Ca2+ signaling in the inner ear

Advances in the Study of Etiology and Molecular Mechanisms of Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL), a multifactorial progressive disorder, results from a complex interplay of genetic and environmental factors, with its underlying mechanisms remaining unclear. Several pathological factors are believed to contribute to SNHL, including genetic factors, ion homeostasis, cell apoptosis, immune inflammatory responses, oxidative stress, hormones, metabolic syndrome, human cytomegalovirus infection, mitochondrial damage, and impaired autophagy. These factors collectively interact and play significant roles in the onset and progression of SNHL. The present review offers a comprehensive overview of the various factors that contribute to SNHL, emphasizes recent developments in understanding its etiology, and explores relevant preventive and intervention measures.

Pharmacological Treatment of Acute Unilateral Vestibulopathy: A Review

There have been few investigations on the epidemiology, etiology, and medical management of acute unilateral vestibulopathy (AUV). Short-term pharmaceutical resolutions include vestibular symptomatic suppressants, anti-emetics, and some cause-based therapies. Anticholinergics, phenothiazines, antihistamines, antidopaminergics, benzodiazepines, and calcium channel antagonists are examples of vestibular suppressants. Some of these medications may show their effects through multiple mechanisms. In contrast, N-acetyl-L-leucine, Ginkgo biloba, and betahistine improve central vestibular compensation. Currently, AUV pathophysiology is poorly understood. Diverse hypotheses have previously been identified which have brought about some causal treatments presently used. According to some publications, acute administration of anti-inflammatory medications may have a deleterious impact on both post-lesional functional recovery and endogenous adaptive plasticity processes. Thus, some authors do not recommend the use of corticosteroids in AUV. Antivirals are even more contentious in the context of AUV treatment. Although vascular theories have been presented, no verified investigations employing anti-clotting or vasodilator medications have been conducted. There are no standardized treatment protocols for AUV to date, and the pharmacological treatment of AUV is still questionable. This review addresses the most current developments and controversies in AUV medical treatment.

Usher syndrome IIIA: a review of the disorder and preclinical research advances in therapeutic approaches

Usher syndrome (USH) is an autosomal recessive disorder characterized by sensorineural hearing loss, progressive pigmentary retinopathy, and vestibular dysfunction. The degree and onset of hearing loss vary among subtypes I, II, and III, while blindness often occurs in the second to fourth decades of life. Usher type III (USH3), characterized by postlingual progressive sensorineural hearing loss, varying levels of vestibular dysfunction, and varying degrees of visual impairment, typically manifests in the first to second decades of life. While USH3 is rare, it is highly prevalent in certain populations. RP61, USH3, and USH3A symbolize the same disorder, with the latter symbol used more frequently in recent literature. This review focuses on the clinical features, epidemiology, molecular genetics, treatment, and research advances for sensory deficits in USH3A.

ATP and ACh Evoked Calcium Transients in the Neonatal Mouse Cochlear and Vestibular Sensory Epithelia

Hair cells in the mammalian inner ear sensory epithelia are surrounded by supporting cells which are essential for function of cochlear and vestibular systems. In mice, support cells exhibit spontaneous intracellular Ca²⁺ transients in both auditory and vestibular organs during the first postnatal week before the onset of hearing. We recorded long lasting (>200 ms) Ca²⁺ transients in cochlear and vestibular support cells in neonatal mice using the genetic calcium indicator GCaMP5. Both cochlear and vestibular support cells exhibited spontaneous intracellular Ca²⁺ transients (GCaMP5 ΔF/F), in some cases propagating as waves from the apical (endolymph facing) to the basolateral surface with a speed of ∼25 μm per second, consistent with inositol trisphosphate dependent calcium induced calcium release (CICR). Acetylcholine evoked Ca²⁺ transients were observed in both inner border cells in the cochlea and vestibular support cells, with a larger change in GCaMP5 fluorescence in the vestibular support cells. Adenosine triphosphate evoked robust Ca²⁺ transients predominantly in the cochlear support cells that included Hensen’s cells, Deiters’ cells, inner hair cells, inner phalangeal cells and inner border cells. A Ca²⁺ event initiated in one inner border cells propagated in some instances longitudinally to neighboring inner border cells with an intercellular speed of ∼2 μm per second, and decayed after propagating along ∼3 cells. Similar intercellular propagation was not observed in the radial direction from inner border cell to inner sulcus cells, and was not observed between adjacent vestibular support cells.

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.

Using the Zebrafish Lateral Line to Understand the Roles of Mitochondria in Sensorineural Hearing Loss

Hair cells are the mechanosensory receptors of the inner ear and can be damaged by noise, aging, and ototoxic drugs. This damage often results in permanent sensorineural hearing loss. Hair cells have high energy demands and rely on mitochondria to produce ATP as well as contribute to intracellular calcium homeostasis. In addition to generating ATP, mitochondria produce reactive oxygen species, which can lead to oxidative stress, and regulate cell death pathways. Zebrafish lateral-line hair cells are structurally and functionally analogous to cochlear hair cells but are optically and pharmacologically accessible within an intact specimen, making the zebrafish a good model in which to study hair-cell mitochondrial activity. Moreover, the ease of genetic manipulation of zebrafish embryos allows for the study of mutations implicated in human deafness, as well as the generation of transgenic models to visualize mitochondrial calcium transients and mitochondrial activity in live organisms. Studies of the zebrafish lateral line have shown that variations in mitochondrial activity can predict hair-cell susceptibility to damage by aminoglycosides or noise exposure. In addition, antioxidants have been shown to protect against noise trauma and ototoxic drug–induced hair-cell death. In this review, we discuss the tools and findings of recent investigations into zebrafish hair-cell mitochondria and their involvement in cellular processes, both under homeostatic conditions and in response to noise or ototoxic drugs. The zebrafish lateral line is a valuable model in which to study the roles of mitochondria in hair-cell pathologies and to develop therapeutic strategies to prevent sensorineural hearing loss in humans.

