Transient expression of an inwardly rectifying potassium conductance in developing inner and outer hair cells along the mouse cochlea

Charge and lipophilicity are required for effective block of the hair-cell mechano-electrical transducer channel by FM1-43 and its derivatives

The styryl dye FM1-43 is widely used to study endocytosis but behaves as a permeant blocker of the mechano-electrical transducer (MET) channel in sensory hair cells, loading rapidly and specifically into the cytoplasm of hair cells in a MET channel-dependent manner. Patch clamp recordings of mouse outer hair cells (OHCs) were used to determine how a series of structural modifications of FM1-43 affect MET channel block. Fluorescence microscopy was used to assess how the modifications influence hair-cell loading in mouse cochlear cultures and zebrafish neuromasts. Cochlear cultures were also used to evaluate otoprotective potential of the modified FM1-43 derivatives. Structure-activity relationships reveal that the lipophilic tail and the cationic head group of FM1-43 are both required for MET channel block in mouse cochlear OHCs; neither moiety alone is sufficient. The extent of MET channel block is augmented by increasing the lipophilicity/bulkiness of the tail, by reducing the number of positive charges in the head group from two to one, or by increasing the distance between the two charged head groups. Loading assays with zebrafish neuromasts and mouse cochlear cultures are broadly in accordance with these observations but reveal a loss of hair-cell specific labelling with increasing lipophilicity. Although FM1-43 and many of its derivatives are generally cytotoxic when tested on cochlear cultures in the presence of an equimolar concentration of the ototoxic antibiotic gentamicin (5 µM), at a 10-fold lower concentration (0.5 µM), two of the derivatives protect OHCs from cell death caused by 48 h-exposure to 5 µM gentamicin.

AAV-mediated rescue of Eps8 expression in vivo restores hair-cell function in a mouse model of recessive deafness

The transduction of acoustic information by hair cells depends upon mechanosensitive stereociliary bundles that project from their apical surface. Mutations or absence of the stereociliary protein EPS8 cause deafness in humans and mice, respectively. Eps8 knockout mice (Eps8 -/- ) have hair cells with immature stereocilia and fail to become sensory receptors. Here, we show that exogenous delivery of Eps8 using Anc80L65 in P1-P2 Eps8 -/- mice in vivo rescued the hair bundle structure of apical-coil hair cells. Rescued hair bundles correctly localize EPS8, WHIRLIN, MYO15, and BAIAP2L2, and generate normal mechanoelectrical transducer currents. Inner hair cells with normal-looking stereocilia re-expressed adult-like basolateral ion channels (BK and KCNQ4) and have normal exocytosis. The number of hair cells undergoing full recovery was not sufficient to rescue hearing in Eps8 -/- mice. Adeno-associated virus (AAV)-transduction of P3 apical-coil and P1-P2 basal-coil hair cells does not rescue hair cells, nor does Anc80L65-Eps8 delivery in adult Eps8 -/- mice. We propose that AAV-induced gene-base therapy is an efficient strategy to recover the complex hair-cell defects in Eps8 -/- mice. However, this therapeutic approach may need to be performed in utero since, at postnatal ages, Eps8 -/- hair cells appear to have matured or accumulated damage beyond the point of repair.

Identification of a series of hair-cell MET channel blockers that protect against aminoglycoside-induced ototoxicity

To identify small molecules that shield mammalian sensory hair cells from the ototoxic side effects of aminoglycoside antibiotics, 10,240 compounds were initially screened in zebrafish larvae, selecting for those that protected lateral-line hair cells against neomycin and gentamicin. When the 64 hits from this screen were retested in mouse cochlear cultures, 8 protected outer hair cells (OHCs) from gentamicin in vitro without causing hair-bundle damage. These 8 hits shared structural features and blocked, to varying degrees, the OHC's mechano-electrical transducer (MET) channel, a route of aminoglycoside entry into hair cells. Further characterization of one of the strongest MET channel blockers, UoS-7692, revealed it additionally protected against kanamycin and tobramycin and did not abrogate the bactericidal activity of gentamicin. UoS-7692 behaved, like the aminoglycosides, as a permeant blocker of the MET channel; significantly reduced gentamicin-Texas red loading into OHCs; and preserved lateral-line function in neomycin-treated zebrafish. Transtympanic injection of UoS-7692 protected mouse OHCs from furosemide/kanamycin exposure in vivo and partially preserved hearing. The results confirmed the hair-cell MET channel as a viable target for the identification of compounds that protect the cochlea from aminoglycosides and provide a series of hit compounds that will inform the design of future otoprotectants.

Developmental and Functional Hair Cell-Like Cells Induced by Atoh1 Overexpression in the Adult Mammalian Cochlea In Vitro

Hair cells (HCs) in the mammalian cochleae cannot spontaneously regenerate once damaged, resulting in permanent hearing loss. It has been shown that Atoh1 overexpression induces hair cell-like cells (HCLCs) in the cochlea of newborn rodents, but this is hard to achieve in adult mammals. In this study, we used a three-dimensional cochlear culture system and an adenoviral-mediated delivery vector to overexpress Atoh1 in adult mouse cochleae. HCLCs were successfully induced from 3 days after virus infection (3 DVI) in vitro, and the number increased with time. HCLCs were myosin7a positive and distinguishable from remnant HCs in a culture environment. Meanwhile, patch-clamp results showed that noninactive outward potassium currents (sustained outward potassium currents) could be recorded in HCLCs and that their magnitude increased with time, similar to normal HCs. Furthermore, transient HCN currents were recorded in some HCLCs, indicating that the HCLCs experienced a developmental stage similar to normal HCs. We also compared the electrophysiological features of HCLCs from adult mice with native HCs and found the HCLCs gradually matured, similar to the normal HCs. Meanwhile, HCLCs from adult mice possessed the same bundles as developmental HCs. However, these HCLCs did not express prestin, which is a special marker for outer hair cells (OHCs), even at 13 DVI. These results demonstrate that Atoh1 overexpression induces HCLC formation in the adult mammalian cochlea and that these HCLCs were functional and experienced a developmental process similar to that of normal HCs.

Generation of Otic Lineages from Integration-Free Human-Induced Pluripotent Stem Cells Reprogrammed by mRNAs

Damage to the sensory hair cells and the spiral ganglion neurons of the cochlea leads to deafness. Induced pluripotent stem cells (iPSCs) are a promising tool to regenerate the cells in the inner ear that have been affected by pathology or have been lost. To facilitate the clinical application of iPSCs, the reprogramming process should minimize the risk of introducing undesired genetic alterations while conferring the cells the capacity to differentiate into the desired cell type. Currently, reprogramming induced by synthetic mRNAs is considered to be one of the safest ways of inducing pluripotency, as the transgenes are transiently delivered into the cells without integrating into the genome. In this study, we explore the ability of integration-free human-induced pluripotent cell lines that were reprogrammed by mRNAs, to differentiate into otic progenitors and, subsequently, into hair cell and neuronal lineages. hiPSC lines were induced to differentiate by culturing them in the presence of fibroblast growth factors 3 and 10 (FGF3 and FGF10). Progenitors were identified by quantitative microscopy, based on the coexpression of otic markers PAX8, PAX2, FOXG1, and SOX2. Otic epithelial progenitors (OEPs) and otic neuroprogenitors (ONPs) were purified and allowed to differentiate further into hair cell-like cells and neurons. Lineages were characterised by immunocytochemistry and electrophysiology. Neuronal cells showed inward Na⁺ (I_Na) currents and outward (I_k) and inward K⁺ (I_K1) currents while hair cell-like cells had inward I_K1 and outward delayed rectifier K⁺ currents, characteristic of developing hair cells. We conclude that human-induced pluripotent cell lines that have been reprogrammed using nonintegrating mRNAs are capable to differentiate into otic cell types.

