Architecture of the mouse utricle: macular organization and hair bundle heights

Contributions of mirror-image hair cell orientation to mouse otolith organ and zebrafish neuromast function

Otolith organs in the inner ear and neuromasts in the fish lateral-line harbor two populations of hair cells oriented to detect stimuli in opposing directions. The underlying mechanism is highly conserved: the transcription factor EMX2 is regionally expressed in just one hair cell population and acts through the receptor GPR156 to reverse cell orientation relative to the other population. In mouse and zebrafish, loss of Emx2 results in sensory organs that harbor only one hair cell orientation and are not innervated properly. In zebrafish, Emx2 also confers hair cells with reduced mechanosensory properties. Here, we leverage mouse and zebrafish models lacking GPR156 to determine how detecting stimuli of opposing directions serves vestibular function, and whether GPR156 has other roles besides orienting hair cells. We find that otolith organs in Gpr156 mouse mutants have normal zonal organization and normal type I-II hair cell distribution and mechano-electrical transduction properties. In contrast, gpr156 zebrafish mutants lack the smaller mechanically evoked signals that characterize Emx2-positive hair cells. Loss of GPR156 does not affect orientation-selectivity of afferents in mouse utricle or zebrafish neuromasts. Consistent with normal otolith organ anatomy and afferent selectivity, Gpr156 mutant mice do not show overt vestibular dysfunction. Instead, performance on two tests that engage otolith organs is significantly altered - swimming and off-vertical-axis rotation. We conclude that GPR156 relays hair cell orientation and transduction information downstream of EMX2, but not selectivity for direction-specific afferents. These results clarify how molecular mechanisms that confer bi-directionality to sensory organs contribute to function, from single hair cell physiology to animal behavior.

Contributions of mirror-image hair cell orientation to mouse otolith organ and zebrafish neuromast function

Otolith organs in the inner ear and neuromasts in the fish lateral-line harbor two populations of hair cells oriented to detect stimuli in opposing directions. The underlying mechanism is highly conserved: the transcription factor EMX2 is regionally expressed in just one hair cell population and acts through the receptor GPR156 to reverse cell orientation relative to the other population. In mouse and zebrafish, loss of Emx2 results in sensory organs that harbor only one hair cell orientation and are not innervated properly. In zebrafish, Emx2 also confers hair cells with reduced mechanosensory properties. Here, we leverage mouse and zebrafish models lacking GPR156 to determine how detecting stimuli of opposing directions serves vestibular function, and whether GPR156 has other roles besides orienting hair cells. We find that otolith organs in Gpr156 mouse mutants have normal zonal organization and normal type I-II hair cell distribution and mechano-electrical transduction properties. In contrast, gpr156 zebrafish mutants lack the smaller mechanically-evoked signals that characterize Emx2-positive hair cells. Loss of GPR156 does not affect orientation-selectivity of afferents in mouse utricle or zebrafish neuromasts. Consistent with normal otolith organ anatomy and afferent selectivity, Gpr156 mutant mice do not show overt vestibular dysfunction. Instead, performance on two tests that engage otolith organs is significantly altered - swimming and off-vertical-axis rotation. We conclude that GPR156 relays hair cell orientation and transduction information downstream of EMX2, but not selectivity for direction-specific afferents. These results clarify how molecular mechanisms that confer bi-directionality to sensory organs contribute to function, from single hair cell physiology to animal behavior.

Comparative biology of the amniote vestibular utricle

The sensory epithelia of the auditory and vestibular systems of vertebrates have shared developmental and evolutionary histories. However, while the auditory epithelia show great variation across vertebrates, the vestibular sensory epithelia appear seemingly more conserved. An exploration of the current knowledge of the comparative biology of the amniote utricle, a vestibular sensory epithelium that senses linear acceleration, shows interesting instances of variability between birds and mammals. The distribution of sensory hair cell types, the position of the line of hair bundle polarity reversal and the properties of supporting cells show marked differences, likely impacting vestibular function and hair cell regeneration potential.

Notch1 is required to maintain supporting cell identity and vestibular function during maturation of the mammalian balance organs

The inner ear houses two sensory modalities: the hearing organ, located in the cochlea, and the balance organs, located throughout the vestibular regions of the ear. Both hearing and vestibular sensory regions are composed of similar cell types, including hair cells and associated supporting cells. Recently, we showed that Notch1 is required for maintaining supporting cell survival postnatally during cochlear maturation. However, it is not known whether Notch1 plays a similar role in the balance organs of the inner ear. To characterize the role of Notch during vestibular maturation, we conditionally deleted Notch1 from Sox2-expressing cells of the vestibular organs in the mouse at P0/P1. Histological analyses showed a dramatic loss of supporting cells accompanied by an increase in type II hair cells without cell death, indicating the supporting cells are converting to hair cells in the maturing vestibular regions. Analysis of 6-week old animals indicate that the converted hair cells survive, despite the reduction of supporting cells. Interestingly, measurements of vestibular sensory evoked potentials (VsEPs), known to be generated in the striolar regions of the vestibular afferents in the maculae, failed to show a response, indicating that NOTCH1 expression is critical for striolar function postnatally. Consistent with this, we find that the specialized type I hair cells in the striola fail to develop the complex calyces typical of these cells. These defects are likely due to the reduction in supporting cells, which have previously been shown to express factors critical for the striolar region. Similar to other mutants that lack proper striolar development, Notch1 mutants do not exhibit typical vestibular behaviors such as circling and head shaking, but do show difficulties in some vestibular tests, including the balance beam and forced swim test. These results indicate that, unlike the hearing organ in which the supporting cells undergo cell death, supporting cells in the balance regions retain the ability to convert to hair cells during maturation, which survive into adulthood despite the reduction in supporting cells.

A New Perspective to Interpret How the Vestibular Efferent System Correlates the Complexity of Routine Balance Maintenance with Management of Emergency Fall Prevention Strategies

Bipedalism is unique among mammals. Until modern times, a fall and resulting leg fracture could be fatal. Balance maintenance after a destabilizing event requires instantaneous decision making. The vestibular system plays an essential role in this process, initiating an emergency response. The afferent otolithic neural response is the first directionally oriented information to reach the cortex, and it can then be used to initiate an appropriate protective response. Some vestibular efferent axons feed directly into type I vestibular hair cells. This allows for rapid vestibular feedback via the striated organelle (STO), which has been largely ignored in most texts. We propose that this structure is essential in emergency fall prevention, and also that the system of sensory detection and resultant motor response works by having efferent movement information simultaneously transmitted to the maculae with the movement commands. This results in the otolithic membrane positioning itself precisely for the planned movement, and any error is due to an unexpected external cause. Error is fed back via the vestibular afferent system. The efferent system causes macular otolithic membrane movement through the STO, which occurs simultaneously with the initiating motor command. As a result, no vestibular afferent activity occurs unless an error must be dealt with.

Live-cell single-molecule fluorescence microscopy for protruding organelles reveals regulatory mechanisms of MYO7A-driven cargo transport in stereocilia of inner ear hair cells

Stereocilia are unidirectional F-actin-based cylindrical protrusions on the apical surface of inner ear hair cells and function as biological mechanosensors of sound and acceleration. Development of functional stereocilia requires motor activities of unconventional myosins to transport proteins necessary for elongating the F-actin cores and to assemble the mechanoelectrical transduction (MET) channel complex. However, how each myosin localizes in stereocilia using the energy from ATP hydrolysis is only partially understood. In this study, we develop a methodology for live-cell single-molecule fluorescence microscopy of organelles protruding from the apical surface using a dual-view light-sheet microscope, diSPIM. We demonstrate that MYO7A, a component of the MET machinery, traffics as a dimer in stereocilia. Movements of MYO7A are restricted when scaffolded by the plasma membrane and F-actin as mediated by MYO7A's interacting partners. Here, we discuss the technical details of our methodology and its future applications including analyses of cargo transportation in various organelles.

