Unbiased stereological estimation of the spiral ligament and stria vascularis volumes in aging and Ménière's disease using archival human temporal bones

Advance and Application of Single-cell Transcriptomics in Auditory Research

Hearing loss and deafness, as a worldwide disability disease, have been troubling human beings. However, the auditory organ of the inner ear is highly heterogeneous and has a very limited number of cells, which are largely uncharacterized in depth. Recently, with the development and utilization of single-cell RNA sequencing (scRNA-seq), researchers have been able to unveil the complex and sophisticated biological mechanisms of various types of cells in the auditory organ at the single-cell level and address the challenges of cellular heterogeneity that are not resolved through by conventional bulk RNA sequencing (bulk RNA-seq). Herein, we reviewed the application of scRNA-seq technology in auditory research, with the aim of providing a reference for the development of auditory organs, the pathogenesis of hearing loss, and regenerative therapy. Prospects about spatial transcriptomic scRNA-seq, single-cell based genome, and Live-seq technology will also be discussed.

Characteristics of spatial protein expression in the mouse cochlear sensory epithelia: Implications for age-related hearing loss

Hair cells in the cochlear sensory epithelia serve as mechanosensory receptors, converting sound into neuronal signals. The basal sensory epithelia are responsible for transducing high-frequency sounds, while the apex handles low-frequency sounds. Age-related hearing loss predominantly affects hearing at high frequencies and is indicative of damage to the basal sensory epithelia. However, the precise mechanism underlying this site-selective injury remains unclear. In this study, we employed a microscale proteomics approach to examine and compare protein expression in different regions of the cochlear sensory epithelia (upper half and lower half) in 1.5-month-old (normal hearing) and 6-month-old (severe high-frequency hearing loss without hair cell loss) C57BL/6J mice. A total of 2,386 proteins were detected, and no significant differences in protein expression were detected in the upper half of the cochlear sensory epithelia between the two age groups. The expression of 20 proteins in the lower half of the cochlear sensory epithelia significantly differed between the two age groups (e.g., MATN1, MATN4, and AQP1). Moreover, there were 311 and 226 differentially expressed proteins between the upper and lower halves of the cochlear sensory epithelia in 1.5-month-old and 6-month-old mice, respectively. The expression levels of selected proteins were validated by Western blotting. These findings suggest that the spatial differences in protein expression within the cochlear sensory epithelia may play a role in determining the susceptibility of cells at different sites of the cochlea to age-related damage.

Emerging Mechanisms in the Pathogenesis of Menière's Disease: Evidence for the Involvement of Ion Homeostatic or Blood-Labyrinthine Barrier Dysfunction in Human Temporal Bones

HYPOTHESIS

Analysis of human temporal bone specimens of patients with Menière's disease (MD) may demonstrate altered expression of gene products related to barrier formation and ionic homeostasis within cochlear structures compared with control specimens.

BACKGROUND

MD represents a challenging otologic disorder for investigation. Despite attempts to define the pathogenesis of MD, there remain many gaps in our understanding, including differences in protein expression within the inner ear. Understanding these changes may facilitate the identification of more targeted therapies for MD.

METHODS

Human temporal bones from patients with MD (n = 8) and age-matched control patients (n = 8) were processed with immunohistochemistry stains to detect known protein expression related to ionic homeostasis and barrier function in the cochlea, including CLDN11, CLU, KCNJ10, and SLC12A2. Immunofluorescence intensity analysis was performed to quantify protein expression in the stria vascularis, organ of Corti, and spiral ganglion neuron (SGN).

RESULTS

Expression of KCNJ10 was significantly reduced in all cochlear regions, including the stria vascularis (9.23 vs 17.52, p = 0.011), OC (14.93 vs 29.16, p = 0.014), and SGN (7.69 vs 18.85, p = 0.0048) in human temporal bone specimens from patients with MD compared with control, respectively. CLDN11 (7.40 vs 10.88, p = 0.049) and CLU (7.80 vs 17.51, p = 0.0051) expression was significantly reduced in the SGN.

