Water channel proteins in the inner ear and their link to hearing impairment and deafness

The inner ear is a fluid-filled sensory organ that transforms mechanical stimuli into the senses of hearing and balance. These neurosensory functions depend on the strict regulation of the volume of the two major extracellular fluid domains of the inner ear, the perilymph and the endolymph. Water channel proteins, or aquaporins (AQPs), are molecular candidates for the precise regulation of perilymph and endolymph volume. Eight AQP subtypes have been identified in the membranous labyrinth of the inner ear. Similar AQP subtypes are also expressed in the kidney, where they function in whole-body water regulation. In the inner ear, AQP subtypes are ubiquitously expressed in distinct cell types, suggesting that AQPs have an important physiological role in the volume regulation of perilymph and endolymph. Furthermore, disturbed AQP function may have pathophysiological relevance and may turn AQPs into therapeutic targets for the treatment of inner ear diseases. In this review, we present the currently available knowledge regarding the expression and function of AQPs in the inner ear. We give special consideration to AQP subtypes AQP2, AQP4 and AQP5, which have been studied most extensively. The potential functions of AQP2 and AQP5 in the resorption and secretion of endolymph and of AQP4 in the equilibration of cell volume are described. The pathophysiological implications of these AQP subtypes for inner ear diseases, that appear to involve impaired fluid regulation, such as Menière's disease and Sj?gren's syndrome, are discussed.     

Structure, function and translational relevance of aquaporin dual water and ion channels

Aquaporins have been assumed to be selective for water alone, and aquaglyceroporins are accepted as carrying water and small uncharged solutes including glycerol. This review presents an expanded view of aquaporins as channels with more complex mechanisms of regulation and diverse repertoires of substrate permeabilities than were originally appreciated in the early establishment of the field. The role of aquaporins as dual water and gated ion channels is likely to have physiological and potentially translational relevance, and can be evaluated with newly developed molecular and pharmacological tools. Ion channel activity has been shown for Aquaporins -0, -1, and -6, Drosphila Big Brain, and plant Nodulin-26. Although the concept of ion channel function in aquaporins remains controversial, research advances are beginning to define not only the ion channel function but also the detailed molecular mechanisms that govern and mediate the multifunctional capabilities. With regard to physiological relevance, the adaptive benefit of expression of ion channel activity in aquaporins, implied by amino acid sequence conservation of the ion channel gating domains, suggests they provide more than water or glycerol and solute transport. Dual ion and water channels are of interest for understanding the modulation of transmembrane fluid gradients, volume regulation, and possible signal transduction in tissues expressing classes of aquaporins that have the dual function capability. Other aquaporin classes might be found in future work to have ion channel activities, pending identification of the possible signaling pathways that could govern activation.     

Combined transport of water and ions through membrane channels

The coupling of ion and water flow through membrane channels is under dispute. Among all human aquaporins only aquaporin-6 exhibits ion channel activity. Whether aquaporin-6 functions also as a water channel cannot yet be determined with confidence. Similarly, a comparison of single-channel water permeabilities from ion channels and aquaporins suggests that ion channels may play a secondary role as water channels. However, the fraction of absorbed fluid that crosses epithelial ion channels still remains to be determined.     

EphB2 guides axons at the midline and is necessary for normal vestibular function

Mice lacking the EphB2 receptor tyrosine kinase display a cell-autonomous, strain-specific circling behavior that is associated with vestibular phenotypes. In mutant embryos, the contralateral inner ear efferent growth cones exhibit inappropriate pathway selection at the midline, while in mutant adults, the endolymph-filled lumen of the semicircular canals is severely reduced. EphB2 is expressed in the endolymph-producing dark cells in the inner ear epithelium, and these cells show ultrastructural defects in the mutants. A molecular link to fluid regulation is provided by demonstrating that PDZ domain-containing proteins that bind the C termini of EphB2 and B-ephrins can also recognize the cytoplasmic tails of anion exchangers and aquaporins. This suggests EphB2 may regulate ionic homeostasis and endolymph fluid production through macromolecular associations with membrane channels that transport chloride, bicarbonate, and water.     

Impaired hearing in mice lacking aquaporin-4 water channels.

