Harmonin enhances voltage-dependent facilitation of Cav1.3 channels and synchronous exocytosis in mouse inner hair cells

Expression and subcellular localization of USH1C/harmonin in human retina provides insights into pathomechanisms and therapy

Usher syndrome (USH) is the most common form of hereditary deaf-blindness in humans. USH is a complex genetic disorder, assigned to three clinical subtypes differing in onset, course and severity, with USH1 being the most severe. Rodent USH1 models do not reflect the ocular phenotype observed in human patients to date; hence, little is known about the pathophysiology of USH1 in the human eye. One of the USH1 genes, USH1C, exhibits extensive alternative splicing and encodes numerous harmonin protein isoforms that function as scaffolds for organizing the USH interactome. RNA-seq analysis of human retinae uncovered harmonin_a1 as the most abundant transcript of USH1C. Bulk RNA-seq analysis and immunoblotting showed abundant expression of harmonin in Müller glia cells (MGCs) and retinal neurons. Furthermore, harmonin was localized in the terminal endfeet and apical microvilli of MGCs, presynaptic region (pedicle) of cones and outer segments (OS) of rods as well as at adhesive junctions between MGCs and photoreceptor cells (PRCs) in the outer limiting membrane (OLM). Our data provide evidence for the interaction of harmonin with OLM molecules in PRCs and MGCs and rhodopsin in PRCs. Subcellular expression and colocalization of harmonin correlate with the clinical phenotype observed in USH1C patients. We also demonstrate that primary cilia defects in USH1C patient-derived fibroblasts could be reverted by the delivery of harmonin_a1 transcript isoform. Our studies thus provide novel insights into PRC cell biology, USH1C pathophysiology and development of gene therapy treatment(s).

Structure of the Harmonin PDZ2 and coiled-coil domains in a complex with CDHR2 tail and its implications

Harmonin is a protein containing multiple PDZ domains and is required for the development and maintenance of hair cell stereocilia and brush border microvilli. Mutations in the USH1C gene can cause Usher syndrome type 1C, a severe inheritable disease characterized by the loss of hearing and vision. Here, by solving the high-resolution crystal structure of Harmonin PDZ2 and coiled-coil domains in a complex with the tail of cadherin-related family member 2, we demonstrated that mutations located in the Harmonin PDZ2 domain and found in patients could affect its stability, and thus, the target binding capability. The structure also implies that the coiled-coil domain could form antiparallel dimers under high concentrations, possibly when Harmonin underwent liquid-liquid phase separation in the upper tip-link density in hair cell stereocilia or microvilli of enterocytes of the intestinal epithelium. The crystal structure, together with the biochemical analysis, provided mechanistic implications for Harmonin mutations causing Usher syndrome, non-syndromic deafness, or enteropathy.

Early disruption of photoreceptor cell architecture and loss of vision in a humanized pig model of usher syndromes

Usher syndrome (USH) is the most common form of monogenic deaf-blindness. Loss of vision is untreatable and there are no suitable animal models for testing therapeutic strategies of the ocular constituent of USH, so far. By introducing a human mutation into the harmonin-encoding USH1C gene in pigs, we generated the first translational animal model for USH type 1 with characteristic hearing defect, vestibular dysfunction, and visual impairment. Changes in photoreceptor architecture, quantitative motion analysis, and electroretinography were characteristics of the reduced retinal virtue in USH1C pigs. Fibroblasts from USH1C pigs or USH1C patients showed significantly elongated primary cilia, confirming USH as a true and general ciliopathy. Primary cells also proved their capacity for assessing the therapeutic potential of CRISPR/Cas-mediated gene repair or gene therapy in vitro. AAV-based delivery of harmonin into the eye of USH1C pigs indicated therapeutic efficacy in vivo.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

Fetal antisense oligonucleotide therapy for congenital deafness and vestibular dysfunction

