Kanamycin-furosemide ototoxicity in the mouse cochlea: a 3-dimensional analysis

Hyperosmotic Sisomicin Infusion: A Mouse Model for Hearing Loss

Hearing impairment arises from the loss of either type of cochlear sensory hair cells. Inner hair cells act as primary sound transducers, while outer hair cells enhance sound-induced vibrations within the organ of Corti. Established models, such as systemic administration of ototoxic aminoglycosides, yield inconsistent and variable hair cell death in mice. Overcoming this limitation, we developed a method involving surgical delivery of a hyperosmotic sisomicin solution into the posterior semicircular canal of adult mice. This procedure induced rapid and synchronous apoptotic demise of outer hair cells within 14 hours, leading to irreversible hearing loss. The combination of sisomicin and hyperosmotic stress caused consistent and synergistic ototoxic damage. Inner hair cells remained intact until three days post-treatment, after which deterioration in structure and number was observed, culminating in cell loss by day seven. This robust animal model provides a valuable tool for otoregenerative research, facilitating single-cell and omics-based studies toward exploring preclinical therapeutic strategies.

Proteomic Analysis Reveals the Composition of Glutamatergic Organelles of Auditory Inner Hair Cells

In the ear, inner hair cells (IHCs) employ sophisticated glutamatergic ribbon synapses with afferent neurons to transmit auditory information to the brain. The presynaptic machinery responsible for neurotransmitter release in IHC synapses includes proteins such as the multi-C2-domain protein otoferlin and the vesicular glutamate transporter 3 (VGluT3). Yet, much of this likely unique molecular machinery remains to be deciphered. The scarcity of material has so far hampered biochemical studies which require large amounts of purified samples. We developed a subcellular fractionation workflow combined with immunoisolation of VGluT3-containing membrane vesicles, allowing for the enrichment of glutamatergic organelles that are likely dominated by synaptic vesicles (SVs) of IHCs. We have characterized their protein composition in mice before and after hearing onset using mass spectrometry and confocal imaging and provide a fully annotated proteome with hitherto unidentified proteins. Despite the prevalence of IHC marker proteins across IHC maturation, the profiles of trafficking proteins differed markedly before and after hearing onset. Among the proteins enriched after hearing onset were VAMP-7, syntaxin-7, syntaxin-8, syntaxin-12/13, SCAMP1, V-ATPase, SV2, and PKCα. Our study provides an inventory of the machinery associated with synaptic vesicle-mediated trafficking and presynaptic activity at IHC ribbon synapses and serves as a foundation for future functional studies.

Oridonin alleviates kanamycin-related hearing loss by inhibiting NLRP3/caspase-1/gasdermin D-induced inflammasome activation and hair cell pyroptosis

Aminoglycoside antibiotic drugs induce hearing loss in children and adults every year; however, the pathological mechanisms remain unknown. Previous studies have shown that the accumulation of reactive oxygen species (ROS) and inflammation in the inner ear may be responsible for kanamycin (KM)-induced hair cell death and hearing loss. Nucleotide-binding oligomerization domain-, leucine-rich repeat-, and pyrin domain-containing protein 3 (NLRP3) is a specific ROS sensor that initiates inflammasome assembly as well as activates caspase-1 and downstream inflammatory factors. Therefore, this study aimed to determine whether NLRP3 inflammasomes are involved in KM-related hearing loss in mice. Compared with the control (saline) group, increased levels of activated caspase-1, interleukin (IL)-1β, IL-18, N-terminal fragment of gasdermin D (GSDMD-N), and NLRP3 were detected by immunofluorescence, western blot, and enzyme-linked immunosorbent assay (ELISA) in the KM-plus-furosemide (LASIX)-treated group. Moreover, we also found that the NLRP3 inhibitor oridonin (Ori) could significantly rescue KM-related hearing loss by inhibiting NLRP3-inflammasome activation and caspase-1/GSDMD-related hair cell pyroptosis. These findings demonstrate that apoptosis, as well as pyroptosis, may be involved in KM-related hearing loss and that the NLRP3/caspase-1/GSDMD pathway may be a new target for treating aminoglycoside-induced hearing loss.