Structural determinants of protocadherin-15 mechanics and function in hearing and balance perception

When sound vibrations reach the inner ear, fine protein filaments called “tip links” stretch and open cochlear hair-cell mechanosensitive channels that trigger sensory perception. Similarly, vestibular hair cells use tip links to sense mechanical stimuli produced by head motions. Tip links are formed by cadherin-23 and protocadherin-15, two large proteins involved in hearing loss and balance disorders. Here we present multiple structures, models, and simulations that depict the lower end of the tip link, including the complete protocadherin-15 ectodomain. These models show an essential connection between cadherin-23 and protocadherin-15 with dual molecular “handshakes” and various protein sites that are mutated in inherited deafness. The simulations also reveal how the tip link responds to force to mediate hearing and balance sensing.

The vertebrate inner ear, responsible for hearing and balance, is able to sense minute mechanical stimuli originating from an extraordinarily broad range of sound frequencies and intensities or from head movements. Integral to these processes is the tip-link protein complex, which conveys force to open the inner-ear transduction channels that mediate sensory perception. Protocadherin-15 and cadherin-23, two atypically large cadherins with 11 and 27 extracellular cadherin (EC) repeats, are involved in deafness and balance disorders and assemble as parallel homodimers that interact to form the tip link. Here we report the X-ray crystal structure of a protocadherin-15 + cadherin-23 heterotetrameric complex at 2.9-Å resolution, depicting a parallel homodimer of protocadherin-15 EC1-3 molecules forming an antiparallel complex with two cadherin-23 EC1-2 molecules. In addition, we report structures for 10 protocadherin-15 fragments used to build complete high-resolution models of the monomeric protocadherin-15 ectodomain. Molecular dynamics simulations and validated crystal contacts are used to propose models for the complete extracellular protocadherin-15 parallel homodimer and the tip-link bond. Steered molecular dynamics simulations of these models suggest conditions in which a structurally diverse and multimodal protocadherin-15 ectodomain can act as a stiff or soft gating spring. These results reveal the structural determinants of tip-link–mediated inner-ear sensory perception and elucidate protocadherin-15’s structural and adhesive properties relevant in disease.

Detection of BDNF-Related Proteins in Human Perilymph in Patients With Hearing Loss

The outcome of cochlear implantation depends on multiple variables including the underlying health of the cochlea. Brain derived neurotrophic factor (BDNF) has been shown to support spiral ganglion neurons and to improve implant function in animal models. Whether endogenous BDNF or BDNF-regulated proteins can be used as biomarkers to predict cochlear health and implant outcome has not been investigated yet. Gene expression of BDNF and downstream signaling molecules were identified in tissue of human cochleae obtained from normal hearing patients (n = 3) during skull base surgeries. Based on the gene expression data, bioinformatic analysis was utilized to predict the regulation of proteins by BDNF. The presence of proteins corresponding to these genes was investigated in perilymph (n = 41) obtained from hearing-impaired patients (n = 38) during cochlear implantation or skull base surgery for removal of vestibular schwannoma by nanoscale liquid chromatography coupled to tandem mass spectrometry (nano LC-MS/MS). Analyzed by mass spectrometry were 41 perilymph samples despite three patients undergoing bilateral cochlear implantation. These particular BDNF regulated proteins were not detectable in any of the perilymph samples. Subsequently, targeted analysis of the perilymph proteome data with Ingenuity Pathway Analysis (IPA) identified further proteins in human perilymph that could be regulated by BDNF. These BDNF regulated proteins were correlated to the presence of residual hearing (RH) prior to implantation and to the performance data with the cochlear implant after 1 year. There was overall a decreased level of expression of BDNF-regulated proteins in profoundly hearing-impaired patients compared to patients with some RH. Phospholipid transfer protein was positively correlated to the preoperative hearing level of the patients. Our data show that combination of gene expression arrays and bioinformatic analysis can aid in the prediction of downstream signaling proteins related to the BDNF pathway. Proteomic analysis of perilymph may help to identify the presence or absence of these molecules in the diseased organ. The impact of such prediction algorithms on diagnosis and treatment needs to be established in further studies.