Hair cell maturation is differentially regulated along the tonotopic axis of the mammalian cochlea

Key points

Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea.

Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset.

We found that OHCs, like IHCs, fire spontaneous Ca²⁺‐induced action potentials (APs) during immature stages of development, which are driven by Ca_V1.3 Ca²⁺ channels.

We also showed that the development of low‐ and high‐frequency hair cells is differentially regulated during pre‐hearing stages, with the former cells being more strongly dependent on experience‐independent Ca²⁺ action potential activity.

Abstract

Sound amplification within the mammalian cochlea depends upon specialized hair cells, the outer hair cells (OHCs), which possess both sensory and motile capabilities. In various altricial rodents, OHCs become functionally competent from around postnatal day 7 (P7), before the primary sensory inner hair cells (IHCs), which become competent at about the onset of hearing (P12). The mechanisms responsible for the maturation of OHCs and their synaptic specialization remain poorly understood. We report that spontaneous Ca²⁺ activity in the immature cochlea, which is generated by Ca_V1.3 Ca²⁺ channels, differentially regulates the maturation of hair cells along the cochlea. Under near‐physiological recording conditions we found that, similar to IHCs, immature OHCs elicited spontaneous Ca²⁺ action potentials (APs), but only during the first few postnatal days. Genetic ablation of these APs in vivo, using Ca_V1.3^−/− mice, prevented the normal developmental acquisition of mature‐like basolateral membrane currents in low‐frequency (apical) hair cells, such as I_K,n (carried by KCNQ4 channels), I_SK2 and I_ACh (α9α10nAChRs) in OHCs and I_K,n and I_K,f (BK channels) in IHCs. Electromotility and prestin expression in OHCs were normal in Ca_V1.3^−/− mice. The maturation of high‐frequency (basal) hair cells was also affected in Ca_V1.3^−/− mice, but to a much lesser extent than apical cells. However, a characteristic feature in Ca_V1.3^−/− mice was the reduced hair cell size irrespective of their cochlear location. We conclude that the development of low‐ and high‐frequency hair cells is differentially regulated during development, with apical cells being more strongly dependent on experience‐independent Ca²⁺ APs.

Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea.

Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset.

We found that OHCs, like IHCs, fire spontaneous Ca²⁺‐induced action potentials (APs) during immature stages of development, which are driven by Ca_V1.3 Ca²⁺ channels.

We also showed that the development of low‐ and high‐frequency hair cells is differentially regulated during pre‐hearing stages, with the former cells being more strongly dependent on experience‐independent Ca²⁺ action potential activity.

Clarin-2 is essential for hearing by maintaining stereocilia integrity and function

Hearing relies on mechanically gated ion channels present in the actin-rich stereocilia bundles at the apical surface of cochlear hair cells. Our knowledge of the mechanisms underlying the formation and maintenance of the sound-receptive structure is limited. Utilizing a large-scale forward genetic screen in mice, genome mapping and gene complementation tests, we identified Clrn2 as a new deafness gene. The Clrn2clarinet/clarinet mice (p.Trp4* mutation) exhibit a progressive, early-onset hearing loss, with no overt retinal deficits. Utilizing data from the UK Biobank study, we could show that CLRN2 is involved in human non-syndromic progressive hearing loss. Our in-depth morphological, molecular and functional investigations establish that while it is not required for initial formation of cochlear sensory hair cell stereocilia bundles, clarin-2 is critical for maintaining normal bundle integrity and functioning. In the differentiating hair bundles, lack of clarin-2 leads to loss of mechano-electrical transduction, followed by selective progressive loss of the transducing stereocilia. Together, our findings demonstrate a key role for clarin-2 in mammalian hearing, providing insights into the interplay between mechano-electrical transduction and stereocilia maintenance.

Gata3 is required for the functional maturation of inner hair cells and their innervation in the mouse cochlea

KEY POINTS

The physiological maturation of auditory hair cells and their innervation requires precise temporal and spatial control of cell differentiation. The transcription factor gata3 is essential for the earliest stages of auditory system development and for survival and synaptogenesis in auditory sensory afferent neurons. We show that during postnatal development in the mouse inner ear gata3 is required for the biophysical maturation, growth and innervation of inner hair cells; in contrast, it is required only for the survival of outer hair cells. Loss of gata3 in inner hair cells causes progressive hearing loss and accounts for at least some of the deafness associated with the human hypoparathyroidism, deafness and renal anomaly (HDR) syndrome. The results show that gata3 is critical for later stages of mammalian auditory system development where it plays distinct, complementary roles in the coordinated maturation of sensory hair cells and their innervation.

ABSTRACT

The zinc finger transcription factor gata3 regulates inner ear development from the formation of the embryonic otic placode. Throughout development, gata3 is expressed dynamically in all the major cochlear cell types. Its role in afferent formation is well established but its possible involvement in hair cell maturation remains unknown. Here, we find that in heterozygous gata3 null mice (gata3+/- ) outer hair cells (OHCs) differentiate normally but their numbers are significantly lower. In contrast, inner hair cells (IHCs) survive normally but they fail to acquire adult basolateral membrane currents, retain pre-hearing current and efferent innervation profiles and have fewer ribbon synapses. Targeted deletion of gata3 driven by otoferlin-cre recombinase (gata3fl/fl otof-cre+/- ) in IHCs does not affect OHCs or the number of IHC afferent synapses but it leads to a failure in IHC maturation comparable to that observed in gata3+/- mice. Auditory brainstem responses in gata3fl/fl otof-cre+/- mice reveal progressive hearing loss that becomes profound by 6-7 months, whilst distortion product otoacoustic emissions are no different to control animals up to this age. Our results, alongside existing data, indicate that gata3 has specific, complementary functions in different cell types during inner ear development and that its continued expression in the sensory epithelium orchestrates critical aspects of physiological development and neural connectivity. Furthermore, our work indicates that hearing loss in human hypoparathyroidism, deafness and renal anomaly (HDR) syndrome arises from functional deficits in IHCs as well as loss of function from OHCs and both afferent and efferent neurons.

Mechanotransduction is required for establishing and maintaining mature inner hair cells and regulating efferent innervation

In the adult auditory organ, mechanoelectrical transducer (MET) channels are essential for transducing acoustic stimuli into electrical signals. In the absence of incoming sound, a fraction of the MET channels on top of the sensory hair cells are open, resulting in a sustained depolarizing current. By genetically manipulating the in vivo expression of molecular components of the MET apparatus, we show that during pre-hearing stages the MET current is essential for establishing the electrophysiological properties of mature inner hair cells (IHCs). If the MET current is abolished in adult IHCs, they revert into cells showing electrical and morphological features characteristic of pre-hearing IHCs, including the re-establishment of cholinergic efferent innervation. The MET current is thus critical for the maintenance of the functional properties of adult IHCs, implying a degree of plasticity in the mature auditory system in response to the absence of normal transduction of acoustic signals.

