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

Efferent synaptic transmission at the vestibular type II hair cell synapse

In the vestibular peripheral organs, type I and type II hair cells (HCs) transmit incoming signals via glutamatergic quantal transmission onto afferent nerve fibers. Additionally, type I HCs transmit via "non-quantal" transmission to calyx afferent fibers, by accumulation of glutamate and potassium in the synaptic cleft. Vestibular efferent inputs originating in the brainstem contact type II HCs and vestibular afferents. Here, synaptic inputs to type II HCs were characterized by using electrical and optogenetic stimulation of efferent fibers combined with in vitro whole cell patch-clamp recording from type II HCs in the rodent vestibular crista. Properties of efferent synaptic currents in type II HCs were similar to those found in cochlear HCs and mediated by activation of α9-containing nicotinic acetylcholine receptors (nAChRs) and small-conductance calcium-activated potassium (SK) channels. While efferents showed a low probability of release at low frequencies of stimulation, repetitive stimulation resulted in facilitation and increased probability of release. Notably, the membrane potential of type II HCs during optogenetic stimulation of efferents showed a strong hyperpolarization in response to single pulses and was further enhanced by repetitive stimulation. Such efferent-mediated inhibition of type II HCs can provide a mechanism to adjust the contribution of signals from type I and type II HCs to vestibular nerve fibers, with a shift of the response to be more like that of calyx-only afferents with faster non-quantal responses.NEW & NOTEWORTHY Type II vestibular hair cells (HCs) receive inputs from efferent neurons in the brain stem. We used in vitro optogenetic and electrical stimulation of vestibular efferent fibers to study their synaptic inputs to type II HCs. Stimulation of efferents inhibited type II HCs, similar to efferent effects on cochlear HCs. We propose that efferent inputs adjust the contribution of signals from type I and II HCs to vestibular nerve fibers.

Characterization of spatial and temporal development of Type I and Type II hair cells in the mouse utricle using new cell-type-specific markers

The utricle of the inner ear, a vestibular sensory structure that mediates perception of linear acceleration, is comprised of two morphologically and physiologically distinct types of mechanosensory hair cells, referred to as Type Is and Type IIs. While these cell types are easily discriminated in an adult utricle, understanding their development has been hampered by a lack of molecular markers that can be used to identify each cell type prior to maturity. Therefore, we collected single hair cells at three different ages and used single cell RNAseq to characterize the transcriptomes of those cells. Analysis of differential gene expression identified Spp1 as a specific marker for Type I hair cells and Mapt and Anxa4 as specific markers for Type II hair cells. Antibody labeling confirmed the specificity of these markers which were then used to examine the temporal and spatial development of utricular hair cells. While Type I hair cells develop in a gradient that extends across the utricle from posterior-medial to anterior-lateral, Type II hair cells initially develop in the central striolar region and then extend uniformly towards the periphery. Finally, by combining these markers with genetic fate mapping, we demonstrate that over 98% of all Type I hair cells develop prior to birth while over 98% of Type II hair cells develop post-natally. These results are consistent with previous findings suggesting that Type I hair cells develop first and refute the hypothesis that Type II hair cells represent a transitional form between immature and Type I hair cells.

Generation of inner ear organoids containing functional hair cells from human pluripotent stem cells

Human inner ear tissue derived from pluripotent stem cells could provide a powerful platform for drug discovery or a source of sound- or motion-sensing cells for patients with hearing loss or balance dysfunction. Here we report a method for differentiating human pluripotent stem cells to inner ear organoids that harbor functional hair cells. Using a three-dimensional culture system, we modulate TGF, BMP, FGF, and Wnt signaling to generate multiple otic vesicle–like structures from a single stem-cell aggregate. Over two months, the vesicles develop into inner ear organoids with sensory epithelia that are innervated by sensory neurons. Additionally, using CRISPR/Cas9, we generate an ATOH1-2A-eGFP cell line to detect hair cell induction and demonstrate that derived hair cells exhibit electrophysiological properties similar to those of native sensory hair cells. Our culture system will be useful for elucidating mechanisms of human inner ear development and testing potential inner ear therapies.

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.

SHIELD: an integrative gene expression database for inner ear research

The inner ear is a highly specialized mechanosensitive organ responsible for hearing and balance. Its small size and difficulty in harvesting sufficient tissue has hindered the progress of molecular studies. The protein components of mechanotransduction, the molecular biology of inner ear development and the genetic causes of many hereditary hearing and balance disorders remain largely unknown. Inner-ear gene expression data will help illuminate each of these areas. For over a decade, our laboratories and others have generated extensive sets of gene expression data for different cell types in the inner ear using various sample preparation methods and high-throughput genome-wide approaches. To facilitate the study of genes in the inner ear by efficient presentation of the accumulated data and to foster collaboration among investigators, we have developed the Shared Harvard Inner Ear Laboratory Database (SHIELD), an integrated resource that seeks to compile, organize and analyse the genomic, transcriptomic and proteomic knowledge of the inner ear. Five datasets are currently available. These datasets are combined in a relational database that integrates experimental data and annotations relevant to the inner ear. The SHIELD has a searchable web interface with two data retrieval options: viewing the gene pages online or downloading individual datasets as data tables. Each retrieved gene page shows the gene expression data and detailed gene information with hyperlinks to other online databases with up-to-date annotations. Downloadable data tables, for more convenient offline data analysis, are derived from publications and are current as of the time of publication. The SHIELD has made published and some unpublished data freely available to the public with the hope and expectation of accelerating discovery in the molecular biology of balance, hearing and deafness.