Targeted single-cell electroporation loading of Ca2+ indicators in the mature hemicochlea preparation

Ca²⁺ is an important intracellular messenger and regulator in both physiological and pathophysiological mechanisms in the hearing organ. Investigation of cellular Ca²⁺ homeostasis in the mature cochlea is hampered by the special anatomy and high vulnerability of the organ. A quick, straightforward and reliable Ca²⁺ imaging method with high spatial and temporal resolution in the mature organ of Corti is missing. Cell cultures or isolated cells do not preserve the special microenvironment and intercellular communication, while cochlear explants are excised from only a restricted portion of the organ of Corti and usually from neonatal pre-hearing murines. The hemicochlea, prepared from hearing mice allows tonotopic experimental approach on the radial perspective in the basal, middle and apical turns of the organ. We used the preparation recently for functional imaging in supporting cells of the organ of Corti after bulk loading of the Ca²⁺ indicator. However, bulk loading takes long time, is variable and non-selective, and causes the accumulation of the indicator in the extracellular space. In this study we show the improved labeling of supporting cells of the organ of Corti by targeted single-cell electroporation in mature mouse hemicochlea. Single-cell electroporation proved to be a reliable way of reducing the duration and variability of loading and allowed subcellular Ca²⁺ imaging by increasing the signal-to-noise ratio, while cell viability was retained during the experiments. We demonstrated the applicability of the method by measuring the effect of purinergic, TRPA1, TRPV1 and ACh receptor stimulation on intracellular Ca²⁺ concentration at the cellular and subcellular level. In agreement with previous results, ATP evoked reversible and repeatable Ca²⁺ transients in Deiters', Hensen's and Claudius' cells. TRPA1 and TRPV1 stimulation by AITC and capsaicin, respectively, failed to induce any Ca²⁺ response in the supporting cells, except in a single Hensen's cell in which AITC evoked transients with smaller amplitude. AITC also caused the displacement of the tissue. Carbachol, agonist of ACh receptors induced Ca²⁺ transients in about a third of Deiters' and fifth of Hensen's cells. Here we have presented a fast and cell-specific indicator loading method allowing subcellular functional Ca²⁺ imaging in supporting cells of the organ of Corti in the mature hemicochlea preparation, thus providing a straightforward tool for deciphering the poorly understood regulation of Ca²⁺ homeostasis in these cells.

Beyond Cell-Cell Adhesion: Sensational Cadherins for Hearing and Balance

Cadherins form a large family of proteins often involved in calcium-dependent cellular adhesion. Although classical members of the family can provide a physical bond between cells, a subset of special cadherins use their extracellular domains to interlink apical specializations of single epithelial sensory cells. Two of these cadherins, cadherin-23 (CDH23) and protocadherin-15 (PCDH15), form extracellular "tip link" filaments that connect apical bundles of stereocilia on hair cells essential for inner-ear mechanotransduction. As these bundles deflect in response to mechanical stimuli from sound or head movements, tip links gate hair-cell mechanosensitive channels to initiate sensory perception. Here, we review the unusual and diverse structural properties of these tip-link cadherins and the functional significance of their deafness-related missense mutations. Based on the structural features of CDH23 and PCDH15, we discuss the elasticity of tip links and models that bridge the gap between the nanomechanics of cadherins and the micromechanics of hair-cell bundles during inner-ear mechanotransduction.

Loss of CIB2 Causes Profound Hearing Loss and Abolishes Mechanoelectrical Transduction in Mice

Calcium and integrin-binding protein 2 (CIB2) belongs to a protein family with four known members, CIB1 through CIB4, which are characterized by multiple calcium-binding EF-hand domains. Among the family members, the Cib1 and Cib2 genes are expressed in mouse cochlear hair cells, and mutations in the human CIB2 gene have been associated with nonsyndromic deafness DFNB48 and syndromic deafness USH1J. To further explore the function of CIB1 and CIB2 in hearing, we established Cib1 and Cib2 knockout mice using the clustered regularly interspaced short palindromic repeat (CRISPR)-associated Cas9 nuclease (CRISPR/Cas9) genome editing technique. We found that loss of CIB1 protein does not affect auditory function, whereas loss of CIB2 protein causes profound hearing loss in mice. Further investigation revealed that hair cell stereocilia development is affected in Cib2 knockout mice. Noticeably, loss of CIB2 abolishes mechanoelectrical transduction (MET) currents in auditory hair cells. In conclusion, we show here that although both CIB1 and CIB2 are readily detected in the cochlea, only loss of CIB2 results in profound hearing loss, and that CIB2 is essential for auditory hair cell MET.

Ca2+ signaling, apoptosis and autophagy in the developing cochlea: Milestones to hearing acquisition

In mammals, the sense of hearing arises through a complex sequence of morphogenetic events that drive the sculpting of the auditory sensory epithelium into its terminally functional three-dimensional shape. While the majority of the underlying mechanisms remain unknown, it has become increasingly clear that Ca²⁺ signaling is at center stage and plays numerous fundamental roles both in the sensory hair cells and in the matrix of non-sensory, epithelial and supporting cells, which embed them and are tightly interconnected by a dense network of gap junctions formed by connexin 26 (Cx26) and connexin 30 (Cx30) protein subunits. In this review, we discuss the intricate interplay between Ca²⁺ signaling, connexin expression and function, apoptosis and autophagy in the crucial steps that lead to hearing acquisition.