Mechanoelectrical transducer (MET) channels on the tips of inner hair cells are essential for transducing auditory sensory information. Here, the authors show that disrupting MET channel function also prevents the preservation of normal inner hair cell identity in adult mice.

TMC2 Modifies Permeation Properties of the Mechanoelectrical Transducer Channel in Early Postnatal Mouse Cochlear Outer Hair Cells

The ability of cochlear hair cells to convert sound into receptor potentials relies on the mechanoelectrical transducer (MET) channels present in their stereociliary bundles. There is strong evidence implying that transmembrane channel-like protein (TMC) 1 contributes to the pore-forming subunit of the mature MET channel, yet its expression is delayed (~>P5 in apical outer hair cells, OHCs) compared to the onset of mechanotransduction (~P1). Instead, the temporal expression of TMC2 coincides with this onset, indicating that it could be part of the immature MET channel. We investigated MET channel properties from OHCs of homo- and heterozygous Tmc2 knockout mice. In the presence of TMC2, the MET channel blocker dihydrostreptomycin (DHS) had a lower affinity for the channel, when the aminoglycoside was applied extracellularly or intracellularly, with the latter effect being more pronounced. In Tmc2 knockout mice OHCs were protected from aminoglycoside ototoxicity during the first postnatal week, most likely due to their small MET current and the lower saturation level for aminoglycoside entry into the individual MET channels. DHS entry through the MET channels of Tmc2 knockout OHCs was lower during the first than in the second postnatal week, suggestive of a developmental change in the channel pore properties independent of TMC2. However, the ability of TMC2 to modify the MET channel properties strongly suggests it contributes to the pore-forming subunit of the neonatal channel. Nevertheless, we found that TMC2, different from TMC1, is not necessary for OHC development. While TMC2 is required for mechanotransduction in mature vestibular hair cells, its expression in the immature cochlea may be an evolutionary remnant.

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.

Inhibition of K+ outward currents by linopirdine in the cochlear outer hair cells of circling mice within the first postnatal week

Inhibition of K⁺ outward currents by linopirdine in the outer hair cells (OHCs) of circling mice (homozygous (cir/cir) mice), an animal model for human deafness (DFNB6 type), was investigated using a whole cell patch clamp technique. Littermate heterozygous (+/cir) and ICR mice of the same age (postnatal day (P) 0 –P6) were used as controls. Voltage steps from –100 mV to 40 mV elicited small inward currents (–100 mV~–70 mV) and slow rising K⁺ outward currents (–60 mV ~40 mV) which activated near –50 mV in all OHCs tested. Linopirdine, a known blocker of K⁺ currents activated at negative potentials (I_K,n), did cause inhibition at varying degree (severe, moderate, mild) in K⁺ outward currents of heterozygous (+/cir) or homozygous (cir/cir) mice OHCs in the concentration range between 1 and 100 µM, while it was apparent only in one ICR mice OHC out of nine OHCs at 100 µM. Although the half inhibition concentrations in heterozygous (+/cir) or homozygous (cir/cir) mice OHCs were close to those reported in I_K,n, biophysical and pharmacological properties of K⁺ outward currents, such as the activation close to –50 mV, small inward currents evoked by hyperpolarizing steps and TEA sensitivity, were not in line with I_K,n reported in other tissues. Our results show that the delayed rectifier type K⁺ outward currents, which are not similar to I_K,n with respect to biophysical and pharmacological properties, are inhibited by linopirdine in the developing (P0~P6) homozygous (cir/cir) or heterozygous (+/cir) mice OHCs.

Distinct capacity for differentiation to inner ear cell types by progenitor cells of the cochlea and vestibular organs

Disorders of hearing and balance are most commonly associated with damage to cochlear and vestibular hair cells or neurons. Although these cells are not capable of spontaneous regeneration, progenitor cells in the hearing and balance organs of the neonatal mammalian inner ear have the capacity to generate new hair cells after damage. To investigate whether these cells are restricted in their differentiation capacity, we assessed the phenotypes of differentiated progenitor cells isolated from three compartments of the mouse inner ear - the vestibular and cochlear sensory epithelia and the spiral ganglion - by measuring electrophysiological properties and gene expression. Lgr5⁺ progenitor cells from the sensory epithelia gave rise to hair cell-like cells, but not neurons or glial cells. Newly created hair cell-like cells had hair bundle proteins, synaptic proteins and membrane proteins characteristic of the compartment of origin. PLP1⁺ glial cells from the spiral ganglion were identified as neural progenitors, which gave rise to neurons, astrocytes and oligodendrocytes, but not hair cells. Thus, distinct progenitor populations from the neonatal inner ear differentiate to cell types associated with their organ of origin.

Functional development of mechanosensitive hair cells in stem cell-derived organoids parallels native vestibular hair cells

Inner ear sensory epithelia contain mechanosensitive hair cells that transmit information to the brain through innervation with bipolar neurons. Mammalian hair cells do not regenerate and are limited in number. Here we investigate the potential to generate mechanosensitive hair cells from mouse embryonic stem cells in a three-dimensional (3D) culture system. The system faithfully recapitulates mouse inner ear induction followed by self-guided development into organoids that morphologically resemble inner ear vestibular organs. We find that organoid hair cells acquire mechanosensitivity equivalent to functionally mature hair cells in postnatal mice. The organoid hair cells also progress through a similar dynamic developmental pattern of ion channel expression, reminiscent of two subtypes of native vestibular hair cells. We conclude that our 3D culture system can generate large numbers of fully functional sensory cells which could be used to investigate mechanisms of inner ear development and disease as well as regenerative mechanisms for inner ear repair.

Sensory hair cells from the mammalian inner ear do not regenerate. Here, the authors induce direct hair cell formation from mouse embryonic stem cells using a three-dimensional culture system and observe differentiation of Type I and Type II vestibular hair cells and establishment of synapses with neurons.

Assessment of the expression and role of the α1-nAChR subunit in efferent cholinergic function during the development of the mammalian cochlea

Hair cell (HC) activity in the mammalian cochlea is modulated by cholinergic efferent inputs from the brainstem. These inhibitory inputs are mediated by calcium-permeable nicotinic acetylcholine receptors (nAChRs) containing α9- and α10-subunits and by subsequent activation of calcium-dependent potassium channels. Intriguingly, mRNAs of α1- and γ-nAChRs, subunits of the "muscle-type" nAChR have also been found in developing HCs (Cai T, Jen HI, Kang H, Klisch TJ, Zoghbi HY, Groves AK. J Neurosci 35: 5870-5883, 2015; Scheffer D, Sage C, Plazas PV, Huang M, Wedemeyer C, Zhang DS, Chen ZY, Elgoyhen AB, Corey DP, Pingault V. J Neurochem 103: 2651-2664, 2007; Sinkkonen ST, Chai R, Jan TA, Hartman BH, Laske RD, Gahlen F, Sinkkonen W, Cheng AG, Oshima K, Heller S. Sci Rep 1: 26, 2011) prompting proposals that another type of nAChR is present and may be critical during early synaptic development. Mouse genetics, histochemistry, pharmacology, and whole cell recording approaches were combined to test the role of α1-nAChR subunit in HC efferent synapse formation and cholinergic function. The onset of α1-mRNA expression in mouse HCs was found to coincide with the onset of the ACh response and efferent synaptic function. However, in mouse inner hair cells (IHCs) no response to the muscle-type nAChR agonists (±)-anatoxin A, (±)-epibatidine, (-)-nicotine, or 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) was detected, arguing against the presence of an independent functional α1-containing muscle-type nAChR in IHCs. In α1-deficient mice, no obvious change of IHC efferent innervation was detected at embryonic day 18, contrary to the hyperinnervation observed at the neuromuscular junction. Additionally, ACh response and efferent synaptic activity were detectable in α1-deficient IHCs, suggesting that α1 is not necessary for assembly and membrane targeting of nAChRs or for efferent synapse formation in IHCs.