Disabling hearing loss impacts ∼466 million individuals worldwide with 34 million children affected. Gene and pharmacotherapeutic strategies to rescue auditory function in mouse models of human deafness are most effective when administered before hearing onset, after which therapeutic efficacy is significantly diminished or lost. We hypothesize that preemptive correction of a mutation in the fetal inner ear prior to maturation of the sensory epithelium will optimally restore sensory function. We previously demonstrated that transuterine microinjection of a splice-switching antisense oligonucleotide (ASO) into the amniotic cavity immediately surrounding the embryo on embryonic day 13-13.5 (E13-13.5) corrected pre-mRNA splicing in the juvenile Usher syndrome type 1c (Ush1c) mouse mutant. Here, we show that this strategy only marginally rescues hearing and partially rescues vestibular function. To improve therapeutic outcomes, we microinjected ASO directly into the E12.5 inner ear. A single intra-otic dose of ASO corrects harmonin RNA splicing, restores harmonin protein expression in sensory hair cell bundles, prevents hair cell loss, improves hearing sensitivity, and ameliorates vestibular dysfunction. Improvements in auditory and vestibular function were sustained well into adulthood. Our results demonstrate that an ASO pharmacotherapeutic administered to a developing organ system in utero preemptively corrects pre-mRNA splicing to abrogate the disease phenotype.

Disease mechanisms and gene therapy for Usher syndrome

Usher syndrome (USH) is a major cause of deaf-blindness in humans, affecting ∼400 000 patients worldwide. Three clinical subtypes, USH1-3, have been defined, with 10 USH genes identified so far. In recent years, in addition to identification of new Usher genes and diagnostic tools, major progress has been made in understanding the role of Usher proteins and how they cooperate through interaction networks to ensure proper development, architecture and function of the stereociliary bundle at the apex of sensory hair cells in the inner ear. Several Usher mouse models of known human Usher genes have been characterized. These mice faithfully reproduce the auditory phenotype associated with Usher syndrome and the vestibular phenotype associated with some mutations in USH genes, particularly USH1. Interestingly, very few mouse models of Usher syndrome recapitulate the retinal phenotype associated with the disease in human. Usher patients can benefit from hearing aids or cochlear implants, which partially alleviate auditory sensory deprivation. However, there are currently no biological treatments available for auditory or visual dysfunction in Usher patients. Development of novel therapies for Usher syndrome has sprouted over the past decade, building on recent progress in gene transfer and new gene editing tools. Promising success demonstrating recovery of hearing and balance functions have been obtained via distinct therapeutic strategies in animal models. Clinical translation to Usher patients, however, calls for further improvements and concerted efforts to overcome the challenges ahead.

Diseases of the ear

In children with normal hearing, inflammatory disorders caused by infections of the middle ear (otitis media) are the most common ear illnesses. Many of older adults experience some level of hearing loss. Several factors can lead to either a partial loss or the total inability to hear (deafness) including exposure to noise, a hereditary predisposition, chronic infections, traumas, medications, and aging.

Hair Cell Afferent Synapses: Function and Dysfunction

To provide a meaningful representation of the auditory landscape, mammalian cochlear hair cells are optimized to detect sounds over an incredibly broad range of frequencies and intensities with unparalleled accuracy. This ability is largely conferred by specialized ribbon synapses that continuously transmit acoustic information with high fidelity and sub-millisecond precision to the afferent dendrites of the spiral ganglion neurons. To achieve this extraordinary task, ribbon synapses employ a unique combination of molecules and mechanisms that are tailored to sounds of different frequencies. Here we review the current understanding of how the hair cell's presynaptic machinery and its postsynaptic afferent connections are formed, how they mature, and how their function is adapted for an accurate perception of sound.

Intrinsic planar polarity mechanisms influence the position-dependent regulation of synapse properties in inner hair cells

Encoding the wide range of audible sounds in the mammalian cochlea is collectively achieved by functionally diverse type I spiral ganglion neurons (SGNs) at each tonotopic position. The firing of each SGN is thought to be driven by an individual active zone (AZ) of a given inner hair cell (IHC). These AZs present distinct properties according to their position within the IHC, to some extent forming a gradient between the modiolar and the pillar IHC side. In this study, we investigated whether signaling involved in planar polarity at the apical surface can influence position-dependent AZ properties at the IHC base. Specifically, we tested the role of Gαi proteins and their binding partner LGN/Gpsm2 implicated in cytoskeleton polarization and hair cell (HC) orientation along the epithelial plane. Using high and superresolution immunofluorescence microscopy as well as patch-clamp combined with confocal Ca²⁺ imaging we analyzed IHCs in which Gαi signaling was blocked by Cre-induced expression of the pertussis toxin catalytic subunit (PTXa). PTXa-expressing IHCs exhibited larger Ca_V1.3 Ca²⁺-channel clusters and consequently greater Ca²⁺ influx at the whole-cell and single-synapse levels, which also showed a hyperpolarized shift of activation. Moreover, PTXa expression collapsed the modiolar-pillar gradients of ribbon size and maximal synaptic Ca²⁺ influx. Finally, genetic deletion of Gαi3 and LGN/Gpsm2 also disrupted the modiolar-pillar gradient of ribbon size. We propose a role for Gαi proteins and LGN in regulating the position-dependent AZ properties in IHCs and suggest that this signaling pathway contributes to setting up the diverse firing properties of SGNs.

