immunohistochemistry results. AQP4 protein level in propofol group was significantly lower than that in sevoflurane group (1.4±0.13 versus 1.7±0.12, P<0.05). Western blot analysis did not show any difference of expression of AQP9 protein between the propofol group and sevoflurane group (2.0±0.13 versus 2.1±0.13, P>0.05, n=6). AQP4 expression was lower in patients of propofol group than that in sevoflurane group. Our results suggested that propofol could inhibit the expression of AQP4.

Metabolic impact of the glycerol channels AQP7 and AQP9 in adipose tissue and liver

Obesity and secondary development of type 2 diabetes (T2D) are major health care problems throughout the developed world. Accumulating evidence suggest that glycerol metabolism contributes to the pathophysiology of obesity and T2D. Glycerol is a small molecule that serves as an important intermediate between carbohydrate and lipid metabolism. It is stored primarily in adipose tissue as the backbone of triglyceride (TG) and during states of metabolic stress, such as fasting and diabetes, it is released for metabolism in other tissues. In the liver, glycerol serves as a gluconeogenic precursor and it is used for the esterification of free fatty acid into TGs. Aquaporin 7 (AQP7) in adipose tissue and AQP9 in the liver are transmembrane proteins that belong to the subset of AQPs called aquaglyceroporins. AQP7 facilitates the efflux of glycerol from adipose tissue and AQP7 deficiency has been linked to TG accumulation in adipose tissue and adult onset obesity. On the other hand, AQP9 expressed in liver facilitates the hepatic uptake of glycerol and thereby the availability of glycerol for de novo synthesis of glucose and TG that both are involved in the pathophysiology of diabetes. The aim of this review was to summarize the current knowledge on the role of the two glycerol channels in controlling glycerol metabolism in adipose tissue and liver.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

MOLEonline 2.0: interactive web-based analysis of biomacromolecular channels

Biomolecular channels play important roles in many biological systems, e.g. enzymes, ribosomes and ion channels. This article introduces a web-based interactive MOLEonline 2.0 application for the analysis of access/egress paths to interior molecular voids. MOLEonline 2.0 enables platform-independent, easy-to-use and interactive analyses of (bio)macromolecular channels, tunnels and pores. Results are presented in a clear manner, making their interpretation easy. For each channel, MOLEonline displays a 3D graphical representation of the channel, its profile accompanied by a list of lining residues and also its basic physicochemical properties. The users can tune advanced parameters when performing a channel search to direct the search according to their needs. The MOLEonline 2.0 application is freely available via the Internet at http://ncbr.muni.cz/mole or http://mole.upol.cz.

Aquaporins from pathogenic protozoan parasites: structure, function and potential for chemotherapy

Infectious diseases, caused by protozoa, such as malaria, sleeping sickness, Chagas' disease or leishmaniasis, are a global threat. The increase in the number of affected individuals and the rapid spread of drug-resistant strains call for specific novel strategies to combat human pathogenic parasites. In the search for novel drug targets, transport proteins for nutrients and metabolites of the parasite-host interface are getting into focus. The present review summarizes and discusses the currently available results on protozoan aquaporins. Various genes coding for aquaporin water and solute channels have been identified in the protozoan genomes and they are probable elements of the parasite's cell membrane. Phylogenetic analysis reveals that individual aquaporin genes are of bacterial or plant origin. So far, six protozoan aquaporins have been cloned and functionally characterized. Typically, these are bifunctional channels and pass water at intermediate to high rates as well as uncharged solutes. In the present review, amino acid compositions of the individual pore entries are compared and permeability properties are attributed to specific protein features. Furthermore, possible physiological roles in osmotic protection and metabolism are discussed. Finally, the potential of protozoan aquaporins for use as a target or entry pathway for chemotherapeutic compounds is reviewed.

Biosilica: Molecular Biology, Biochemistry and Function in Demosponges as well as its Applied Aspects for Tissue Engineering