CONCLUSION

The results of this study support that there may be differences in the expression of proteins related to ionic homeostasis and barrier function within the cochlea, potentially supporting the role of targeted therapies to treat MD.

In Silico Localization of Perilymph Proteins Enriched in Meńier̀e Disease Using Mammalian Cochlear Single-cell Transcriptomics

Hypothesis

Proteins enriched in the perilymph proteome of Meńier̀e disease (MD) patients may identify affected cell types. Utilizing single-cell transcriptome datasets from the mammalian cochlea, we hypothesize that these enriched perilymph proteins can be localized to specific cochlear cell types.

Background

The limited understanding of human inner ear pathologies and their associated biomolecular variations hinder efforts to develop disease-specific diagnostics and therapeutics. Perilymph sampling and analysis is now enabling further characterization of the cochlear microenvironment. Recently, enriched inner ear protein expression has been demonstrated in patients with MD compared to patients with other inner ear diseases. Localizing expression of these proteins to cochlear cell types can further our knowledge of potential disease pathways and subsequent development of targeted therapeutics.

Methods

We compiled previously published data regarding differential perilymph proteome profiles amongst patients with MD, otosclerosis, enlarged vestibular aqueduct, sudden hearing loss, and hearing loss of undefined etiology (controls). Enriched proteins in MD were cross-referenced against published single-cell/single-nucleus RNA-sequencing datasets to localize gene expression to specific cochlear cell types.

Results

In silico analysis of single-cell transcriptomic datasets demonstrates enrichment of a unique group of perilymph proteins associated with MD in a variety of intracochlear cells, and some exogeneous hematologic and immune effector cells. This suggests that these cell types may play an important role in the pathology associated with late MD, suggesting potential future areas of investigation for MD pathophysiology and treatment.

Conclusions

Perilymph proteins enriched in MD are expressed by specific cochlear cell types based on in silico localization, potentially facilitating development of disease-specific diagnostic markers and therapeutics.

Identification of Potential Meniere's Disease Targets in the Adult Stria Vascularis

The stria vascularis generates the endocochlear potential and is involved in processes that underlie ionic homeostasis in the cochlear endolymph, both which play essential roles in hearing. The histological hallmark of Meniere's disease (MD) is endolymphatic hydrops, which refers to the bulging or expansion of the scala media, which is the endolymph-containing compartment of the cochlea. This histologic hallmark suggests that processes that disrupt ion homeostasis or potentially endocochlear potential may underlie MD. While treatments exist for vestibular symptoms related to MD, effective therapies for hearing fluctuation and hearing loss seen in MD remain elusive. Understanding the potential cell types involved in MD may inform the creation of disease mouse models and provide insight into underlying mechanisms and potential therapeutic targets. For these reasons, we compare published datasets related to MD in humans with our previously published adult mouse stria vascularis single-cell and single-nucleus RNA-Seq datasets to implicate potentially involved stria vascularis (SV) cell types in MD. Finally, we provide support for these implicated cell types by demonstrating co-expression of select candidate genes for MD within SV cell types.

Pathology and mechanisms of cochlear aging

Presbycusis, or age-related hearing loss (ARHL), occurs in most mammals with variations in the age of onset, rate of decline, and magnitude of degeneration in the central nervous system and inner ear. The affected cochlear structures include the stria vascularis and its vasculature, spiral ligament, sensory hair cells and auditory neurons. Dysfunction of the stria vascularis results in a reduced endocochlear potential. Without this potential, the cochlear amplification provided by the electro-motility of the outer hair cells is insufficient, and a high-frequency hearing-loss results. Degeneration of the sensory cells, especially the outer hair cells also leads to hearing loss due to lack of amplification. Neuronal degeneration, another hallmark of ARHL, most likely underlies difficulties with speech discrimination, especially in noisy environments. Noise exposure is a major cause of ARHL. It is well-known to cause sensory cell degeneration, especially the outer hair cells at the high frequency end of the cochlea. Even loud, but not uncomfortable, sound levels can lead to synaptopathy and ultimately neuronal degeneration. Even in the absence of a noisy environment, aged cells degenerate. This pathology most likely results from damage to mitochondria and contributes to degenerative changes in the stria vascularis, hair cells, and neurons. The genetic underpinnings of ARHL are still unknown and most likely involve various combinations of genes. At present, the only effective strategy for reducing ARHL is prevention of noise exposure. If future strategies can improve mitochondrial activity and reduce oxidative damage in old age, these should also bring relief.

