Mice with altered KCNQ4 K+ channels implicate sensory outer hair cells in human progressive deafness

Phosphodiesterase-5 (PDE-5) Inhibitors and Ototoxicity: A Systematic Review

OBJECTIVE

This study explores the current literature regarding associations between phosphodiesterase-5 (PDE-5) inhibitors and ototoxicity and provides a detailed summary and discussion of the findings.

DATA SOURCES

A comprehensive electronic search of PubMed/MEDLINE, Scopus, and Cochrane Library for studies published from database inception through March 21, 2018.

STUDY SELECTION

Basic science articles, epidemiological studies, randomized controlled trials, cohort studies, case reports, reviews, meta-analyses, press releases, and newsletters were included. The PRISMA search strategy was used to select papers. Search terms are included in the appendix (http://links.lww.com/MAO/A733).

RESULTS

Twenty-two articles met the inclusion criteria. Among case reports, there were a total of nine patients, all male, with an average age of 57.4 years (37-79 years, SD = 13.87 years). Of the cases of hearing loss, 25% (2/8 cases) were bilateral and 75% (6/8) were unilateral; 22% (2/9) were associated with tinnitus; and 33% (3/9) had accompanying vestibular symptoms (including vertigo and dizziness). Among multipatient studies, all prospective studies failed to find a significant association between ototoxicity and PDE-5 inhibitor use. Results of the retrospective studies were also heterogeneous. Many key molecules in the PDE-5 inhibition pathway have been demonstrated to exist in the cochlea. However, mirroring the clinical studies, the basic science mechanisms have suggested both ototoxic and otoprotective effects.

CONCLUSIONS

Currently, the literature is inconclusive regarding the interaction between PDE-5 inhibitor use and ototoxicity. Future study such as a double-blinded placebo controlled randomized trial with audiometric assessment would provide more sound evidence. Similarly, a unified molecular model is necessary.

Molecular basis and restoration of function deficiencies of Kv7.4 variants associated with inherited hearing loss

Deafness non-syndromic autosomal dominant 2 (DFNA2) is characterized by symmetric, predominantly high-frequency sensorineural hearing loss that is progressive across all frequencies. The disease is associated with variants of a potassium voltage-gated channel subfamily Q member 4 gene, KCNQ4 (Kv7.4). Here, we studied nine recently identified Kv7.4 variants in DFNA2 pedigrees, including V230E, E260K, D262V, Y270H, W275R, G287R, P291L, P291S and S680F. We proved that the variant S680F did not alter the channel function while the other eight variants resulted in function deficiencies. We further proved that the two variants E260K and P291S showed reduced cell membrane expressions while the other seven variants showed moderate cell surface expressions. Thus, trafficking deficiency is not a common mechanism underlying channel dysfunction. Next, we studied two variants, V230E and G287R, using molecular dynamics simulation. We showed that V230E stabilized Kv7.4 channel in the closed state by forming an additional hydrogen bond with a basic residue K325, while G287R distorted the selectivity filter and blocked the pore region of Kv7.4 channel. Moreover, by co-expressing wild-type (WT) and variant proteins in vitro, we demonstrated that the heterogeneous Kv7.4 channel currents were reduced compared to the WT channel currents and the reduction could be rescued by a Kv7.4 opener retigabine. Our study provided the underlying mechanisms and suggested a potential alternative therapeutic approach for DFNA2.

Transcriptomic Analyses of Inner Ear Sensory Epithelia in Zebrafish

Analysis of gene expression has the potential to assist in the understanding of multiple cellular processes including proliferation, cell-fate specification, senesence, and activity in both healthy and disease states. Zebrafish model has been increasingly used to understand the process of hearing and the development of the vertebrate auditory system. Within the zebrafish inner ear, there are three otolith organs, each containing a sensory macula of hair cells. The saccular macula is primarily involved in hearing, the utricular macula is primarily involved in balance and the function of the lagenar macula is not completely understood. The goal of this study is to understand the transcriptional differences in the sensory macula associated with different otolith organs with the intention of understanding the genetic mechanisms responsible for the distinct role each organ plays in sensory perception. The sensory maculae of the saccule, utricle, and lagena were dissected out of adult Et(krt4:GFP)^sqet4 zebrafish expressing green fluorescent protein in hair cells for transcriptional analysis. The total RNAs of the maculae were isolated and analyzed by RNA GeneChip microarray. Several of the differentially expressed genes are known to be involved in deafness, otolith development and balance. Gene expression among these otolith organs was very well conserved with less than 10% of genes showing differential expression. Data from this study will help to elucidate which genes are involved in hearing and balance. Furthermore, the findings of this study will assist in the development of the zebrafish model for human hearing and balance disorders. Anat Rec, 303:527-543, 2020. © 2019 American Association for Anatomy.

Kv7.4 Channel Contribute to Projection-Specific Auto-Inhibition of Dopamine Neurons in the Ventral Tegmental Area

Dopaminergic neurons in the ventral tegmental area (VTA) encode behavioral patterns important in reward and drug addiction as well as in emotional disorders. These functions of dopamine neurons are directly related to the release of dopamine in the targeted regions of the brain which are, thus, controlled by the excitability of dopamine neurons. One mechanism for modulation of dopamine neuronal excitability is mediated by the auto dopamine type 2 (D2) receptors, through activation of a Kir3/GIRK K⁺ channel which inhibits the firing of dopamine neurons. In this study, we provide evidence that Kv7.4, in addition to Kir3.2 channels, contributes to dopamine (DA)-mediated auto-inhibition of DA activity projecting to NAc and to basolateral amygdale (BLA). Furthermore, we demonstrate that D2 receptors enhance Kv7.4 currents through G_i/o protein and redox-dependent cellular pathway. Finally, we show this D2 mediated auto-inhibition is blunted in a social defeat mice model of depression, a phenomenon that may contribute to the altered excitability of VTA DA neurons in depressed animals. These results provide a new perspective for understanding the molecular mechanism of the excitability of VTA DA neurons and for potential new strategies against mental disorders involving altered excitability of DA neurons, such as major depression and drug addictions.

Kv2.1 voltage-gated potassium channels in developmental perspective

Kv2.1 voltage-gated potassium channels consist of two types of α-subunits: (a) electrically-active Kcnb1 α-subunits and (b) silent or modulatory α-subunits plus β-subunits that, similar to silent α-subunits, also regulate electrically-active subunits. Voltage-gated potassium channels were traditionally viewed, mainly by electrophysiologists, as regulators of the electrical activity of the plasma membrane in excitable cells, a role that is performed by transmembrane protein domains of α-subunits that form the electric pore. Genetic studies revealed a role for this region of α-subunits of voltage-gated potassium channels in human neurodevelopmental disorders, such as epileptic encephalopathy. The N- and C-terminal domains of α-subunits interact to form the cytoplasmic subunit of heterotetrameric potassium channels that regulate electric pores. Subsequent animal studies revealed the developmental functions of Kcnb1-containing voltage-gated potassium channels and illustrated their role during brain development and reproduction. These functions of potassium channels are discussed in this review in the context of regulatory interactions between electrically-active and regulatory subunits.