Advances in genetic hearing loss: CIB2 gene

Hearing plays a crucial role in human development. Receiving and processing sounds are essential for the advancement of the speech ability during the early childhood and for a proper functioning in the society. Hearing loss is one of the most frequent disabilities that affect human senses. It can be caused by genetic or environmental factors or both of them. Calcium- and integrin-binding protein 2 (CIB2) is one of the recently identified genes, involved in HI pathogenesis. CIB2 is widely expressed in various human and animal tissues, mainly in skeletal muscle, nervous tissue, inner ear, and retina. The CIB2 protein is responsible for maintaining Ca2+ homeostasis in cells and interacting with integrins-transmembrane receptors essential for cell adhesion, migration, and activation of signaling pathways. Calcium signaling pathway is crucial for signal transduction in the inner ear, and integrins regulate hair cell differentiation and maturation of the stereocilia. To date, mutations detected in CIB2 are causative for nonsyndromic hearing loss (DFNB48) or Usher syndrome type 1 J. Patients harboring biallelic CIB2 mutations suffer from bilateral, early onset, moderate to profound HI. In the paper, we summarize the current status of the research on CIB2.

Insights into electrosensory organ development, physiology and evolution from a lateral line-enriched transcriptome

The anamniote lateral line system, comprising mechanosensory neuromasts and electrosensory ampullary organs, is a useful model for investigating the developmental and evolutionary diversification of different organs and cell types. Zebrafish neuromast development is increasingly well understood, but neither zebrafish nor Xenopus is electroreceptive and our molecular understanding of ampullary organ development is rudimentary. We have used RNA-seq to generate a lateral line-enriched gene-set from late-larval paddlefish (Polyodon spathula). Validation of a subset reveals expression in developing ampullary organs of transcription factor genes critical for hair cell development, and genes essential for glutamate release at hair cell ribbon synapses, suggesting close developmental, physiological and evolutionary links between non-teleost electroreceptors and hair cells. We identify an ampullary organ-specific proneural transcription factor, and candidates for the voltage-sensing L-type Ca_v channel and rectifying K_v channel predicted from skate (cartilaginous fish) ampullary organ electrophysiology. Overall, our results illuminate ampullary organ development, physiology and evolution.

ATP-Evoked Intracellular Ca²⁺ Signaling of Different Supporting Cells in the Hearing Mouse Hemicochlea

Hearing and its protection is regulated by ATP-evoked Ca(2+) signaling in the supporting cells of the organ of Corti, however, the unique anatomy of the cochlea hampers observing these mechanisms. For the first time, we have performed functional ratiometric Ca(2+) imaging (fura-2) in three different supporting cell types in the hemicochlea preparation of hearing mice to measure purinergic receptor-mediated Ca(2+) signaling in pillar, Deiters' and Hensen's cells. Their resting [Ca(2+)]i was determined and compared in the same type of preparation. ATP evoked reversible, repeatable and dose-dependent Ca(2+) transients in all three cell types, showing desensitization. Inhibiting the Ca(2+) signaling of the ionotropic P2X (omission of extracellular Ca(2+)) and metabotropic P2Y purinergic receptors (depletion of intracellular Ca(2+) stores) revealed the involvement of both receptor types. Detection of P2X2,3,4,6,7 and P2Y1,2,6,12,14 receptor mRNAs by RT-PCR supported this finding and antagonism by PPADS suggested different functional purinergic receptor population in pillar versus Deiters' and Hensen's cells. The sum of the extra- and intracellular Ca(2+)-dependent components of the response was about equal with the control ATP response (linear additivity) in pillar cells, and showed supralinearity in Deiters' and Hensen's cells. Calcium-induced calcium release might explain this synergistic interaction. The more pronounced Ca(2+) leak from the endoplasmic reticulum in Deiters' and Hensen's cells, unmasked by cyclopiazonic acid, may also suggests the higher activity of the internal stores in Ca(2+) signaling in these cells. Differences in Ca(2+) homeostasis and ATP-induced Ca(2+) signaling might reflect the distinct roles these cells play in cochlear function and pathophysiology.

ATP hydrolysis is critically required for function of CaV1.3 channels in cochlear inner hair cells via fueling Ca2+ clearance

Sound encoding is mediated by Ca(2+) influx-evoked release of glutamate at the ribbon synapse of inner hair cells. Here we studied the role of ATP in this process focusing on Ca(2+) current through CaV1.3 channels and Ca(2+) homeostasis in mouse inner hair cells. Patch-clamp recordings and Ca(2+) imaging demonstrate that hydrolyzable ATP is essential to maintain synaptic Ca(2+) influx in inner hair cells via fueling Ca(2+)-ATPases to avoid an increase in cytosolic [Ca(2+)] and subsequent Ca(2+)/calmodulin-dependent inactivation of CaV1.3 channels.

Multiple indices of the 'bounce' phenomenon obtained from the same human ears

Loud low-frequency sounds can induce temporary oscillatory changes in cochlear sensitivity, which have been termed the 'bounce' phenomenon. The origin of these sensitivity changes has been attributed to slow fluctuations in cochlear homeostasis, causing changes in the operating points of the outer hair cell mechano-electrical and electro-mechanical transducers. Here, we acquired three objective and subjective measures resulting in a comprehensive dataset of the bounce phenomenon in each of 22 normal-hearing human subjects. We analysed the level and phase of cubic and quadratic distortion product otoacoustic emissions and the auditory thresholds before and after presentation of a low-frequency stimulus (30 Hz sine wave, 120 dB SPL, 90 s) as a function of time. In addition, the perceived loudness of temporary, tinnitus-like sensations occurring in all subjects after cessation of the low-frequency stimulus was tracked over time. The majority of the subjects (70 %) showed a significant, biphasic change of quadratic, but not cubic, distortion product otoacoustic emissions of about 3-4 dB. Eighty-six percent of the tested subjects showed significant alterations of hearing thresholds after low-frequency stimulation. Four different types of threshold changes were observed, namely monophasic desensitisations (the majority of cases), monophasic sensitisations, biphasic alterations with initial sensitisation and biphasic alterations with initial desensitisation. The similar duration of the three bounce phenomenon measures indicates a common origin. The current findings are consistent with the hypothesis that slow oscillations of homeostatic control mechanisms and associated operating point shifts within the cochlea are the source of the bounce phenomenon.