Single Cell and Single Nucleus RNA-Seq Reveal Cellular Heterogeneity and Homeostatic Regulatory Networks in Adult Mouse Stria Vascularis

The stria vascularis (SV) generates the endocochlear potential (EP) in the inner ear and is necessary for proper hair cell mechanotransduction and hearing. While channels belonging to SV cell types are known to play crucial roles in EP generation, relatively little is known about gene regulatory networks that underlie the ability of the SV to generate and maintain the EP. Using single cell and single nucleus RNA-sequencing, we identify and validate known and rare cell populations in the SV. Furthermore, we establish a basis for understanding molecular mechanisms underlying SV function by identifying potential gene regulatory networks as well as druggable gene targets. Finally, we associate known deafness genes with adult SV cell types. This work establishes a basis for dissecting the genetic mechanisms underlying the role of the SV in hearing and will serve as a basis for designing therapeutic approaches to hearing loss related to SV dysfunction.

Forgotten Fibrocytes: A Neglected, Supporting Cell Type of the Cochlea With the Potential to be an Alternative Therapeutic Target in Hearing Loss

Cochlear fibrocytes are a homeostatic supporting cell type embedded in the vascularized extracellular matrix of the spiral ligament, within the lateral wall. Here, they participate in the connective tissue syncytium that enables potassium recirculation into the scala media to take place and ensures development of the endolymphatic potential that helps drive current into hair cells during acoustic stimulation. They have also been implicated in inflammatory responses in the cochlea. Some fibrocytes interact closely with the capillaries of the vasculature in a way which suggests potential involvement, together with the stria vascularis, also in the blood-labyrinth barrier. Several lines of evidence suggests that pathology of the fibrocytes, along with other degenerative changes in this region, contribute to metabolic hearing loss (MHL) during aging that is becoming recognized as distinct from, and potentially a precursor for, sensorineural hearing loss (SNHL). This pathology may underlie a significant proportion of cases of presbycusis. Some evidence points also to an association between fibrocyte degeneration and Ménière’s disease (MD). Fibrocytes are mesenchymal; this characteristic, and their location, make them amenable to potential cell therapy in the form of cell replacement or genetic modification to arrest the process of degeneration that leads to MHL. This review explores the properties and roles of this neglected cell type and suggests potential therapeutic approaches, such as cell transplantation or genetic engineering of fibrocytes, which could be used to prevent this form of presbycusis or provide a therapeutic avenue for MD.

The aging cochlea: Towards unraveling the functional contributions of strial dysfunction and synaptopathy

Strial dysfunction is commonly observed as a key consequence of aging in the cochlea. A large body of animal research, especially in the quiet-aged Mongolian gerbil, shows specific histopathological changes in the cochlear stria vascularis and the putatively corresponding effects on endocochlear potential and auditory nerve responses. However, recent work suggests that synaptopathy, or the loss of inner hair cell-auditory nerve fiber synapses, also presents as a consequence of aging. It is now believed that the loss of synapses is the earliest age-related degenerative event. The present review aims to integrate classic and novel research on age-related pathologies of the inner ear. First, we summarize current knowledge on age-related strial dysfunction and synaptopathy. We describe how these cochlear pathologies fit into the categories for presbyacusis, as first defined by Schuknecht in the '70s. Further, we discuss how strial dysfunction and synaptopathy affect sound coding by the auditory nerve and how they can be experimentally induced to study their specific contributions to age-related hearing deficits. As such, we aim to give an overview of the current literature on age-related cochlear pathologies and hope to inspire further research on the role of cochlear aging in age-related hearing deficits.