Genetic Correction of Induced Pluripotent Stem Cells From a Deaf Patient With MYO7A Mutation Results in Morphologic and Functional Recovery of the Derived Hair Cell-Like Cells

The genetic correction of induced pluripotent stem cells (iPSCs) induced from somatic cells of patients with sensorineural hearing loss (caused by hereditary factors) is a promising method for its treatment. The correction of gene mutations in iPSCs could restore the normal function of cells and provide a rich source of cells for transplantation. In the present study, iPSCs were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T; P-iPSCs), the asymptomatic father of the patient (MYO7A c.1184G>A mutation; CF-iPSCs), and a normal donor (MYO7A(WT/WT); C-iPSCs). One of MYO7A mutation sites (c.4118C>T) in the P-iPSCs was corrected using CRISPR/Cas9. The corrected iPSCs (CP-iPSCs) retained cell pluripotency and normal karyotypes. Hair cell-like cells induced from CP-iPSCs showed restored organization of stereocilia-like protrusions; moreover, the electrophysiological function of these cells was similar to that of cells induced from C-iPSCs and CF-iPSCs. These results might facilitate the development of iPSC-based gene therapy for genetic disorders.

SIGNIFICANCE

Induced pluripotent stem cells (iPSCs) were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T). One of the MYO7A mutation sites (c.4118C>T) in the iPSCs was corrected using CRISPR/Cas9. The genetic correction of MYO7A mutation resulted in morphologic and functional recovery of hair cell-like cells derived from iPSCs. These findings confirm the hypothesis that MYO7A plays an important role in the assembly of stereocilia into stereociliary bundles. Thus, the present study might provide further insight into the pathogenesis of sensorineural hearing loss and facilitate the development of therapeutic strategies against monogenic disease through the genetic repair of patient-specific iPSCs.

Induction of Functional Hair-Cell-Like Cells from Mouse Cochlear Multipotent Cells

In this paper, we developed a two-step-induction method of generating functional hair cells from inner ear multipotent cells. Multipotent cells from the inner ear were established and induced initially into progenitor cells committed to the inner ear cell lineage on the poly-L-lysine substratum. Subsequently, the committed progenitor cells were cultured on the mitotically inactivated chicken utricle stromal cells and induced into hair-cell-like cells containing characteristic stereocilia bundles. The hair-cell-like cells exhibited rapid permeation of FM1-43FX. The whole-cell patch-clamp technique was used to measure the membrane currents of cells differentiated for 7 days on chicken utricle stromal cells and analyze the biophysical properties of the hair-cell-like cells by recording membrane properties of cells. The results suggested that the hair-cell-like cells derived from inner ear multipotent cells were functional following differentiation in an enabling environment.

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.

In vivo and in vitro biophysical properties of hair cells from the lateral line and inner ear of developing and adult zebrafish

Hair cells detect and process sound and movement information, and transmit this with remarkable precision and efficiency to afferent neurons via specialized ribbon synapses. The zebrafish is emerging as a powerful model for genetic analysis of hair cell development and function both in vitro and in vivo. However, the full exploitation of the zebrafish is currently limited by the difficulty in obtaining systematic electrophysiological recordings from hair cells under physiological recording conditions. Thus, the biophysical properties of developing and adult zebrafish hair cells are largely unknown. We investigated potassium and calcium currents, voltage responses and synaptic activity in hair cells from the lateral line and inner ear in vivo and using near-physiological in vitro recordings. We found that the basolateral current profile of hair cells from the lateral line becomes more segregated with age, and that cells positioned in the centre of the neuromast show more mature characteristics and those towards the edge retain a more immature phenotype. The proportion of mature-like hair cells within a given neuromast increased with zebrafish development. Hair cells from the inner ear showed a developmental change in current profile between the juvenile and adult stages. In lateral line hair cells from juvenile zebrafish, exocytosis also became more efficient and required less calcium for vesicle fusion. In hair cells from mature zebrafish, the biophysical characteristics of ion channels and exocytosis resembled those of hair cells from other lower vertebrates and, to some extent, those in the immature mammalian vestibular and auditory systems. We show that although the zebrafish provides a suitable animal model for studies on hair cell physiology, it is advisable to consider that the age at which the majority of hair cells acquire a mature-type configuration is reached only in the juvenile lateral line and in the inner ear from >2 months after hatching.

TRPC3 and TRPC6 are essential for normal mechanotransduction in subsets of sensory neurons and cochlear hair cells

Transient receptor potential (TRP) channels TRPC3 and TRPC6 are expressed in both sensory neurons and cochlear hair cells. Deletion of TRPC3 or TRPC6 in mice caused no behavioural phenotype, although loss of TRPC3 caused a shift of rapidly adapting (RA) mechanosensitive currents to intermediate-adapting currents in dorsal root ganglion sensory neurons. Deletion of both TRPC3 and TRPC6 caused deficits in light touch and silenced half of small-diameter sensory neurons expressing mechanically activated RA currents. Double TRPC3/TRPC6 knock-out mice also showed hearing impairment, vestibular deficits and defective auditory brain stem responses to high-frequency sounds. Basal, but not apical, cochlear outer hair cells lost more than 75 per cent of their responses to mechanical stimulation. FM1-43-sensitive mechanically gated currents were induced when TRPC3 and TRPC6 were co-expressed in sensory neuron cell lines. TRPC3 and TRPC6 are thus required for the normal function of cells involved in touch and hearing, and are potential components of mechanotransducing complexes.

Presynaptic maturation in auditory hair cells requires a critical period of sensory-independent spiking activity

The development of neural circuits relies on spontaneous electrical activity that occurs during immature stages of development. In the developing mammalian auditory system, spontaneous calcium action potentials are generated by inner hair cells (IHCs), which form the primary sensory synapse. It remains unknown whether this electrical activity is required for the functional maturation of the auditory system. We found that sensory-independent electrical activity controls synaptic maturation in IHCs. We used a mouse model in which the potassium channel SK2 is normally overexpressed, but can be modulated in vivo using doxycycline. SK2 overexpression affected the frequency and duration of spontaneous action potentials, which prevented the development of the Ca(2+)-sensitivity of vesicle fusion at IHC ribbon synapses, without affecting their morphology or general cell development. By manipulating the in vivo expression of SK2 channels, we identified the "critical period" during which spiking activity influences IHC synaptic maturation. Here we provide direct evidence that IHC development depends upon a specific temporal pattern of calcium spikes before sound-driven neuronal activity.