DPOAE Intensity Increase at Individual Dominant Frequency after Short-Term Auditory Exposure

Previous experiments suggested the possibility of a short-term sound stimulus-evoked and transient increase in DPOAE amplitudes. This phenomenon is possibly due to the complexity of the outer hair cells and their efferent control system and the different time scales of regulatory processes. A total of 100 healthy subjects ranging from 18 to 40 years of age with normal hearing and normal DPOAE values in the range of 781–4000 Hz were recruited in the study. Diagnostic DPOAE measurements were performed after short-term sound exposure. We proposed a 10 sec, 50 dB sound impulse as the most effective stimulus for clinical practice between 40 and 60 sec poststimulus time to detect the aforementioned transient DPOAE increase. We developed a procedure for detection of this transient increase in DPOAE by the application of a short-term sound exposure. The phenomenon was consistent and well detectable. Based on our findings, a new aspect of cochlear adaptation can be established that might be introduced as a routine clinical diagnostic tool. A mathematical model was provided that summarizes various factors that determine electromotility of OHCs and serves as a possible clinical application using this phenomenon for the prediction of individual noise susceptibility.

A new Atp2b2 deafwaddler allele, dfw(i5), interacts strongly with Cdh23 and other auditory modifiers

Tight regulation of calcium (Ca2+) concentrations in the stereocilia bundles of auditory hair cells of the inner ear is critical to normal auditory transduction. The plasma membrane Ca2+ ATPase 2 (PMCA2), encoded by the Atp2b2 gene, is the primary mechanism for clearance of Ca2+ from auditory stereocilia, keeping intracellular levels low, and also contributes to maintaining adequate levels of extracellular Ca2+ in the endolymph. This study characterizes a novel null Atp2b2 allele, dfw(i5), by examining cochlear anatomy, vestibular function and auditory physiology in mutant mice. Loss of auditory function in PMCA2 mutants can be attributed to dysregulation of intracellular Ca2+ inside the stereocilia bundles. However, extracellular Ca2+ ions surrounding the stereocilia are also required for rigidity of cadherin 23, a component of the stereocilia tip-link encoded by the Cdh23 gene. This study further resolves the interaction between Atp2b2 and Cdh23 in a gene dosage and frequency-dependent manner, and finds that low frequencies are significantly affected by the interaction. In +/dfw(i5) mice, one mutant copy of Cdh23 is sufficient to cause broad frequency hearing impairment. Additionally, we report another modifying interaction with Atp2b2 on auditory sensitivity, possibly caused by an unidentified hearing loss gene in mice.

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Reduced phosphatidylinositol 4,5-bisphosphate synthesis impairs inner ear Ca2+ signaling and high-frequency hearing acquisition

Phosphatidylinositol phosphate kinase type 1γ (PIPKIγ) is a key enzyme in the generation of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] and is expressed at high levels in the nervous system. Homozygous knockout mice lacking this enzyme die postnatally within 24 h, whereas PIPKIγ(+/-) siblings breed normally and have no reported phenotype. Here we show that adult PIPKIγ(+/-) mice have dramatically elevated hearing thresholds for high-frequency sounds. During the first postnatal week we observed a reduction of ATP-dependent Ca(2+) signaling activity in cochlear nonsensory cells. Because Ca(2+) signaling under these conditions depends on inositol-1,4,5-trisphosphate generation from phospholipase C (PLC)-dependent hydrolysis of PI(4,5)P(2), we conclude that (i) PIPKIγ is primarily responsible for the synthesis of the receptor-regulated PLC-sensitive PI(4,5)P(2) pool in the cell syncytia that supports auditory hair cells; (ii) spatially graded impairment of this signaling pathway in cochlear nonsensory cells causes a selective alteration in the acquisition of hearing in PIPKIγ(+/-) mice. This mouse model also suggests that PIPKIγ may determine the level of gap junction contribution to cochlear development.

Calcium signaling in the cochlea - Molecular mechanisms and physiopathological implications

Calcium ions (Ca2+) regulate numerous and diverse aspects of cochlear and vestibular physiology. This review focuses on the Ca2+ control of mechanotransduction and synaptic transmission in sensory hair cells, as well as on Ca2+ signalling in non-sensory cells of the developing cochlea.