Estimation of Volume of Stria Vascularis and the Length of Its Capillaries in the Human Cochlea

Background:

The stria vascularis (SV) is a vascularized epithelium that secretes endolymph and is located on the lateral wall of the membranous cochlea. The capillaries of SV directly influence the composition of the endolymph and hence the generation of impulses by the hair-cells that are auditory receptors and thus affect hearing. Therefore, the real morphology of the SV would be very important for understanding the hearing system. There are few reliable reports of the morphology of the human SV.

Aims and Objectives:

In this research, we have estimated the volume of the SV and total length of strial capillaries in the apical, middle and basal turns of the human cochlea by updated stereological techniques.

Methods:

The point-counting Cavalieri's method and hemispherical volume probes were applied on stained, 40 μm-thick serial sections of five celloidin-embedded, decalcified cochleae.

Results:

The mean age of persons at the time of death was 51 ± 15.25 years, the mean volume of the SV was 0.56 ± 0.054 mm3 and the mean length of the SV capillaries was 289.08 ± 72.96 mm. We also estimated the same parameters with different stereological parameters, probes and in differently stained sections and checked the relationship and limits of agreement between different methods by paired t-test and Bland-Altman plot. We found agreement in our results.

Conclusion:

We provide reliable baseline data on the real morphology of the human SV.

Age-related hearing impairment and the triad of acquired hearing loss

Understanding underlying pathological mechanisms is prerequisite for a sensible design of protective therapies against hearing loss. The triad of age-related, noise-generated, and drug-induced hearing loss displays intriguing similarities in some cellular responses of cochlear sensory cells such as a potential involvement of reactive oxygen species (ROS) and apoptotic and necrotic cell death. On the other hand, detailed studies have revealed that molecular pathways are considerably complex and, importantly, it has become clear that pharmacological protection successful against one form of hearing loss will not necessarily protect against another. This review will summarize pathological and pathophysiological features of age-related hearing impairment (ARHI) in human and animal models and address selected aspects of the commonality (or lack thereof) of cellular responses in ARHI to drugs and noise.

Is allergy related to Meniere's disease?

Meniere's disease (MD) is characterized by episodic rotational vertigo, fluctuating sensorineural hearing loss, aural pressure, and tinnitus. The cause of MD is thought to be multifactorial, with anatomic and genetic contributions. Allergy is thought to be one of the possible extrinsic factors that, when combined with underlying intrinsic factors, may lead to MD. We review the epidemiologic associations of MD and allergy and review the recent literature on the association of allergy and MD.

Head bobber: an insertional mutation causes inner ear defects, hyperactive circling, and deafness

The head bobber transgenic mouse line, produced by pronuclear integration, exhibits repetitive head tilting, circling behavior, and severe hearing loss. Transmitted as an autosomal recessive trait, the homozygote has vestibular and cochlea inner ear defects. The space between the semicircular canals is enclosed within the otic capsule creating a vacuous chamber with remnants of the semicircular canals, associated cristae, and vestibular organs. A poorly developed stria vascularis and endolymphatic duct is likely the cause for Reissner's membrane to collapse post-natally onto the organ of Corti in the cochlea. Molecular analyses identified a single integration of ~3 tandemly repeated copies of the transgene, a short duplicated segment of chromosome X and a 648 kb deletion of chromosome 7(F3). The three known genes (Gpr26, Cpxm2, and Chst15) in the deleted region are conserved in mammals and expressed in the wild-type inner ear during vestibular and cochlea development but are absent in homozygous mutant ears. We propose that genes critical for inner ear patterning and differentiation are lost at the head bobber locus and are candidate genes for human deafness and vestibular disorders.

Variability in the perilymphatic diffusion of gadolinium does not predict the outcome of intratympanic gentamicin in patients with Ménière's disease

OBJECTIVES/HYPOTHESIS

To assess the utility of imaging in planning intratympanic (IT) gentamicin (Gent) treatment in Ménière's disease (MD), we compared the dosage and outcomes of ITGent with the severity and extent of endolymphatic hydrops (EH), as evaluated by three-dimensional fluid-attenuated inversion recovery (3D-FLAIR) sequence in a 3-T magnetic resonance imaging (MRI) unit, after IT gadolinium administration.

