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.     

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.     

What did Morganucodon hear?

The structure of the middle and inner ear of Morganucodon , one of the oldest known mammals, is reviewed and compared to the structure of the ears of extant mammals, reptiles and birds with known auditory capabilities. Specifically, allometric relationships between ear dimensions (basilar-membrane length, tympanic-membrane area and stapes-footplate area) and specific features of the audiogram are defined in extant ears. These relationships are then used to make several predictions of auditory function in Morganucodon. The results point out that the ear structures of Morganucodon–Art similar in dimensions to ear structures in both extant small mammals–with predominantly high-frequency (10 kHz) auditory capabilities, and reptiles and birds- with better low and middle-frequency hearing (< 5 kHz). Although the allometric analysis cannot by itself determine whether Morganucodon heard more like present-day small mammals, or birds and reptiles, the apparent stiffness of the Morganucodon middle ear is both more consistent with the high-frequency mammalian middle ear and would act to decrease the sensitivity of a bird-reptile middle ear to low-frequency sound. Several likely hearing scenarios for Morganucodon are defined, including a scenario in which these animals had ears like those of modern small mammals that are selectively sensitive to high-frequency sounds, and a second scenario in which the Morganucodon ear was moderately sensitive to sounds of a narrow middle-frequency range (5–7 kHz) and relatively insensitive to sounds of higher or lower frequency. The evidence needed to substantiate either scenario includes some objective measure of the stiffness of the Morganucodon ossicular system, while a key datum needed to distinguish between the two hypotheses includes confirmation of the presence or absence of a cochlear lamina in the Morganucodon inner ear.     

How Transmembrane Inner Ear (TMIE) plays role in the auditory system: A mystery to us

Different cellular mechanisms contribute to the hearing sense, so it is obvious that any disruption in such processes leads to hearing impairment that greatly influences the global economy and quality of life of the patients and their relatives. In the past two decades, transmembrane inner ear (TMIE) protein has received a great deal of research interest because its impairments cause hereditary deafness in humans. This evolutionarily conserved membrane protein contributes to a fundamental complex that plays role in the maintenance and function of the sensory hair cells. Although the critical roles of the TMIE in mechanoelectrical transduction or hearing procedures have been discussed, there are little to no review papers summarizing the roles of the TMIE in the auditory system. In order to fill this gap, herein, we discuss the important roles of this protein in the auditory system including its role in mechanotransduction, olivocochlear synapse, morphology and different signalling pathways; we also review the genotype-phenotype correlation that can per se show the possible roles of this protein in the auditory system.     

Aquaporin-mediated fluid regulation in the inner ear

1. The sensory functions of the inner ear (hearing and balance) critically depend on the precise regulation of two fluid compartments of highly desparate ion composition, i.e., the endolymph and the perilymph.2. The parameters volume, ion composition, and pH need to be held athomeostasis irrespective of the hydration status of the total organism.3. Specific cellular water channels, aquaporins, have been shown to be essential for the fluid regulation of several organs, e.g., kidney, lung, and brain.4. Because of functional similarities of water regulation in the kidney and inner ear this review initially summarizes some aquaporin functions in the kidney and then focuses on 6 out of 11 mammalian aquaporins that are present in the inner ear (AQP1-6).5. Their potential role in the inner ear fluid control will be discussed on the basis of the respective expression patterns and individual pore properties.6. Further, a working model is presented of how the endolymphatic sac may contribute to inner ear fluid regulation.     

Sphingosine 1-phosphate-mediated activation of ezrin-radixin-moesin proteins contributes to cytoskeletal remodeling and changes of membrane properties in epithelial otic vesicle progenitors

Hearing loss is among the most prevalent sensory impairments in humans. Cochlear implantable devices represent the current therapies for hearing loss but have various shortcomings. ERM (ezrin- radixin -moesin) are a family of adaptor proteins that link plasma membrane with actin cytoskeleton, playing a crucial role in cell morphology and in the formation of membrane protrusions. Recently, bioactive sphingolipids have emerged as regulators of ERM proteins. Sphingosine 1-phosphate (S1P) is a pleiotropic sphingolipid which regulates fundamental cellular functions such as proliferation, survival, migration as well as processes such as development and inflammation mainly via ligation to its specific receptors S1PR (S1P_1–5). Experimental findings demonstrate a key role for S1P signaling axis in the maintenance of auditory function. Preservation of cellular junctions is a fundamental function both for S1P and ERM proteins, crucial for the maintenance of cochlear integrity. In the present work, S1P was found to activate ERM in a S1P₂-dependent manner in murine auditory epithelial progenitors US/VOT-E36. S1P-induced ERM activation potently contributed to actin cytoskeletal remodeling and to the appearance of ionic currents and membrane passive properties changes typical of more differentiated cells. Moreover, PKC and Akt activation was found to mediate S1P-induced ERM phosphorylation. The obtained findings contribute to demonstrate the role of S1P signaling pathway in inner ear biology and to disclose potential innovative therapeutical approaches in the field of hearing loss prevention and treatment.     