Identifying targets to prevent aminoglycoside ototoxicity

Aminoglycosides are potent antibiotics that are commonly prescribed worldwide. Their use carries significant risks of ototoxicity by directly causing inner ear hair cell degeneration. Despite their ototoxic side effects, there are currently no approved antidotes. Here we review recent advances in our understanding of aminoglycoside ototoxicity, mechanisms of drug transport, and promising sites for intervention to prevent ototoxicity.

SOD2 Alleviates Hearing Loss Induced by Noise and Kanamycin in Mitochondrial DNA4834-deficient Rats by Regulating PI3K/MAPK Signaling

Superoxide dismutase 2 (SOD2)-mediated gene therapy has significant protective effects against kanamycin-induced hearing loss and hair cell loss in the inner ear, but the underlying mechanisms are still unclear. Herein, an in vivo aging model of mitochondrial DNA (mtDNA)4834 deletion mutation was established using D-galactose, and the effects of noise or kanamycin on inner ear injury was investigated. Rats subjected to mtDNA4834 mutation via D-galactose administration showed hearing loss characterized by the disruption of inner ear structure (abnormal cell morphology, hair cell lysis, and the absence of the organ of Corti), increased SOD2 promoter methylation, and an increase in the degree of apoptosis. Exposure to noise or kanamycin further contributed to the effects of D-galactose. SOD2 overexpression induced by viral injection accordingly counteracted the effects of noise and kanamycin and ameliorated the symptoms of hearing loss, suggesting the critical involvement of SOD2 in preventing deafness and hearing-related conditions. The PI3K and MAPK signaling pathways were also regulated by noise/kanamycin exposure and/or SOD2 overexpression, indicating that they may be involved in the therapeutic effect of SOD2 against age-related hearing loss.

Effects of combined gentamicin and furosemide treatment on cochlear ribbon synapses

It is well-established that aminoglycoside antibiotics are ototoxic, and the toxicity can be drastically enhanced by the addition of loop diuretics, resulting in rapid irreversible hair cell damage. Using both electrophysiologic and morphological approaches, we investigated whether this combined treatment affected the cochlea at the region of ribbon synapses, consequently resulting in auditory synaptopathy. A series of varied gentamicin and furosemide doses were applied to C57BL/6 mice, and auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE) were measured to assess ototoxic damage within the cochlea. In brief, the treatment effectively induced cochlear damage and promoted a certain reorganization of synaptic ribbons, while a reduction of ribbon density only occurred after a substantial loss of outer hair cells. In addition, both the ABR wave I amplitude and the ribbon density were elevated in low-dose treatment conditions, but a correlation between the two events was not significant for individual cochleae. In sum, combined gentamicin and furosemide treatment, at titrated doses below those that produce hair cell damage, typically triggers synaptic plasticity rather than a permanent synaptic loss.

Three-dimensional imaging of intact porcine cochlea using tissue clearing and custom-built light-sheet microscopy

Hearing loss is a prevalent disorder that affects people of all ages. On top of the existing hearing aids and cochlear implants, there is a growing effort to regenerate functional tissues and restore hearing. However, studying and evaluating these regenerative medicine approaches in a big animal model (e.g. pigs) whose anatomy, physiology, and organ size are similar to a human is challenging. In big animal models, the cochlea is bulky, intricate, and veiled in a dense and craggy otic capsule. These facts complicate 3D microscopic analysis that is vital in the cochlea, where structure-function relation is time and again manifested. To allow 3D imaging of an intact cochlea of newborn and juvenile pigs with a volume up to ∼ 250 mm3, we adapted the BoneClear tissue clearing technique, which renders the bone transparent. The transparent cochleae were then imaged with cellular resolution and in a timely fashion, which prevented bubble formation and tissue degradation, using an adaptive custom-built light-sheet fluorescence microscope. The adaptive light-sheet microscope compensated for deflections of the illumination beam by changing the angles of the beam and translating the detection objective while acquiring images. Using this combination of techniques, macroscopic and microscopic properties of the cochlea were extracted, including the density of hair cells, frequency maps, and lower frequency limits. Consequently, the proposed platform could support the growing effort to regenerate cochlear tissues and assist with basic research to advance cures for hearing impairments.