Synaptic transfer from outer hair cells to type II afferent fibers in the rat cochlea

Type II cochlear afferents receive glutamatergic synaptic excitation from outer hair cells (OHCs) in the rat cochlea. However, it remains uncertain whether this connection is capable of providing auditory information to the brain. The functional efficacy of this connection depends in part on the number of presynaptic OHCs, their probability of transmitter release, and the effective electrical distance for spatial summation in the type II fiber. The present work addresses these questions using whole-cell recordings from the spiral process of type II afferents that run below OHCs in the apical turn of young (5-9 d postnatal) rat cochlea. A "high potassium puffer" was used to elicit calcium action potentials from individual OHCs and thereby show that the average probability of transmitter release was 0.26 (range 0.02-0.73). Electron microscopy showed relatively few vesicles tethered to ribbons in equivalent OHCs. A "receptive field" map for individual type II fibers was constructed by successively puffing onto OHCs along the cochlear spiral, up to 180 μm from the recording pipette. These revealed a conservative estimate of 7 presynaptic OHCs per type II fiber (range 1-11). EPSCs evoked from presynaptic OHCs separated by >100 μm did not differ in amplitude or waveform, implying that the type II fiber's length constant exceeded the length of the synaptic input zone. Together these data suggest that type II fibers could communicate centrally by maximal activation of their entire pool of presynaptic OHCs.

Functional features of trans-differentiated hair cells mediated by Atoh1 reveals a primordial mechanism

Evolution has transformed a simple ear with few vestibular maculae into a complex three-dimensional structure consisting of nine distinct endorgans. It is debatable whether the sensory epithelia underwent progressive segregation or emerged from distinct sensory patches. To address these uncertainties we examined the morphological and functional phenotype of trans-differentiated rat hair cells to reveal their primitive or endorgan-specific origins. Additionally, it is uncertain how Atoh1-mediated trans-differentiated hair cells trigger the processes that establish their neural ranking from the vestibulocochlear ganglia. We have demonstrated that the morphology and functional expression of ionic currents in trans-differentiated hair cells resemble those of "ancestral" hair cells, even at the lesser epithelia ridge aspects of the cochlea. The structures of stereociliary bundles of trans-differentiated hair cells were in keeping with cells in the vestibule. Functionally, the transient expression of Na⁺ and I(h) currents initiates and promotes evoked spikes. Additionally, Ca²⁺ current was expressed and underwent developmental changes. These events correlate well with the innervation of ectopic hair cells. New "born" hair cells at the abneural aspects of the cochlea are innervated by spiral ganglion neurons, presumably under the tropic influence of chemoattractants. The disappearance of inward currents coincides well with the attenuation of evoked electrical activity, remarkably recapitulating the development of hair cells. Ectopic hair cells underwent stepwise changes in the magnitude and kinetics of transducer currents. We propose that Atoh1 mediates trans-differentiation of morphological and functional "ancestral" hair cells that are likely to undergo diversification in an endorgan-specific manner.

The function and molecular identity of inward rectifier channels in vestibular hair cells of the mouse inner ear

Inner ear hair cells respond to mechanical stimuli with graded receptor potentials. These graded responses are modulated by a host of voltage-dependent currents that flow across the basolateral membrane. Here, we examine the molecular identity and the function of a class of voltage-dependent ion channels that carries the potassium-selective inward rectifier current known as I(K1). I(K1) has been identified in vestibular hair cells of various species, but its molecular composition and functional contributions remain obscure. We used quantitative RT-PCR to show that the inward rectifier gene, Kir2.1, is highly expressed in mouse utricle between embryonic day 15 and adulthood. We confirmed Kir2.1 protein expression in hair cells by immunolocalization. To examine the molecular composition of I(K1), we recorded voltage-dependent currents from type II hair cells in response to 50-ms steps from -124 to -54 in 10-mV increments. Wild-type cells had rapidly activating inward currents with reversal potentials close to the K(+) equilibrium potential and a whole-cell conductance of 4.8 ± 1.5 nS (n = 46). In utricle hair cells from Kir2.1-deficient (Kir2.1(-/-)) mice, I(K1) was absent at all stages examined. To identify the functional contribution of Kir2.1, we recorded membrane responses in current-clamp mode. Hair cells from Kir2.1(-/-) mice had significantly (P < 0.001) more depolarized resting potentials and larger, slower membrane responses than those of wild-type cells. These data suggest that Kir2.1 is required for I(K1) in type II utricle hair cells and contributes to hyperpolarized resting potentials and fast, small amplitude receptor potentials in response to current inputs, such as those evoked by hair bundle deflections.

Functional assembly of mammalian cochlear hair cells

Hair cells in the mammalian inner ear convert sound into electrical signals that are relayed to the nervous system by the chemical neurotransmitter glutamate. Electrical information encoding sound is then passed through the central nervous system to the higher auditory centres in the brain, where it is used to construct a temporally and spatially accurate representation of the auditory landscape. To achieve this, hair cells must encode fundamental properties of sound stimuli at extremely high rates, not only during mechano-electrical transduction, which occurs in the hair bundles at the cell apex, but also during electrochemical transduction at the specialized ribbon synapses at the cell base. How is the development of such a sophisticated cell regulated? More specifically, to what extent does physiological activity contribute to the progression of the intrinsic genetic programmes that drive cell differentiation? Hair cell differentiation takes about 3 weeks in most rodents, from terminal mitosis during embryonic development to the onset of hearing around 2 weeks after birth. Until recent years, most of the molecules involved in hair cell development and function were unknown, which was mainly due to difficulties in working with the mammalian cochlea and the very small number of hair cells, about 16,000 in humans, present in the auditory organ. Recent advances in the ability to record from the acutely isolated cochlea maintained in near-physiological conditions, combined with the use of genetically modified mouse models, has allowed the identification of several proteins and molecular mechanisms that are crucial for the maturation and function of hair cells. In this article, I highlight recent findings from my laboratory that have furthered our understanding of how developing hair cells acquire the remarkable sensitivity of adult auditory sensory receptors.

Eps8 regulates hair bundle length and functional maturation of mammalian auditory hair cells

Hair cells of the mammalian cochlea are specialized for the dynamic coding of sound stimuli. The transduction of sound waves into electrical signals depends upon mechanosensitive hair bundles that project from the cell's apical surface. Each stereocilium within a hair bundle is composed of uniformly polarized and tightly packed actin filaments. Several stereociliary proteins have been shown to be associated with hair bundle development and function and are known to cause deafness in mice and humans when mutated. The growth of the stereociliar actin core is dynamically regulated at the actin filament barbed ends in the stereociliary tip. We show that Eps8, a protein with actin binding, bundling, and barbed-end capping activities in other systems, is a novel component of the hair bundle. Eps8 is localized predominantly at the tip of the stereocilia and is essential for their normal elongation and function. Moreover, we have found that Eps8 knockout mice are profoundly deaf and that IHCs, but not OHCs, fail to mature into fully functional sensory receptors. We propose that Eps8 directly regulates stereocilia growth in hair cells and also plays a crucial role in the physiological maturation of mammalian cochlear IHCs. Together, our results indicate that Eps8 is critical in coordinating the development and functionality of mammalian auditory hair cells.