Biomineralization, biosilicification in particular (i.e. the formation of biogenic silica, SiO(2)), has become an exciting source of inspiration for the development of novel bionic approaches following 'nature as model'. Siliceous sponges are unique among silica-forming organisms in their ability to catalyze silica formation using a specific enzyme termed silicatein. In this study, we review the present state of knowledge on silicatein-mediated 'biosilica' formation in marine demosponges, the involvement of further molecules in silica metabolism and their potential applications in nano-biotechnology and bio-medicine. While most forms of multicellular life have developed a calcium-based skeleton, a few specialized organisms complement their body plan with silica. Only sponges (phylum Porifera) are able to polymerize silica enzymatically mediated in order to generate massive siliceous skeletal elements (spicules) during a unique reaction, at ambient temperature and pressure. During this biomineralization process (i.e. biosilicification), hydrated, amorphous silica is deposited within highly specialized sponge cells, ultimately resulting in structures that range in size from micrometres to metres. This peculiar phenomenon has been comprehensively studied in recent years, and in several approaches, the molecular background was explored to create tools that might be employed for novel bioinspired biotechnological and biomedical applications. Thus, it was discovered that spiculogenesis is mediated by the enzyme silicatein and starts intracellularly. The resulting silica nanoparticles fuse and subsequently form concentric lamellar layers around a central protein filament, consisting of silicatein and the scaffold protein silintaphin-1. Once the growing spicule is extruded into the extracellular space, it obtains final size and shape. Again, this process is mediated by silicatein and silintaphin-1/silintaphin-1, in combination with other molecules such as galectin and collagen. The molecular toolbox generated so far allows the fabrication of novel micro- and nano-structured composites, contributing to the economical and sustainable synthesis of biomaterials with unique characteristics. In this context, first bioinspired approaches implement recombinant silicatein and silintaphin-1 for applications in the field of biomedicine (biosilica-mediated regeneration of tooth and bone defects) with promising results.

Molecular dynamics study of the archaeal aquaporin AqpM

Background

Aquaporins are a large family of transmembrane channel proteins that are present throughout all domains of life and are implicated in human disorders. These channels, allow the passive but selective movement of water and other small neutral solutes across cell membranes. Aquaporins have been classified into two sub-families: i) strict aquaporins that only allow the passage of water and ii) the less selective aquaglyceroporins that transport water and other neutral solutes, such as glycerol, CO₂ or urea. Recently, the identification and characterization of a number of archaeal and bacterial aquaporins suggested the existence of a third sub-family; one that is neither a strict aquaporin nor an aquaglyceroporin. The function and phylogeny of this third family is still a matter of debate.

Results

Twenty nanosecond molecular dynamics (MD) simulation of a fully hydrated tetramer of AqpM embedded in a lipid bilayer permitted predictions to be made of key biophysical parameters including: single channel osmotic permeability constant (p_f), single channel diffusive permeability constant (p_d), channel radius, potential water occupancy of the channel and water orientation inside the pore. These properties were compared with those of well characterized representatives of the two main aquaporin sub-families. Results show that changes in the amino acid composition of the aromatic/arginine region affect the size and polarity of the selectivity filter (SF) and could help explain the difference in water permeability between aquaporins. In addition, MD simulation results suggest that AqpM combines characteristics of strict aquaporins, such as the narrow SF and channel radius, with those of aquaglyceroporins, such as a more hydrophobic and less polar SF.

Conclusions

MD simulations of AqpM extend previous evidence that this archaeal aquaporin exhibits hybrid features intermediate between the two known aquaporin sub-families, supporting the idea that it may constitute a member of a novel class of aquaporins.

Sponge biosilica formation involves syneresis following polycondensation in vivo

Syneresis is a process observed during the maturation/aging of silica gels obtained by sol-gel synthesis that results in shrinkage and expulsion of water due to a rearrangement and increase in the number of bridging siloxane bonds. Here we describe how the process of biosilica deposition during spicule ("biosilica" skeleton of the siliceous sponges) formation involves a phase of syneresis that occurs after the enzyme-mediated polycondensation reaction. Primmorphs from the demosponge Suberites domuncula were used to study syneresis and the inhibition of this mechanism. We showed by scanning electron microscopy that spicules added to primmorphs that have been incubated with manganese sulfate fuse together through the deposition of silica spheres and bridges. Energy-dispersive X-ray mapping of the newly formed deposits showed high silicon and oxygen content. These biosilica deposits contain a comparably higher percentage of water than mature/aged spicules. Quantitative real-time polymerase chain reaction analyses revealed that the addition of silicate to primmorph cultures resulted in a marked upregulation of the expression of the aquaporin gene and of the genes encoding the silica anabolic enzyme silicatein-α and the silica catabolic enzyme silicase. On the other hand, addition of manganese sulfate, either alone or together with silicate, caused a strong reduction in the level of aquaporin transcripts, although this metal ion did not essentially affect the silicate-induced increase in silicatein-α and silicase gene expression. We conclude that the secondary silica deposits formed on spicules under physiological conditions in the presence of silicate fuse together and subsequently undergo syneresis, which is facilitated by the removal of water through aquaporin channels. In growing spicules, these processes of biosilica formation and syneresis in the lamellar monolithic structures precede the final step of "biosintering" during which the massive biosilica rods of the spicules are formed.