β-Secretase BACE1 Is Required for Normal Cochlear Function

Cleavage of amyloid precursor protein (APP) by β-secretase BACE1 initiates the production and accumulation of neurotoxic amyloid-β peptides, which is widely considered an essential pathogenic mechanism in Alzheimer's disease (AD). Here, we report that BACE1 is essential for normal auditory function. Compared with wild-type littermates, BACE1^-/- mice of either sex exhibit significant hearing deficits, as indicated by increased thresholds and reduced amplitudes in auditory brainstem responses (ABRs) and decreased distortion product otoacoustic emissions (DPOAEs). Immunohistochemistry revealed aberrant synaptic organization in the cochlea and hypomyelination of auditory nerve fibers as predominant neuropathological substrates of hearing loss in BACE1^-/- mice. In particular, we found that fibers of spiral ganglion neurons (SGN) close to the organ of Corti are disorganized and abnormally swollen. BACE1 deficiency also engenders organization defects in the postsynaptic compartment of SGN fibers with ectopic overexpression of PSD95 far outside the synaptic region. During postnatal development, auditory fiber myelination in BACE1^-/- mice lags behind dramatically and remains incomplete into adulthood. We relate the marked hypomyelination to the impaired processing of Neuregulin-1 when BACE1 is absent. To determine whether the cochlea of adult wild-type mice is susceptible to AD treatment-like suppression of BACE1, we administered the established BACE1 inhibitor NB-360 for 6 weeks. The drug suppressed BACE1 activity in the brain, but did not impair hearing performance and, upon neuropathological examination, did not produce the characteristic cochlear abnormalities of BACE1^-/- mice. Together, these data strongly suggest that the hearing loss of BACE1 knock-out mice represents a developmental phenotype.SIGNIFICANCE STATEMENT Given its crucial role in the pathogenesis of Alzheimer's disease (AD), BACE1 is a prime pharmacological target for AD prevention and therapy. However, the safe and long-term administration of BACE1-inhibitors as envisioned in AD requires a comprehensive understanding of the various physiological functions of BACE1. Here, we report that BACE1 is essential for the processing of auditory signals in the inner ear, as BACE1-deficient mice exhibit significant hearing loss. We relate this deficit to impaired myelination and aberrant synapse formation in the cochlea, which manifest during postnatal development. By contrast, prolonged pharmacological suppression of BACE1 activity in adult wild-type mice did not reproduce the hearing deficit or the cochlear abnormalities of BACE1 null mice.

Rare KCNQ4 variants found in public databases underlie impaired channel activity that may contribute to hearing impairment

KCNQ4 is frequently mutated in autosomal dominant non-syndromic hearing loss (NSHL), a typically late-onset, initially high-frequency loss that progresses over time (DFNA2). Most KCNQ4 mutations linked to hearing loss are clustered around the pore region of the protein and lead to loss of KCNQ4-mediated potassium currents. To understand the contribution of KCNQ4 variants to NSHL, we surveyed public databases and found 17 loss-of-function and six missense KCNQ4 variants affecting amino acids around the pore region. The missense variants have not been reported as pathogenic and are present at a low frequency (minor allele frequency < 0.0005) in the population. We examined the functional impact of these variants, which, interestingly, induced a reduction in potassium channel activity without altering expression or trafficking of the channel protein, being functionally similar to DFNA2-associated KCNQ4 mutations. Therefore, these variants may be risk factors for late-onset hearing loss, and individuals harboring any one of these variants may develop hearing loss during adulthood. Reduced channel activity could be rescued by KCNQ activators, suggesting the possibility of medical intervention. These findings indicate that KCNQ4 variants may contribute more to late-onset NSHL than expected, and therefore, genetic screening for this gene is important for the prevention and treatment of NSHL.

Inner Hair Cell and Neuron Degeneration Contribute to Hearing Loss in a DFNA2-Like Mouse Model

DFNA2 is a progressive deafness caused by mutations in the voltage-activated potassium channel KCNQ4. Hearing loss develops with age from a mild increase in the hearing threshold to profound deafness. Studies using transgenic mice for Kcnq4 expressed in a mixed background demonstrated the implication of outer hair cells at the initial phase. However, it could not explain the last phase mechanisms of the disease. Genetic backgrounds are known to influence disease expressivity. To unmask the cause of profound deafness phenotype, we backcrossed the Kcnq4 knock-out allele to the inbred strain C3H/HeJ and investigated inner and outer hair cell and spiral ganglion neuron degeneration across the lifespan. In addition to the already reported outer hair cell death, the C3H/HeJ strain also exhibited inner hair cell and spiral ganglion neuron death. We tracked the spatiotemporal survival of cochlear cells by plotting cytocochleograms and neuronal counts at different ages. Cell loss progressed from basal to apical turns with age. Interestingly, the time-course of cell degeneration was different for each cell-type. While for outer hair cells it was already present by week 3, inner hair cell and neuronal loss started 30 weeks later. We also established that outer hair cell loss kinetics slowed down from basal to apical regions correlating with KCNQ4 expression pattern determined in wild-type mice. Our findings indicate that KCNQ4 plays differential roles in each cochlear cell-type impacting in their survival ability. Inner hair cell and spiral ganglion neuron death generates severe hearing loss that could be associated with the last phase of DFNA2.

Inhibition of Histone Methyltransferase G9a Attenuates Noise-Induced Cochlear Synaptopathy and Hearing Loss

Posttranslational modification of histones alters their interaction with DNA and nuclear proteins, influencing gene expression and cell fate. In this study, we investigated the effect of G9a (KMT1C, EHMT2), a major histone lysine methyltransferase encoded by the human EHMT2 gene and responsible for histone H3 lysine 9 dimethylation (H3K9me2) on noise-induced permanent hearing loss (NIHL) in adult CBA/J mice. The conditions of noise exposure used in this study led to losses of cochlear synapses and outer hair cells (OHCs) and permanent auditory threshold shifts. Inhibition of G9a with its specific inhibitor BIX 01294 or with siRNA significantly attenuated these pathological features. Treatment with BIX 01294 also prevented the noise-induced decrease of KCNQ4 immunolabeling in OHCs. Additionally, G9a was increased in cochlear cells, including both outer and inner sensory hair cells, some spiral ganglion neurons (SGNs), and marginal cells, 1 h after the completion of the noise exposure. Also subsequent to noise exposure, immunoreactivity for H3K9me2 appeared in some nuclei of OHCs following a high-to-low frequency gradient with more labeled OHCs in the 45-kHz than the 32-kHz region, as well as in the marginal cells and in some SGNs of the basal turn. These findings suggest that epigenetic modifications of H3K9me2 are involved in NIHL and that pharmacological targeting of G9a may offer a strategy for protection against cochlear synaptopathy and NIHL.