Nitric oxide--a versatile key player in cochlear function and hearing disorders

Nitric oxide (NO) is a signaling molecule which can generally be formed by three nitric oxide synthases (NOS). Two of them, the endothelial nitric oxide synthase (eNOS) and the neural nitric oxide synthase (nNOS), are calcium/calmodulin-dependent and constitutively expressed in many cell types. Both isoforms are found in the vertebrate cochlea. The inducible nitric oxide synthase (iNOS) is independent of calcium and normally not detectable in the un-stimulated cochlea. In the inner ear, as in other tissues, NO was identified as a multitask molecule involved in various processes such as neurotransmission and neuromodulation. In addition, increasing evidence demonstrates that the NO-dependent processes of cell protection or, alternatively, cell destruction seem to depend, among other things, on changes in the local cochlear NO-concentration. These alterations can occur at the cellular level or within a distinct cell population both leading to an NO-imbalance within the hearing organ. This dysfunction can result in hearing loss or even in deafness. In cases of cochlear malfunction, regulatory systems such as the gap junction system, the blood vessels or the synaptic region might be affected temporarily or permanently by an altered NO-level. This review discusses potential cellular mechanisms how NO might contribute to different forms of hearing disorders. Approaches of NO-reduction are evaluated and the transfer of results obtained from experimental animal models to human medication is discussed.

Expression patterns of Ca(V)1.3 channels in the rat cochlea

Although Ca(V)1.3 channels are known to be essential for neuronal excitation and signal transduction in the auditory system, their expression patterns in the cochlea are still not fully understood, particularly in the regions where non-sensory cells are located. We performed immunohistochemistry, western blotting and reverse transcription-polymerase chain reaction (RT-PCR) to identify the expression and distribution of Ca(V)1.3 channels in the rat cochlea. Immunohistochemistry revealed that Ca(V)1.3 channels were localized in the outer hair cells (OHCs), inner hair cells (IHCs), limbus laminae spiralis, spiral ganglion cell, spiral ligament (SL), and stria vascularis (STV). The results of RT-PCR and western blotting demonstrated Ca(V)1.3 channels had a tissue-specific expression pattern. Ca(V)1.3 mRNA and protein were intensively expressed in the basilar membrane and spiral ganglion while moderate level of Ca(V)1.3 channels was observed in SL and STV. Our study preliminarily revealed the expression patterns of Ca(V)1.3 channels in the rat cochlea, providing a theoretical basis for further research on the role of Ca(V)1.3 channels in the periphery auditory system.

[Possible molecular mechanisms of spontaneous remission in sudden idiopathic hearing loss]

According to current knowledge, it must be assumed that temporary idiopathic hearing loss and its spontaneous remission are based on mechanical and/or pathological alterations in the inner ear. The causal mechanisms might be based on inter-individual variations. Induced by dose-dependent activators, temporary as well as permanent damage might occur. Sudden hearing loss may be initiated by an increase in the local nitric oxide (NO) concentration. Spontaneous remission, i.e. functional restoration, can be explained by a local decrease in the NO concentration. In this context, regulatory systems such as the gap-junction system, blood vessels or synapses might be affected. In addition, alterations in the hormone level of estrogen and mineralocorticoids, as well as cellular glutathione and vitamin levels, might lead to temporary alterations in the inner ear. Recent experimental findings indicate a role for the shuttle protein Survivin in the spontaneous remission of sudden hearing loss.

Expression and localization of ryanodine receptors in the frog semicircular canal

Several experiments suggest an important role for store-released Ca²⁺ in hair cell organs: drugs targeting IP₃ and ryanodine (RyRs) receptors affect release from hair cells, and stores are thought to be involved in vesicle recycling at ribbon synapses. In this work we investigated the semicircular canal distribution of RyRs by immunofluorescence, using slice preparations of the sensory epithelium (to distinguish cell types) and flat mounts of the simpler nonsensory regions. RyRs were present in hair cells, mostly in supranuclear spots, but not in supporting cells; as regards nonsensory regions, they were also localized in dark cells and cells from the ductus. No labeling was found in nerve terminals, although nerve branches could be observed in proximity to hair cell RyR spots. The differential expression of RyR isoforms was studied by RT-PCR and immunoblotting, showing the presence of RyRα in both ampulla and canal arm and RyRβ in the ampulla only.

Oncomodulin identifies different hair cell types in the mammalian inner ear

The tight regulation of Ca(2+) is essential for inner ear function, and yet the role of Ca(2+) binding proteins (CaBPs) remains elusive. By using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR), we investigated the expression of oncomodulin (Ocm), a member of the parvalbumin family, relative to other EF-hand CaBPs in cochlear and vestibular organs in the mouse. In the mouse cochlea, Ocm is found only in outer hair cells and is localized preferentially to the basolateral outer hair cell membrane and to the base of the hair bundle. Developmentally, Ocm immunoreactivity begins as early as postnatal day (P) 2 and shows preferential localization to the basolateral membrane and hair bundle after P8. Unlike the cochlea, Ocm expression is substantially reduced in vestibular tissues at older adult ages. In vestibular organs, Ocm is found in type I striolar or central hair cells, and has a more diffuse subcellular localization throughout the hair cell body. Additionally, Ocm immunoreactivity in vestibular hair cells is present as early as E18 and is not obviously affected by mutations that cause a disruption of hair bundle polarity. We also find Ocm expression in striolar hair cells across mammalian species. These data suggest that Ocm may have distinct functional roles in cochlear and vestibular hair cells.