Chemosensory function of amphibian skin: integrating epithelial transport, capillary blood flow and behaviour

Terrestrial anuran amphibians absorb water across specialized regions of skin on the posterioventral region of their bodies. Rapid water absorption is mediated by the insertion of aquaporins into the apical membrane of the outermost cell layer. Water moves out of the epithelium via aquaglyceroporins in the basolateral membrane and into the circulation in conjunction with increased capillary blood flow to the skin and aquaporins in the capillary endothelial cells. These physiological responses are activated by intrinsic stimuli relating to the animals' hydration status and extrinsic stimuli relating to the detection of osmotically available water. The integration of these processes has been studied using behavioural observations in conjunction with neurophysiological recordings and studies of epithelial transport. These studies have identified plasma volume and urinary bladder stores as intrinsic stimuli that activate the formation of angiotensin II (AII) to stimulate water absorption behaviour. The coordinated increase in water permeability and capillary blood flow appears to be mediated primarily by sympathetic stimulation of beta adrenergic receptors, although the neurohypopyseal hormone arginine vasotocin (AVT) may also play a role. Extrinsic stimuli relate primarily to the ionic and osmotic properties of hydration sources. Toads avoid NaCl solutions that have been shown to be harmful in acute exposure, approx. 200-250 mm. The avoidance is partially attenuated by amiloride raising the hypothesis that the mechanism for salt detection by toads resembles that for salt taste in mammals that take in water by mouth. In this model, depolarization of the basolateral membrane of taste cells is coupled to afferent neural stimulation. In toad skin we have identified innervation of skin epithelial cells by branches of spinal nerves and measured neural responses to NaCl solutions that elicit behavioural avoidance. These same concentrations produce depolarization of the basolateral membrane in isolated epithelial preparations. As with salt taste in mammals, the neural responses and depolarization of basolateral membrane potential are partially inhibited by amiloride. In addition, toads are more tolerant of sodium gluconate solution which is consistent with the phenomenon in mammalian taste physiology termed the anion paradox in which sodium salts with larger molecular weight anions produce a reduced intensity of salt taste. Finally, toads also avoid concentrated solutions of a non-electrolyte, mannitol, which differs from NaCl solutions in not affecting transepithelial conductance and requires a longer time to depolarize the basolateral membrane. Osmotic stimuli may mediate sensory processes for longer term detection of conditions with low water potential while ionic stimuli are more important for shorter term analysis of rehydration sources.

Design and characterization of genetically engineered zebrafish aquaporin-3 mutants highly permeable to the cryoprotectant ethylene glycol

Background

Increasing cell membrane permeability to water and cryoprotectants is critical for the successful cryopreservation of cells with large volumes. Artificial expression of water-selective aquaporins or aquaglyceroporins (GLPs), such as mammalian aquaporin-3 (AQP3), enhances cell permeability to water and cryoprotectants, but it is known that AQP3-mediated water and solute permeation is limited and pH dependent. To exploit further the possibilities of using aquaporins in cryobiology, we investigated the functional properties of zebrafish (Danio rerio) GLPs.

Results

Water, glycerol, propylene glycol and ethylene glycol permeability of zebrafish Aqp3a, -3b, -7, -9a, -9b, -10a and -10b, and human AQP3, was examined. Expression in Xenopus laevis oocytes indicated that the permeability of DrAqp3a and -3b to ethylene glycol was higher than for glycerol or propylene glycol under isotonic conditions, unlike other zebrafish GLPs and human AQP3, which were more permeable to glycerol. In addition, dose-response experiments and radiolabeled ethylene glycol uptake assays suggested that oocytes expressing DrAqp3b were permeated by this cryoprotectant more efficiently than those expressing AQP3. Water and ethylene glycol transport through DrAqp3a and -3b were, however, highest at pH 8.5 and completely abolished at pH 6.0. Point mutations in the DrAqp3b amino acid sequence rendered two constructs, DrAqp3b-T85A showing higher water and ethylene glycol permeability at neutral and alkaline pH, and DrAqp3b-H53A/G54H/T85A, no longer inhibited at acidic pH but less permeable than the wild type. Finally, calculation of permeability coefficients for ethylene glycol under concentration gradients confirmed that the two DrAqp3b mutants were more permeable than wild-type DrAqp3b and/or AQP3 at neutral pH, resulting in a 2.6- to 4-fold increase in the oocyte intracellular concentration of ethylene glycol.