Diagnosis, Intervention, and Prevention of Genetic Hearing Loss

It is estimated that at least 50% of congenital or childhood hearing loss is attributable to genetic causes. In non-syndromic hearing loss, which accounts for 70% of genetic hearing loss, approximately 80% of cases are autosomal recessive, 15% autosomal dominant, and 1-2% mitochondrial or X-linked. In addition, 30% of genetic hearing loss is syndromic. The genetic causes of hearing loss are highly heterogeneous. So far, more than 140 deafness-related genes have been discovered. Studies on those genes tremendously increased our understanding of the inner ear functions at the molecular level. It also offers important information for the patients and allows personalized and accurate genetic counseling. In many cases, genetic diagnosis of hearing loss can help to avoid unnecessary and costly clinical testing, offer prognostic information, and guide future medical management. On the other hand, a variety of gene therapeutic approaches have been developed aiming to relieve or converse the hearing loss due to genetic causes. Prevention of genetic hearing loss is feasible through prepregnancy and prenatal genetic diagnosis and counseling.

Whole-exome sequencing identifies two novel mutations in KCNQ4 in individuals with nonsyndromic hearing loss

Mutations in potassium voltage-gated channel subfamily Q member 4 (KCNQ4) are etiologically linked to a type of nonsyndromic hearing loss, deafness nonsyndromic autosomal dominant 2 (DFNA2). We performed whole-exome sequencing for 98 families with hearing loss and found mutations in KCNQ4 in five families. In this study, we characterized two novel mutations in KCNQ4: a missense mutation (c.796G>T; p.Asp266Tyr) and an in-frame deletion mutation (c.259_267del; p.Val87_Asn89del). p.Asp266Tyr located in the channel pore region resulted in early onset and moderate hearing loss, whereas p.Val87_Asn89del located in the N-terminal cytoplasmic region resulted in late onset and high frequency-specific hearing loss. When heterologously expressed in HEK 293 T cells, both mutant proteins did not show defects in protein trafficking to the plasma membrane or in interactions with wild-type (WT) KCNQ4 channels. Patch-clamp analysis demonstrated that both p.Asp266Tyr and p.Val87_Asn89del mutant channels lost conductance and were completely unresponsive to KCNQ activators, such as retigabine, zinc pyrithione, and ML213. Channels assembled from WT-p.Asp266Tyr concatemers, like those from WT-WT concatemers, exhibited conductance and responsiveness to KCNQ activators. However, channels assembled from WT-p.Val87_Asn89del concatemers showed impaired conductance, suggesting that p.Val87_Asn89del caused complete loss-of-function with a strong dominant-negative effect on functional WT channels. Therefore, the main pathological mechanism may be related to loss of K⁺ channel activity, not defects in trafficking.

Pore- and voltage sensor-targeted KCNQ openers have distinct state-dependent actions

Ion channels encoded by KCNQ2-5 generate a prominent K⁺ conductance in the central nervous system, referred to as the M current, which is controlled by membrane voltage and PIP2. The KCNQ2-5 voltage-gated potassium channels are targeted by a variety of activating compounds that cause negative shifts in the voltage dependence of activation. The underlying pharmacology of these effects is of growing interest because of possible clinical applications. Recent studies have revealed multiple binding sites and mechanisms of action of KCNQ activators. For example, retigabine targets the pore domain, but several compounds have been shown to influence the voltage-sensing domain. An important unexplored feature of these compounds is the influence of channel gating on drug binding or effects. In the present study, we compare the state-dependent actions of retigabine and ICA-069673 (ICA73, a voltage sensor-targeted activator). We assess drug binding to preopen states by applying drugs to homomeric KCNQ2 channels at different holding voltages, demonstrating little or no association of ICA73 with resting states. Using rapid solution switching, we also demonstrate that the rate of onset of ICA73 correlates with the voltage dependence of channel activation. Retigabine actions differ significantly, with prominent drug effects seen at very negative holding voltages and distinct voltage dependences of drug binding versus channel activation. Using similar approaches, we investigate the mechanistic basis for attenuation of ICA73 actions by the voltage-sensing domain mutation KCNQ2[A181P]. Our findings demonstrate different state-dependent actions of pore- versus voltage sensor-targeted KCNQ channel activators, which highlight that subtypes of this drug class operate with distinct mechanisms.

GC-B Deficient Mice With Axon Bifurcation Loss Exhibit Compromised Auditory Processing

Sensory axon T-like branching (bifurcation) in neurons from dorsal root ganglia and cranial sensory ganglia depends on the molecular signaling cascade involving the secreted factor C-type natriuretic peptide, the natriuretic peptide receptor guanylyl cyclase B (GC-B; also known as Npr2) and cGMP-dependent protein kinase I (cGKI, also known as PKGI). The bifurcation of cranial nerves is suggested to be important for information processing by second-order neurons in the hindbrain or spinal cord. Indeed, mice with a spontaneous GC-B loss of function mutation (Npr2^cn/cn ) display an impaired bifurcation of auditory nerve (AN) fibers. However, these mice did not show any obvious sign of impaired basal hearing. Here, we demonstrate that mice with a targeted inactivation of the GC-B gene (Npr2 ^lacZ/lacZ , GC-B KO mice) show an elevation of audiometric thresholds. In the inner ear, the cochlear hair cells in GC-B KO mice were nevertheless similar to those from wild type mice, justified by the typical expression of functionally relevant marker proteins. However, efferent cholinergic feedback to inner and outer hair cells was reduced in GC-B KO mice, linked to very likely reduced rapid efferent feedback. Sound-evoked AN responses of GC-B KO mice were elevated, a feature that is known to occur when the efferent axo-dendritic feedback on AN is compromised. Furthermore, late sound-evoked brainstem responses were significantly delayed in GC-B KO mice. This delay in sound response was accompanied by a weaker sensitivity of the auditory steady state response to amplitude-modulated sound stimuli. Finally, the acoustic startle response (ASR) - one of the fastest auditory responses - and the prepulse inhibition of the ASR indicated significant changes in temporal precision of auditory processing. These findings suggest that GC-B-controlled axon bifurcation of spiral ganglion neurons is important for proper activation of second-order neurons in the hindbrain and is a prerequisite for proper temporal auditory processing likely by establishing accurate efferent top-down control circuits. These data hypothesize that the bifurcation pattern of cranial nerves is important to shape spatial and temporal information processing for sensory feedback control.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

[Molecular Understanding of Hearing - How Does This Matter to the Hearing Impaired?]