Damage-induced cell-cell communication in different cochlear cell types via two distinct ATP-dependent Ca waves

Intercellular Ca(2+) waves can coordinate the action of large numbers of cells over significant distances. Recent work in many different systems has indicated that the release of ATP is fundamental for the propagation of most Ca(2+) waves. In the organ of hearing, the cochlea, ATP release is involved in critical signalling events during tissue maturation. ATP-dependent signalling is also implicated in the normal hearing process and in sensing cochlear damage. Here, we show that two distinct Ca(2+) waves are triggered during damage to cochlear explants. Both Ca(2+) waves are elicited by extracellular ATP acting on P2 receptors, but they differ in their source of Ca(2+), their velocity, their extent of spread and the cell type through which they propagate. A slower Ca(2+) wave (14 mum/s) communicates between Deiters' cells and is mediated by P2Y receptors and Ca(2+) release from IP(3)-sensitive stores. In contrast, a faster Ca(2+) wave (41 mum/s) propagates through sensory hair cells and is mediated by Ca(2+) influx from the external environment. Using inhibitors and selective agonists of P2 receptors, we suggest that the faster Ca(2+) wave is mediated by P2X(4) receptors. Thus, in complex tissues, the expression of different receptors determines the propagation of distinct intercellular communication signals.

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The online version of this article (doi:10.1007/s11302-010-9193-8) contains supplementary material, which is available to authorized users.

Role of nitric oxide on purinergic signalling in the cochlea

In the inner ear, there is considerable evidence that extracellular adenosine 5'-triphosphate (ATP) plays an important role in auditory neurotransmission as a neurotransmitter or a neuromodulator, although the potential role of adenosine signalling in the modulation of auditory neurotransmission has also been reported. The activation of ligand-gated ionotropic P2X receptors and G protein-coupled metabotropic P2Y receptors has been reported to induce an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) in inner hair cells (IHCs), outer hair cells (OHCs), spiral ganglion neurons (SGNs), and supporting cells in the cochlea. ATP may participate in auditory neurotransmission by modulating [Ca(2+)](i) in the cochlear cells. Recent studies showed that extracellular ATP induced nitric oxide (NO) production in IHCs, OHCs, and SGNs, which affects the ATP-induced Ca(2+) response via the NO-cGMP-PKG pathway in those cells by a feedback mechanism. A cross-talk between NO and ATP may therefore exist in the auditory signal transduction. In the present article, I review the role of NO on the ATP-induced Ca(2+) signalling in IHCs and OHCs. I also consider the possible role of NO in the ATP-induced Ca(2+) signalling in SGNs and supporting cells.

ATP-mediated cell-cell signaling in the organ of Corti: the role of connexin channels

Connexin 26 (Cx26) and connexin 30 (Cx30) form hemichannels that release ATP from the endolymphatic surface of cochlear supporting and epithelial cells and also form gap junction (GJ) channels that allow the concomitant intercellular diffusion of Ca(2+) mobilizing second messengers. Released ATP in turn activates G-protein coupled P2Y(2) and P2Y(4) receptors, PLC-dependent generation of IP(3), release of Ca(2+) from intracellular stores, instigating the regenerative propagation of intercellular Ca(2+) signals (ICS). The range of ICS propagation is sensitive to the concentration of extracellular divalent cations and activity of ectonucleotidases. Here, the expression patterns of Cx26 and Cx30 were characterized in postnatal cochlear tissues obtained from mice aged between P5 and P6. The expression gradient along the longitudinal axis of the cochlea, decreasing from the basal to the apical cochlear turn (CT), was more pronounced in outer sulcus (OS) cells than in inner sulcus (IS) cells. GJ-mediated dye coupling was maximal in OS cells of the basal CT, inhibited by the nonselective connexin channel blocker carbenoxolone (CBX) and absent in hair cells. Photostimulating OS cells with caged inositol (3,4,5) tri-phosphate (IP(3)) resulted in transfer of ICS in the lateral direction, from OS cells to IS cells across the hair cell region (HCR) of medial and basal CTs. ICS transfer in the opposite (medial) direction, from IS cells photostimulated with caged IP(3) to OS cells, occurred mostly in the basal CT. In addition, OS cells displayed impressive rhythmic activity with oscillations of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) coordinated by the propagation of Ca(2+) wavefronts sweeping repeatedly through the same tissue area along the coiling axis of the cochlea. Oscillations evoked by uncaging IP(3) or by applying ATP differed greatly, by as much as one order of magnitude, in frequency and waveform rise time. ICS evoked by direct application of ATP propagated along convoluted cellular paths in the OS, which often branched and changed dynamically over time. Potential implications of these findings are discussed in the context of developmental regulation and cochlear pathophysiology.

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The online version of this article (doi:10.1007/s11302-010-9192-9) contains supplementary material, which is available to authorized users.