Conclusion

By single or triple point mutations in the DrAqp3b amino acid sequence, we constructed one mutant with enhanced ethylene glycol permeability and another with reduced pH sensitivity. The DrAqp3b and the two mutant constructs may be useful for application in cryobiology.

Plant aquaporins with non-aqua functions: deciphering the signature sequences

Research in recent years on plant Major Intrinsic Proteins (MIPs), commonly referred to as 'aquaporins', has seen a vast expansion in the substrates found to be transported via these membrane channels. The diversity in sizes, chemical nature and physiological significance of these substrates has meant a need to critically analyse the possible structural and biochemical properties of MIPs that transport these, in order to understand their roles. In this work we have undertaken a comprehensive analysis of all plant MIPs, coming from different families, that have been proven to transport ammonia, boron, carbon dioxide, hydrogen peroxide, silicon and urea. The sequences were analysed for all primary selectivity-related motifs (NPA motifs, ar/R filter, P1-P5 residues). In addition, the putative regulatory phosphorylation and glycosylation sites and mechanistic regulators such as loop lengths have been analysed. Further, nine specificity-determining positions (SDPs) were predicted for each group. The results show the ar/R filter residues, P2-P4 positions and some of the SDPs are characteristic for certain groups, and O-glycosylation sites are unique to a subgroup while N-glycosylation was characteristic of the other MIPs. Certain residues, especially in loop C, and structural parameters such as loop lengths also contribute to the uniqueness of groups. The comprehensive analysis makes significant inroads into appraising the intriguing diversity of plant MIPs and their roles in fundamental life processes, and provides tools for plant selections, protein engineering and transgenics.

Structure and stability of the spinach aquaporin SoPIP2;1 in detergent micelles and lipid membranes

Background

SoPIP2;1 constitutes one of the major integral proteins in spinach leaf plasma membranes and belongs to the aquaporin family. SoPIP2;1 is a highly permeable and selective water channel that has been successfully overexpressed and purified with high yields. In order to optimize reconstitution of the purified protein into biomimetic systems, we have here for the first time characterized the structural stability of SoPIP2;1.

Methodology/Principal Finding

We have characterized the protein structural stability after purification and after reconstitution into detergent micelles and proteoliposomes using circular dichroism and fluorescence spectroscopy techniques. The structure of SoPIP2;1 was analyzed either with the protein solubilized with octyl-β-D-glucopyranoside (OG) or reconstituted into lipid membranes formed by E. coli lipids, diphytanoylphosphatidylcholine (DPhPC), or reconstituted into lipid membranes formed from mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE), 1-palmitoyl-2oleoyl-phosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-phosphatidylserine (POPS), and ergosterol. Generally, SoPIP2;1 secondary structure was found to be predominantly α-helical in accordance with crystallographic data. The protein has a high thermal structural stability in detergent solutions, with an irreversible thermal unfolding occurring at a melting temperature of 58°C. Incorporation of the protein into lipid membranes increases the structural stability as evidenced by an increased melting temperature of up to 70°C.

Conclusion/Significance

The results of this study provide insights into SoPIP2;1 stability in various host membranes and suggest suitable choices of detergent and lipid composition for reconstitution of SoPIP2;1 into biomimetic membranes for biotechnological applications.

A novel mutation in MIP associated with congenital nuclear cataract in a Chinese family

PURPOSE

To identify the underlying genetic defect in a Chinese family affected with autosomal dominant congenital nuclear cataract.

METHODS

A four-generation Chinese family with inherited nuclear cataract phenotype was recruited. Detailed family history and clinical data were recorded. All reported nuclear cataract-related candidate genes were screened for causative mutations by direct DNA sequencing. Effects of amino acid changes on the structure and function of protein were predicted by bioinformatics analysis.

RESULTS

All affected individuals in this family showed nuclear cataracts. Sequencing of the candidate genes revealed a heterozygous c.559C>T change in the coding region of the major intrinsic protein (MIP), which caused a substitution of highly conserved arginine by cysteine at codon 187 (p.R187C). This mutation co-segregated with all affected individuals and was not observed in unaffected family members or 110 ethnically matched controls. Bioinformatics analysis showed that the mutation was predicted to affect the function and secondary structure of MIP protein.