Mechanism of spontaneous activity in afferent neurons of the zebrafish lateral-line organ

Many auditory, vestibular, and lateral-line afferent neurons display spontaneous action potentials. This spontaneous spiking is thought to result from hair-cell glutamate release in the absence of stimuli. Spontaneous release at hair-cell resting potentials presumably results from Ca(V)1.3 L-type calcium channel activity. Here, using intact zebrafish larvae, we recorded robust spontaneous spiking from lateral-line afferent neurons in the absence of external stimuli. Consistent with the above assumptions, spiking was absent in mutants that lacked either Vesicular glutamate transporter 3 (Vglut3) or Ca(V)1.3. We then tested the hypothesis that spontaneous spiking resulted from sustained Ca(V)1.3 activity due to depolarizing currents that are active at rest. Mechanotransduction currents (I(MET)) provide a depolarizing influence to the resting potential. However, following block of I(MET), spontaneous spiking persisted and was characterized by longer interspike intervals and increased periods of inactivity. These results suggest that an additional depolarizing influence maintains the resting potential within the activation range of Ca(V)1.3. To test whether the hyperpolarization-activated cation current, I(h) participates in setting the resting potential, we applied I(h) antagonists. Both ZD7288 and DK-AH 269 reduced spontaneous activity. Finally, concomitant block of I(MET) and I(h) essentially abolished spontaneous activity, ostensibly by hyperpolarization outside of the activation range for Ca(V)1.3. Together, our data support a mechanism for spontaneous spiking that results from Ca(2+)-dependent neurotransmitter release at hair-cell resting potentials that are maintained within the activation range of Ca(V)1.3 channels through active I(MET) and I(h).

miR-96 regulates the progression of differentiation in mammalian cochlear inner and outer hair cells

MicroRNAs (miRNAs) are small noncoding RNAs able to regulate a broad range of protein-coding genes involved in many biological processes. miR-96 is a sensory organ-specific miRNA expressed in the mammalian cochlea during development. Mutations in miR-96 cause nonsyndromic progressive hearing loss in humans and mice. The mouse mutant diminuendo has a single base change in the seed region of the Mir96 gene leading to widespread changes in the expression of many genes. We have used this mutant to explore the role of miR-96 in the maturation of the auditory organ. We found that the physiological development of mutant sensory hair cells is arrested at around the day of birth, before their biophysical differentiation into inner and outer hair cells. Moreover, maturation of the hair cell stereocilia bundle and remodelling of auditory nerve connections within the cochlea fail to occur in miR-96 mutants. We conclude that miR-96 regulates the progression of the physiological and morphological differentiation of cochlear hair cells and, as such, coordinates one of the most distinctive functional refinements of the mammalian auditory system.

Developmental regulation of spontaneous activity in the Mammalian cochlea

Neurons in the developing auditory system fire bursts of action potentials before the onset of hearing. This spontaneous activity promotes the survival and maturation of auditory neurons and the refinement of synaptic connections in auditory nuclei; however, the mechanisms responsible for initiating this activity remain uncertain. Previous studies indicate that inner supporting cells (ISCs) in the developing cochlea periodically release ATP, which depolarizes inner hair cells (IHCs), leading to bursts of action potentials in postsynaptic spiral ganglion neurons (SGNs). To determine when purinergic signaling appears in the developing cochlea and whether it is responsible for initiating auditory neuron activity throughout the prehearing period, we examined spontaneous activity from ISCs, IHCs, and SGNs in cochleae acutely isolated from rats during the first three postnatal weeks. We found that ATP was released from ISCs within the cochlea from birth until the onset of hearing, which led to periodic inward currents, Ca(2+) transients, and morphological changes in these supporting cells. This spontaneous release of ATP also depolarized IHCs and triggered bursts of action potentials in SGNs for most of the postnatal prehearing period, beginning a few days after birth as IHCs became responsive to ATP, until the onset of hearing when ATP was no longer released from ISCs. When IHCs were not subject to purinergic excitation, SGNs exhibited little or no activity. These results suggest that supporting cells in the cochlea provide the primary excitatory stimulus responsible for initiating bursts of action potentials in auditory nerve fibers before the onset of hearing.

Mechanosensitive hair cell-like cells from embryonic and induced pluripotent stem cells

Mechanosensitive sensory hair cells are the linchpin of our senses of hearing and balance. The inability of the mammalian inner ear to regenerate lost hair cells is the major reason for the permanence of hearing loss and certain balance disorders. Here, we present a stepwise guidance protocol starting with mouse embryonic stem and induced pluripotent stem cells, which were directed toward becoming ectoderm capable of responding to otic-inducing growth factors. The resulting otic progenitor cells were subjected to varying differentiation conditions, one of which promoted the organization of the cells into epithelial clusters displaying hair cell-like cells with stereociliary bundles. Bundle-bearing cells in these clusters responded to mechanical stimulation with currents that were reminiscent of immature hair cell transduction currents.

Persistence of Ca(v)1.3 Ca2+ channels in mature outer hair cells supports outer hair cell afferent signaling

Outer hair cells (OHCs) are innervated by type II afferent fibers of as yet unknown function. It is still a matter of debate whether OHCs perform exocytosis. If so, they would require presynaptic Ca2+ channels at their basal poles where the type II fibers make contacts. Here we show that L-type Ca2+ channel currents (charge carrier, 10 mM Ba2+) present in neonatal OHCs [postnatal day 1 (P1) to P7] decreased from approximately 170 to approximately 50 pA at approximately the onset of hearing. Ba2+ currents could hardly be measured in mature mouse OHCs because of their high fragility, whereas in the rat, the average Ba2+ current amplitude of apical OHCs was 58 +/- 9 pA (n = 20, P19-P30) compared with that of the inner hair cells (IHCs) of 181 +/- 50 pA (n = 24, P17-P30). Properties of Ba2+ currents of mature OHCs resembled those of neonatal OHCs. One exception was the voltage dependence of activation that shifted between birth and P12 by +9 mV toward positive voltages in OHCs, whereas it remained constant in the IHCs. Ca(v)1.3-specific mRNA was detected in mature OHCs using cell-specific reverse transcription (RT)-PCR and in situ hybridization. Ca(v)1.3 protein was stained exclusively at the base of mature OHCs, in colocalization with the ribbon synapse protein CtBP2 (C-terminal binding protein 2)/RIBEYE. When current sizes were normalized to the estimated number of afferent fibers or presynaptic ribbons, comparable values for IHCs and OHCs were obtained, a finding that together with the colocalization of Ca(v)1.3 and CtBP2/RIBEYE protein strongly suggests a role for Ca(v)1.3 channels in exocytosis of mature OHCs.

Genetic deletion of SK2 channels in mouse inner hair cells prevents the developmental linearization in the Ca2+ dependence of exocytosis

Inner hair cells (IHCs), the primary sensory receptors of the mammalian cochlea, fire spontaneous Ca(2+) action potentials (APs) only before the onset of hearing. Although a role for APs in the developing auditory system has not been determined it could, by analogy with other sensory systems, guide the functional maturation of the cochlea before experience-driven activity begins. Spontaneous APs in immature IHCs are shaped by a variety of ion channels including that of the small conductance Ca(2+)-activated K(+) current (SK2), which is only transiently expressed in immature cells. Using SK2 knockout mice we found that SK2 channels are not required for generating APs but are essential for sustaining continuous repetitive spontaneous AP activity in pre-hearing IHCs. Therefore we used this mutant mouse as a model to study possible developmental implications of disrupted AP activity. Immature mutant IHCs showed impaired exocytotic responses, which are likely to be due to the expression of fewer Ca(2+) channels. Exocytosis was also impaired in adult mutant IHCs, although in this case it resulted from a reduced Ca(2+) efficiency and increased Ca(2+) dependence of the synaptic machinery. Since SK2 channels can only have a functional influence on IHCs during immature development and are not directly involved in neurotransmitter release, the altered Ca(2+) dependence of exocytosis in adult IHCs is likely to be a consequence of their disrupted AP activity at immature stages.