How is the highly positive endocochlear potential formed? The specific architecture of the stria vascularis and the roles of the ion-transport apparatus

Cochlear endolymph, an extracellular solution containing 150 mM K(+), exhibits a positive potential of +80 mV. This is called the endocochlear potential (EP) and is essential for audition. The mechanism responsible for formation of the EP has been an enigma for the half century since its first measurement. A key element is the stria vascularis, which displays a characteristic tissue structure and expresses multiple ion-transport apparatus. The stria comprises two epithelial layers: a layer of marginal cells and one composed of intermediate and basal cells. Between the two layers lies an extracellular space termed the intrastrial space (IS), which is thus surrounded by the apical membranes of intermediate cells and the basolateral membranes of marginal cells. The fluid in the IS exhibits a low concentration of K(+) and a positive potential similar to the EP. We have demonstrated that the IS is electrically isolated from the neighboring extracellular fluids, perilymph, and endolymph, which allows the IS to sustain its positive potential. This IS potential is generated by K(+) diffusion across the apical membranes of intermediate cells, where inwardly rectifying Kir4.1 channels are localized. The low K(+) concentration in the IS, which is mandatory for the large K(+)-diffusion potential, is maintained by Na(+),K(+)-ATPases and Na(+),K(+),2Cl(-)-cotransporters expressed at the basolateral membranes of marginal cells. An additional K(+)-diffusion potential formed by KCNQ1/KCNE1-K(+) channels at the apical membranes of marginal cells also contributes to the EP. Therefore, the EP depends on an electrically isolated space and two K(+)-diffusion potentials in the stria vascularis.

Calbindin and S100 protein expression in the developing inner ear in mice

Calbindin (CB) and S100 are calcium-binding proteins expressed in the inner ear in vertebrates. Information about their developmental roles is incomplete. This study investigated the expression patterns of CB and S100 in C3H mice using immunohistochemistry, from embryonic day 11 (E11) to postnatal day 10 (P10). CB was expressed in the otocyst and vestibulocochlear ganglion (VCG) from E11. In the cochlea at E17, CB immunoreactivity clearly labeled the VCG, the outer and inner hair cells, and the stria vascularis. CB staining was also present in the vestibular sensory cells, including their nerve fibers. Two days later, to this expression pattern was added the labeling of Kölliker's organ. Early postnatal CB expression encompassed VCG neurons, auditory hair cells, their afferent nerve fibers, and cells of the cochlear lateral wall. The first signs of S100 immunostaining of cochlear and vestibular epithelial cells appeared at E14. At E17 S100 immunoreactivity was found in a restricted expression pattern in the cochlea. Immunostaining was also present in the sacculus and utriculus and their afferent fibers. The Deiters', pillar and inner hair cells, and the VCG were S100-positive from E19. Postnatally, S100 staining also appeared in the inner hair cells and Deiters' cells, in some VCG neurons, and, in addition, in the spiral limbus, the spiral prominence, and the intermediate cells of the stria vascularis. This study demonstrates that the sites of CB and S100 expression in the mouse inner ear during embryonic and early postnatal development do not overlap and signal independent developmental patterns.

The molecular architecture of ribbon presynaptic terminals

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

Molecular mechanisms of BK channel activation

Large conductance, Ca(2+)-activated potassium (BK) channels are widely expressed throughout the animal kingdom and play important roles in many physiological processes, such as muscle contraction, neural transmission and hearing. These physiological roles derive from the ability of BK channels to be synergistically activated by membrane voltage, intracellular Ca(2+) and other ligands. Similar to voltage-gated K(+) channels, BK channels possess a pore-gate domain (S5-S6 transmembrane segments) and a voltage-sensor domain (S1-S4). In addition, BK channels contain a large cytoplasmic C-terminal domain that serves as the primary ligand sensor. The voltage sensor and the ligand sensor allosterically control K(+) flux through the pore-gate domain in response to various stimuli, thereby linking cellular metabolism and membrane excitability. This review summarizes the current understanding of these structural domains and their mutual interactions in voltage-, Ca(2+)- and Mg(2+)-dependent activation of the channel.

Coordinated control of connexin 26 and connexin 30 at the regulatory and functional level in the inner ear

Connexin 26 (Cx26) and connexin 30 (Cx30) are encoded by two genes (GJB2 and GJB6, respectively) that are found within 50 kb in the same complex deafness locus, DFNB1. Immunocytochemistry and quantitative PCR analysis of Cx30 KO mouse cultures revealed that Cx26 is downregulated at the protein level and at the mRNA level in nonsensory cells located between outer hair cells and the stria vascularis. To explore connexin coregulation, we manipulated gene expression using the bovine adeno-associated virus. Overexpression of Cx30 in the Cx30 KO mouse by transduction with bovine adeno-associated virus restored Cx26 expression, permitted the formation of functional gap junction channels, and rescued propagating Ca(2+) signals. Ablation of Cx26 by transduction of Cx26(loxP/loxP) cultures with a Cre recombinase vector caused concurrent downregulation of Cx30 and impaired intercellular communication. The coordinated regulation of Cx26 and Cx30 expression appears to occur as a result of signaling through PLC and the NF-kappaB pathway, because activation of IP(3)-mediated Ca(2+) responses by stimulation of P2Y receptors for 20 min with 20 nM ATP increased the levels of Cx26 transcripts in Cx30 KO cultures. This effect was inhibited by expressing a stable form of the IkappaB repressor protein that prevents activation/translocation of NF-kappaB. Thus, our data reveal a Ca(2+)-dependent control in the expression of inner ear connexins implicated in hereditary deafness as well as insight into the hitherto unexplained observation that some deafness-associated DFNB1 alleles are characterized by hereditable reduction of both GJB2 and GJB6 expression.