Single-Channel Resolution of the Interaction between C-Terminal CaV1.3 Isoforms and Calmodulin

Voltage-dependent calcium (Ca_V) 1.3 channels are involved in the control of cellular excitability and pacemaking in neuronal, cardiac, and sensory cells. Various proteins interact with the alternatively spliced channel C-terminus regulating gating of Ca_V1.3 channels. Binding of a regulatory calcium-binding protein calmodulin (CaM) to the proximal C-terminus leads to the boosting of channel activity and promotes calcium-dependent inactivation (CDI). The C-terminal modulator domain (CTM) of Ca_V1.3 channels can interfere with the CaM binding, thereby inhibiting channel activity and CDI. Here, we compared single-channel gating behavior of two natural Ca_V1.3 splice isoforms: the long Ca_V1.3₄₂ with the full-length CTM and the short Ca_V1.3_42A with the C-terminus truncated before the CTM. We found that CaM regulation of Ca_V1.3 channels is dynamic on a minute timescale. We observed that at equilibrium, single Ca_V1.3₄₂ channels occasionally switched from low to high open probability, which perhaps reflects occasional binding of CaM despite the presence of CTM. Similarly, when the amount of the available CaM in the cell was reduced, the short Ca_V1.3_42A isoform showed patterns of the low channel activity. CDI also underwent periodic changes with corresponding kinetics in both isoforms. Our results suggest that the competition between CTM and CaM is influenced by calcium, allowing further fine-tuning of Ca_V1.3 channel activity for particular cellular needs.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Role of a conserved glutamine in the function of voltage-gated Ca2+ channels revealed by a mutation in human CACNA1D

Voltage-gated Ca_v Ca²⁺ channels play crucial roles in regulating gene transcription, neuronal excitability, and synaptic transmission. Natural or pathological variations in Ca_v channels have yielded rich insights into the molecular determinants controlling channel function. Here, we report the consequences of a natural, putatively disease-associated mutation in the CACNA1D gene encoding the pore-forming Ca_v1.3 α₁ subunit. The mutation causes a substitution of a glutamine residue that is highly conserved in the extracellular S1-S2 loop of domain II in all Ca_v channels with a histidine and was identified by whole-exome sequencing of an individual with moderate hearing impairment, developmental delay, and epilepsy. When introduced into the rat Ca_v1.3 cDNA, Q558H significantly decreased the density of Ca²⁺ currents in transfected HEK293T cells. Gating current analyses and cell-surface biotinylation experiments suggested that the smaller current amplitudes caused by Q558H were because of decreased numbers of functional Ca_v1.3 channels at the cell surface. The substitution also produced more sustained Ca²⁺ currents by weakening voltage-dependent inactivation. When inserted into the corresponding locus of Ca_v2.1, the substitution had similar effects as in Ca_v1.3. However, the substitution introduced in Ca_v3.1 reduced current density, but had no effects on voltage-dependent inactivation. Our results reveal a critical extracellular determinant of current density for all Ca_v family members and of voltage-dependent inactivation of Ca_v1.3 and Ca_v2.1 channels.