TMC function, dysfunction, and restoration in mouse vestibular organs

Tmc1 and Tmc2 are essential pore-forming subunits of mechanosensory transduction channels localized to the tips of stereovilli in auditory and vestibular hair cells of the inner ear. To investigate expression and function of Tmc1 and Tmc2 in vestibular organs, we used quantitative polymerase chain reaction (qPCR), fluorescence in situ hybridization - hairpin chain reaction (FISH-HCR), immunostaining, FM1-43 uptake and we measured vestibular evoked potentials (VsEPs) and vestibular ocular reflexes (VORs). We found that Tmc1 and Tmc2 showed dynamic developmental changes, differences in regional expression patterns, and overall expression levels which differed between the utricle and saccule. These underlying changes contributed to unanticipated phenotypic loss of VsEPs and VORs in Tmc1 KO mice. In contrast, Tmc2 KO mice retained VsEPs despite the loss of the calcium buffering protein calretinin, a characteristic biomarker of mature striolar calyx-only afferents. Lastly, we found that neonatal Tmc1 gene replacement therapy is sufficient to restore VsEP in Tmc1 KO mice for up to six months post-injection.

Vestibular Testing-New Physiological Results for the Optimization of Clinical VEMP Stimuli

Both auditory and vestibular primary afferent neurons can be activated by sound and vibration. This review relates the differences between them to the different receptor/synaptic mechanisms of the two systems, as shown by indicators of peripheral function-cochlear and vestibular compound action potentials (cCAPs and vCAPs)-to click stimulation as recorded in animal studies. Sound- and vibration-sensitive type 1 receptors at the striola of the utricular macula are enveloped by the unique calyx afferent ending, which has three modes of synaptic transmission. Glutamate is the transmitter for both cochlear and vestibular primary afferents; however, blocking glutamate transmission has very little effect on vCAPs but greatly reduces cCAPs. We suggest that the ultrafast non-quantal synaptic mechanism called resistive coupling is the cause of the short latency vestibular afferent responses and related results-failure of transmitter blockade, masking, and temporal precision. This "ultrafast" non-quantal transmission is effectively electrical coupling that is dependent on the membrane potentials of the calyx and the type 1 receptor. The major clinical implication is that decreasing stimulus rise time increases vCAP response, corresponding to the increased VEMP response in human subjects. Short rise times are optimal in human clinical VEMP testing, whereas long rise times are mandatory for audiometric threshold testing.

The proper timing of Atoh1 expression is pivotal for hair cell subtype differentiation and the establishment of inner ear function

Atoh1 overexpression is essential for hair cell (HC) regeneration in the sensory epithelium of mammalian auditory and vestibular organs. However, Atoh1 overexpression alone cannot induce fully mature and functional HCs in the mammalian inner ear. In the current study, we investigated the effect of Atoh1 constitutive overexpression in native HCs by manipulating Atoh1 expression at different developmental stages. We demonstrated that constitutive overexpression of Atoh1 in native vestibular HCs did not affect cell survival but did impair vestibular function by interfering with the subtype differentiation of HCs and hair bundle development. In contrast, Atoh1 overexpression in cochlear HCs impeded their maturation, eventually leading to gradual HC loss in the cochlea and hearing dysfunction. Our study suggests that time-restricted Atoh1 expression is essential for the differentiation and survival of HCs in the inner ear, and this is pivotal for both hearing and vestibular function re-establishment through Atoh1 overexpression-induced HC regeneration strategies.

Cross-species analysis and comparison of the inner ear between chickens and mice

The inner ear of mammals includes the cochlea and vestibule, which house specialized hair cells that are responsible for hearing and balance, respectively. While cochlear hair cells fail to regenerate following damage, those of the utricle, which is part of the vestibular apparatus, show partial regeneration. In birds, the macula lagena, a unique ear structure in this clade, has the ability to regenerate hair cells similarly to the utricle. Many studies have sought to explain regeneration in terms of evolution and species differences. However, it remains unclear what the cellular and molecular basis is behind the differences in inner ear structures and between avians and mammals. In the present study, we first investigated the anatomical structures of the inner ear of both chickens and rodents. We then performed RNA sequencing (RNA-Seq) and made cross-species analyses of the expression of homologous genes obtained from the inner ear tissue from both chickens and mice. Finally, we focused on the lagena, the basilar papilla, and the utricle in chickens and identified differentially expressed genes between tissues and determined the expression patterns of genes involved in inner ear structure formation by single-cell RNA sequencing and bulk RNA-Seq. We concluded that the cellular and molecular composition of the lagena is more similar to that of the utricle than the cochlea. Taken together, our study provides a valuable resource for the study of inner ear evolution and development.

Quantitative analysis and correlative evaluation of video-oculography, micro-computed tomography, and histopathology in Pendrin-null mice

Patients with SLC26A4 mutations exhibit highly variable hearing loss and vestibular dysfunction. Although Slc26a4 mutant mice similarly exhibit vestibular deficits, including circling behavior, head tilting, and torticollis, the underlying pathogenesis of the vestibular symptoms remains unclear, hindering its effective management for patients with SLC26A4 mutations. In this study, we evaluated the equilibrium function using the inspection equipment, which can record eye movements against rotational, gravitational, and thermal stimulations. Moreover, we correlated the degree of functional impairment with the morphological alterations observed in Slc26a4^Δ/Δ mice. The rotational stimulus and ice water caloric tests revealed considerable impairment of the semicircular canal, while the tilted gravitational stimulus test showed a severe functional decline of the otolithic system in Slc26a4^Δ/Δ mice. Generally, the degree of impairment was more severe in circling Slc26a4^Δ/Δ mice than in non-circling Slc26a4^Δ/Δ mice. In non-circling Slc26a4^Δ/Δ mice, the semicircular canal function was normal. Micro-computed tomography results showed enlargement of the vestibular aqueduct and bony semicircular canals but no correlative relationship between the severity of the caloric response and the size of bony labyrinths. Giant otoconia and a significant decrease in total otolith volume in the saccule and utricle were observed in Slc26a4^Δ/Δ mice. However, the giant otoconia were not overly dislocated in the bony otolithic system and ectopic otoconia were absent in the semicircular canal. The number and morphology of the utricular hair cells in Slc26a4^Δ/Δ mice were not significantly reduced compared to those in Slc26a4^Δ/+ mice. Collectively, we can conclude that vestibular impairments are mainly associated with otoconia formation and morphology rather than hair cell degeneration. In addition, severe disturbances of semicircular canals cause circling behavior in Slc26a4^Δ/Δ mice. Our comprehensive morphological and functional assessments apply to mouse models of other genetic diseases with vestibular impairment.

Magnetic actuation of otoliths allows behavioral and brain-wide neuronal exploration of vestibulo-motor processing in larval zebrafish

The vestibular system in the inner ear plays a central role in sensorimotor control by informing the brain about the orientation and acceleration of the head. However, most experiments in neurophysiology are performed using head-fixed configurations, depriving animals of vestibular inputs. To overcome this limitation, we decorated the utricular otolith of the vestibular system in larval zebrafish with paramagnetic nanoparticles. This procedure effectively endowed the animal with magneto-sensitive capacities: applied magnetic field gradients induced forces on the otoliths, resulting in robust behavioral responses comparable to those evoked by rotating the animal by up to 25°. We recorded the whole-brain neuronal response to this fictive motion stimulation using light-sheet functional imaging. Experiments performed in unilaterally injected fish revealed the activation of a commissural inhibition between the brain hemispheres. This magnetic-based stimulation technique for larval zebrafish opens new perspectives to functionally dissect the neural circuits underlying vestibular processing and to develop multisensory virtual environments, including vestibular feedback.