STUDY DESIGN

Retrospective review.

METHODS

A total of 18 patients (10 males and 8 females; age, 28-78 years; median age, 53.2 years) with definite MD participated in the investigation. The duration of the disease ranged from 8 months to 9 years (median, 2 years), with a prevalence of vertigo spells ranging from 0.8 to 8 per month (median, 2.2), as calculated in the last 6 months. A 3D-FLAIR MRI was performed 24 hours after IT injection of diluted gadobutrol. ITGent injection was performed within a variable period of time, from 1 week to 3 weeks after 3D-FLAIR MRI. The degree and extension of EH as evaluated by 3D-FLAIR MRI were compared with the number of injections necessary to cure vertigo attacks. Vertigo results, functional level scale modifications, variations in caloric excitability, and pure-tone average modifications.

RESULTS

No statistically significant correlation was observed between severity of EH and outcomes of ITGent administration.

CONCLUSIONS

The hypothesis of a reduced effect of Gent administered intratympanically in the presence of severe EH, owing to obstacled diffusion along the perilymphatic compartments, has not been confirmed in the present investigation.

Reliability of magnetic resonance imaging performed after intratympanic administration of gadolinium in the identification of endolymphatic hydrops in patients with Ménière's disease

OBJECTIVE

To evaluate the reliability of magnetic resonance imaging performed after intratympanic gadolinium administration in evidencing endolymphatic hydrops in patients with Ménière's disease (MD).

PATIENTS

A total of 26 patients (18 male and 8 female subjects, aged 25-78 yr; median age, 56 yr) with definite MD and 12 subjects (8 male and 4 female subjects, aged 31-75 yr; median age, 51 yr) with various unilateral non-MD disorders of the inner ear were examined.

INTERVENTION

A 0.6-ml solution of gadobutrol (1 mmol/ml), diluted 1:7 in saline, was injected in the affected ear through the inferior-posterior quadrant of the tympanic membrane, using a 22-gauge spinal needle. In 9 MD patients, the contralateral ear also was injected. The patient was kept with the head rotated 45 degrees contralaterally for 30 minutes after each injection. Twenty-four hours later, a 3-dimensional fluid-attenuated inversion recovery magnetic resonance imaging using a 3 Tesla unit was performed.

MAIN OUTCOME MEASURE

Perilymphatic enhancement was evaluated in different portions of the labyrinth in MD ears and compared with the outcomes obtained in the non-MD ears.

RESULTS

All MD ears showed impaired perilymphatic enhancement of variable degrees. No enhancement defects could be observed in all examined contralateral unaffected ear of the patients with MD, as well as in 11 of the 12 ears of the subjects with various unilateral non-MD disorders.

CONCLUSION

Perilymphatic enhancement defect of variable degrees is observed in the pathologic ear of every patient with MD. The consistency of this phenomenon in MD ears and the complete enhancement in most of the ears without MD safely enable to attribute these findings to endolymphatic hydrops. It is likely in the near future that imaging may be used to achieve a certain diagnosis of MD in life.

Immunocytochemical distribution of WARP (von Willebrand A domain-related protein) in the inner ear

The basic components of the epithelial, perineural, and perivascular basement membranes in the inner ear have been well-documented in several animal models and in the human inner ear. The von Willebrand A domain-related protein (WARP) is an extracellular matrix molecule with restricted expression in cartilage, and a subset of basement membranes in peripheral nerves, muscle, and central nervous system vasculature. It has been suggested that WARP has an important role in maintaining the blood-brain barrier. To date no studies on WARP distribution have been performed in the inner ear, which is equipped with an intricate vasculature network. In the present study, we determined the distribution of WARP by immunocytochemistry in the human inner ear using auditory and vestibular endorgans microdissected from human temporal bones obtained at autopsy. All subjects (n=5, aged 55-87years old) had documented normal auditory and vestibular function. We also determined the WARP immunolocalization in the mouse inner ear. WARP immunoreactivity localized to the vasculature throughout the stroma of the cristae ampullaris, the maculae utricle, and saccule in the human and mouse. In the human and mouse inner ear, WARP immunoreactivity delineated blood vessels located in the stria vascularis, spiral ligament, sub-basilar region, stromal tissue, and the spiral and vestibular ganglia. The distinct localization of WARP in the inner ear vasculature suggests an important role in maintaining its integrity. In addition, WARP allows delineation of microvessels in the inner ear allowing the study of vascular pathology in the development of otological diseases.