Thyroid hormone deficiency affects postnatal spiking activity and expression of Ca2+ and K+ channels in rodent inner hair cells

Thyroid hormone (TH) is essential for the development of hearing. Lack of TH in a critical developmental period from embryonic day 17 to postnatal day 12 (P12) in rats and mice leads to morphological and functional deficits in the organ of Corti and the auditory pathway. We investigated the effects of TH on inner hair cells (IHCs) using patch-clamp recordings, capacitance measurements, and immunocytochemistry in hypothyroid rats and athyroid Pax8-/- mice. Spontaneous and evoked Ca2+ action potentials (APs) were present in control IHCs from P3-P11 rats and vanished in parallel with the expression of a rapidly activating Ca2+- and voltage-activated K+ (BK) conductance. IHCs of hypothyroid rats and athyroid Pax8-/- mice displayed APs until the end of the third postnatal week because of threefold elevated Ca2+ currents and missing expression of BK currents. After the fourth postnatal week, some IHCs showed BK currents whereas adjacent IHCs did not, demonstrated by electrophysiology and immunocytochemistry. To test whether the prolonged spiking activity during TH deficiency may be transmitted at IHC synapses, capacitance measurements were performed in parallel to analysis of otoferlin expression, a protein thought to play an essential role in exocytosis of IHCs. Strikingly, otoferlin was absent from IHCs of hypothyroid rats but not of Pax8-/- mice, although both cell types showed exocytosis with an efficiency typical for immature IHCs. These results demonstrate for the first time a TH-dependent control of IHC spiking activity before the onset of hearing attributable to effects of TH on Ca2+ and BK channels. Moreover, they question an indispensable role of otoferlin for exocytosis in IHCs.

Tmc1 is necessary for normal functional maturation and survival of inner and outer hair cells in the mouse cochlea

The deafness (dn) and Beethoven (Bth) mutant mice are models for profound congenital deafness (DFNB7/B11) and progressive hearing loss (DFNA36), respectively, caused by recessive and dominant mutations of transmembrane cochlear-expressed gene 1 (TMC1), which encodes a transmembrane protein of unknown function. In the mouse cochlea Tmc1 is expressed in both outer (OHCs) and inner (IHCs) hair cells from early stages of development. Immature hair cells of mutant mice seem normal in appearance and biophysical properties. From around P8 for OHCs and P12 for IHCs, mutants fail to acquire (dn/dn) or show reduced expression (Bth/Bth and, to a lesser extent Bth/+) of the K+ currents which contribute to their normal functional maturation (the BK-type current IK,f in IHCs, and the delayed rectifier IK,n in both cell types). Moreover, the exocytotic machinery in mutant IHCs does not develop normally as judged by the persistence of immature features of the Ca2+ current and exocytosis into adulthood. Mutant mice exhibited progressive hair cell damage and loss. The compound action potential (CAP) thresholds of Bth/+ mice were raised and correlated with the degree of hair cell loss. Homozygous mutants (dn/dn and Bth/Bth) never showed CAP responses, even at ages where many hair cells were still present in the apex of the cochlea, suggesting their hair cells never function normally. We propose that Tmc1 is involved in trafficking of molecules to the plasma membrane or serves as an intracellular regulatory signal for differentiation of immature hair cells into fully functional auditory receptors.

Developmental acquisition of voltage-dependent conductances and sensory signaling in hair cells of the embryonic mouse inner ear

How and when sensory hair cells acquire the remarkable ability to detect and transmit mechanical information carried by sound and head movements has not been illuminated. Previously, we defined the onset of mechanotransduction in embryonic hair cells of mouse vestibular organs to be at approximately embryonic day 16 (E16). Here we examine the functional maturation of hair cells in intact sensory epithelia excised from the inner ears of embryonic mice. Hair cells were studied at stages between E14 and postnatal day 2 using the whole-cell, tight-seal recording technique. We tracked the developmental acquisition of four voltage-dependent conductances. We found a delayed rectifier potassium conductance that appeared as early as E14 and grew in amplitude over the subsequent prenatal week. Interestingly, we also found a low-voltage-activated potassium conductance present at E18, approximately 1 week earlier than reported previously. An inward rectifier conductance appeared at approximately E15 and doubled in size over the next few days. We also noted transient expression of a voltage-gated sodium conductance that peaked between E16 and E18 and then declined to near zero at birth. We propose that hair cells undergo a stereotyped developmental pattern of ion channel acquisition and that the precise pattern may underlie other developmental processes such as synaptogenesis and functional differentiation into type I and type II hair cells. In addition, we find that the developmental acquisition of basolateral conductances shapes the hair cell receptor potential and therefore comprises an important step in the signal cascade from mechanotransduction to neurotransmission.

Characterization of the chicken inward rectifier K+ channel IRK1/Kir2.1 gene

Background

Inward rectifier potassium channels (IRK) contribute to the normal function of skeletal and cardiac muscle cells. The chick inward rectifier K⁺ channel cIRK1/Kir2.1 is expressed in skeletal muscle, heart, brain, but not in liver; a distribution similar but not identical to that of mouse Kir2.1. We set out to explore regulatory domains of the cIRK1 promoter that enhance or inhibit expression of the gene in different cell types.

Results

We cloned and characterized the 5'-flanking region of cIRK1. cIRK1 contains two exons with splice sites in the 5'-untranslated region, a structure similar to mouse and human orthologs. cIRK1 has multiple transcription initiation sites, a feature also seen in mouse. However, while the chicken and mouse promoter regions share many regulatory motifs, cIRK1 possesses a GC-richer promoter and a putative TATA box, which appears to positively regulate gene expression. We report here the identification of several candidate cell/tissue specific cIRK1 regulatory domains by comparing promoter activities in expressing (Qm7) and non-expressing (DF1) cells using in vitro transcription assays.

Conclusion

While multiple transcription initiation sites and the combinatorial function of several domains in activating cIRK1 expression are similar to those seen in mKir2.1, the cIRK1 promoter differs by the presence of a putative TATA box. In addition, several domains that regulate the gene's expression differentially in muscle (Qm7) and fibroblast cells (DF1) were identified. These results provide fundamental data to analyze cIRK1 transcriptional mechanisms. The control elements identified here may provide clues to the tissue-specific expression of this K⁺ channel.