This review addresses the advances of our molecular understanding of hearing and how this benefits the hearing impaired. Classical biochemical methods usually fall short in contributing to the analysis of the molecular mechanisms of hearing e. g. in the cochlea, the auditory part of the inner ear, due to the scarcity of the cells of interest. Genetics, molecular cell biology, and physiology, on the other hand, have elucidated the intricate molecular and cellular mechanisms that bring about the outstanding performance of the auditory system. Many of those mechanisms are quite unique and specialized to serve the specific needs of hearing. Hence, their defects often spare other organs and lead to specific non-syndromic deafness. High throughput sequencing can reveal causes of sporadic deafness when combined with careful bioinformatics. Molecular approaches are also helpful for understanding more common forms of hearing impairment such as noise-induced hearing impairment. While molecular therapies are not yet clinically available, careful molecular genetic analysis helps to counsel the hearing impaired subjects.

Acute blockade of inner ear marginal and dark cell K+ secretion: Effects on gravity receptor function

Specific pharmacological blockade of KCNQ (Kv7) channels with XE991 rapidly (within 20 min) and profoundly alters inner ear gravity receptor responses to head motion (Lee et al., 2017). We hypothesized that these effects were attributable to the suppression of K⁺ secretion following blockade of KCNQ1-KCNE1 channels in vestibular dark cells and marginal cells. To test this hypothesis, K⁺ secretion was independently inhibited by blocking the Na⁺-K⁺-2Cl^- cotransporter (NKCC1, Slc12a2) rather than KCNQ1-KCNE1 channels. Acute blockade of NKCC1 with ethacrynic acid (40 mg/kg) eliminated auditory responses (ABRs) within approximately 70 min of injection, but had no effect on vestibular gravity receptor function (VsEPs) over a period of 2 h in the same animals. These findings show that, vestibular gravity receptors are highly resistant to acute disruption of endolymph secretion unlike the auditory system. Based on this we argue that acute suppression of K⁺ secretion alone does not likely account for the rapid profound effects of XE991 on gravity receptors. Instead the effects of XE991 likely require additional action at KCNQ channels located within the sensory epithelium itself.

Selective Modulation of K+ Channel Kv7.4 Significantly Affects the Excitability of DRN 5-HT Neurons

The serotonin (5-HT) system originating in the dorsal raphe nucleus (DRN) is implicated in various mood- and emotion-related disorders, such as anxiety, fear and stress. Abnormal activity of DRN 5-HT neurons is the key factor in the development of these disorders. Here, we describe a crucial role for the Kv7.4 potassium channel in modulating DRN 5-HT neuronal excitability. We demonstrate that Kv7.4 is selectively expressed in 5-HT neurons of the DRN. Using selective Kv7.4 opener fasudil and Kv7.4 knock-out mice, we demonstrate that Kv7.4 is a potent modulator of DRN 5-HT neuronal excitability. Furthermore, we demonstrate that the cellular redox signaling mechanism is involved in this 5-HT activation of Kv7.4. The current study suggests a new strategy for treating psychiatric disorders related to altered activity of DRN 5-HT neurons using K⁺ channel modulators.

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.

Coordinated Expression of Two Types of Low-Threshold K+ Channels Establishes Unique Single Spiking of Mauthner Cells among Segmentally Homologous Neurons in the Zebrafish Hindbrain

Expression of different ion channels permits homologously-generated neurons to acquire different types of excitability and thus code various kinds of input information. Mauthner (M) series neurons in the teleost hindbrain consist of M cells and their morphological homologs, which are repeated in adjacent segments and share auditory inputs. When excited, M cells generate a single spike at the onset of abrupt stimuli, while their homologs encode input intensity with firing frequency. Our previous study in zebrafish showed that immature M cells burst phasically at 2 d postfertilization (dpf) and acquire single spiking at 4 dpf by specific expression of auxiliary Kvβ2 subunits in M cells in association with common expression of Kv1.1 channels in the M series. Here, we further reveal the ionic mechanisms underlying this functional differentiation. Pharmacological blocking of Kv7/KCNQ in addition to Kv1 altered mature M cells to fire tonically, similar to the homologs. In contrast, blocking either channel alone caused M cells to burst phasically. M cells at 2 dpf fired tonically after blocking Kv7. In situ hybridization revealed specific Kv7.4/KCNQ4 expression in M cells at 2 dpf. Kv7.4 and Kv1.1 channels expressed in Xenopus oocytes exhibited low-threshold outward currents with slow and fast rise times, while coexpression of Kvβ2 accelerated and increased Kv1.1 currents, respectively. Computational models, modified from a mouse cochlear neuron model, demonstrated that Kv7.4 channels suppress repetitive firing to produce spike-frequency adaptation, while Kvβ2-associated Kv1.1 channels increase firing threshold and decrease the onset latency of spiking. Altogether, coordinated expression of these low-threshold K⁺ channels with Kvβ2 functionally differentiates M cells among homologous neurons.

Selective targeting of M-type potassium Kv 7.4 channels demonstrates their key role in the regulation of dopaminergic neuronal excitability and depression-like behaviour

BACKGROUND AND PURPOSE

The mesolimbic dopamine system originating in the ventral tegmental area (VTA) is involved in the development of depression, and firing patterns of VTA dopaminergic neurons are key determinants in this process. Here, we describe a crucial role for the M-type K_v 7.4 channels in modulating excitability of VTA dopaminergic neurons and in the development of depressive behaviour in mice.

EXPERIMENTAL APPROACH

We used K_v 7.4 channel knockout mice and a social defeat model of depression in combination with electrophysiological techniques (patch clamp recording and in vivo single-unit recordings), immunohistochemistry, single-cell PCR and behavioural analyses (social interaction time and glucose preference tests) to investigate VTA circuits involved in the development of depression-like behaviour.

KEY RESULTS

Among the K_v 7 channels, K_v 7.4 channels are selectively expressed in dopaminergic neurons of the VTA. Using a newly identified selective K_v 7.4 channel activator, fasudil, and K_v 7.4 channel knockout mice, we demonstrate that these channels are a dominant modulator of excitability of VTA dopaminergic neurons, in vitro and in vivo. Down-regulation of K_v 7.4 channels could be a causal factor of the altered excitability of VTA dopaminergic neurons and depression-like behaviour. The selective K_v 7.4 channel activator, fasudil, strongly alleviated depression-like behaviour in the social defeat mouse model of depression.