Clarin-1 gene transfer rescues auditory synaptopathy in model of Usher syndrome

Clarin-1, a tetraspan-like membrane protein defective in Usher syndrome type IIIA (USH3A), is essential for hair bundle morphogenesis in auditory hair cells. We report a new synaptic role for clarin-1 in mouse auditory hair cells elucidated by characterization of Clrn1 total (Clrn1ex4-/-) and postnatal hair cell-specific conditional (Clrn1ex4fl/fl Myo15-Cre+/-) knockout mice. Clrn1ex4-/- mice were profoundly deaf, whereas Clrn1ex4fl/fl Myo15-Cre+/- mice displayed progressive increases in hearing thresholds, with, initially, normal otoacoustic emissions and hair bundle morphology. Inner hair cell (IHC) patch-clamp recordings for the 2 mutant mice revealed defective exocytosis and a disorganization of synaptic F-actin and CaV1.3 Ca2+ channels, indicative of a synaptopathy. Postsynaptic defects were also observed, with an abnormally broad distribution of AMPA receptors associated with a loss of afferent dendrites and defective electrically evoked auditory brainstem responses. Protein-protein interaction assays revealed interactions between clarin-1 and the synaptic CaV1.3 Ca2+ channel complex via the Cavβ2 auxiliary subunit and the PDZ domain-containing protein harmonin (defective in Usher syndrome type IC). Cochlear gene therapy in vivo, through adeno-associated virus-mediated Clrn1 transfer into hair cells, prevented the synaptic defects and durably improved hearing in Clrn1ex4fl/fl Myo15-Cre+/- mice. Our results identify clarin-1 as a key organizer of IHC ribbon synapses, and suggest new treatment possibilities for USH3A patients.

Inframe deletion of human ESPN is associated with deafness, vestibulopathy and vision impairment

BACKGROUND

Usher syndrome (USH) is a neurosensory disorder characterised by deafness, variable vestibular areflexia and vision loss. The aim of the study was to identify the genetic defect in a Pakistani family (PKDF1051) segregating USH.

METHODS

Genome-wide linkage analysis was performed by using an Illumina linkage array followed by Sanger and exome sequencing. Heterologous cells and mouse organ of Corti explant-based transfection assays were used for functional evaluations. Detailed clinical evaluations were performed to characterise the USH phenotype.

RESULTS

Through homozygosity mapping, we genetically linked the USH phenotype segregating in family PKDF1051 to markers on chromosome 1p36.32-p36.22. The locus was designated USH1M. Using a combination of Sanger sequencing and exome sequencing, we identified a novel homozygous 18 base pair inframe deletion in ESPN. Variants of ESPN, encoding the actin-bundling protein espin, have been previously associated with deafness and vestibular areflexia in humans with no apparent visual deficits. Our functional studies in heterologous cells and in mouse organ of Corti explant cultures revealed that the six deleted residues in affected individuals of family PKDF1051 are essential for the actin bundling function of espin demonstrated by ultracentrifugation actin binding and bundling assays. Funduscopic examination of the affected individuals of family PKDF1051 revealed irregular retinal contour, temporal flecks and disc pallor in both eyes. ERG revealed diminished rod photoreceptor function among affected individuals.

CONCLUSION

Our study uncovers an additional USH gene, assigns the USH1 phenotype to a variant of ESPN and provides a 12th molecular component to the USH proteome.

Emerging Gene Therapies for Genetic Hearing Loss

Gene therapy, or the treatment of human disease using genetic material, for inner ear dysfunction is coming of age. Recent progress in developing gene therapy treatments for genetic hearing loss has demonstrated tantalizing proof-of-principle in animal models. While successful translation of this progress into treatments for humans awaits, there is growing interest from patients, scientists, clinicians, and industry. Nonetheless, it is clear that a number of hurdles remain, and expectations for total restoration of auditory function should remain tempered until these challenges have been overcome. Here, we review progress, prospects, and challenges for gene therapy in the inner ear. We focus on technical aspects, including routes of gene delivery to the inner ear, choice of vectors, promoters, inner ear targets, therapeutic strategies, preliminary success stories, and points to consider for translating of these successes to the clinic.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

Densin-180 Controls the Trafficking and Signaling of L-Type Voltage-Gated Cav1.2 Ca2+ Channels at Excitatory Synapses