CIB2 and CIB3 Regulate Stereocilia Maintenance and Mechanoelectrical Transduction in Mouse Vestibular Hair Cells

The mechanoelectrical transduction (MET) protein complex in the inner-ear hair cells is essential for hearing and balance perception. Calcium and integrin-binding protein 2 (CIB2) has been reported to be a component of MET complex, and loss of CIB2 completely abolishes MET currents in auditory hair cells, causing profound congenital hearing loss. However, loss of CIB2 does not affect MET currents in vestibular hair cells (VHCs) as well as general balance function. Here, we show that CIB2 and CIB3 act redundantly to regulate MET in VHCs, as MET currents are completely abolished in the VHCs of Cib2/Cib3 double knock-out mice of either sex. Furthermore, we show that Cib2 and Cib3 transcripts have complementary expression patterns in the vestibular maculae, and that they play different roles in stereocilia maintenance in VHCs. Cib2 transcripts are highly expressed in the striolar region, and knock-out of Cib2 affects stereocilia maintenance in striolar VHCs. In contrast, Cib3 transcripts are highly expressed in the extrastriolar region, and knock-out of Cib3 mainly affects stereocilia maintenance in extrastriolar VHCs. Simultaneous knock-out of Cib2 and Cib3 affects stereocilia maintenance in all VHCs and leads to severe balance deficits. Taken together, our present work reveals that CIB2 and CIB3 are important for stereocilia maintenance as well as MET in mouse VHCs.SIGNIFICANCE STATEMENT Calcium and integrin-binding protein 2 (CIB2) is an important component of mechanoelectrical transduction (MET) complex, and loss of CIB2 completely abolishes MET in auditory hair cells. However, MET is unaffected in Cib2 knock-out vestibular hair cells (VHCs). In the present work, we show that CIB3 could compensate for the loss of CIB2 in VHCs, and Cib2/Cib3 double knock-out completely abolishes MET in VHCs. Interestingly, CIB2 and CIB3 could also regulate VHC stereocilia maintenance in a nonredundant way. Cib2 and Cib3 transcripts are highly expressed in the striolar and extrastriolar regions, respectively. Stereocilia maintenance and balance function are differently affected in Cib2 or Cib3 knock-out mice. In conclusion, our data suggest that CIB2 and CIB3 are important for stereocilia maintenance and MET in mouse VHCs.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

Single-cell transcriptomic profiling of the zebrafish inner ear reveals molecularly distinct hair cell and supporting cell subtypes

A major cause of human deafness and vestibular dysfunction is permanent loss of the mechanosensory hair cells of the inner ear. In non-mammalian vertebrates such as zebrafish, regeneration of missing hair cells can occur throughout life. While a comparative approach has the potential to reveal the basis of such differential regenerative ability, the degree to which the inner ears of fish and mammals share common hair cells and supporting cell types remains unresolved. Here, we perform single-cell RNA sequencing of the zebrafish inner ear at embryonic through adult stages to catalog the diversity of hair cells and non-sensory supporting cells. We identify a putative progenitor population for hair cells and supporting cells, as well as distinct hair and supporting cell types in the maculae versus cristae. The hair cell and supporting cell types differ from those described for the lateral line system, a distributed mechanosensory organ in zebrafish in which most studies of hair cell regeneration have been conducted. In the maculae, we identify two subtypes of hair cells that share gene expression with mammalian striolar or extrastriolar hair cells. In situ hybridization reveals that these hair cell subtypes occupy distinct spatial domains within the three macular organs, the utricle, saccule, and lagena, consistent with the reported distinct electrophysiological properties of hair cells within these domains. These findings suggest that primitive specialization of spatially distinct striolar and extrastriolar hair cells likely arose in the last common ancestor of fish and mammals. The similarities of inner ear cell type composition between fish and mammals validate zebrafish as a relevant model for understanding inner ear-specific hair cell function and regeneration.

Tiltable objective microscope visualizes selectivity for head motion direction and dynamics in zebrafish vestibular system

Spatio-temporal information about head orientation and movement is fundamental to the sense of balance and motion. Hair cells (HCs) in otolith organs of the vestibular system transduce linear acceleration, including head tilt and vibration. Here, we build a tiltable objective microscope in which an objective lens and specimen tilt together. With in vivo Ca2+ imaging of all utricular HCs and ganglion neurons during 360° static tilt and vibration in pitch and roll axes, we reveal the direction- and static/dynamic stimulus-selective topographic responses in larval zebrafish. We find that head vibration is preferentially received by striolar HCs, whereas static tilt is preferentially transduced by extrastriolar HCs. Spatially ordered direction preference in HCs is consistent with hair-bundle polarity and is preserved in ganglion neurons through topographic innervation. Together, these results demonstrate topographically organized selectivity for direction and dynamics of head orientation/movement in the vestibular periphery.

Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration in adult mice

Sox2 is a transcription factor that is necessary in the mammalian inner ear for development of sensory hair cells and supporting cells. Sox2 is expressed in supporting cells of adult mammals, but its function in this context is poorly understood. Given its role in the developing inner ear, we hypothesized that Sox2 is required in vestibular supporting cells for regeneration of type II hair cells after damage. Using adult mice, we deleted Sox2 from Sox9-CreER-expressing supporting cells prior to diphtheria toxin-mediated hair cell destruction and used fate-mapping to assess regeneration. In utricles of control mice with normal Sox2 expression, supporting cells regenerated nearly 200 hair cells by 3 weeks post-damage, which doubled by 12 weeks. In contrast, mice with Sox2 deletion from supporting cells had approximately 20 fate-mapped hair cells at 3 weeks post-damage, and this number did not change significantly by 12 weeks, indicating regeneration was dramatically curtailed. We made similar observations for saccules and ampullae. We found no evidence that supporting cells lacking Sox2 had altered cellular density, morphology, or ultrastructure. However, some Sox2-negative supporting cell nuclei appeared to migrate apically but did not turn on hair cell markers, and type I hair cell survival was higher. Sox2 heterozygotes also had reduced regeneration in utricles, but more hair cells were replaced than mice with Sox2 deletion. Our study determined that Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration but found no other major requirements for Sox2 in adult supporting cells.

CDK2 regulates aminoglycoside-induced hair cell death through modulating c-Jun activity: Inhibiting CDK2 to preserve hearing

Sensory hair cell death caused by the ototoxic side effects of many clinically used drugs leads to permanent sensorineural hearing loss in patients. Aminoglycoside antibiotics are widely used and well-known for their ototoxicity, but the molecular mechanisms of aminoglycoside-induced hair cell death are not well understood. This creates challenges in our attempts to alleviate or prevent such adverse side effects. Here, we report a regulatory role of CDK2 in aminoglycoside-induced hair cell death. Utilizing organotypic cultures of cochleae from neonatal mice, we show that blocking CDK2 activity by either pharmaceutical inhibition or by Cdk2 gene knockout protects hair cells against the ototoxicity of gentamicin—one of the most commonly used aminoglycoside antibiotics—by interfering with intrinsic programmed cell death processes. Specifically, we show that CDK2 inhibition delays the collapse of mitochondria and the activation of a caspase cascade. Furthermore, at the molecular level, inhibition of CDK2 activity influences proapoptotic JNK signaling by reducing the protein level of c-Jun and suppressing the gentamicin-induced upregulation of c-Jun target genes Jun and Bim. Our in vivo studies reveal that Cdk2 gene knockout animals are significantly less sensitive to gentamicin ototoxicity compared to wild-type littermates. Altogether, our work ascertains the non-cell cycle role of CDK2 in regulating aminoglycoside-induced hair cell apoptosis and sheds lights on new potential strategies for hearing protection against ototoxicity.