CONCLUSION AND IMPLICATIONS

Because expression of K_v 7.4 channels in the CNS is limited, selectively targeting this M channel subunit is likely to produce less on-target side effects than non-selective M channel modulators. Thus, K_v 7.4 channels may offer alternative targets in treatment of depression.

Hair Cell Transduction, Tuning, and Synaptic Transmission in the Mammalian Cochlea

Sound pressure fluctuations striking the ear are conveyed to the cochlea, where they vibrate the basilar membrane on which sit hair cells, the mechanoreceptors of the inner ear. Recordings of hair cell electrical responses have shown that they transduce sound via submicrometer deflections of their hair bundles, which are arrays of interconnected stereocilia containing the mechanoelectrical transducer (MET) channels. MET channels are activated by tension in extracellular tip links bridging adjacent stereocilia, and they can respond within microseconds to nanometer displacements of the bundle, facilitated by multiple processes of Ca2+-dependent adaptation. Studies of mouse mutants have produced much detail about the molecular organization of the stereocilia, the tip links and their attachment sites, and the MET channels localized to the lower end of each tip link. The mammalian cochlea contains two categories of hair cells. Inner hair cells relay acoustic information via multiple ribbon synapses that transmit rapidly without rundown. Outer hair cells are important for amplifying sound-evoked vibrations. The amplification mechanism primarily involves contractions of the outer hair cells, which are driven by changes in membrane potential and mediated by prestin, a motor protein in the outer hair cell lateral membrane. Different sound frequencies are separated along the cochlea, with each hair cell being tuned to a narrow frequency range; amplification sharpens the frequency resolution and augments sensitivity 100-fold around the cell's characteristic frequency. Genetic mutations and environmental factors such as acoustic overstimulation cause hearing loss through irreversible damage to the hair cells or degeneration of inner hair cell synapses. © 2017 American Physiological Society. Compr Physiol 7:1197-1227, 2017.

Effect of M-current modulation on mammalian vestibular responses to transient head motion

The precise role and mechanisms underlying efferent modulation of peripheral vestibular afferent function are not well understood in mammals. Clarifying the details of efferent action may lead to new strategies for clinical management of debilitating disturbances in vestibular and balance function. Recent evidence in turtle indicates that efferent modulation of M-currents is likely one mechanism for modifying afferent discharge. M-currents depend in part on KCNQ potassium conductances (Kv7), which can be adjusted through efferent activation of M1, M3, and/or M5 muscarinic acetylcholine receptors (mAChRs). How KCNQ channels and altered M-currents affect vestibular afferent function in vivo is unclear, and whether such a mechanism operates in mammals is unknown. In this study we used the KCNQ antagonist XE991 and the KCNQ activator retigabine in anesthetized mice to evaluate the effects of M-current modulation on peripheral vestibular responses to transient head motion. At low doses of XE991, responses were modestly enhanced, becoming larger in amplitude and shorter in latency. Higher doses of XE991 produced transient response enhancement, followed by steady-state suppression where latencies and thresholds increased and amplitudes decreased. Retigabine produced opposite effects. Auditory function was also impacted, based on results of companion auditory brain stem response testing. We propose that closure of KCNQ channels transforms vestibular afferent behavior by suppressing responses to transient high-frequency stimuli while simultaneously enhancing responses to sustained low-frequency stimulation. Our results clearly demonstrate that KCNQ channels are critical for normal mammalian vestibular function and suggest that efferent action may utilize these mechanisms to modulate the dynamic characteristics and gain of vestibular afferent responses.NEW & NOTEWORTHY The role of calyceal KCNQ channels and associated M-current in normal mammalian vestibular function is unknown. Our results show that calyceal KCNQ channels are critical for normal vestibular function in the intact mammal. The findings provide evidence that efferent modulation of M-currents may act normally to differentially adjust the sensitivity of vestibular neurons to transient and tonic stimulation and that such mechanisms may be targeted to achieve effective clinical management of vestibular disorders.

Connexin-Mediated Signaling in Nonsensory Cells Is Crucial for the Development of Sensory Inner Hair Cells in the Mouse Cochlea

Mutations in the genes encoding for gap junction proteins connexin 26 (Cx26) and connexin 30 (Cx30) have been linked to syndromic and nonsyndromic hearing loss in mice and humans. The release of ATP from connexin hemichannels in cochlear nonsensory cells has been proposed to be the main trigger for action potential activity in immature sensory inner hair cells (IHCs), which is crucial for the refinement of the developing auditory circuitry. Using connexin knock-out mice, we show that IHCs fire spontaneous action potentials even in the absence of ATP-dependent intercellular Ca²⁺ signaling in the nonsensory cells. However, this signaling from nonsensory cells was able to increase the intrinsic IHC firing frequency. We also found that connexin expression is key to IHC functional maturation. In Cx26 conditional knock-out mice (Cx26^Sox10-Cre), the maturation of IHCs, which normally occurs at approximately postnatal day 12, was partially prevented. Although Cx30 has been shown not to be required for hearing in young adult mice, IHCs from Cx30 knock-out mice exhibited a comprehensive brake in their development, such that their basolateral membrane currents and synaptic machinery retain a prehearing phenotype. We propose that IHC functional differentiation into mature sensory receptors is initiated in the prehearing cochlea provided that the expression of either connexin reaches a threshold level. As such, connexins regulate one of the most crucial functional refinements in the mammalian cochlea, the disruption of which contributes to the deafness phenotype observed in mice and DFNB1 patients.

SIGNIFICANCE STATEMENT The correct development and function of the mammalian cochlea relies not only on the sensory hair cells, but also on the surrounding nonsensory cells. Although the nonsensory cells have been largely implicated in the general homeostasis in the mature cochlea, their involvement in the initial functional differentiation of the sensory inner hair cells is less clear. Using mutant mouse models for the most common form of congenital deafness in humans, which are knock-outs for the gap-junction channels connexin 26 and connexin 30 genes, we show that defects in nonsensory cells prevented the functional maturation of inner hair cells. In connexin knock-outs, inner hair cells remained stuck at a prehearing stage of development and, as such, are unable to process sound information.

A novel pore-region mutation, c.887G > A (p.G296D) in KCNQ4, causing hearing loss in a Chinese family with autosomal dominant non-syndromic deafness 2

BACKGROUND

Hereditary non-syndromic hearing loss is the most common inherited sensory defect in humans. The KCNQ4 channel belongs to a family of potassium ion channels that play crucial roles in physiology and disease. Mutations in KCNQ4 underlie deafness non-syndromic autosomal dominant 2, a subtype of autosomal dominant, progressive, high-frequency hearing loss.