Light sheet microscopy of the gerbil cochlea

Light sheet fluorescence microscopy (LSFM) provides a rapid and complete three-dimensional image of the cochlea. The method retains anatomical relationships-on a micrometer scale-between internal structures such as hair cells, basilar membrane (BM), and modiolus with external surface structures such as the round and oval windows. Immunolabeled hair cells were used to visualize the spiraling BM in the intact cochlea without time intensive dissections or additional histological processing; yet material prepared for LSFM could be rehydrated, the BM dissected out and reimaged at higher resolution with the confocal microscope. In immersion-fixed material, details of the cochlear vasculature were seen throughout the cochlea. Hair cell counts (both inner and outer) as well as frequency maps of the BM were comparable to those obtained by other methods, but with the added dimension of depth. The material provided measures of angular, linear, and vector distance between characteristic frequency regions along the BM. Thus, LSFM provides a unique ability to rapidly image the entire cochlea in a manner applicable to model and interpret physiological results. Furthermore, the three-dimensional organization of the cochlea can be studied at the organ and cellular level with LSFM, and this same material can be taken to the confocal microscope for detailed analysis at the subcellular level.

The Novel Peptide Vaccine GV1001 Protects Hearing in a Kanamycin-induced Ototoxicity Mouse Model

HYPOTHESIS

We tested whether GV1001 has any ototoxic side effects at different doses and whether it protects hearing in an aminoglycoside-induced ototoxicity mouse model.

BACKGROUND

GV1001, a novel peptide vaccine currently being examined in a Phase 3 clinical trial to treat pancreatic cancer, also has anti-inflammatory and antioxidant effects.

METHODS

In the first experiment, C57/BL6 mice were injected with GV1001 preparations at concentrations of 0.1 to 100 mg/kg for 7 days to evaluate the toxicity of GV1001 on the inner ear and kidneys. In the second experiment, the protective effect of GV1001 was tested in an ototoxicity mouse model that was generated by injecting 800 mg/kg kanamycin (KM) for 2 weeks. The hearing threshold and hair cell loss were compared between the KM + GV1001 group (treated with 10 mg/kg GV1001 for 2 wk) and the KM + saline group. The hearing threshold was measured before, and 7, 14, and 21 days after the initial treatment. The blood urea nitrogen level was measured.

RESULTS

No ototoxicity or renal toxicity was found following treatment with different doses of GV1001 (0.1-100 mg/kg). The KM + saline group showed impaired auditory function and markedly disoriented and missing cochlear hair cells, while the KM + GV1001 group showed significant hearing and hair cell preservation in comparison (p < 0.05).

CONCLUSION

GV1001 itself did not have any detrimental effects on the inner ear or kidney. In the KM induced ototoxicity model, concomitant administration of GV1001 protected against cochlear hair cell damage and preserve hearing.

Generation of new hair cells by DNA methyltransferase (Dnmt) inhibitor 5-azacytidine in a chemically-deafened mouse model

Regeneration of mature mammalian inner ear hair cells remains to be a challenge. This study aims to evaluate the ability of DNA methyltransferase (Dnmt) inhibitor 5-azacytidine (5-aza) to generate outer hair cells (OHCs) in a chemically-deafened adult mouse model. 5-aza was administrated into the mouse inner ear via the round window. Immunofluorescence was used to examine the expression of hair cell specific proteins following 5-aza treatment. The results showed that in the chemically-deafened mouse cochlea, new OHCs were found post 5-aza treatment, whereas OHCs were completely lost in saline-treated mice. New hair cells expressed multiple hair cell markers included Myosin VIIa, Pou4f3 and Myosin VI. Newly-generated hair cells presented in three cochlear turns and were able to survive for at least six weeks. The effects of new hair cells generation by 5-aza were concentration dependent. Quantitative PCR study indicates that 5-aza may function through Dnmt1 inhibition. The results of this report suggest that the Dnmt inhibitor 5-aza may promote hair cell regeneration in a chemically-deafened mouse model.

A Simple Model for Inducing Optimal Increase of SDF-1 with Aminoglycoside Ototoxicity

Objectives

As a homing factor of stem cell, stromal derived factor-1 (SDF-1) is important for the regenerative research in ototoxicity. Mice models with aminoglycoside ototoxicity have been widely used to study the regeneration capacity of MSCs in repair of cochlear injury. We developed a mouse model with maximal increase in SDF-1 levels in the inner ear, according to the “one-shot” doses of kanamycin and furosemide.