Organization of the gravity-sensing system in zebrafish

Motor circuits develop in sequence from those governing fast movements to those governing slow. Here we examine whether upstream sensory circuits are organized by similar principles. Using serial-section electron microscopy in larval zebrafish, we generated a complete map of the gravity-sensing (utricular) system spanning from the inner ear to the brainstem. We find that both sensory tuning and developmental sequence are organizing principles of vestibular topography. Patterned rostrocaudal innervation from hair cells to afferents creates an anatomically inferred directional tuning map in the utricular ganglion, forming segregated pathways for rostral and caudal tilt. Furthermore, the mediolateral axis of the ganglion is linked to both developmental sequence and neuronal temporal dynamics. Early-born pathways carrying phasic information preferentially excite fast escape circuits, whereas later-born pathways carrying tonic signals excite slower postural and oculomotor circuits. These results demonstrate that vestibular circuits are organized by tuning direction and dynamics, aligning them with downstream motor circuits and behaviors.

Synaptic transmission at the vestibular hair cells of amniotes

A harmonized interplay between the central nervous system and the five peripheral end organs is how the vestibular system helps organisms feel a sense of balance and motion in three-dimensional space. The receptor cells of this system, much like their cochlear equivalents, are the specialized hair cells. However, research over the years has shown that the vestibular endorgans and hair cells evolved very differently from their cochlear counterparts. The structurally unique calyceal synapse, which appeared much later in the evolutionary time scale, and continues to intrigue researchers, is now known to support several forms of synaptic neurotransmission. The conventional quantal transmission is believed to employ the ribbon structures, which carry several tethered vesicles filled with neurotransmitters. However, the field of vestibular hair cell synaptic molecular anatomy is still at a nascent stage and needs further work. In this review, we will touch upon the basic structure and function of the peripheral vestibular system, with the focus on the various modes of neurotransmission at the type I vestibular hair cells. We will also shed light on the current knowledge about the molecular anatomy of the vestibular hair cell synapses and vestibular synaptopathy.

Vestibular Organ Dissection and Whole-Mount Immunolabeling in Mouse

The vestibular sensory apparatus contained in the inner ear is a marvelous evolutionary adaptation for sensing movement in 3 dimensions and is essential for an animal's sense of orientation in space, head movement, and balance. Damage to these systems through injury or disease can lead to vertigo, Meniere's disease, and other disorders that are profoundly debilitating. One challenge in studying vestibular organs is their location within the boney inner ear and their small size, especially in mice, which have become an advantageous mammalian model. This protocol describes the dissection procedure of the five vestibular organs from the inner ear of adult mice, followed by immunohistochemical labeling of a whole mount preparation using antibodies to label endogenous proteins such as calretinin to label Type I hair cells or to amplify genetically expressed fluorescent proteins for confocal microscopic imaging. Using typical lab equipment and reagents, a patient technician, student, or postdoc can learn to dissect and immunolabel mouse vestibular organs to investigate their structure in health and disease.

Mechanotransduction in mammalian sensory hair cells

In the inner ear, the auditory and vestibular systems detect and translate sensory information regarding sound and balance. The sensory cells that transform mechanical input into an electrical signal in these systems are called hair cells. A specialized organelle on the apical surface of hair cells called the hair bundle detects mechanical signals. Displacement of the hair bundle causes mechanotransduction channels to open. The morphology and organization of the hair bundle, as well as the properties and characteristics of the mechanotransduction process, differ between the different hair cell types in the auditory and vestibular systems. These differences likely contribute to maximizing the transduction of specific signals in each system. This review will discuss the molecules essential for mechanotransduction and the properties of the mechanotransduction process, focusing our attention on recent data and differences between the auditory and vestibular systems.

Vestibular Deficits in Deafness: Clinical Presentation, Animal Modeling, and Treatment Solutions

The inner ear is responsible for both hearing and balance. These functions are dependent on the correct functioning of mechanosensitive hair cells, which convert sound- and motion-induced stimuli into electrical signals conveyed to the brain. During evolution of the inner ear, the major changes occurred in the hearing organ, whereas the structure of the vestibular organs remained constant in all vertebrates over the same period. Vestibular deficits are highly prevalent in humans, due to multiple intersecting causes: genetics, environmental factors, ototoxic drugs, infections and aging. Studies of deafness genes associated with balance deficits and their corresponding animal models have shed light on the development and function of these two sensory systems. Bilateral vestibular deficits often impair individual postural control, gaze stabilization, locomotion and spatial orientation. The resulting dizziness, vertigo, and/or falls (frequent in elderly populations) greatly affect patient quality of life. In the absence of treatment, prosthetic devices, such as vestibular implants, providing information about the direction, amplitude and velocity of body movements, are being developed and have given promising results in animal models and humans. Novel methods and techniques have led to major progress in gene therapies targeting the inner ear (gene supplementation and gene editing), 3D inner ear organoids and reprograming protocols for generating hair cell-like cells. These rapid advances in multiscale approaches covering basic research, clinical diagnostics and therapies are fostering interdisciplinary research to develop personalized treatments for vestibular disorders.

Microtubule and auditory function - an underestimated connection

The organ of Corti, located in the cochlea within the inner ear is the receptor organ for hearing. It converts auditory signals into neuronal action potentials that are transmitted to the brain for further processing. The mature organ of Corti consists of a variety of highly differentiated sensory cells that fulfil unique tasks in the processing of auditory signals. The actin and microtubule cytoskeleton play essential function in hearing, however so far, more attention has been paid to the role of actin. Microtubules play important roles in maintaining cellular structure and intracellular transport in virtually all eukaryotic cells. Their functions are controlled by interactions with a large variety of microtubule-associated proteins (MAPs) and molecular motors. Current advances show that tubulin posttranslational modifications, as well as tubulin isotypes could play key roles in modulating microtubule properties and functions in cells. These mechanisms could have various effects on the stability and functions of microtubules in the highly specialised cells of the cochlea. Here, we review the current understanding of the role of microtubule-regulating mechanisms in the function of the cochlea and their implications for hearing, which highlights the importance of microtubules in the field of hearing research.

Semi-automated Quantification of Hair Cells in the Mature Mouse Utricle

The mouse utricle model system is the best-characterized ex vivo preparation for studies of mature mammalian hair cells (HCs). Despite the many advantages of this model system, efficient and reliable quantification of HCs from cultured utricles has been a persistent challenge with this model system. Utricular HCs are commonly quantified by counting immunolabeled HCs in regions of interest (ROIs) placed over an image of the utricle. Our data indicate that the accuracy of HC counts obtained using this method can be impacted by variability in HC density across different regions of the utricle. In addition, the commonly used HC marker myosin 7a results in a diffuse cytoplasmic stain that is not conducive to automated quantification and must be quantified manually, a labor-intensive task. Furthermore, myosin 7a immunoreactivity is retained in dead HCs, resulting in inaccurate quantification of live HCs using this marker. Here we have developed a method for semi-automated quantification of surviving HCs that combines immunoreactivity for the HC-specific transcription factor Pou4f3 with labeling of activated caspase 3/7 (AC3/7) to detect apoptotic HCs. The discrete nuclear Pou4f3 signal allowed us to utilize the binary or threshold function within ImageJ to automate HC quantification. To further streamline this process, we created an ImageJ macro that automates the process from raw image loading to a final quantified image that can be immediately evaluated for accuracy. Within this quantified image, the user can manually correct the quantification via an image overlay indicating the counted HC nuclei. Pou4f3-positive HCs that also express AC3/7 are subtracted to yield accurate counts of surviving HCs. Overall, we present a semi-automated method that is faster than manual HC quantification and identifies surviving HCs with high accuracy.