METHODS

A six-generation Chinese family from Hebei Province with autosomal dominantly inherited, sensorineural, postlingual, progressive hearing loss was enrolled in this study. Mutation screening of 129 genes associated with hearing loss was performed in five family members by next-generation sequencing (NGS). We also carried out variant analysis on DNA from 531 Chinese individuals with normal hearing as controls.

RESULTS

This family exhibits postlingual, progressive, symmetrical, bilateral, non-syndromic sensorineural hearing loss. NGS, bioinformatic analysis, and Sanger sequencing confirmed the co-segregation of a novel mutation [c.887G > A (p.G296D)] in KCNQ4 with the disease phenotype in this family. This mutation leads to a glycine-to-aspartic acid substitution at position 296 in the pore region of the KCNQ4 channel. This mutation affects a highly conserved glutamic acid. NGS is a highly efficient tool for identifying gene mutations causing heritable disease.

CONCLUSIONS

Progressive hearing loss is common in individuals with KCNQ4 mutations. NGS together with Sanger sequencing confirmed that the five affected members of this Chinese family inherited a missense mutation, c.887G > A (p.G296D), in exon 6 of KCNQ4. Our results increase the number of identified KCNQ4 mutations.

Conditional deletion of pejvakin in adult outer hair cells causes progressive hearing loss in mice

Mutations in the Pejvakin (Pjvk) gene cause autosomal recessive hearing loss DFNB59 with audiological features of auditory neuropathy spectrum disorder (ANSD) or cochlear dysfunction. The precise mechanisms underlying the variable clinical phenotypes of DFNB59 remain unclear. Here, we demonstrate that mice with conditional ablation of the Pjvk gene in all sensory hair cells or only in outer hair cells (OHCs) show similar auditory phenotypes with early-onset profound hearing loss. By contrast, loss of Pjvk in adult OHCs causes a slowly progressive hearing loss associated with OHC degeneration and delayed loss of inner hair cells (IHCs), indicating a primary role for pejvakin in regulating OHC function and survival. Consistent with this model, synaptic transmission at the IHC ribbon synapse is largely unaffected in sirtaki mice that carry a C-terminal deletion mutation in Pjvk. Using the C-terminal domain of pejvakin as bait, we identified in a cochlear cDNA library ROCK2, an effector for the small GTPase Rho, and the scaffold protein IQGAP1, involved in modulating actin dynamics. Both ROCK2 and IQGAP1 associate via their coiled-coil domains with pejvakin. We conclude that pejvakin is required to sustain OHC activity and survival in a cell-autonomous manner likely involving regulation of Rho signaling.

Towards the Prevention of Aminoglycoside-Related Hearing Loss

Aminoglycosides are potent antibiotics deployed worldwide despite their known side-effect of sensorineural hearing loss. The main etiology of this sensory deficit is death of inner ear sensory hair cells selectively triggered by aminoglycosides. For decades, research has sought to unravel the molecular events mediating sensory cell demise, emphasizing the roles of reactive oxygen species and their potentials as therapeutic targets. Studies in recent years have revealed candidate transport pathways including the mechanotransducer channel for drug entry into sensory cells. Once inside sensory cells, intracellular targets of aminoglycosides, such as the mitochondrial ribosomes, are beginning to be elucidated. Based on these results, less ototoxic aminoglycoside analogs are being generated and may serve as alternate antimicrobial agents. In this article, we review the latest findings on mechanisms of aminoglycoside entry into hair cells, their intracellular actions and potential therapeutic targets for preventing aminoglycoside ototoxicity.

Organ of Corti and Stria Vascularis: Is there an Interdependence for Survival?

Cochlear hair cells and the stria vascularis are critical for normal hearing. Hair cells transduce mechanical stimuli into electrical signals, whereas the stria is responsible for generating the endocochlear potential (EP), which is the driving force for hair cell mechanotransduction. We questioned whether hair cells and the stria interdepend for survival by using two mouse models. Atoh1 conditional knockout mice, which lose all hair cells within four weeks after birth, were used to determine whether the absence of hair cells would affect function and survival of stria. We showed that stria morphology and EP remained normal for long time despite a complete loss of all hair cells. We then used a mouse model that has an abnormal stria morphology and function due to mutation of the Mitf gene to determine whether hair cells are able to survive and transduce sound signals without a normal electrochemical environment in the endolymph. A strial defect, reflected by missing intermediate cells in the stria and by reduction of EP, led to systematic outer hair cell death from the base to the apex after postnatal day 18. However, an 18-mV EP was sufficient for outer hair cell survival. Surprisingly, inner hair cell survival was less vulnerable to reduction of the EP. Our studies show that normal function of the stria is essential for adult outer hair cell survival, while the survival and normal function of the stria vascularis do not depend on functional hair cells.

Modulation of Kv7 channels and excitability in the brain

Neuronal Kv7 channels underlie a voltage-gated non-inactivating potassium current known as the M-current. Due to its particular characteristics, Kv7 channels show pronounced control over the excitability of neurons. We will discuss various factors that have been shown to drastically alter the activity of this channel such as protein and phospholipid interactions, phosphorylation, calcium, and numerous neurotransmitters. Kv7 channels locate to key areas for the control of action potential initiation and propagation. Moreover, we will explore the dynamic surface expression of the channel modulated by neurotransmitters and neural activity. We will also focus on known principle functions of neural Kv7 channels: control of resting membrane potential and spiking threshold, setting the firing frequency, afterhyperpolarization after burst firing, theta resonance, and transient hyperexcitability from neurotransmitter-induced suppression of the M-current. Finally, we will discuss the contribution of altered Kv7 activity to pathologies such as epilepsy and cognitive deficits.

Alternative splicing of inner-ear-expressed genes

Alternative splicing plays a fundamental role in the development and physiological function of the inner ear. Inner-ear-specific gene splicing is necessary to establish the identity and maintain the function of the inner ear. For example, exon 68 of Cadherin 23 (Cdh23) gene is subject to inner-ear-specific alternative splicing, and as a result, Cdh23(+ 68) is only expressed in inner ear hair cells. Alternative splicing along the tonotopic axis of the cochlea contributes to frequency tuning, particularly in lower vertebrates, such as chickens and turtles. Differential splicing of Kcnma1, which encodes for the α subunit of the Ca(2+)-activated K(+) channel (BK channel), has been suggested to affect the channel gating properties and is important for frequency tuning. Consequently, deficits in alternative splicing have been shown to cause hearing loss, as we can observe in Bronx Waltzer (bv) mice and Sfswap mutant mice. Despite the advances in this field, the regulation of alternative splicing in the inner ear remains elusive. Further investigation is also needed to clarify the mechanism of hearing loss caused by alternative splicing deficits.