A role for aquaporins (AQPs) in hearing has been suggested from the specific expression of aquaporins in inner ear and the need for precise volume regulation in epithelial cells involved in acoustic signal transduction. Using mice deficient in selected aquaporins as controls, we localized AQP1 in fibrocytes in the spiral ligament and AQP4 in supporting epithelial cells (Hensen's, Claudius, and inner sulcus cells) in the organ of Corti. To determine whether aquaporins play a role in hearing, auditory brain stem response (ABR) thresholds were compared in wild-type mice and transgenic null mice lacking (individually) AQP1, AQP3, AQP4, and AQP5. In 4-5-week-old mice in a CD1 genetic background, ABR thresholds in response to a click stimulus were remarkably increased by >12 db in AQP4 null mice compared with wild-type mice (p < 0.001), whereas ABR thresholds were not affected by AQP1, AQP3, or AQP5 deletion. In a C57/bl6 background, nearly all AQP4 null mice were deaf, whereas ABRs could be elicited in wild-type controls. ABRs in AQP4 null CD1 mice measured in response to tone bursts (4-20 kHz) indicated a frequency-independent hearing deficit. Light microscopy showed no differences in cochlear morphology of wild-type versus AQP4 null mice. These results provide the first direct evidence that an aquaporin water channel plays a role in hearing. AQP4 may facilitate rapid osmotic equilibration in epithelial cells in the organ of Corti, which are subject to large K(+) fluxes during mechano-electric signal transduction.     

Developmental regulation of water uptake in wheat

The discovery of aquaporins has provided a new basis for studying and interpreting water relations in plants. However, slow progress has been made in elucidating the functional facets of the aquaporin-mediated water pathway in whole plant systems. While increasing experimental evidence suggests that these proteins are directly involved in mediating water homeostasis at varying environmental conditions, only a few attempts have been made to understand their contribution to overall water transport at different developmental stages. By using a chemical inhibitor (HgCl(2)) of aquaporins function, here we present in planta evidence for both diurnal and developmental regulation of aquaporin activity in wheat. We demonstrate that the greatest sensitivity of water flux to pharmacological blockage occurs at the stage of ear emergence and does not coincide with the phenological stage at which the greatest plant water uptake occurs (milky ripeness). The relationship transpiration flux (Q) vs. soil-leaf water potential difference (DeltaPsi(soil-leaves)) revealed a gradual decrease of plant resistance to water flux from tillering to milky ripeness, both in HgCl(2)-treated and untreated control plants. However, the mercury-inhibition of water flux began to gradually increase at ear emergence, suggesting that a larger portion of water moves through aquaporins from this developmental stage on. Although the intercept of the DeltaPsi(soil-leaves)/Q regression line, i.e. the DeltaPsi required to initiate the water flux through the soil-plant-air continuum, was generally not affected by mercury treatment, a significant mercury effect on the intercept was observed at the stage of ear formation. These findings may have important implications for predicting which strategy plants utilize to optimize water use during their life cycle.     

Aquaporin molecular characterization in the sea-bass (Dicentrarchus labrax): the effect of salinity on AQP1 and AQP3 expression

Euryhaline fish possess the ability to compensate for environmental salinity changes through hydro-mineral regulation. A number of proteins have been studied in order to understand water and ion exchanges, known as fish osmoregulation. Sea-bass (Dicentrarchus labrax) cDNA sequences encoding a homologue of mammalian aquaporin (termed AQP1) and a homologue of mammalian aquaglyceroporin (termed AQP3) have been isolated and sequenced. The aquaporin amino acid sequences share respectively more than 60% and 65% identity with other known aquaporins. We have shown that salinity influences aquaporin expression levels in the gill, kidney and digestive tract, the main osmoregulatory organs. AQP1 may have a major osmoregulatory role in water transport in kidney and gut in SW-acclimated fish, whereas AQP3 could be implicated in gill water transport in FW-acclimated fish.     