A Reversal in Hair Cell Orientation Organizes Both the Auditory and Vestibular Organs

Sensory hair cells detect mechanical stimuli with their hair bundle, an asymmetrical brush of actin-based membrane protrusions, or stereocilia. At the single cell level, stereocilia are organized in rows of graded heights that confer the hair bundle with intrinsic directional sensitivity. At the organ level, each hair cell is precisely oriented so that its intrinsic directional sensitivity matches the direction of mechanical stimuli reaching the sensory epithelium. Coordinated orientation among neighboring hair cells usually ensures the delivery of a coherent local group response. Accordingly, hair cell orientation is locally uniform in the auditory and vestibular cristae epithelia in birds and mammals. However, an exception to this rule is found in the vestibular macular organs, and in fish lateral line neuromasts, where two hair cell populations show opposing orientations. This mirror-image hair cell organization confers bidirectional sensitivity at the organ level. Here I review our current understanding of the molecular machinery that produces mirror-image organization through a regional reversal of hair cell orientation. Interestingly, recent evidence suggests that auditory hair cells adopt their normal uniform orientation through a global reversal mechanism similar to the one at work regionally in macular and neuromast organs. Macular and auditory organs thus appear to be patterned more similarly than previously appreciated during inner ear development.

Similarities and Differences Between Vestibular and Cochlear Systems - A Review of Clinical and Physiological Evidence

The evoked response to repeated brief stimuli, such as clicks or short tone bursts, is used for clinical evaluation of the function of both the auditory and vestibular systems. One auditory response is a neural potential - the Auditory Brainstem Response (ABR) - recorded by surface electrodes on the head. The clinical analogue for testing the otolithic response to abrupt sounds and vibration is the myogenic potential recorded from tensed muscles - the vestibular evoked myogenic potential (VEMP). VEMPs have provided clinicians with a long sought-after tool - a simple, clinically realistic indicator of the function of each of the 4 otolithic sensory regions. We review the basic neural evidence for VEMPs and discuss the similarities and differences between otolithic and cochlear receptors and afferents. VEMPs are probably initiated by sound or vibration selectively activating afferent neurons with irregular resting discharge originating from the unique type I receptors at a specialized region of the otolithic maculae (the striola). We review how changes in VEMP responses indicate the functional state of peripheral vestibular function and the likely transduction mechanisms allowing otolithic receptors and afferents to trigger such very short latency responses. In section "ELECTROPHYSIOLOGY" we show how cochlear and vestibular receptors and afferents have many similar electrophysiological characteristics [e.g., both generate microphonics, summating potentials, and compound action potentials (the vestibular evoked potential, VsEP)]. Recent electrophysiological evidence shows that the hydrodynamic changes in the labyrinth caused by increased fluid volume (endolymphatic hydrops), change the responses of utricular receptors and afferents in a way which mimics the changes in vestibular function attributed to endolymphatic hydrops in human patients. In section "MECHANICS OF OTOLITHS IN VEMPS TESTING" we show how the major VEMP results (latency and frequency response) follow from modeling the physical characteristics of the macula (dimensions, stiffness etc.). In particular, the structure and mechanical operation of the utricular macula explains the very fast response of the type I receptors and irregular afferents which is the very basis of VEMPs and these structural changes of the macula in Menière's Disease (MD) predict the upward shift of VEMP tuning in these patients.

The Transcription Factor Sox2 Is Required to Maintain the Cell Type-Specific Properties and Innervation of Type II Vestibular Hair Cells in Adult Mice

The sense of balance relies on vestibular hair cells, which detect head motions. Mammals have two types of vestibular hair cell, I and II, with unique morphological, molecular, and physiological properties. Furthermore, each hair cell type signals to a unique form of afferent nerve terminal. Little is known about the mechanisms in mature animals that maintain the specific features of each hair cell type or its postsynaptic innervation. We found that deletion of the transcription factor Sox2 from Type II hair cells in adult mice of both sexes caused many cells in utricles to acquire features unique to Type I hair cells and to lose Type II-specific features. This cellular transdifferentiation, which included changes in nuclear size, chromatin condensation, soma and stereocilium morphology, and marker expression, resulted in a significantly higher proportion of Type I-like hair cells in all epithelial zones. Furthermore, Sox2 deletion from Type II hair cells triggered non-cell autonomous changes in vestibular afferent neurons; they retracted bouton terminals (normally present on only Type II cells) from transdifferentiating hair cells and replaced them with a calyx terminal (normally present on only Type I cells). These changes were accompanied by significant expansion of the utricle's central zone, called the striola. Our study presents the first example of a transcription factor required to maintain the type-specific hair cell phenotype in adult inner ears. Furthermore, we demonstrate that a single genetic change in Type II hair cells is sufficient to alter the morphology of their postsynaptic partners, the vestibular afferent neurons.SIGNIFICANCE STATEMENT The sense of balance relies on two types of sensory cells in the inner ear, Type I and Type II hair cells. These two cell types have unique properties. Furthermore, their postsynaptic partners, the vestibular afferent neurons, have differently shaped terminals on Type I versus Type II hair cells. We show that the transcription factor Sox2 is required to maintain the cell-specific features of Type II hair cells and their postsynaptic terminals in adult mice. This is the first evidence of a molecule that maintains the phenotypes of hair cells and, non-cell autonomously, their postsynaptic partners in mature animals.

Spatiotemporal dynamics of inner ear sensory and non-sensory cells revealed by single-cell transcriptomics

The utricle is a vestibular sensory organ that requires mechanosensitive hair cells to detect linear acceleration. In neonatal mice, new hair cells are derived from non-sensory supporting cells, yet cell type diversity and mechanisms of cell addition remain poorly characterized. Here, we perform computational analyses on single-cell transcriptomes to categorize cell types and resolve 14 individual sensory and non-sensory subtypes. Along the periphery of the sensory epithelium, we uncover distinct groups of transitional epithelial cells, marked by Islr, Cnmd, and Enpep expression. By reconstructing de novo trajectories and gene dynamics, we show that as the utricle expands, Islr⁺ transitional epithelial cells exhibit a dynamic and proliferative phase to generate new supporting cells, followed by coordinated differentiation into hair cells. Taken together, our study reveals a sequential and coordinated process by which non-sensory epithelial cells contribute to growth of the postnatal mouse sensory epithelium.

The postnatal mouse utricle expands by more than 35% and doubles its number of hair cells during the first 8 days. Using single-cell transcriptomics, Jan et al. show that the surrounding transitional epithelial cells proliferate and contribute to the expansion of the sensory epithelium through a stepwise differentiation mechanism.

Annexin A4 Is Dispensable for Hair Cell Development and Function

Annexin A4 (ANXA4) is a Ca²⁺-dependent phospholipid-binding protein that is specifically expressed in the cochlear and vestibular hair cells, but its function in the hair cells remains unknown. In the present study, we show that besides localizing on the plasma membrane, ANXA4 immunoreactivity is also localized at the tips of stereocilia in the hair cells. In order to investigate the role of ANXA4 in the hair cells, we established Anxa4 knockout mice using CRISPR/Cas9 technique. Unexpectedly, the development of both cochlear and vestibular hair cells is normal in Anxa4 knockout mice. Moreover, stereocilia morphology of Anxa4 knockout mice is normal, so is the mechano-electrical transduction (MET) function. Consistently, the auditory and vestibular functions are normal in the knockout mice. In conclusion, we show here that ANXA4 is dispensable for the development and function of hair cells, which might result from functional redundancy between ANXA4 and other annexin(s) in the hair cells.

Canonical Wnt Signaling Pathway on Polarity Formation of Utricle Hair Cells

As part of the inner ear, the vestibular system is responsible for sense of balance, which consists of three semicircular canals, the utricle, and the saccule. Increasing evidence has indicated that the noncanonical Wnt/PCP signaling pathway plays a significant role in the development of the polarity of the inner ear. However, the role of canonical Wnt signaling in the polarity of the vestibule is still not completely clear. In this study, we found that canonical Wnt pathway-related genes are expressed in the early stage of development of the utricle and change dynamically. We conditionally knocked out β-catenin, a canonical Wnt signaling core protein, and found that the cilia orientation of hair cells was disordered with reduced number of hair cells in the utricle. Moreover, regulating the canonical Wnt pathway (Licl and IWP2) in vitro also affected hair cell polarity and indicated that Axin2 may be important in this process. In conclusion, our results not only confirm that the regulation of canonical Wnt signaling affects the number of hair cells in the utricle but also provide evidence for its role in polarity development.