CONCLUSIONS

This study identified a novel disease-causing mutation p.R187C in MIP in a Chinese cataract family, expanding the mutation spectrum of MIP causing congenital cataract.

Boron toxicity tolerance in barley through reduced expression of the multifunctional aquaporin HvNIP2;1

Boron (B) toxicity is a significant limitation to cereal crop production in a number of regions worldwide. Here we describe the cloning of a gene from barley (Hordeum vulgare), underlying the chromosome 6H B toxicity tolerance quantitative trait locus. It is the second B toxicity tolerance gene identified in barley. Previously, we identified the gene Bot1 that functions as an efflux transporter in B toxicity-tolerant barley to move B out of the plant. The gene identified in this work encodes HvNIP2;1, an aquaporin from the nodulin-26-like intrinsic protein (NIP) subfamily that was recently described as a silicon influx transporter in barley and rice (Oryza sativa). Here we show that a rice mutant for this gene also shows reduced B accumulation in leaf blades compared to wild type and that the mutant protein alters growth of yeast (Saccharomyces cerevisiae) under high B. HvNIP2;1 facilitates significant transport of B when expressed in Xenopus oocytes compared to controls and to another NIP (NOD26), and also in yeast plasma membranes that appear to have relatively high B permeability. We propose that tolerance to high soil B is mediated by reduced expression of HvNIP2;1 to limit B uptake, as well as by increased expression of Bot1 to remove B from roots and sensitive tissues. Together with Bot1, the multifunctional aquaporin HvNIP2;1 is an important determinant of B toxicity tolerance in barley.

Identification and functional characterisation of aquaporins in the grapevine, Vitis vinifera

Plant aquaporins belong to a large superfamily of conserved proteins called the major intrinsic proteins (MIPs). There is limited information about the diversity of MIPs in grapevine, and their water transport capacity. The aim of the present study was to identify MIPs from grapevine and functionally characterise water transport of a subset of MIPs. Candidate genes were identified, by screening a Vitis vinifera L. (cv. Cabernet Sauvignon) cDNA library with gene specific probes, for aquaporin cDNAs encoding members of the plasma membrane intrinsic protein (PIP) and tonoplast intrinsic protein (TIP) subfamilies. The screen resulted in the identification of 11 full-length and two partial length aquaporin cDNAs. VvTIP2;1 isoforms had different 3' UTRs, immediately upstream of the poly(A) tail, suggesting the presence of multiple cleavage sites for polyadenylation. Using published genome sequences of grapevine, we conducted a phylogenetic analysis of the MIPs with previously characterised MIPs from Arabidopsis. We identified 23 full-length MIP genes from the V. vinifera genome sequence of a near homozygous line (PN40024) that cluster into the four main subfamilies (and subgroups within) identified in other species. However, based on the identification of PIP2 genes in Cabernet Sauvignon that were not present in the PN40024 genome, there are likely to be more than 23 MIP genes in other heterozygous grapevine cultivars. Water transport capacity was determined for several PIPs and TIPs, by expression in Xenopus oocytes. Only VvPIP2 and VvTIP proteins function as water channels with the exception of VvPIP2;5. VvPIP2;5 differs from the water conducting VvPIP2;1 by the substitution of two highly conserved amino acids in Loop B (G97S, G100W), which was shown by homology modelling to likely form a hydrophobic block of the water pore.

Expression and functional characterization of NPA motif-null aquaporin-1 mutations

The asparagine-proline-alanine sequences (NPA motifs) in Loops B and E of aquaporin are highly conserved. To investigate the role of two NPA motifs in the structure and function of aquaporin water channels, we generated human aquaporins (AQP)-1 mutations with NPA1 deletion, NPA2 deletion and NPA1,2 double deletion. Immunoblotting and immunofluorescence analysis indicated that all the three human AQP1 mutants possessed identical protein pattern and similar plasma membrane expression pattern compared to wild-type AQP1. Plasma membrane osmotic water permeability analysis, measured by YFP-based fluorescence quenching method and Xenopus oocyte expression assays, demonstrated that NPA1 or NPA2 deletion significantly reduced human AQP1 water permeability nearly 50% compared to wild-type AQP1, while NPA1,2 double deletion had little effect on human AQP1 water permeability. These results provide evidence that NPA motifs are important for water permeation but not essential for the expression, intracellular processing and the basic structure of human aquaporin 1.