Differences between the negatively activating potassium conductances of Mammalian cochlear and vestibular hair cells

Cochlear and type I vestibular hair cells of mammals express negatively activating potassium (K(+)) conductances, called g(K,n) and g(K,L) respectively, which are important in setting the hair cells' resting potentials and input conductances. It has been suggested that the channels underlying both conductances include KCNQ4 subunits from the KCNQ family of K(+) channels. In whole-cell recordings from rat hair cells, we found substantial differences between g(K,n) and g(K,L) in voltage dependence, kinetics, ionic permeability, and stability during whole-cell recording. Relative to g(K,L), g(K,n) had a significantly broader and more negative voltage range of activation and activated with less delay and faster principal time constants over the negative part of the activation range. Deactivation of g(K,n) had an unusual sigmoidal time course, while g(K,L) deactivated with a double-exponential decay. g(K,L), but not g(K,n), had appreciable permeability to Cs(+). Unlike g(K,L), g(K,n)'s properties did not change ("wash out") during the replacement of cytoplasmic solution with pipette solution during ruptured-patch recordings. These differences in the functional expression of g(K,n) and g(K,L) channels suggest that there are substantial differences in their molecular structure as well.

A transiently expressed SK current sustains and modulates action potential activity in immature mouse inner hair cells

From just after birth, mouse inner hair cells (IHCs) expressed a Ca(2+)-activated K(+) current that was reduced by intracellular BAPTA at concentrations >or= 1 mM. The block of this current by nifedipine suggests the direct involvement of Ca(v)1.3 Ca(2+) channels in its activation. On the basis of its high sensitivity to apamin (K(D) 360 pM) it was identified as a small-conductance Ca(2+)-activated K(+) current (SK), probably SK2. A similar current was also found in outer hair cells (OHCs) from the beginning of the second postnatal week. In both cell types the appearance of the SK current coincided with their becoming responsive to acetylcholine (ACh), the main efferent neurotransmitter in the cochlea. The effect of ACh on IHCs was abolished when they were simultaneously superfused with strychnine, consistent with the presence of nicotinic ACh receptors (nAChRs). Extracellular Ca(2+) either potentiated or blocked the nAChR current depending on its concentration, as previously reported for the recombinant alpha9alpha10 nAChR. Outward currents activated by ACh were reduced by blocking the SK current with apamin or by preventing SK current activation with intracellular BAPTA (>or= 10 mM). The endogenous mobile Ca(2+) buffer concentration was estimated to be equivalent to about 1 mM BAPTA, suggesting that in physiological conditions the SK channel is significantly activated by Ca(2+) influx through both Ca(v)1.3 Ca(2+) channels and alpha9alpha10 nAChRs. Current clamp experiments showed that in IHCs the SK current is required for sustaining a train of action potentials and also modulates their frequency when activated by ACh.

Cav1.3 (alpha1D) Ca2+ currents in neonatal outer hair cells of mice

Outer hair cells (OHC) serve as electromechanical amplifiers that guarantee the unique sensitivity and frequency selectivity of the mammalian cochlea. It is unknown whether the afferent fibres connected to adult OHCs are functional. If so, voltage-activated Ca2+ channels would be required for afferent synaptic transmission. In neonatal OHCs, Ca2+ channels seem to play a role in maturation since OHCs from Cav1.3-deficient (Cav1.3-/-) mice degenerate shortly after the onset of hearing. We therefore studied whole-cell Ca2+ currents in outer hair cells aged between postnatal day 1 (P1) and P8. OHCs showed a rapidly activating inward current that was 1.8 times larger with 10 mM Ba2+ as charge carrier (IBa) than with equimolar Ca2+ (ICa). IBa started activating at -50 mV with Vmax = -1.9 +/- 6.9 mV, V0.5 = -15.0 +/- 7.1 mV and k = 8.2 +/- 1.1 mV (n = 34). The peak IBa showed negligible inactivation (3.6 % after 300 ms) whereas the ICa (10 mM Ca2+) was inactivated by 50.7 %. OHC IBa was reduced by 33.5 +/- 10.3 % (n = 14) with 10 microM nifedipine and increased to 178.5 +/- 57.8 % (n = 14) by 5 microM Bay K 8644. A dose-response curve for nifedipine revealed an IC50 of 2.3 microM, a Hill coefficient of 2.7 and a maximum block of 36 %. Average IBa density in OHCs was 24.4 +/- 10.8 pA pF-1 (n = 105) which is only 38 % of the value in inner hair cells. Single cell RT-PCR revealed expression of Cav1.3 in OHCs. In OHCs from Cav1.3-/- mice, Ba2+ current density was reduced to 0.6 +/- 0.5 pA pF-1 (n = 9) indicating that > 97 % of the Ca2+ channel current in OHCs flows through Cav1.3.

Developmental changes in the expression of potassium currents of embryonic, neonatal and mature mouse inner hair cells

Developmental changes in electrophysiological membrane properties of mouse cochlear inner hair cells (IHCs) were studied from just after terminal differentiation up to functional maturity. As early as embryonic day 14.5 (E14.5) newly differentiated IHCs express a very small outward K+ current that is largely insensitive to 4-aminopyridine (4-AP). One day later the inward rectifier, IK1, is first observed. These immature cells initially exhibit only slow graded voltage responses under current clamp. From E17.5 spontaneous action potentials occur. During the first week of postnatal development, the outward K+ current steadily increases in size and a progressively larger fraction of the current is sensitive to 4-AP. During the second postnatal week, the activation of the 4-AP-sensitive current, by now contributing about half of the outward K+ current, shifts in the hyperpolarizing direction. Together with an increase in size of IK1, this hyperpolarizes the cell, thus inhibiting the spontaneous spike activity, although spikes could still be evoked upon depolarizing current injection. Starting at about the onset of hearing (postnatal day 12, P12) immature IHCs make the final steps towards fully functional sensory receptors with fast graded voltage responses. This is achieved mainly by the expression of the large-conductance Ca2+-activated K+ current IK,f, but also of a current indistinguishable from the negatively activating IK,n previously described in mature outer hair cells (OHCs). The 4-AP-sensitive current continues to increase after the onset of hearing to form the major part of the mature delayed rectifier, IK,s. By P20 IHCs appear mature in terms of their complement of K+ conductances.

Developmental expression of the potassium current IK,n contributes to maturation of mouse outer hair cells

1. The expression of K+ currents in mouse outer hair cells (OHCs) was investigated as a function of developmental age between postnatal day (P) 0 and P26, using whole-cell patch clamp. 2. During the first postnatal week, a slow outward K+ current (IK,neo) was expressed by all OHCs from the apical coil of the cochlea. The amplitude of this current increased greatly between P0 and P6. Then, at the beginning of the second postnatal week, IK,neo decreased. At the same time, from P8 onwards, IK,n, a K+ current characteristic of mature OHCs, was rapidly expressed. 3. The expression of IK,n coincided with the onset of electromotility of the cell body of the OHCs, which could also be detected from P8 onwards and increased substantially in size thereafter. 4. IK,n was reversibly blocked by linopirdine, an inhibitor of members of the KCNQ family of K+ channels, with a KD of 0.7 microM. In the cochlea, KCNQ4 is only expressed in OHCs and is responsible for a form of non-syndromic autosomal dominant deafness. Linopirdine had no effect on other OHC K+ currents at concentrations up to 200 microM. We conclude that ion channels underlying IK,n contain the KCNQ4 subunit. 5. In current clamp, depolarizing current injections from the resting potential triggered action potentials in OHCs during the first postnatal week. Thereafter, more rapid and graded voltage responses occurred from more negative resting potentials. Thus, OHCs mature rapidly from P8 onwards, and IK,n contributes to this maturation.