Three-dimensional image analysis of the mouse cochlea

The mouse has proven to be an essential model system for studying hearing loss. A key advantage of the mouse is the ability to image the sensory cells in the cochlea. Many different protocols exist for the dissection and imaging of the cochlea. Here we describe a method that utilizes confocal imaging of whole-mount preparations followed by 3D analysis using the Imaris software. The 3D analysis of confocal stacks has been successfully used for investigating a number of mouse tissues and developmental processes. We propose that this method is also a valuable tool to analyze the cellular and tissue organization of the sensory hair cells in the cochlea.

KCNQ potassium channels in sensory system and neural circuits

M channels, an important regulator of neural excitability, are composed of four subunits of the Kv7 (KCNQ) K(+) channel family. M channels were named as such because their activity was suppressed by stimulation of muscarinic acetylcholine receptors. These channels are of particular interest because they are activated at the subthreshold membrane potentials. Furthermore, neural KCNQ channels are drug targets for the treatments of epilepsy and a variety of neurological disorders, including chronic and neuropathic pain, deafness, and mental illness. This review will update readers on the roles of KCNQ channels in the sensory system and neural circuits as well as discuss their respective mechanisms and the implications for physiology and medicine. We will also consider future perspectives and the development of additional pharmacological models, such as seizure, stroke, pain and mental illness, which work in combination with drug-design targeting of KCNQ channels. These models will hopefully deepen our understanding of KCNQ channels and provide general therapeutic prospects of related channelopathies.

KCNK5 channels mostly expressed in cochlear outer sulcus cells are indispensable for hearing

In the cochlea, K(+) is essential for mechano-electrical transduction. Here, we explore cochlear structure and function in mice lacking K(+) channels of the two-pore domain family. A profound deafness associated with a decrease in endocochlear potential is found in adult Kcnk5(-/-) mice. Hearing occurs around postnatal day 19 (P19), and completely disappears 2 days later. At P19, Kcnk5(-/-) mice have a normal endolymphatic [K(+)] but a partly lowered endocochlear potential. Using Lac-Z as a gene reporter, KCNK5 is mainly found in outer sulcus Claudius', Boettcher's and root cells. Low levels of expression are also seen in the spiral ganglion, Reissner's membrane and stria vascularis. Essential channels (KCNJ10 and KCNQ1) contributing to K(+) secretion in stria vascularis have normal expression in Kcnk5(-/-) mice. Thus, KCNK5 channels are indispensable for the maintenance of hearing. Among several plausible mechanisms, we emphasize their role in K(+) recycling along the outer sulcus lateral route.

Characteristics of K(+) Outward Currents in the Cochlear Outer Hair Cells of Circling Mice within the First Postnatal Week

K⁺ outward currents in the outer hair cells (OHCs) of circling mice (homozygous (cir/cir) mice), an animal model for human deafness (DFNB6 type), were investigated using a whole cell patch clamp technique. Littermate heterozygous (+/cir) mice of the same age (postnatal day (P) 0 -P6) were used as controls. Similar slow rising K⁺ currents were observed in both genotypes, but their biophysical and pharmacological properties were quite different. The values of V_half for activation were significantly different in the heterozygous (+/cir) and homozygous (cir/cir) mice (-8.1±2.2 mV, heterozygous (+/cir) mice (n=7) and -17.2±4.2 mV, homozygous (cir/cir) mice (n=5)). The inactivation curve was expressed by a single first order Boltzmann equation in the homozygous (cir/cir) mice, while it was expressed by a sum of two first order Boltzmann equations in the heterozygous (+/cir) mice. The K⁺ current of homozygous (cir/cir) mice was more sensitive to TEA in the 1 to 10 mM range, while the 4-AP sensitivities were not different between the two genotypes. Removal of external Ca²⁺ did not affect the K⁺ currents in either genotype, indicating that the higher sensitivity of K⁺ current to TEA in the homozygous (cir/cir) mice was not due to an early expression of Ca²⁺ activated K⁺ channels. Our results suggest that the K⁺ outward current of developing homozygous (cir/cir) mice OHCs is different in both biophysical and pharmacological aspects than that of heterozygous (+/cir) mice.

Hearing Loss and Otopathology Following Systemic and Intracerebroventricular Delivery of 2-Hydroxypropyl-Beta-Cyclodextrin

Cyclodextrins are simple yet powerful molecules widely used in medicinal formulations and industry for their ability to stabilize and solubilize guest compounds. However, recent evidence shows that 2-hydroxypropyl-β-cyclodextrin (HPβCD) causes severe hearing loss in mice, selectively killing outer hair cells (OHC) within 1 week of subcutaneous drug treatment. In the current study, the impact of HPβCD on auditory physiology and pathology was explored further as a function of time and route of administration. When administered subcutaneously or directly into cerebrospinal fluid, single injections of HPβCD caused up to 60 dB threshold shifts and widespread OHC loss in a dose-dependent manner. Combined dosing caused no greater deficit, suggesting a common mode of action. After drug treatment, OHC loss progressed over time, beginning in the base and extending toward the apex, creating a sharp transition between normal and damaged regions of the cochlea. Administration into cerebrospinal fluid caused rapid ototoxicity when compared to subcutaneous delivery. Despite the devastating effect on the cochlea, HPβCD was relatively safe to other peripheral and central organ systems; specifically, it had no notable nephrotoxicity in contrast to other ototoxic compounds like aminoglycosides and platinum-based drugs. As cyclodextrins find expanding medicinal applications, caution should be exercised as these drugs possess a unique, poorly understood, ototoxic mechanism.

Potassium channel activator attenuates salicylate-induced cochlear hearing loss potentially ameliorating tinnitus

High dose sodium salicylate causes moderate, reversible hearing loss and tinnitus. Salicylate-induced hearing loss is believed to arise from a reduction in the electromotile response of outer hair cells (OHCs) and/or reduction of KCNQ4 potassium currents in OHCs, which decreases the driving force for the transduction current. Therefore, enhancing OHC potassium currents could potentially prevent salicylate-induced temporary hearing loss. In this study, we tested whether opening voltage-gated potassium channels using ICA-105665, a novel small molecule that opens KCNQ2/3 and KCNQ3/5 channels, can reduce salicylate-induced hearing loss. We found that systemic application of ICA-105665 at 10 mg/kg prevented the salicylate-induced amplitude reduction and threshold shift in the compound action potentials recorded at the round window of the cochlea. ICA-105665 also prevented the salicylate-induced reduction of distortion-product otoacoustic emission. These results suggest that ICA-105665 partially compensates for salicylate-induced cochlear hearing loss by enhancing KCNQ2/3 and KCNQ3/5 potassium currents and the motility of OHCs.