Voltage-gated Ca_v1.2 and Ca_v1.3 (L-type) Ca²⁺ channels regulate neuronal excitability, synaptic plasticity, and learning and memory. Densin-180 (densin) is an excitatory synaptic protein that promotes Ca²⁺-dependent facilitation of voltage-gated Ca_v1.3 Ca²⁺ channels in transfected cells. Mice lacking densin (densin KO) exhibit defects in synaptic plasticity, spatial memory, and increased anxiety-related behaviors-phenotypes that more closely match those in mice lacking Ca_v1.2 than Ca_v1.3. Therefore, we investigated the functional impact of densin on Ca_v1.2. We report that densin is an essential regulator of Ca_v1.2 in neurons, but has distinct modulatory effects compared with its regulation of Ca_v1.3. Densin binds to the N-terminal domain of Ca_v1.2, but not that of Ca_v1.3, and increases Ca_v1.2 currents in transfected cells and in neurons. In transfected cells, densin accelerates the forward trafficking of Ca_v1.2 channels without affecting their endocytosis. Consistent with a role for densin in increasing the number of postsynaptic Ca_v1.2 channels, overexpression of densin increases the clustering of Ca_v1.2 in dendrites of hippocampal neurons in culture. Compared with wild-type mice, the cell surface levels of Ca_v1.2 in the brain, as well as Ca_v1.2 current density and signaling to the nucleus, are reduced in neurons from densin KO mice. We conclude that densin is an essential regulator of neuronal Ca_v1 channels and ensures efficient Ca_v1.2 Ca²⁺ signaling at excitatory synapses.SIGNIFICANCE STATEMENT The number and localization of voltage-gated Ca_v Ca²⁺ channels are crucial determinants of neuronal excitability and synaptic transmission. We report that the protein densin-180 is highly enriched at excitatory synapses in the brain and enhances the cell surface trafficking and postsynaptic localization of Ca_v1.2 L-type Ca²⁺ channels in neurons. This interaction promotes coupling of Ca_v1.2 channels to activity-dependent gene transcription. Our results reveal a mechanism that may contribute to the roles of Ca_v1.2 in regulating cognition and mood.

Motor Neurons

Motor neurons translate synaptic input from widely distributed premotor networks into patterns of action potentials that orchestrate motor unit force and motor behavior. Intercalated between the CNS and muscles, motor neurons add to and adjust the final motor command. The identity and functional properties of this facility in the path from synaptic sites to the motor axon is reviewed with emphasis on voltage sensitive ion channels and regulatory metabotropic transmitter pathways. The catalog of the intrinsic response properties, their underlying mechanisms, and regulation obtained from motoneurons in in vitro preparations is far from complete. Nevertheless, a foundation has been provided for pursuing functional significance of intrinsic response properties in motoneurons in vivo during motor behavior at levels from molecules to systems. © 2017 American Physiological Society. Compr Physiol 7:463-484, 2017.

Ca2+-binding protein 2 inhibits Ca2+-channel inactivation in mouse inner hair cells

Ca²⁺-binding protein 2 (CaBP2) inhibits the inactivation of heterologously expressed voltage-gated Ca²⁺ channels of type 1.3 (Ca_V1.3) and is defective in human autosomal-recessive deafness 93 (DFNB93). Here, we report a newly identified mutation in CABP2 that causes a moderate hearing impairment likely via nonsense-mediated decay of CABP2-mRNA. To study the mechanism of hearing impairment resulting from CABP2 loss of function, we disrupted Cabp2 in mice (Cabp2^LacZ/LacZ ). CaBP2 was expressed by cochlear hair cells, preferentially in inner hair cells (IHCs), and was lacking from the postsynaptic spiral ganglion neurons (SGNs). Cabp2^LacZ/LacZ mice displayed intact cochlear amplification but impaired auditory brainstem responses. Patch-clamp recordings from Cabp2^LacZ/LacZ IHCs revealed enhanced Ca²⁺-channel inactivation. The voltage dependence of activation and the number of Ca²⁺ channels appeared normal in Cabp2^LacZ/LacZ mice, as were ribbon synapse counts. Recordings from single SGNs showed reduced spontaneous and sound-evoked firing rates. We propose that CaBP2 inhibits Ca_V1.3 Ca²⁺-channel inactivation, and thus sustains the availability of Ca_V1.3 Ca²⁺ channels for synaptic sound encoding. Therefore, we conclude that human deafness DFNB93 is an auditory synaptopathy.

Gene therapy restores auditory and vestibular function in a mouse model of Usher syndrome type 1c

Because there are currently no biological treatments for deafness, we sought to advance gene therapy approaches to treat genetic deafness. We reasoned that gene delivery systems that target auditory and vestibular sensory cells with high efficiency would be required to restore complex auditory and balance function. We focused on Usher Syndrome, a devastating genetic disorder that causes blindness, balance disorders and profound deafness, and used a knock-in mouse model, Ush1c c.216G>A, which carries a cryptic splice site mutation found in French-Acadian patients with Usher Syndrome type IC (USH1C). Following delivery of wild-type Ush1c into the inner ears of neonatal Ush1c c.216G>A mice, we find recovery of gene and protein expression, restoration of sensory cell function, rescue of complex auditory function and recovery of hearing and balance behavior to near wild-type levels. The data represent unprecedented recovery of inner ear function and suggest that biological therapies to treat deafness may be suitable for translation to humans with genetic inner ear disorders.