Methods

C57BL/6 mice had kanamycin (420, 550, and 600 mg/kg) dissolved in PBS, followed by an intraperitoneal injection of furosemide (130 mg/kg). The injuries of inner ear were measured with hearing thresholds, histology, and outer hair cell counts at 0, 3, 5, 7, 10, and 14 days before the sacrifice. The levels of SDF-1 in the inner ear were tested by real-time RT-PCR and immunohistochemistry.

Results

There were a significant reduction in hearing thresholds and a maximal increase of SDF-1 levels in the furosemide 130 mg/kg + kanamycin 550 mg/kg group, but severe hearing deterioration over time was observed in the furosemide 130 mg/kg + kanamycin 600 mg/kg group and four mice were dead. SDF-1 was detected mostly in the stria vascularis and organ of Corti showing the highest increase in expression.

Conclusion

We observed optimal induction of the stem cell homing factor in the newly generated aminoglycoside-induced ototoxicity mouse model using a “one-shot” protocol. This study regarding high SDF-1 levels in our mouse model of ototoxicity would play a major role in the development of therapeutic agents using MSC homing.

Genetic disruption of fractalkine signaling leads to enhanced loss of cochlear afferents following ototoxic or acoustic injury

Cochlear hair cells are vulnerable to a variety of insults like acoustic trauma and ototoxic drugs. Such injury can also lead to degeneration of spiral ganglion neurons (SGNs), but this occurs over a period of months to years. Neuronal survival is necessary for the proper function of cochlear prosthetics, therefore, it is of great interest to understand the mechanisms that regulate neuronal survival in deaf ears. We have recently demonstrated that selective hair cell ablation is sufficient to attract leukocytes into the spiral ganglion, and that fractalkine signaling plays a role in macrophage recruitment and in the survival of auditory neurons. Fractalkine (CX₃ CL1), a chemokine that regulates adhesion and migration of leukocytes is expressed by SGNs and signals to leukocytes via its receptor CX₃ CR1. The present study has extended the previous findings to more clinically relevant conditions of sensorineural hearing loss by examining the role of fractalkine signaling after aminoglycoside ototoxicity or acoustic trauma. Both aminoglycoside treatment and acoustic overstimulation led to the loss of hair cells as well as prolonged increase in the numbers of cochlear leukocytes. Lack of CX₃ CR1 did not affect macrophage recruitment after injury, but resulted in increased loss of SGNs and enhanced expression of the inflammatory cytokine interleukin-1β, when compared to mice with intact CX₃ CR1. These data indicate that the dysregulation of macrophage response caused by the absence of CX₃ CR1 may contribute to inflammation-mediated neuronal loss in the deafened ear, suggesting a key role for inflammation in the long-term survival of target-deprived afferent neurons.

Usefulness of Intravital Multiphoton Microscopy in Visualizing Study of Mouse Cochlea and Volume Changes in the Scala Media

Conventional microscopy has limitations in viewing the cochlear microstructures due to three-dimensional spiral structure and the overlying bone. But these issues can be overcome by imaging the cochlea in vitro with intravital multiphoton microscopy (MPM). By using near-infrared lasers for multiphoton excitation, intravital MPM can detect endogenous fluorescence and second harmonic generation of tissues. In this study, we used intravital MPM to visualize various cochlear microstructures without any staining and non-invasively analyze the volume changes of the scala media (SM) without removing the overlying cochlear bone. The intravital MPM images revealed various tissue types, ranging from thin membranes to dense bone, as well as the spiral ganglion beneath the cochlear bone. The two-dimensional, cross-sectional, and serial z-stack intravital MPM images also revealed the spatial dilation of the SM in the temporal bone of pendrin-deficient mice. These findings suggest that intravital MPM might serve as a new method for obtaining microanatomical information regarding the cochlea, similar to standard histopathological analyses in the animal study for the cochlea. Given the capability of intravital MPM for detecting an increase in the volume of the SM in pendrin-deficient mice, it might be a promising new tool for assessing the pathophysiology of hearing loss in the future.

Endolymph movement visualized with light sheet fluorescence microscopy in an acute hydrops model

There are a variety of techniques available to investigate endolymph dynamics, primarily seeking to understand the cause of endolymphatic hydrops. Here we have taken the novel approach of injecting, via a glass micropipette, fluorescein isothiocyanate-dextran (FITC-dex) and artificial endolymph into scala media of anaesthetized guinea pigs, with subsequent imaging of the inner ear using Light Sheet Fluorescence Microscopy (LSFM) as a means to obtain highly resolved 3D visualization of fluid movements. Our results demonstrate endolymph movement into the utricle, semicircular canals and endolymphatic duct and sac when more than 2.5 μl of fluid had been injected into scala media, with no apparent movement of fluid into the perilymphatic compartments. There was no movement of endolymph into these compartments when less than 2.5 μl was injected. The remarkable uptake of the FITC-dex into the endolymphatic duct, including an absorption into the periductal channels surrounding the endolymphatic duct, highlights the functional role this structure plays in endolymph volume regulation.