Database URL:https://shield.hms.harvard.edu

Gene Expression by Mouse Inner Ear Hair Cells during Development

Hair cells of the inner ear are essential for hearing and balance. As a consequence, pathogenic variants in genes specifically expressed in hair cells often cause hereditary deafness. Hair cells are few in number and not easily isolated from the adjacent supporting cells, so the biochemistry and molecular biology of hair cells can be difficult to study. To study gene expression in hair cells, we developed a protocol for hair cell isolation by FACS. With nearly pure hair cells and surrounding cells, from cochlea and utricle and from E16 to P7, we performed a comprehensive cell type-specific RNA-Seq study of gene expression during mouse inner ear development. Expression profiling revealed new hair cell genes with distinct expression patterns: some are specific for vestibular hair cells, others for cochlear hair cells, and some are expressed just before or after maturation of mechanosensitivity. We found that many of the known hereditary deafness genes are much more highly expressed in hair cells than surrounding cells, suggesting that genes preferentially expressed in hair cells are good candidates for unknown deafness genes.

Mechanotransduction and hyperpolarization-activated currents contribute to spontaneous activity in mouse vestibular ganglion neurons

The hyperpolarization-activated, cyclic nucleotide-sensitive current, Ih, is present in vestibular hair cells and vestibular ganglion neurons, and is required for normal balance function. We sought to identify the molecular correlates and functional relevance of Ih in vestibular ganglion neurons. Ih is carried by channels consisting of homo- or heteromeric assemblies of four protein subunits from the Hcn gene family. The relative expression of Hcn1-4 mRNA was examined using a quantitative reverse transcription PCR (RT-PCR) screen. Hcn2 was the most highly expressed subunit in vestibular neuron cell bodies. Immunolocalization of HCN2 revealed robust expression in cell bodies of all vestibular ganglion neurons. To characterize Ih in vestibular neuron cell bodies and at hair cell-afferent synapses, we developed an intact, ex vivo preparation. We found robust physiological expression of Ih in 89% of cell bodies and 100% of calyx terminals. Ih was significantly larger in calyx terminals than in cell bodies; however, other biophysical characteristics were similar. Ih was absent in calyces lacking Hcn1 and Hcn2, but small Ih was still present in cell bodies, which suggests expression of an additional subunit, perhaps Hcn4. To determine the contributions of hair cell mechanotransduction and Ih to the firing patterns of calyx terminals, we recorded action potentials in current-clamp mode. Mechanotransduction currents were modulated by hair bundle defection and application of calcium chelators to disrupt tip links. Ih activity was modulated using ZD7288 and cAMP. We found that both hair cell transduction and Ih contribute to the rate and regularity of spontaneous action potentials in the vestibular afferent neurons. We propose that modulation of Ih in vestibular ganglion neurons may provide a mechanism for modulation of spontaneous activity in the vestibular periphery.

Human fetal inner ear involvement in congenital cytomegalovirus infection

Background

Congenital cytomegalovirus (CMV) infection is a leading cause of sensorineural hearing loss (SNHL). The mechanisms of pathogenesis of CMV-related SNHL are still unclear. The aim is to study congenital CMV-related damage in the fetal inner ear, in order to better understand the underlying pathophysiology behind CMV-SNHL.

Results

We studied inner ears and brains of 20 human fetuses, all at 21 week gestational age, with a high viral load in the amniotic fluid, with and without ultrasound (US) brain abnormalities. We evaluated histological brain damage, inner ear infection, local inflammatory response and tissue viral load.

Immunohistochemistry revealed that CMV was positive in 14/20 brains (70%) and in the inner ears of 9/20 fetuses (45%). In the cases with inner ear infection, the marginal cell layer of the stria vascularis was always infected, followed by infection in the Reissner’s membrane. The highest tissue viral load was observed in the inner ear with infected Organ of Corti. Vestibular labyrinth showed CMV infection of sensory cells in the utricle and in the crista ampullaris.

US cerebral anomalies were detected in 6 cases, and in all those cases, the inner ear was always involved. In the other 14 cases with normal brain scan, histological brain damage was present in 8 fetuses and 3 of them presented inner ear infection.

Conclusions

CMV-infection of the marginal cell layer of the stria vascularis may alter potassium and ion circulation, dissipating the endocochlear potential with consequent SNHL. Although abnormal cerebral US is highly predictive of brain and inner ear damage, normal US findings cannot exclude them either.