PoreWalker: a novel tool for the identification and characterization of channels in transmembrane proteins from their three-dimensional structure

Transmembrane channel proteins play pivotal roles in maintaining the homeostasis and responsiveness of cells and the cross-membrane electrochemical gradient by mediating the transport of ions and molecules through biological membranes. Therefore, computational methods which, given a set of 3D coordinates, can automatically identify and describe channels in transmembrane proteins are key tools to provide insights into how they function.

Herein we present PoreWalker, a fully automated method, which detects and fully characterises channels in transmembrane proteins from their 3D structures. A stepwise procedure is followed in which the pore centre and pore axis are first identified and optimised using geometric criteria, and then the biggest and longest cavity through the channel is detected. Finally, pore features, including diameter profiles, pore-lining residues, size, shape and regularity of the pore are calculated, providing a quantitative and visual characterization of the channel. To illustrate the use of this tool, the method was applied to several structures of transmembrane channel proteins and was able to identify shape/size/residue features representative of specific channel families. The software is available as a web-based resource at http://www.ebi.ac.uk/thornton-srv/software/PoreWalker/.

Transmembrane channel proteins are responsible for the transport of ions and molecules through biological membranes and are pivotal for the physiology of the cell. In fact, their incorrect functioning is involved or related to several diseases (diabetes, myotonia, Parkinson's disease, etc.). Moreover, their specificity and selectivity to different ions or molecules have been hypothesized and sometimes shown to strongly depend on the shape and size or amino acid composition of the channel. Therefore, computational methods to identify and quantitatively characterise channel geometry in transmembrane protein structures are key tools to better understand how they function. We have developed PoreWalker, a new method to detect and describe the geometry of these channels in transmembrane proteins from their 3D structures. The method is fully automated, very user-friendly, identifies the location of the channel and derives a number of channel features: diameter profiles at given heights along the channel, all the residues lining the channel walls, size, shape and regularity of the channel. These features can be very helpful in the study of how these channels might function. We have applied PoreWalker to several channel protein structures and were able to identify shape/size/residue features that were representative of specific channel families.

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.

Protozoan parasite aquaporins

Protozoan parasites are a major threat to human health with millions of fatalities worldwide, especially in nonindustrialized countries. Currently, there is no cure for many of these parasitic diseases. Consequently, there is an imperative to find treatment targets and develop novel drugs based on the proteins encoded in the genomes of these parasites. Aquaporins, members of membrane proteins discovered and characterized within the past 20 years, are the mechanism through which water is transported through living membranes. The presence of aquaporins explains disease etiology related to water physiology and presents new pharmacogenomic targets. In this article, we review the literature on aquaporins found in Apicomplexan, Kinetoplastida and Microsporidia parasites as potential drug targets. Furthermore, by analyzing protein motion dynamics, we identify impediments that need to be surmounted for developing effective drugs targeting the aquaglyceroporin of Plasmodium falciparum, the causative agent of the most fatal form of human malaria.

pH regulated anion permeability of aquaporin-6

The kidney is a model organ for transport physiology (Nielsen 1996). AQPs are well-characterized in mammalian kidneys, where they facilitate transepithelial water reabsorption. Most renal AQPs are expressed either in proximal tubule cells or in collecting duct principal cells, which are known as sites for water reabsorption. AQP1 is present in both apical and basolateral membranes of proximal tubules, and in descending limbs of Henle's loop where 70% of filtrated water is isoosmotically reabsorbed (King and Agre 1996). AQP2 is expressed in principal cells of the collecting duct; in response to vasopressin, AQP2 translocates from intracellular vesicles to the apical plasma membranes, thereby increasing water permeability to concentrate urine (Nielsen et al. 1993, 1995; Knepper 1997; Schrier 2006). AQP3 and AQP4 reside in the basolateral membranes of collecting duct principal cells, where they may provide the exit pathways for urine. AQP7, AQP8, and AQP11 are also present in the proximal tubules (Nielsen et al. 1998).A rat cDNA clone encoding AQP6 was isolated by PCR-based homologous cloning from a rat kidney cDNA library (Ma et al. 1993; Yasui et al. 1999). AQP6 has high sequence homology to AQP0, AQP2, and AQP5. A human AQP6 was also cloned (Ma et al. 1996). Interestingly, the genes encoding AQP2, AQP5, and AQP6 are mapped to chromosome band 12q13 as a family gene cluster at this locus (Ma et al. 1997). Nevertheless, AQP6 is distinct from AQP0, AQP2, and AQP5 in terms of function. Among the renal aquaporins mentioned above, AQP6 has a unique distribution and a distinct function.

Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts

Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.

Projection structure of a member of the amino acid/polyamine/organocation transporter superfamily

The L-arginine/agmatine antiporter AdiC is a key component of the arginine-dependent extreme acid resistance system of Escherichia coli. Phylogenetic analysis indicated that AdiC belongs to the amino acid/polyamine/organocation (APC) transporter superfamily having sequence identities of 15-17% to eukaryotic and human APC transporters. For functional and structural characterization, we cloned, overexpressed, and purified wild-type AdiC and the point mutant AdiC-W293L, which is unable to bind and consequently transport L-arginine. Purified detergent-solubilized AdiC particles were dimeric. Reconstitution experiments yielded two-dimensional crystals of AdiC-W293L diffracting beyond 6 angstroms resolution from which we determined the projection structure at 6.5 angstroms resolution. The projection map showed 10-12 density peaks per monomer and suggested mainly tilted helices with the exception of one distinct perpendicular membrane spanning alpha-helix. Comparison of AdiC-W293L with the projection map of the oxalate/formate antiporter from Oxalobacter formigenes, a member from the major facilitator superfamily, indicated different structures. Thus, two-dimensional crystals of AdiC-W293L yielded the first detailed view of a transport protein from the APC superfamily at sub-nanometer resolution.

AQP1 expression analysis in human diseases: implications for proteomic characterization

Aquaporin (AQP)1 belongs to a ubiquitous family of water channel proteins characterized by sequence similarity and the presence of two NPA (Asp-Pro-Ala) motifs existing in almost all organs and tissues. Currently, 13 human AQPs are known and they are divided into two subgroups according to their ability to transport only water molecules, such as AQP1, or also glycerol and other small solutes. The genomic, structural and functional aspects of AQP1 are briefly described. An in-depth discussion is devoted to proteomic approaches that are useful for identifying and characterizing AQP1, mainly through electrophoretic techniques combined with different extraction procedures followed by mass spectrometry analysis. Moreover, the relevance of AQP1 in human diseases is also explained. Its role in human tumors and, in particular, those of the kidney (e.g., clear cell renal carcinoma) is discussed.

Localization of a Drosophila DRIP-like aquaporin in the malpighian tubules of the house cricket, Acheta domesticus

Malpighian tubules (Mt) are the primary excretory and osmoregulatory organs of insects, capable of rapidly transporting extraordinary volumes of fluid when stimulated by diuretic factors. In the house cricket, Acheta domesticus, the Mt are composed of three morphologically distinct regions (proximal, mid, and distal). Unlike the dipteran Mt, which have both primary and stellate cells, each region of the Acheta Mt consists of a morphologically uniform cell type. The mid and distal regions are both secretory in function and increase secretion rate in response to dibutyryl cAMP (cAMP). Achetakinin-2, while acting synergistically with cAMP on the mid-Mt, inhibits secretion by the distal Mt, and the effects can be reversed by cAMP. Using an antibody to the water-specific Drosophila aquaporin (DRIP), we demonstrated that DRIP-like immunoreactivity was found in both the distal and mid-Mt. The distribution of the aquaporin altered in response to stimulation and was consistent with the secretory data. The regulation of secretion in Acheta Mt is quite different from that of Drosophila, with both cation and anion/water transport occurring in the same cells. This is the first demonstration of the presence of an insect aquaporin, namely DRIP, in the Mt of an order other than the Diptera.

Aquaporin 7 is a beta-cell protein and regulator of intraislet glycerol content and glycerol kinase activity, beta-cell mass, and insulin production and secretion