Summating potentials from the utricular macula of anaesthetized guinea pigs

The Summating Potential (SP) was first recorded in the cochlea in the 1950s and represents an objective measure of cochlear hair cell function, in vivo. Despite being a regular tool in hearing research, a similar response has not yet been recorded from the vestibular system. This is mainly due to the lack of experimental techniques available to record electrical vestibular hair cell responses in isolation from the much larger cochlear potentials. Here we demonstrate the first recordings of the vestibular SP, evoked by Bone-Conducted Vibration (BCV) and Air-Conducted Sound (ACS) stimuli, in anaesthetized guinea pigs. Field potential measurements were taken from the basal surface of the utricular macula, and from the facial nerve canal following surgical or chemical ablation of the cochlea. SPs were evoked by stimuli with frequencies above ~200 Hz, and only with moderate to high intensity (~0.005-0.05 g) BCV and ACS (~120-140 dB SPL). Neural blockade abolished the Vestibular short-latency Evoked Potential (VsEP) and Vestibular Nerve Neurophonic (VNN) from the facial nerve canal recordings but did not abolish the vestibular SP nor the vestibular microphonic. Importantly, the vestibular SP was irreversibly abolished from the utricle and facial nerve canal recordings following local gentamicin application, highlighting its hair cell origin. This is the first study to record the Summating Potential from the mammalian vestibular system, in vivo, providing a novel research tool to assess vestibular hair cell function during experimental manipulations and animal models of disease.

Utricular Sensitivity during Hydrodynamic Displacements of the Macula

To explore the effects of cochlear hair cell displacement, researchers have previously monitored functional and mechanical responses during low-frequency (LF) acoustic stimulation of the cochlea. The induced changes are believed to result from modulation of the conductance of mechano-electrical transduction (MET) channels on cochlear hair cells, along with receptor potential modulation. It is less clear how, or if, vestibular hair cell displacement affects vestibular function. Here, we have used LF (<20 Hz) hydrodynamic modulation of the utricular macula position, whilst recording functional and mechanical responses, to investigate the effects of utricular macula displacement. Measured responses included the Utricular Microphonic (UM), the vestibular short-latency evoked potential (VsEP), and laser Doppler vibrometry recordings of macular position. Over 1 cycle of the LF bias, the UM amplitude and waveform were cyclically modulated, with Boltzmann analysis suggesting a cyclic modulation of the vestibular MET gating. The VsEP amplitude was cyclically modulated throughout the LF bias, demonstrating a relative increase (~20-50 %; re baseline) and decrease (~10-20 %; re baseline), which is believed to be related to the MET conductance and vestibular hair cell sensitivity. The relationship between macular displacement and changes in UM and VsEP responses was consistent within and across animals. These results suggest that the sensory structures underlying the VsEP, often thought to be a cranial jerk-sensitive response, are at least partially sensitive to LF (and possibly static) pressures or motion. Furthermore, these results highlight the possibility that some of the vestibular dysfunction related to endolymphatic hydrops may be due to altered vestibular transduction following mechanical (or morphological) changes in the labyrinth.

Vestibular suppression of normal bodily sounds

Background: Vestibular-evoked myogenic potentials (VEMPs) are present in almost everyone. The proximity of the cochlear and vestibular end organs strongly indicates an overlap of the stimuli to which they respond.Aims/objectives: To determine the loudness of shouting compared to the external auditory canal (EAC) and explore how hyperstimulation of the cochlea and vestibular structures might be prevented.Materials and methods: We compared the loudness and frequency response of sound during shouting, biting and chewing, as well as the measurement of the loudness and frequency of sound in the external auditory canal (EAC) when a calibrated minishaker is applied to the forehead and to the upper incisor teeth.Results: We showed that vibratory sounds produced by vocalizations and oral activities were attenuated when they reached the ear, so that these sounds would not act as vestibular stimulants.Conclusions and significance: Chewing is known to cause a stapedius reflex which suppresses internal sounds to optimize audition of external sounds, while at the same time suppressing vestibular stimulation, which serves to optimize the sensitivity of the vestibular system, in order that it may respond precisely to a threat.

Calbindin expression in adult vestibular epithelia

The mammalian vestibular epithelia exhibit a remarkably stereotyped organization featuring cellular characteristics under planar cell polarity (PCP) control. PCP mechanisms are responsible for the organization of hair cell morphologic polarization vectors, and are thought to be responsible for the postsynaptic expression of the calcium-binding protein calretinin that defines the utricular striola and cristae central zone. However, recent analyses revealed that subtle differences in the topographic expression of oncomodulin, another calcium-binding protein, reflects heterogeneous factors driving the subtle variations in expression. Calbindin represents a third calcium-binding protein that has been previously described to be expressed in both hair cells and afferent calyces in proximity to the utricular striola and crista central zone. The objective of the present investigation was to determine calbindin's topographic pattern of expression to further elucidate the extent to which PCP mechanisms might exert control over the organization of vestibular neuroepithelia. The findings revealed that calbindin exhibited an expression pattern strikingly similar to oncomodulin. However, within calyces of the central zone calbindin was colocalized with calretinin. These results indicate that organizational features of vestibular epithelia are governed by a suite of factors that include PCP mechanisms as well others yet to be defined.

EGF and a GSK3 Inhibitor Deplete Junctional E-cadherin and Stimulate Proliferation in the Mature Mammalian Ear

Sensory hair cell losses underlie the vast majority of permanent hearing and balance deficits in humans, but many nonmammalian vertebrates can fully recover from hearing impairments and balance dysfunctions because supporting cells (SCs) in their ears retain lifelong regenerative capacities that depend on proliferation and differentiation as replacement hair cells. Most SCs in vertebrate ears stop dividing during embryogenesis; and soon after birth, vestibular SCs in mammals transition to lasting quiescence as they develop massively thickened circumferential F-actin bands at their E-cadherin-rich adherens junctions. Here, we report that treatment with EGF and a GSK3 inhibitor thinned the circumferential F-actin bands throughout the sensory epithelium of cultured utricles that were isolated from adult mice of either sex. That treatment also caused decreases in E-cadherin, β-catenin, and YAP in the striola, and stimulated robust proliferation of mature, normally quiescent striolar SCs. The findings suggest that E-cadherin-rich junctions, which are not present in the SCs of the fish, amphibians, and birds which readily regenerate hair cells, are responsible in part for the mammalian ear's vulnerability to permanent balance and hearing deficits.SIGNIFICANCE STATEMENT Millions of people are affected by hearing and balance deficits that arise when loud sounds, ototoxic drugs, infections, and aging cause hair cell losses. Such deficits are permanent for humans and other mammals, but nonmammals can recover hearing and balance after supporting cells regenerate replacement hair cells. Mammalian supporting cells lose the capacity to proliferate around the time they develop unique, exceptionally reinforced, E-cadherin-rich intercellular junctions. Here, we report the discovery of a pharmacological treatment that thins F-actin bands, depletes E-cadherin, and stimulates proliferation in long-quiescent supporting cells within a balance epithelium from adult mice. The findings suggest that high E-cadherin in those supporting cell junctions may be responsible, in part, for the permanence of hair cell loss in mammals.