New insights into cochlear sound encoding

The inner ear uses specialized synapses to indefatigably transmit sound information from hair cells to spiral ganglion neurons at high rates with submillisecond precision. The emerging view is that hair cell synapses achieve their demanding function by employing an unconventional presynaptic molecular composition. Hair cell active zones hold the synaptic ribbon, an electron-dense projection made primarily of RIBEYE, which tethers a halo of synaptic vesicles and is thought to enable a large readily releasable pool of vesicles and to contribute to its rapid replenishment. Another important presynaptic player is otoferlin, coded by a deafness gene, which assumes a multi-faceted role in vesicular exocytosis and, when disrupted, causes auditory synaptopathy. A functional peculiarity of hair cell synapses is the massive heterogeneity in the sizes and shapes of excitatory postsynaptic currents. Currently, there is controversy as to whether this reflects multiquantal release with a variable extent of synchronization or uniquantal release through a dynamic fusion pore. Another important question in the field has been the precise mechanisms of coupling presynaptic Ca ²⁺ channels and vesicular Ca ²⁺ sensors. This commentary provides an update on the current understanding of sound encoding in the cochlea with a focus on presynaptic mechanisms.

Regulation of Postsynaptic Stability by the L-type Calcium Channel CaV1.3 and its Interaction with PDZ Proteins

Alterations in dendritic spine morphology and postsynaptic structure are a hallmark of neurological disorders. Particularly spine pruning of striatal medium spiny neurons and aberrant rewiring of corticostriatal synapses have been associated with the pathology of Parkinson’s disease and L-DOPA induced dyskinesia, respectively. Owing to its low activation threshold the neuronal L-type calcium channel Ca_V1.3 is particularly critical in the control of neuronal excitability and thus in the calcium-dependent regulation of neuronal functions. Ca_V1.3 channels are located in dendritic spines and contain a C-terminal class 1 PDZ domain-binding sequence. Until today the postsynaptic PDZ domain proteins shank, densin-180, and erbin have been shown to interact with Ca_V1.3 channels and to modulate their current properties. Interestingly experimental evidence suggests an involvement of all three PDZ proteins as well as Ca_V1.3 itself in regulating dendritic and postsynaptic morphology. Here we briefly review the importance of Ca_V1.3 and its proposed interactions with PDZ proteins for the stability of dendritic spines. With a special focus on the pathology associated with Parkinson’s disease, we discuss the hypothesis that Ca_V1.3 L-type calcium channels may be critical modulators of dendritic spine stability.

Hair cells use active zones with different voltage dependence of Ca2+ influx to decompose sounds into complementary neural codes

For sounds of a given frequency, spiral ganglion neurons (SGNs) with different thresholds and dynamic ranges collectively encode the wide range of audible sound pressures. Heterogeneity of synapses between inner hair cells (IHCs) and SGNs is an attractive candidate mechanism for generating complementary neural codes covering the entire dynamic range. Here, we quantified active zone (AZ) properties as a function of AZ position within mouse IHCs by combining patch clamp and imaging of presynaptic Ca(2+) influx and by immunohistochemistry. We report substantial AZ heterogeneity whereby the voltage of half-maximal activation of Ca(2+) influx ranged over ∼20 mV. Ca(2+) influx at AZs facing away from the ganglion activated at weaker depolarizations. Estimates of AZ size and Ca(2+) channel number were correlated and larger when AZs faced the ganglion. Disruption of the deafness gene GIPC3 in mice shifted the activation of presynaptic Ca(2+) influx to more hyperpolarized potentials and increased the spontaneous SGN discharge. Moreover, Gipc3 disruption enhanced Ca(2+) influx and exocytosis in IHCs, reversed the spatial gradient of maximal Ca(2+) influx in IHCs, and increased the maximal firing rate of SGNs at sound onset. We propose that IHCs diversify Ca(2+) channel properties among AZs and thereby contribute to decomposing auditory information into complementary representations in SGNs.