Fractalkine Signaling Regulates Macrophage Recruitment into the Cochlea and Promotes the Survival of Spiral Ganglion Neurons after Selective Hair Cell Lesion

Macrophages are recruited into the cochlea in response to injury caused by acoustic trauma or ototoxicity, but the nature of the interaction between macrophages and the sensory structures of the inner ear remains unclear. The present study examined the role of fractalkine signaling in regulating the injury-evoked behavior of macrophages following the selective ablation of cochlear hair cells. We used a novel transgenic mouse model in which the human diphtheria toxin receptor (huDTR) is selectively expressed under the control of Pou4f3, a hair cell-specific transcription factor. Administration of diphtheria toxin (DT) to these mice resulted in nearly complete ablation of cochlear hair cells, with no evident pathology among supporting cells, spiral ganglion neurons, or cells of the cochlear lateral wall. Hair cell death led to an increase in macrophages associated with the sensory epithelium of the cochlea. Their numbers peaked at 14 days after DT and then declined at later survival times. Increased macrophages were also observed within the spiral ganglion, but their numbers remained elevated for (at least) 56 d after DT. To investigate the role of fractalkine signaling in macrophage recruitment, we crossed huDTR mice to a mouse line that lacks expression of the fractalkine receptor (CX3CR1). Disruption of fractalkine signaling reduced macrophage recruitment into both the sensory epithelium and spiral ganglion and also resulted in diminished survival of spiral ganglion neurons after hair cell death. Our results suggest a fractalkine-mediated interaction between macrophages and the neurons of the cochlea.

SIGNIFICANCE STATEMENT

It is known that damage to the inner ear leads to recruitment of inflammatory cells (macrophages), but the chemical signals that initiate this recruitment and the functions of macrophages in the damaged ear are unclear. Here we show that fractalkine signaling regulates macrophage recruitment into the cochlea and also promotes the survival of cochlear afferents after selective hair cell lesion. Because these afferent neurons carry sound information from the cochlea to the auditory brainstem, their survival is a key determinant of the success of cochlear prosthetics. Our data suggest that fractalkine signaling in the cochlea is neuroprotective, and reveal a previously uncharacterized interaction between cells of the cochlea and the innate immune system.

Auditory Pathology in a Transgenic mtTFB1 Mouse Model of Mitochondrial Deafness

The A1555G mutation in the 12S rRNA gene of human mitochondrial DNA causes maternally inherited, nonsyndromic deafness, an extreme case of tissue-specific mitochondrial pathology. A transgenic mouse strain that robustly overexpresses the mitochondrial 12S ribosomal RNA methyltransferase TFB1M (Tg-mtTFB1 mice) exhibits progressive hearing loss that we proposed models aspects of A1555G-related pathology in humans. Although our previous studies of Tg-mtTFB1 mice implicated apoptosis in the spiral ganglion and stria vascularis because of mitochondrial reactive oxygen species-mediated activation of AMP kinase (AMPK) and the nuclear transcription factor E2F1, detailed auditory pathology was not delineated. Herein, we show that Tg-mtTFB1 mice have reduced endocochlear potential, indicative of significant stria vascularis dysfunction, but without obvious signs of strial atrophy. We also observed decreased auditory brainstem response peak 1 amplitude and prolonged wave I latency, consistent with apoptosis of spiral ganglion neurons. Although no major loss of hair cells was observed, there was a mild impairment of voltage-dependent electromotility of outer hair cells. On the basis of these results, we propose that these events conspire to produce the progressive hearing loss phenotype in Tg-mtTFB1 mice. Finally, genetically reducing AMPK α1 rescues hearing loss in Tg-mtTFB1 mice, confirming that aberrant up-regulation of AMPK signaling promotes the observed auditory pathology. The relevance of these findings to human A1555G patients and the potential therapeutic value of reducing AMPK activity are discussed.