Retinoic acid degradation shapes zonal development of vestibular organs and sensitivity to transient linear accelerations

Each vestibular sensory epithelium in the inner ear is divided morphologically and physiologically into two zones, called the striola and extrastriola in otolith organ maculae, and the central and peripheral zones in semicircular canal cristae. We found that formation of striolar/central zones during embryogenesis requires Cytochrome P450 26b1 (Cyp26b1)-mediated degradation of retinoic acid (RA). In Cyp26b1 conditional knockout mice, formation of striolar/central zones is compromised, such that they resemble extrastriolar/peripheral zones in multiple features. Mutants have deficient vestibular evoked potential (VsEP) responses to jerk stimuli, head tremor and deficits in balance beam tests that are consistent with abnormal vestibular input, but normal vestibulo-ocular reflexes and apparently normal motor performance during swimming. Thus, degradation of RA during embryogenesis is required for formation of highly specialized regions of the vestibular sensory epithelia with specific functions in detecting head motions.

A review of mechanical and synaptic processes in otolith transduction of sound and vibration for clinical VEMP testing

Older studies of mammalian otolith physiology have focused mainly on sustained responses to low-frequency (<50 Hz) or maintained linear acceleration. So the otoliths have been regarded as accelerometers. Thus evidence of otolithic activation and high-precision phase locking to high-frequency sound and vibration appears to be very unusual. However, those results are exactly in accord with a substantial body of knowledge of otolith function in fish and frogs. It is likely that phase locking of otolith afferents to vibration is a general property of all vertebrates. This review examines the literature about the activation and phase locking of single otolithic neurons to air-conducted sound and bone-conducted vibration, in particular the high precision of phase locking shown by mammalian irregular afferents that synapse on striolar type I hair cells by calyx endings. Potassium in the synaptic cleft between the type I hair cell receptor and the calyx afferent ending may be responsible for the tight phase locking of these afferents even at very high discharge rates. Since frogs and fish do not possess full calyx endings, it is unlikely that they show phase locking with such high precision and to such high frequencies as has been found in mammals. The high-frequency responses have been modeled as the otoliths operating in a seismometer mode rather than an accelerometer mode. These high-frequency otolithic responses constitute the neural basis for clinical vestibular-evoked myogenic potential tests of otolith function.

On the Legacy of Genetically Altered Mouse Models to Explore Vestibular Function: Distribution of Vestibular Hair Cell Phenotypes in the Otoferlin-Null Mouse

Objectives:

Early in his career, David Lim recognized the scientific impact of genetically anomalous mice exhibiting otoconia agenesis as models of drastically compromised vestibular function. While these studies focused on the mutant pallid mouse, contemporary genetic tools have produced other models with engineered functional modifications. Lim and colleagues foresaw the need to analyze vestibular epithelia from pallid mice to verify the absence of downstream consequences that might be secondary to the altered load represented by otoconial agenesis. More generally, however, such comparisons also contribute to an understanding of the susceptibility of labyrinthine sensory epithelia to more widespread cellular changes associated with what may appear as isolated modifications.

Methods:

Our laboratory utilizes a model of vestibular hypofunction produced through genetic alteration, the otoferlin-null mouse, which has been shown to exhibit severely compromised stimulus-evoked neurotransmitter release in type I hair cells of the utricular striola. The present study, reminiscent of early investigations of Lim and colleagues that explored the utility of a genetically altered mouse to explore its utility as a model of vestibular hypofunction, endeavored to compare the expression of the hair cell marker oncomodulin in vestibular epithelia from wild-type and otoferlin-null mice.

Results:

We found that levels of oncomodulin expression were much greater in type I than type II hair cells, though were similar across the 3 genotypes examined (ie, including heterozygotes).

Conclusion:

These findings support the notion that modifications resulting in a specific component of vestibular hypofunction are not accompanied by widespread morphologic and cellular changes in the vestibular sensory epithelia.

Multiscale modeling of mechanotransduction in the utricle

We review recent progress in using numerical models to relate utricular hair bundle and otoconial membrane (OM) structure to the functional requirements imposed by natural behavior in turtles. The head movements section reviews the evolution of experimental attempts to understand vestibular system function with emphasis on turtles, including data showing that accelerations occurring during natural head movements achieve higher magnitudes and frequencies than previously assumed. The structure section reviews quantitative anatomical data documenting topographical variation in the structures underlying macromechanical and micromechanical responses of the turtle utricle to head movement: hair bundles, OM, and bundle-OM coupling. The macromechanics section reviews macromechanical models that incorporate realistic anatomical and mechanical parameters and reveal that the system is significantly underdamped, contrary to previous assumptions. The micromechanics: hair bundle motion and met currents section reviews work based on micromechanical models, which demonstrates that topographical variation in the structure of hair bundles and OM, and their mode of coupling, result in regional specializations for signaling of low frequency (or static) head position and high frequency head accelerations. We conclude that computational models based on empirical data are especially promising for investigating mechanotransduction in this challenging sensorimotor system.

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.

Regulation of actin-based apical structures on epithelial cells

Cells of transporting epithelia are characterized by the presence of abundant F-actin-based microvilli on their apical surfaces. Likewise, auditory hair cells have highly reproducible rows of apical stereocilia (giant microvilli) that convert mechanical sound into an electrical signal. Analysis of mutations in deaf patients has highlighted the critical components of tip links between stereocilia, and related structures that contribute to the organization of microvilli on epithelial cells have been found. Ezrin/radixin/moesin (ERM) proteins, which are activated by phosphorylation, provide a critical link between the plasma membrane and underlying actin cytoskeleton in surface structures. Here, we outline recent insights into how microvilli and stereocilia are built, and the roles of tip links. Furthermore, we highlight how ezrin is locally regulated by phosphorylation, and that this is necessary to maintain polarity. Localized phosphorylation is achieved through an intricate coincidence detection mechanism that requires the membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and the apically localized ezrin kinase, lymphocyte-oriented kinase (LOK, also known as STK10) or Ste20-like kinase (SLK). We also discuss how ezrin-binding scaffolding proteins regulate microvilli and how, despite these significant advances, it remains to be discovered how the cell polarity program ultimately interfaces with these processes.

Fbxo2VHC mouse and embryonic stem cell reporter lines delineate in vitro-generated inner ear sensory epithelia cells and enable otic lineage selection and Cre-recombination

While the mouse has been a productive model for inner ear studies, a lack of highly specific genes and tools has presented challenges. The absence of definitive otic lineage markers and tools is limiting in vitro studies of otic development, where innate cellular heterogeneity and disorganization increase the reliance on lineage-specific markers. To address this challenge in mice and embryonic stem (ES) cells, we targeted the lineage-specific otic gene Fbxo2 with a multicistronic reporter cassette (Venus/Hygro/CreER = VHC). In otic organoids derived from ES cells, Fbxo2^VHC specifically delineates otic progenitors and inner ear sensory epithelia. In mice, Venus expression and CreER activity reveal a cochlear developmental gradient, label the prosensory lineage, show enrichment in a subset of type I vestibular hair cells, and expose strong expression in adult cerebellar granule cells. We provide a toolbox of multiple spectrally distinct reporter combinations for studies that require use of fluorescent reporters, hygromycin selection, and conditional Cre-mediated recombination.

Morphology of the utricular otolith organ in the toadfish, Opsanus tau

The utricle provides the vestibular reflex pathways with the sensory codes of inertial acceleration of self-motion and head orientation with respect to gravity to control balance and equilibrium. Here we present an anatomical description of this structure in the adult oyster toadfish and establish a morphological basis for interpretation of subsequent functional studies. Light, scanning, and transmission electron microscopy techniques were applied to visualize the sensory epithelium at varying levels of detail, its neural innervation and its synaptic organization. Scanning electron microscopy was used to visualize otolith mass and morphological polarization patterns of hair cells. Afferent nerve fibers were visualized following labeling with biocytin, and light microscope images were used to make three-dimensional (3-D) reconstructions of individual labeled afferents to identify dendritic morphology with respect to epithelial location. Transmission electron micrographs were compiled to create a serial 3-D reconstruction of a labeled afferent over a segment of its dendritic field and to examine the cell-afferent synaptic contacts. Major observations are: a well-defined striola, medial and lateral extra-striolar regions with a zonal organization of hair bundles; prominent lacinia projecting laterally; dependence of hair cell density on macular location; narrow afferent dendritic fields that follow the hair bundle polarization; synaptic specializations issued by afferents are typically directed towards a limited number of 7-13 hair cells, but larger dendritic fields in the medial extra-striola can be associated with > 20 hair cells also; and hair cell synaptic bodies can be confined to only an individual afferent or can synapse upon several afferents.