Molecularly and structurally distinct synapses mediate reliable encoding and processing of auditory information

Hearing impairment is the most common human sensory deficit. Considering the sophisticated anatomy and physiology of the auditory system, disease-related failures frequently occur. To meet the demands of the neuronal circuits responsible for processing auditory information, the synapses of the lower auditory pathway are anatomically and functionally specialized to process acoustic information indefatigably with utmost temporal precision. Despite sharing some functional properties, the afferent synapses of the cochlea and of auditory brainstem differ greatly in their morphology and employ distinct molecular mechanisms for regulating synaptic vesicle release. Calyceal synapses of the endbulb of Held and the calyx of Held profit from a large number of release sites that project onto one principal cell. Cochlear inner hair cell ribbon synapses exhibit a unique one-to-one relation of the presynaptic active zone to the postsynaptic cell and use hair-cell-specific proteins such as otoferlin for vesicle release. The understanding of the molecular physiology of the hair cell ribbon synapse has been advanced by human genetics studies of sensorineural hearing impairment, revealing human auditory synaptopathy as a new nosological entity.

Usher syndrome: Hearing loss, retinal degeneration and associated abnormalities

Usher syndrome (USH), clinically and genetically heterogeneous, is the leading genetic cause of combined hearing and vision loss. USH is classified into three types, based on the hearing and vestibular symptoms observed in patients. Sixteen loci have been reported to be involved in the occurrence of USH and atypical USH. Among them, twelve have been identified as causative genes and one as a modifier gene. Studies on the proteins encoded by these USH genes suggest that USH proteins interact among one another and function in multiprotein complexes in vivo. Although their exact functions remain enigmatic in the retina, USH proteins are required for the development, maintenance and function of hair bundles, which are the primary mechanosensitive structure of inner ear hair cells. Despite the unavailability of a cure, progress has been made to develop effective treatments for this disease. In this review, we focus on the most recent discoveries in the field with an emphasis on USH genes, protein complexes and functions in various tissues as well as progress toward therapeutic development for USH.

Clarin-1 acts as a modulator of mechanotransduction activity and presynaptic ribbon assembly

Clarin-1 is a four-transmembrane protein expressed by hair cells and photoreceptors. Mutations in its corresponding gene are associated with Usher syndrome type 3, characterized by late-onset and progressive hearing and vision loss in humans. Mice carrying mutations in the clarin-1 gene have hair bundle dysmorphology and a delay in synapse maturation. In this paper, we examined the expression and function of clarin-1 in zebrafish hair cells. We observed protein expression as early as 1 d postfertilization. Knockdown of clarin-1 resulted in inhibition of FM1-43 incorporation, shortening of the kinocilia, and mislocalization of ribeye b clusters. These phenotypes were fully prevented by co-injection with clarin-1 transcript, requiring its C-terminal tail. We also observed an in vivo interaction between clarin-1 and Pcdh15a. Altogether, our results suggest that clarin-1 is functionally important for mechanotransduction channel activity and for proper localization of synaptic components, establishing a critical role for clarin-1 at the apical and basal poles of hair cells.

Complexes of Usher proteins preassemble at the endoplasmic reticulum and are required for trafficking and ER homeostasis

Usher syndrome (USH), the leading cause of hereditary combined hearing and vision loss, is characterized by sensorineural deafness and progressive retinal degeneration. Mutations in several different genes produce USH, but the proximal cause of sensory cell death remains mysterious. We adapted a proximity ligation assay to analyze associations among three of the USH proteins, Cdh23, Harmonin and Myo7aa, and the microtubule-based transporter Ift88 in zebrafish inner ear mechanosensory hair cells. We found that the proteins are in close enough proximity to form complexes and that these complexes preassemble at the endoplasmic reticulum (ER). Defects in any one of the three USH proteins disrupt formation and trafficking of the complex and result in diminished levels of the other proteins, generalized trafficking defects and ER stress that triggers apoptosis. ER stress, thus, contributes to sensory hair cell loss and provides a new target to explore for protective therapies for USH.

Usher proteins in inner ear structure and function

Usher syndrome (USH) is a neurosensory disorder affecting both hearing and vision in humans. Linkage studies of families of USH patients, studies in animals, and characterization of purified proteins have provided insight into the molecular mechanisms of hearing. To date, 11 USH proteins have been identified, and evidence suggests that all of them are crucial for the function of the mechanosensory cells of the inner ear, the hair cells. Most USH proteins are localized to the stereocilia of the hair cells, where mechano-electrical transduction (MET) of sound-induced vibrations occurs. Therefore, elucidation of the functions of USH proteins in the stereocilia is a prerequisite to understanding the exact mechanisms of MET.