Correlations between auditory structures and hearing sensitivity in non‐human primates

Primates show distinctions in hearing sensitivity and auditory morphology that generally follow phylogenetic patterns. However, few previous studies have attempted to investigate how differences in primate hearing are directly related to differences in ear morphology. This research helps fill this void by exploring the form‐to‐function relationships of the auditory system in a phylogenetically broad sample of non‐human primates. Numerous structures from the outer, middle, and inner ears were measured in taxa with known hearing capabilities. The structures investigated include the overall size and shape of the pinna, the areas of the tympanic membrane and stapedial footplate, the masses and lever arm lengths of the ossicles, the volumes of the middle ear cavities, and the length of the cochlea. The results demonstrate that a variety of auditory structures show significant correlations with certain aspects of hearing (particularly low‐frequency sensitivity). Although the majority of these relationships agree with expectations from auditory theory, some traditional (and possibly outdated) ideas were not supported. For example, the common misconception that higher middle ear transformer ratios (e.g., impedance transformer ratio) result in increased hearing sensitivity was not supported. Although simple correlations between form and function do not necessarily imply causality, the relationships defined in this study not only increase our understanding of auditory patterns in extant taxa but also lay the foundation to begin investigating the hearing in fossil primates. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

All Akt Isoforms (Akt1, Akt2, Akt3) Are Involved in Normal Hearing,but Only Akt2 and Akt3 Are Involved in Auditory Hair Cell Survival in the Mammalian Inner Ear

The kinase Akt is a key downstream mediator of the phosphoinositide-3-kinase signaling pathway and participates in a variety of cellular processes. Akt comprises three isoforms each encoded by a separate gene. There is evidence to indicate that Akt is involved in the survival and protection of auditory hair cells in vitro. However, little is known about the physiological role of Akt in the inner ear—especially in the intact animal. To elucidate this issue, we first analyzed the mRNA expression of the three Akt isoforms in the inner ear of C57/BL6 mice by real-time PCR. Next, we tested the susceptibility to gentamicin-induced auditory hair cell loss in isoform-specific Akt knockout mice compared to wild-types (C57/BL6) in vitro. To analyze the effect of gene deletion in vivo, hearing and cochlear microanatomy were evaluated in Akt isoform knockout animals. In this study, we found that all three Akt isoforms are expressed in the cochlea. Our results further indicate that Akt2 and Akt3 enhance hair cell resistance to ototoxicity, while Akt1 does not. Finally, we determined that untreated Akt1 and Akt2/Akt3 double knockout mice display significant hearing loss, indicating a role for these isoforms in normal hearing. Taken together, our results indicate that each of the Akt isoforms plays a distinct role in the mammalian inner ear.     

STRUCTURE AND FUNCTION IN WHALE EARS

ABSTRACT

Ultrasonic echolocation abilities are well documented in several dolphin species, but hearing characteristics are unknown for most whales. Vocalization data suggest whale hearing spans infra- to ultrasonic ranges. This paper presents an overview of whale ear anatomy and analyzes 1) how whale ears are adapted for underwater hearing and 2) how inner ear differences relate to different hearing capacities among whales.

Whales have adaptations for rapid, deep diving and long submersion; e.g., broad- bore Eustachian tubes, no pinnae, and no air-filled external canals, that impact sound reception. In odontocetes, two soft tissue channels conduct sound to the ear. In mysticetes, bone and soft tissue conduction are likely. The middle ear is air-filled but has an extensible mucosa. Cochlear structures are hypertrophied and vestibular components are reduced. Auditory ganglion cell densities are double land mammal averages (2000–4000/mm). Basilar membrane lengths range 20–70 mm; gradients are larger than in terrestrial mammals. Odontocetes have 20–60% bony membrane support and basal ratios >0.6, consistent with hearing >150 kHz. Mysticetes have apical ratios <0.002 and no bony lateral support, implying acute infrasonic hearing. Cochlear hypertrophy may be adaptive for high background noise. Vestibular loss is consistent with cervical fusion. Exceptionally high auditory fiber counts suggest both mysticetes and odontocetes have ears “wired” for more complex signal processing mechanisms than most land mammals.     

Hearing impairment: a panoply of genes and functions

Research in the genetics of hearing and deafness has evolved rapidly over the past years, providing the molecular foundation for different aspects of the mechanism of hearing. Considered to be the most common sensory disorder, hearing impairment is genetically heterogeneous. The multitude of genes affected encode proteins associated with many different functions, encompassing overarching areas of research. These include, but are not limited to, developmental biology, cell biology, physiology, and neurobiology. In this review, we discuss the broad categories of genes involved in hearing and deafness. Particular attention is paid to a subgroup of genes associated with inner ear gene regulation, fluid homeostasis, junctional complex and tight junctions, synaptic transmission, and auditory pathways. Overall, studies in genetics have provided research scientists and clinicians with insight regarding practical implications for the hearing impaired, while heralding hope for future development of therapeutics.     