The endolymphatic sac: its roles in the inner ear

The endolymphatic sac is a non-sensory organ of the inner ear. It is connected to the endolymphatic compartment that is filled with endolymph, a potassium-rich fluid that bathes the apical side of inner ear sensory cells. The main functions ascribed to the endolymphatic sac are the regulation of the volume and pressure of endolymph, the immune response of the inner ear, and the elimination of endolymphatic waste products by phagocytosis. Functional alteration of these functions, leading to deficient endolymph homeostasis and/or altered inner ear immune response, may participate to the pathophysiology of Ménière's disease, an inner ear pathology that causes episodes of vertigo, sensorineural hearing loss and tinnitus, and is characterized by an increase in volume of the cochleo-vestibular endolymph (endolymphatic hydrops).     

Renal aquaporins and sodium transporters with special focus on urinary tract obstruction

The discovery of aquaporin-1 (AQP1) by Agre and colleagues explained the long-standing biophysical question of how water specifically crosses biological membranes. These studies led to the discovery and identification of a whole new family of membrane proteins, the aquaporins. At present, at least seven aquaporins are expressed at distinct sites in the kidney and 4 members of this family (AQP1-4) have been demonstrated to play pivotal roles in the physiology and pathophysiology for renal regulation of body water balance. Osmotic equilibration via renal aquaporins is maintained by active transport of NaCl. The major sodium transporters and channels in the individual renal tubule segments have been identified and the regulation of these transporters and channels are fundamental for renal sodium reabsorption and for establishing the driving force. In this mini-review the role of renal aquaporins and sodium transporters and channels is briefly described and their key role for the impaired urinary concentrating capacity in response to urinary tract obstruction is reviewed. Thus this review updates previous detailed reviews (1-5).     

水曲柳幼苗根系在不同浓度NH4NO3溶液中水流导度的变化

水分吸收过程是根系重要的生理过程。水孔蛋白在根系水分径向运输中起着重要的作用,根系水流导度(L_p)的测定是研究水孔蛋白的重要途径。该研究采用压力流的方法,对相同生长条件下的水曲柳(Fraxinus mandshurica)幼苗根系进行研究,测定了根系在去离子水和不同浓度NH₄NO₃溶液中的L_p。结果表明:未经处理的水曲柳幼苗根系,L_p随NH₄NO₃浓度的增加而上升,而且NH₄NO₃溶液中的L_p比去离子水中的L_p平均高77%;经HgCl₂处理后,水曲柳幼苗根系的L_p仍然随NH₄NO₃浓度的增加而增大,但是根系L_p在去离子水下降了22%,而在NH₄NO₃溶液中下降了68%,与以前的研究相比发现,经HgCl₂处理后,以营养液为吸水基质的根系L_p的降低值普遍高于以去离子水为基质的试验。因此,基质中养分离子的存在对根系中水孔蛋白活性产生了重要的影响,进而影响根系水分的吸收过程。     

Albumin-like protein is the major protein constituent of luminal fluid in the human endolymphatic sac

The endolymphatic sac (ES) is an inner ear organ that is connected to the cochleo-vestibular system through the endolymphatic duct. The luminal fluid of the ES contains a much higher concentration of proteins than any other compartment of the inner ear. This high protein concentration likely contributes to inner ear fluid volume regulation by creating an osmotic gradient between the ES lumen and the interstitial fluid. We characterized the protein profile of the ES luminal fluid of patients (n = 11) with enlarged vestibular aqueducts (EVA) by proteomics. In addition, we investigated differences in the protein profiles between patients with recent hearing deterioration and patients without hearing deterioration. The mean total protein concentration of the luminal fluid was 554.7±94.6 mg/dl. A total of 58 out of 517 spots detected by 2-DE were analyzed by MALDI-TOF MS. The protein profile of the luminal fluid was different from the profile of plasma. Proteins identified from 29 of the spots were also present in the MARC-filtered human plasma; however, the proteins identified from the other 25 spots were not detected in the MARC-filtered human plasma. The most abundant protein in the luminal fluid was albumin-like proteins, but most of them were not detected in MARC-filtered human plasma. The concentration of albumin-like proteins was higher in samples from patients without recent hearing deterioration than in patients with recent hearing deterioration. Consequently, the protein of ES luminal fluid is likely to be originated from both the plasma and the inner ear and considering that inner ear fluid volumes increase abnormally in patients with EVA following recent hearing deterioration, it is tempting to speculate that albumin-like proteins may be involved in the regulation of inner ear fluid volume through creation of an osmotic gradient during pathological conditions such as endolymphatic hydrops.     