Divergent aging characteristics in CBA/J and CBA/CaJ mouse cochleae

Two inbred mouse strains, CBA/J and CBA/CaJ, have been used nearly interchangeably as 'good hearing' standards for research in hearing and deafness. We recently reported, however, that these two strains diverge after 1 year of age, such that CBA/CaJ mice show more rapid elevation of compound action potential (CAP) thresholds at high frequencies (Ohlemiller, Brain Res. 1277: 70-83, 2009). One contributor is progressive decline in endocochlear potential (EP) that appears only in CBA/CaJ. Here, we explore the cellular bases of threshold and EP disparities in old CBA/J and CBA/CaJ mice. Among the major findings, both strains exhibit a characteristic age (∼18 months in CBA/J and 24 months in CBA/CaJ) when females overtake males in sensitivity decline. Strain differences in progression of hearing loss are not due to greater hair cell loss in CBA/CaJ, but instead appear to reflect greater neuronal loss, plus more pronounced changes in the lateral wall, leading to EP decline. While both male and female CBA/CaJ show these pathologies, they are more pronounced in females. A novel feature that differed sharply by strain was moderate loss of outer sulcus cells (or 'root' cells) in spiral ligament of the upper basal turn in old CBA/CaJ mice, giving rise to deep indentations and void spaces in the ligament. We conclude that CBA/CaJ mice differ both quantitatively and qualitatively from CBA/J in age-related cochlear pathology, and model different types of presbycusis.

Histopathological and ultrastructural analysis of vestibular endorgans in Meniere's disease reveals basement membrane pathology

BACKGROUND

We report the systematic analysis of the ultrastructural and cytological histopathology of vestibular endorgans acquired from labyrinthectomy in Meniere's disease.

METHODS

17 subjects with intractable Meniere's disease and ipsilateral non-serviceable hearing presenting to the Neurotology Clinic from 1997 to 2006 who chose ablative labyrinthectomy (average age = 62 years; range 29-83 years) participated. The average duration of symptoms prior to surgery was 7 years (range 1-20 years).

RESULTS

Nearly all vestibular endorgans demonstrated varying degrees of degeneration. A monolayer of epithelial cells occurred significantly more frequently in the horizontal cristae (12/13 = 92%) (p < 0.001), the superior cristae (5/5 = 100%) (p < 0.005), the posterior cristae (2/2) compared with the utricular maculae (4/17 = 24%). Basement membrane (BM) thickening was more common in all of the cristae ampullares (18 out of 20) than the utricular maculae. Although only four saccular maculae were obtained, 3 out of 4 exhibited BM thickening and monolayer degeneration. Monolayer degeneration was highly significantly correlated with the presence of BM thickening (p < 0.001). Other degenerative changes noted equally among the five vestibular endorgans which were not significantly correlated with BM thickening or monolayer degeneration included hair cell vacuolization and stereocilia loss, microvesicles in the supporting cells, and increased stromal intercellular spaces. Transmission electron microscopy demonstrated disorganization of the BM collagen-like fibrils, and normal ultrastructural morphology of the nerve terminals and myelinated fibers. Stromal fibroblasts and endothelial cells of stromal blood vessels demonstrated vacuolization, and stromal perivascular BMs were also thickened.

CONCLUSION

Systematic histopathological analysis of the vestibular endorgans from Meniere's disease demonstrated neuroepithelial degeneration which was highly correlated with an associated BM thickening. Other findings included hair cell and supporting cell microvessicles, increased intercellular clear spaces in the stroma, and endothelial cell vacuolization and stromal perivascular BM thickening.