Inherited connexin mutations associated with hearing loss

One of the most dramatic discoveries in the field of hereditary hearing loss is the association of this sensory defect with connexin mutations. Most significant is the large proportion, 30-50%, of inherited hearing loss that is due to mutations in connexin 26. The proteins these genes encode are expressed in the cochlear duct, in regions containing gap junctions. Together, these findings suggest a crucial role for gap junction proteins in the mammalian inner ear. Mouse models with specific connexin mutations leading to deafness will help resolve the many questions regarding the role of these gap junction proteins in the inner ear.     

Preoperative computed tomography of temporal bones and its use in selecting patients with sensorineural deafness for cochlear implantation

Cochlear implantation (CI) involves the insertion of electrode systems in the inner ear in order to restore hearing in patients with sensorineural deafness. Positive CI results are closely related with careful selection of candidates. Preoperative computed tomography (CT) and its results are decisive in this selection. Temporal bone CT was made in 40 patients aged 1 to 57 years who had sensorineural deafness. The following changes were detected: malformations of the inner ear, the inner auditory canal, meningititis-induced cochlear obliteration, post-traumatic temporal bone changes, local auditory ossicle malformation, inflammatory middle ear changes, the high position of the jugular vein bulb, and its diverticula. Temporal bone CT plays an important role in selecting candidates for CI and circumvents potential difficulties and complications at CI.     

Six1基因与听觉系统发生及疾病的关系

同源异形框基因Six1与哺乳动物多种器官发育有关,近年来随着分子生物学研究技术的进步,研究表明Six1与听觉系统的发生、分化及疾病关系密切。本文主要综述Six1在耳发育中的作用和Six1与耳相关疾病的关系研究进展。     

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. The National Institutes of Health (grants AG09693-10, DC005224, 00140-02, and DC05187-01) supported this work.     

Molecular screens for inner ear genes

Identification of the genes that encode proteins that are important for proper function of specific inner ear cell types is central to our understanding of the molecular basis of hearing and balance. Whereas the combination of electrophysiology and biophysics has resulted in an exquisite understanding of inner ear function, little is known about the proteins that confer these properties at the cellular level. Furthermore, the genes that control inner ear development, susceptibility to wear and tear, regeneration from damage, and age-related degeneration, are largely unknown. This review discusses tools that have been developed during the past few years to address this imbalance between a thorough physiologic characterization of inner ear function and a detailed understanding at a molecular level of the proteins involved in these functions. Creation of inner ear cDNA libraries has laid the foundation for the discovery of genes that are specifically expressed by cell types of the inner ear and that encode proteins that are important for molecular processes in these cells. In conjunction with expressed sequence tag database analysis, cDNA subtraction, and DNA arrays, functionally important genes, whose specific expression patterns are usually verified by gene expression analysis, can be identified. Discussion of these techniques takes into account the specific characteristics of the inner ear in relation to its study using molecular biological approaches.     

Towards a molecular understanding of Drosophila hearing

The Drosophila auditory system is presented as a powerful new genetic model system for understanding the molecular aspects of development and physiology of hearing organs. The fly's ear resides in the antenna, with Johnston's organ serving as the mechanoreceptor. New approaches using electrophysiology and laser vibrometry have provided useful tools to apply to the study of mutations that disrupt hearing. The fundamental developmental processes that generate the peripheral nervous system are fairly well understood, although specific variations of these processes for chordotonal organs (CHO) and especially for Johnston's organ require more scrutiny. In contrast, even the fundamental physiologic workings of mechanosensitive systems are still poorly understood, but rapid recent progress is beginning to shed light. The identification and analysis of mutations that affect auditory function are summarized here, and prospects for the role of the Drosophila auditory system in understanding both insect and vertebrate hearing are discussed.     

Auditory rehabilitation

Auditory rehabilitation depends of the cause and the severity of the hearing loss (or deafness). Hearing losses dues to middle ear pathologies can beneficiate of medical or surgical treatments, by ossicular prostheses, if it is necessary to restore the function of the ossicles chain. In the sensorineural hearing losses, with inner ear pathology, the use of auditory aid is immediately considered. In the cases for which they are insufficient because of severity of the hearing loss or not suitable because of local non-tolerance, it is possible to use middle ear implant or cochlear implant. The indications of the auditory brainstern implants remain at this day limited to the total bilateral hearing losses due to a complete destruction of cochleae and auditory nerves. These therapeutic orientations are selected after a multidisciplinary evaluation of the deaf person, evaluation that allows the characterization of the hearing loss and its repercussion. In all the cases, the restoration of a bilateral hearing has to be done if possible, making an improvement of the speech comprehension, mainly in the noisy situations, as well as the localization of the sound sources.