Why do microorganisms have aquaporins?

Aquaporins are channel proteins that enhance the permeability of cell membranes for water. They have been found in Bacteria, Archaea and Eukaryotes. However, their absence in many microorganisms suggests that aquaporins do not fulfill a broad role such as turgor regulation or osmoadaptation but, instead, fulfill a role that enables microorganisms to have specific lifestyles. The recent discovery that aquaporins enhance cellular tolerance against rapid freezing suggests that they have ecological relevance. We have identified several examples of large-scale freeze-thawing of microbes in nature and we also draw attention to alternative lifestyle-related functions for aquaporins, which will be a focus of future research.     

Nano-environmental changes by KCNE proteins modify KCNQ channel function

The KCNQ1 channel is a voltage-dependent potassium channel, which is widely expressed in various tissues of the human body including heart, inner ear, intestine, kidney and pancreas. The ion channel properties of KCNQ1 change remarkably when auxiliary subunit KCNE proteins co-exist. The mechanisms of KCNQ1 channel regulation by KCNE proteins are of longstanding interest but are still far from being fully understood. The pore region (S5-S6 segments) of KCNQ1 is thought to be the main interaction site for KCNE proteins. However, some recent reports showed that the voltage-sensing domain (S1-S4 segments) is critically involved in the regulation of KCNQ1 by KCNE proteins. In addition, we recently re-examined the stoichiometry of the KCNQ1-KCNE1 complex and found that the stoichiometry is not fixed but rather flexible and the KCNQ1 channel can have up to four associated KCNE1 proteins. We will review these recent findings concerning the mechanisms of KCNQ1 regulation by KCNE proteins.     

Nano-environmental changes by KCNE proteins modify KCNQ channel function

The KCNQ1 channel is a voltage-dependent potassium channel, which is widely expressed in various tissues of the human body including heart, inner ear, intestine, kidney and pancreas. The ion channel properties of KCNQ1 change remarkably when auxiliary subunit KCNE proteins co-exist. The mechanisms of KCNQ1 channel regulation by KCNE proteins are of longstanding interest but are still far from being fully understood. The pore region (S5-S6 segments) of KCNQ1 is thought to be the main interaction site for KCNE proteins. However, some recent reports showed that the voltage-sensing domain (S1-S4 segments) is critically involved in the regulation of KCNQ1 by KCNE proteins. In addition, we recently re-examined the stoichiometry of the KCNQ1-KCNE1 complex and found that the stoichiometry is not fixed but rather flexible and the KCNQ1 channel can have up to four associated KCNE1 proteins. We will review these recent findings concerning the mechanisms of KCNQ1 regulation by KCNE proteins.     

Ion transporting proteins of human bronchial epithelium

The electrolyte transport system across human airway epithelium followed by water movement is essential for the normal mucociliary clearance that allows the maintenance of the aseptic condition of the respiratory tract. The function of epithelial cells is to control and regulate ionic composition and volume of fluids in the airways. Various types of proteins taking part in assuring effective ions and water transport in apical and basolateral membranes of the airway epithelium have been found (e.g., CFTR, ENaC, CaCC, ORCC, potassium channels, NaKATPase, aquaporins). The paper reviews the current state of the art in the field of ion channels, transporters, and other signaling proteins identified in the human bronchial epithelium.     

Possible functional implications of aquaporin water channels in reproductive physiology and medically assisted procreation.

Spermatogenesis, the maturation of spermatozoa and their concentration and storage in the seminiferous vessels are associated with considerable fluid secretion or absorption in the male reproductive tract. These fluid movements are in total agreement with the presence of multiple aquaporin (AQP) water channel proteins in germ cells and other tissues within the male reproductive tract. A series of functions of prime importance have already been hypothesized for aquaporins in the physiology of male reproduction. Aquaporins could be involved in the early stages of spermatogenesis, in the secretion of tubular liquid and in the concentration and storage of spermatozoa in the epididymis. In the male reproductive tract, alterations in the expression and functionality and/or regulation of aquaporins have already been demonstrated to be at the basis of forms of male infertility. Indeed, rats with reduced reabsorption of seminiferous fluid in the efferent ducts have been shown to be sub-fertile or infertile. Functions have also been suggested in the fertilization process, where aquaporins may play a role in maintaining osmotic homeostasis in gametes during fertilization. Aquaporins have also been suggested to mediate water movement into antral follicles and to be the pathway for transtrophectodermal water movement during cavitation. Aquaporins are the subject of considerable technological interest for cryopreservation used in medically assisted procreation, as they could be the molecular pathway by which water and/or solutes move across the plasma membrane during the process of freezing/thawing gametes and embryos. Indeed, artificial expression ofAQP3 has been showed to improve the survival of mouse oocytes after cryopreservation.     