The role of the cytosolic HSP70 chaperone system in diseases caused by misfolding and aberrant trafficking of ion channels

Protein-folding diseases are an ongoing medical challenge. Many diseases within this group are genetically determined, and have no known cure. Among the examples in which the underlying cellular and molecular mechanisms are well understood are diseases driven by misfolding of transmembrane proteins that normally function as cell-surface ion channels. Wild-type forms are synthesized and integrated into the endoplasmic reticulum (ER) membrane system and, upon correct folding, are trafficked by the secretory pathway to the cell surface. Misfolded mutant forms traffic poorly, if at all, and are instead degraded by the ER-associated proteasomal degradation (ERAD) system. Molecular chaperones can assist the folding of the cytosolic domains of these transmembrane proteins; however, these chaperones are also involved in selecting misfolded forms for ERAD. Given this dual role of chaperones, diseases caused by the misfolding and aberrant trafficking of ion channels (referred to here as ion-channel-misfolding diseases) can be regarded as a consequence of insufficiency of the pro-folding chaperone activity and/or overefficiency of the chaperone ERAD role. An attractive idea is that manipulation of the chaperones might allow increased folding and trafficking of the mutant proteins, and thereby partial restoration of function. This Review outlines the roles of the cytosolic HSP70 chaperone system in the best-studied paradigms of ion-channel-misfolding disease--the CFTR chloride channel in cystic fibrosis and the hERG potassium channel in cardiac long QT syndrome type 2. In addition, other ion channels implicated in ion-channel-misfolding diseases are discussed.

Targeted high-throughput sequencing identifies pathogenic mutations in KCNQ4 in two large Chinese families with autosomal dominant hearing loss

Autosomal dominant non-syndromic hearing loss (ADNSHL) is highly heterogeneous, among them, KCNQ4 is one of the most frequent disease-causing genes. More than twenty KCNQ4 mutations have been reported, but none of them were detected in Chinese mainland families. In this study, we identified a novel KCNQ4 mutation in a five generation Chinese family with 84 members and a known KCNQ4 mutation in a six generation Chinese family with 66 members. Mutation screening of 30 genes for ADNSHL was performed in the probands from thirty large Chinese families with ADNSHL by targeted region capture and high-throughput sequencing. The candidate variants and the co-segregation of the phenotype were verified by polymerase chain reaction (PCR) amplification and Sanger sequencing in all ascertained family members. Then we identified a novel KCNQ4 mutation p.W275R in exon 5 and a known KCNQ4 mutation p.G285S in exon 6 in two large Chinese ADNSHL families segregating with post-lingual high frequency-involved and progressive sensorineural hearing loss. This is the first report of KCNQ4 mutation in Chinese mainland families. KCNQ4, a member of voltage-gated potassium channel family, is likely to be a common gene in Chinese patients with ADNSHL. The results also support that the combination of targeted enrichment and high-throughput sequencing is a valuable molecular diagnostic tool for autosomal dominant hereditary deafness.

Genetics of auditory mechano-electrical transduction

The hair bundles of cochlear hair cells play a central role in the auditory mechano-electrical transduction (MET) process. The identification of MET components and of associated molecular complexes by biochemical approaches is impeded by the very small number of hair cells within the cochlea. In contrast, human and mouse genetics have proven to be particularly powerful. The study of inherited forms of deafness led to the discovery of several essential proteins of the MET machinery, which are currently used as entry points to decipher the associated molecular networks. Notably, MET relies not only on the MET machinery but also on several elements ensuring the proper sound-induced oscillation of the hair bundle or the ionic environment necessary to drive the MET current. Here, we review the most significant advances in the molecular bases of the MET process that emerged from the genetics of hearing.

Spatiotemporal expression of TRPM4 in the mouse cochlea

The present study was conducted to elucidate the presence of the transient receptor potential cation channel subfamily M member 4, TRPM4, in the mouse inner ear. TRPM4 immunoreactivity (IR) was found in the cell body of inner hair cells (IHCs) in the organ of Corti in the apical side of marginal cells of the stria vascularis, in the apical portion of the dark cells of the vestibule, and in a subset of the type II neurons in the spiral ganglion. Subsequently, changes in the distribution and expression of TRPM4 in the inner ear during embryonic and postnatal developments were also evaluated. Immunohistochemical localization demonstrated that the emergence of the TRPM4-IR in IHCs occurs shortly before the onset of hearing, whereas that in the marginal cells happens earlier, at the time of birth, coinciding with the onset of endolymph formation. Furthermore, semiquantitative real-time PCR assay showed that expressions of TRPM4 in the organ of Corti and in the stria vascularis increased dramatically at the onset of hearing. Because TRPM4 is a Ca(2+) -activated monovalent-selective cation channel, these findings imply that TRPM4 contributes to potassium ion transport, essential for the signal transduction in IHCs and the formation of endolymph by marginal cells.

Stretch-activation of angiotensin II type 1a receptors contributes to the myogenic response of mouse mesenteric and renal arteries

RATIONALE

Vascular wall stretch is the major stimulus for the myogenic response of small arteries to pressure. The molecular mechanisms are elusive, but recent findings suggest that G protein-coupled receptors can elicit a stretch response.

OBJECTIVE

To determine whether angiotensin II type 1 receptors (AT1R) in vascular smooth muscle cells exert mechanosensitivity and identify the downstream ion channel mediators of myogenic vasoconstriction.

METHODS AND RESULTS

We used mice deficient in AT1R signaling molecules and putative ion channel targets, namely AT1R, angiotensinogen, transient receptor potential channel 6 (TRPC6) channels, or several subtypes of the voltage-gated K+ (Kv7) gene family (KCNQ3, 4, or 5). We identified a mechanosensing mechanism in isolated mesenteric arteries and in the renal circulation that relies on coupling of the AT1R subtype a to a Gq/11 protein as a critical event to accomplish the myogenic response. Arterial mechanoactivation occurs after pharmacological block of AT1R and in the absence of angiotensinogen or TRPC6 channels. Activation of AT1R subtype a by osmotically induced membrane stretch suppresses an XE991-sensitive Kv channel current in patch-clamped vascular smooth muscle cells, and similar concentrations of XE991 enhance mesenteric and renal myogenic tone. Although XE991-sensitive KCNQ3, 4, and 5 channels are expressed in vascular smooth muscle cells, XE991-sensitive K+ current and myogenic contractions persist in arteries deficient in these channels.

CONCLUSIONS

Our results provide definitive evidence that myogenic responses of mouse mesenteric and renal arteries rely on ligand-independent, mechanoactivation of AT1R subtype a. The AT1R subtype a signal relies on an ion channel distinct from TRPC6 or KCNQ3, 4, or 5 to enact vascular smooth muscle cell activation and elevated vascular resistance.