To investigate if intracellular glycerol content plays a role in the regulation of insulin secretion in pancreatic beta cells, we studied the expression of the glycerol channels, or aquaglyceroporins, encoded by the aquaporin 3 (Aqp3), Aqp7, and Aqp9 genes in mouse islets. We found expression of Aqp7 only, not that of Aqp3 or Aqp9, in the endocrine pancreas at both the mRNA (by reverse transcription-PCR) and protein (by immunohistochemistry) levels. Immunohistochemistry revealed a complete overlap between insulin and Aqp7 immunostaining in the pancreatic islet. Inactivation of Aqp7 by gene targeting produced viable and healthy mice. Aqp7-/- mice harbored an increased intraislet glycerol concentration with a concomitant increase of the glycerol kinase transcript level and enzyme activity. The islet triglyceride content in the Aqp7-/- mice was also increased compared to that in the Aqp7+/+ mice. Interestingly, Aqp7-/- mice displayed reduced beta-cell mass and insulin content but increased insulin-1 and insulin-2 mRNAs. The reduction of beta-cell mass in Aqp7-/- mice can be explained at least in part by a reduction in cell proliferation through protein kinase C and the c-myc cascade, with a reduction in the transcript levels of these two genes. Concomitantly, there was a decreased rate of apoptosis, as reflected by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling and caspase 3 and Bax expression in Aqp7-/- mice. Compared with Aqp7+/+ islets, islets isolated from Aqp7-/- mice secreted insulin at a higher rate under basal low-glucose conditions and on exposure to a high (450 mg/dl) glucose concentration. Aqp7-/- mice exhibited normal fasting blood glucose levels but elevated blood insulin levels. Their plasma glucose response to an intraperitoneal (i.p.) glucose tolerance test was normal, but their plasma insulin concentrations were higher than those of wild-type mice during the 2-h test. An i.p. insulin tolerance test showed similar plasma glucose lowering in Aqp7-/- and Aqp7+/+ mice, with no evidence of insulin resistance. In conclusion, we found that pancreatic beta cells express AQP7, which appears to be a key regulator of intraislet glycerol content as well as insulin production and secretion.

Immunohistochemical localization of aquaporins in the human inner ear

We report the immunolocalization of aquaporins (AQPs) 1, 4, and 6 in the human auditory and vestibular endorgans. A rapid protocol was applied to audiovestibular endorgans microdissected from postmortem human temporal bones from six subjects (ages ranging from 75 to 97 years) with no history of audiovestibular disease. Temporal bones were fixed in formalin, and the endorgans were immediately microdissected. Cryostat sections were obtained from audiovestibular endorgans and were subjected to double-immunohistochemical staining with antibodies against AQPs and several cellular markers. In the human cochlea, AQP1 immunoreactivity was localized to the fibrocytes of the spiral ligament and the sub-basilar tympanic cells; AQP4 immunoreactivity was localized to the outer sulcus cells, Hensen's cells, and Claudius' cells; AQP6 immunoreactivity was localized to the apical portion of interdental cells in the spiral limbus. In the vestibular endorgans (macula utriculi and cristae), AQP1 was localized to fibrocytes and blood vessels of the underlying stroma and trabecular perilymphatic tissue; AQP4 immunoreactivity was localized to the basal pole of vestibular supporting cells; AQP6 was localized to the apical portion of vestibular supporting cells. Cochlear and vestibular hair cells and nerve fibers were not immunoreactive for any AQP. Supporting cells were identified with antibodies against glial fibrilar acidic protein. Nerve fibers and terminals were identified with antibodies against neurofilaments and Na(+)K(+)ATPase. The high degree of conservation of AQP expression in the human inner ear suggests that AQPs play a critical role in inner ear water homeostasis.

Proteomic knowledge of human aquaporins

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

Induction of aquaporin 1 by dexamethasone in lipid rafts in immortalized brain microvascular endothelial cells

Water homeostasis in the brain is essential for brain function. We have studied how aquaporin (AQP) 1 expression in GP8 immortalized rat brain microvascular endothelial cells is regulated by glucocorticoid. AQP1 protein level was raised by dexamethasone treatment in a time- and concentration-dependent manner. The up-regulation of AQP1 protein by dexamethasone was associated with an increase of AQP1 mRNA level, with no change in the degradation rate of AQP1 mRNA. AQP1 was concentrated in detergent-insoluble fractions in the cells treated with or without dexamethasone, suggesting that function/trafficking of AQP1 may be regulated via the interaction with lipid rafts. Since glucocorticoid therapy has well known beneficial effects in the treatment of brain edema, the induction of AQP1 by dexamethasone raises a possibility that AQP1 plays a role in ameliorating brain edema.

Aquaporins and Meniere's disease

PURPOSE OF REVIEW

Review of the role of aquaporins in inner ear homeostasis and potential role in the pathogenesis of Meniere's disease.

RECENT FINDINGS

Recent findings include the immunolocalization of aquaporins in the inner ear of mouse, rat, and human to cell types that are likely to undergo high ionic perturbances (e.g. potassium flux) and to putative areas of endolymph resorption or cycling.

SUMMARY

The expression of aquaporins and related proteins in the human cochlea and vestibular periphery resembles the distribution found in animal models, suggesting a critical role of aquaporins in inner ear water homeostasis and their potential role in the pathogenesis of Meniere's disease.

Function of aquaporins in female and male reproductive systems

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