Aquaporin Membrane Channels: Biophysics, Classification, Functions, and Possible Biotechnological Applications

Water represents the major component of all living organisms. To make the cell to adapt to the surrounding environment and carry out its biological functions, water has to move into and out of the cell interior driven by osmotic forces. For over a century, scientists studying the movement of fluid into and out of the cell struggled with a difficult biophysical question: how does water pass through the cell? Movement of water across the membrane was indicated almost as soon as the lipid bilayer was recognized as being the plasma membrane of cells, back in the 1920s. Simple diffusion of water across the lipid bilayer occurs through all biological membranes. However, its low velocity and finite extent soon became apparent, suggesting the existence of additional pathways for water moving through the membrane. In spite of the enormous amount of work carried out in this area, the precise and complete answer only came relatively recently with the discovery of aquaporins, transmembrane channel proteins making the membrane tenfold to 100-fold more permeable to water than membranes lacking such pores. The water conductance featured by the aquaporins is astonishing: in fact, each single aquaporin pore can conduct billions of water molecules per second. A branch of the aquaporin family, the aquaglyceroporins, features conductance to small neutral solutes in addition to water. This review summarizes recent updates on the molecular structure, regulation, biophysics, and biological functions of aquaporins. Possible biotechnological applications of aquaporins are also described.     

Functions of water channels in male and female reproductive systems

Twelve water channels (aquaporins) are expressed in mammalian reproductive systems, and play very important roles in maintaining water homeostasis in reproductive cells. Impairment of their functions can result in attenuated male and female fertility. Alteration of AQPs expression is also found in reproductive tissues of the patients with polycystic ovarian syndrome, endometriosis or endometrium carcinoma. A lot of data have increased understanding of the functions and mechanisms of regulation of aquaporins at both the molecular and the clinical level. Researches have also focused on aquaporins as therapeutic targets. This review discusses recent advances in uncovering the physiological and pathophysiological roles of aquaporins in the reproductive systems.     

Advances in functional regulation mechanisms of plant aquaporins: Their diversity,gene expression,localization, structure and roles in plant soil-water relations (Review)

Aquaporins are important molecules that control the moisture level of cells and water flow in plants. Plant aquaporins are present in various tissues, and play roles in water transport, cell differentiation and cell enlargement involved in plant growth and water relations. The insights into aquaporins’ diversity, structure, expression, post-translational modification, permeability properties, subcellular location, etc., from considerable studies, can lead to an understanding of basic features of the water transport mechanism and increased illumination into plant water relations. Recent important advances in determining the structure and activity of different aquaporins give further details on the mechanism of functional regulation. Therefore, the current paper mainly focuses on aquaporin structure-function relationships, in order to understand the function and regulation of aquaporins at the cellular level and in the whole plant subjected to various environmental conditions. As a result, the straightforward view is that most aquaporins in plants are to regulate water flow mainly at cellular scale, which is the most widespread general interpretation of the physiological and functional assays in plants.     

The role of aquaporins in hearing function and dysfunction

The inner ear is composed by tiny and complex structures that, together with peripheral and central auditory pathways, are responsible for hearing processing. However, not only the anatomy of the cochlea, its compartments and related structures are complex. The mechanisms involved in the regulation of homeostasis in the inner ear fluid, which determines the ionic gradient necessary for hearing and balancing sensory excitability, is an intricate phenomenon that involves several molecules. Among them, Aquaporins (AQP) play a significant role in this process. AQP are part of a family of small, integral membrane proteins that regulate different processes, including bidirectional water and ionic flow in the inner ear. Changes in the expression of these proteins are essential to auditory physiology and several pathophysiological processes in the inner ear. This review focuses on the role of AQP in health and disease of the auditory system.