Oncomodulin Expression Reveals New Insights into the Cellular Organization of the Murine Utricle Striola

Oncomodulin (OCM, aka β-parvalbumin) is an EF-hand calcium binding protein that is expressed in a restricted set of hair cells in the peristriolar region of the mammalian utricle. In the present study, we determined the topologic distribution of OCM among hair cell phenotypes to advance our understanding of the cellular organization of the striola and the relationship of these phenotypes with characteristics of tissue polarity. The distributions of OCM-positive (OCM+) hair cells were quantified in utricles of mature C57Bl/6 mice. Immunohistochemistry was conducted using antibodies to OCM, calretinin, and β3-tubulin. Fluorophore-conjugated phalloidin was used to label hair cell stereocilia, which provided the basis for determining hair cell counts and morphologic polarizations. We found OCM expression in striolar types I and II hair cells, though the distributions were dissimilar to the native striolar type I and II distributions, favoring type I hair cells. The distribution of OCM immunoreactivity among striolar type I hair cells also reflected nonrandom distribution among type I_c and I_d phenotypes (i.e., those receiving calretinin-positive and calretinin-negative calyces, respectively). However, many OCM+ hair cells were found lateral to the striola, and within the epithelial region encompassing OCM+ hair cells, the distributions of OCM+ types I_c and I_d hair cells were similar to the native distributions of I_c and I_d in this region. Summarily, these data provide a quantitative perspective supporting the existence of different underlying factors driving the topologic expression of OCM in hair cells than those responsible for tissue polarity characteristics associated within the utricular striola, including calretinin expression in afferent calyces.

Partial Aminoglycoside Lesions in Vestibular Epithelia Reveal Broad Sensory Dysfunction Associated with Modest Hair Cell Loss and Afferent Calyx Retraction

Although the effects of aminoglycoside antibiotics on hair cells have been investigated for decades, their influences on the dendrites of primary afferent neurons have not been widely studied. This is undoubtedly due to the difficulty in disassociating pathology to dendritic processes from that resulting from loss of the presynaptic hair cell. This was overcome in the present investigation through development of a preparation using Chinchilla laniger that enabled direct perilymphatic infusion. Through this strategy we unmasked gentamicin's potential effects on afferent calyces. The pathophysiology of the vestibular neuroepithelia after post-administration durations of 0.5 through 6 months was assessed using single-neuron electrophysiology, immunohistochemistry, and confocal microscopy. Hair cell densities within cristae central zones (0.5-, 1-, 2-, and 6-months) and utricle peri- and extrastriola (6-months) regions were determined, and damage to calretinin-immunoreactive calyces was quantified. Gentamicin-induced hair cell loss exhibited a profile that reflected elimination of a most-sensitive group by 0.5-months post-administration (18.2%), followed by loss of a second group (20.6%) over the subsequent 5.5 months. The total hair cell loss with this gentamicin dose (approximately 38.8%) was less than the estimated fraction of type I hair cells in the chinchilla's crista central zone (approximately 60%), indicating that viable type I hair cells remained. Extensive lesions to afferent calyces were observed at 0.5-months, though stimulus-evoked modulation was intact at this post-administration time. Widespread compromise to calyx morphology and severe attenuation of stimulus-evoked afferent discharge modulation was found at 1 month post-administration, a condition that persisted in preparations examined through the 6-month post-administration interval. Spontaneous discharge was robust at all post-administration intervals. All calretinin-positive calyces had retracted at 2 and 6 months post-administration. We found no evidence of morphologic or physiologic recovery. These results indicate that gentamicin-induced partial lesions to vestibular epithelia include hair cell loss (ostensibly reflecting an apoptotic effect) that is far less extensive than the compromise to stimulus-evoked afferent discharge modulation and retraction of afferent calyces (reflecting non-apoptotic effects). Additionally, calyx retraction cannot be completely accounted for by loss of type I hair cells, supporting the possibility for direct action of gentamicin on the afferent dendrite.

An operating principle of the turtle utricle to detect wide dynamic range

The utricle encodes both static information such as head orientation, and dynamic information such as vibrations. It is not well understood how the utricle can encode both static and dynamic information for a wide dynamic range (from <0.05 to >2 times the gravitational acceleration; from DC to > 1000 Hz vibrations). Using computational models of the hair cells in the turtle utricle, this study presents an explanation on how the turtle utricle encodes stimulations over such a wide dynamic range. Two hair bundles were modeled using the finite element method-one representing the striolar hair cell (Cell S), and the other representing the medial extrastriolar hair cell (Cell E). A mechano-transduction (MET) channel model was incorporated to compute MET current (i_MET) due to hair bundle deflection. A macro-mechanical model of the utricle was used to compute otoconial motions from head accelerations (a_Head). According to known anatomical data, Cell E has a long kinocilium that is embedded into the stiff otoconial layer. Unlike Cell E, the hair bundle of Cell S falls short of the otoconial layer. Considering such difference in the mechanical connectivity between the hair cell bundle and the otoconial layer, three cases were simulated: Cell E displacement-clamped, Cell S viscously-coupled, and Cell S displacement-clamped. Head accelerations at different amplitude levels and different frequencies were simulated for the three cases. When a realistic head motion was simulated, Cell E was responsive to head orientation, while the viscously-coupled Cell S was responsive to fast head motion imitating the feeding strike of a turtle.

Electrophysiological Measurements of Peripheral Vestibular Function-A Review of Electrovestibulography

Electrocochleography (EcochG), incorporating the Cochlear Microphonic (CM), the Summating Potential (SP), and the cochlear Compound Action Potential (CAP), has been used to study cochlear function in humans and experimental animals since the 1930s, providing a simple objective tool to assess both hair cell (HC) and nerve sensitivity. The vestibular equivalent of ECochG, termed here Electrovestibulography (EVestG), incorporates responses of the vestibular HCs and nerve. Few research groups have utilized EVestG to study vestibular function. Arguably, this is because stimulating the cochlea in isolation with sound is a trivial matter, whereas stimulating the vestibular system in isolation requires significantly more technical effort. That is, the vestibular system is sensitive to both high-level sound and bone-conducted vibrations, but so is the cochlea, and gross electrical responses of the inner ear to such stimuli can be difficult to interpret. Fortunately, several simple techniques can be employed to isolate vestibular electrical responses. Here, we review the literature underpinning gross vestibular nerve and HC responses, and we discuss the nomenclature used in this field. We also discuss techniques for recording EVestG in experimental animals and humans and highlight how EVestG is furthering our understanding of the vestibular system.

Development and regeneration of vestibular hair cells in mammals

Vestibular sensation is essential for gaze stabilization, balance, and perception of gravity. The vestibular receptors in mammals, Type I and Type II hair cells, are located in five small organs in the inner ear. Damage to hair cells and their innervating neurons can cause crippling symptoms such as vertigo, visual field oscillation, and imbalance. In adult rodents, some Type II hair cells are regenerated and become re-innervated after damage, presenting opportunities for restoring vestibular function after hair cell damage. This article reviews features of vestibular sensory cells in mammals, including their basic properties, how they develop, and how they are replaced after damage. We discuss molecules that control vestibular hair cell regeneration and highlight areas in which our understanding of development and regeneration needs to be deepened.