Aminoglycoside ototoxicity: permeant drugs cause permanent hair cell loss

Structural and Functional Basis of GenB2 Isomerase Activity from Gentamicin Biosynthesis

Aminoglycosides are essential antibiotics used to treat severe infections caused mainly by Gram-negative bacteria. Gentamicin is an aminoglycoside and, despite its toxicity, is clinically used to treat several pulmonary and urinary infections. The commercial form of gentamicin is a mixture of five compounds with minor differences in the methylation of one of their aminosugars. In the case of two compounds, gentamicin C2 and C2a, the only difference is the stereochemistry of the methyl group attached to C-6'. GenB2 is the enzyme responsible for this epimerization and is one of the four PLP-dependent enzymes encoded by the gentamicin biosynthetic gene cluster. Herein, we have determined the structure of GenB2 in its holo form in complex with PMP and also in the ternary complex with gentamicin X2 and G418, two substrate analogues. Based on the structural analysis, we were able to identify the structural basis for the catalytic mechanism of this enzyme, which was also studied by site-directed mutagenesis. Unprecedently, GenB2 is a PLP-dependent enzyme from fold I, which is able to catalyze an epimerization but with a mechanism distinct from that of fold III PLP-dependent epimerases using a cysteine residue near the N-terminus. The substitution of this cysteine residue for serine or alanine completely abolished the epimerase function of the enzyme, confirming its involvement. This study not only contributes to the understanding of the enzymology of gentamicin biosynthesis but also provides valuable details for exploring the enzymatic production of new aminoglycoside derivatives.

Are Aminoglycoside Antibiotics TRPing Your Metabolic Switches?

Transient receptor potential (TRP) channels are broadly implicated in the developmental programs of most tissues. Amongst these tissues, skeletal muscle and adipose are noteworthy for being essential in establishing systemic metabolic balance. TRP channels respond to environmental stimuli by supplying intracellular calcium that instigates enzymatic cascades of developmental consequence and often impinge on mitochondrial function and biogenesis. Critically, aminoglycoside antibiotics (AGAs) have been shown to block the capacity of TRP channels to conduct calcium entry into the cell in response to a wide range of developmental stimuli of a biophysical nature, including mechanical, electromagnetic, thermal, and chemical. Paradoxically, in vitro paradigms commonly used to understand organismal muscle and adipose development may have been led astray by the conventional use of streptomycin, an AGA, to help prevent bacterial contamination. Accordingly, streptomycin has been shown to disrupt both in vitro and in vivo myogenesis, as well as the phenotypic switch of white adipose into beige thermogenic status. In vivo, streptomycin has been shown to disrupt TRP-mediated calcium-dependent exercise adaptations of importance to systemic metabolism. Alternatively, streptomycin has also been used to curb detrimental levels of calcium leakage into dystrophic skeletal muscle through aberrantly gated TRPC1 channels that have been shown to be involved in the etiology of X-linked muscular dystrophies. TRP channels susceptible to AGA antagonism are critically involved in modulating the development of muscle and adipose tissues that, if administered to behaving animals, may translate to systemwide metabolic disruption. Regenerative medicine and clinical communities need to be made aware of this caveat of AGA usage and seek viable alternatives, to prevent contamination or infection in in vitro and in vivo paradigms, respectively.

Murine cochlear damage models in the context of hair cell regeneration research

Understanding the complex pathologies associated with hearing loss is a significant motivation for conducting inner ear research. Lifelong exposure to loud noise, ototoxic drugs, genetic diversity, sex, and aging collectively contribute to human hearing loss. Replicating this pathology in research animals is challenging because hearing impairment has varied causes and different manifestations. A central aspect, however, is the loss of sensory hair cells and the inability of the mammalian cochlea to replace them. Researching therapeutic strategies to rekindle regenerative cochlear capacity, therefore, requires the generation of animal models in which cochlear hair cells are eliminated. This review discusses different approaches to ablate cochlear hair cells in adult mice. We inventoried the cochlear cyto- and histo-pathology caused by acoustic overstimulation, systemic and locally applied drugs, and various genetic tools. The focus is not to prescribe a perfect damage model but to highlight the limitations and advantages of existing approaches and identify areas for further refinement of damage models for use in regenerative studies.

The Piezo channel is a mechano-sensitive complex component in the mammalian inner ear hair cell

The inner ear is the hub where hair cells (HCs) transduce sound, gravity, and head acceleration stimuli to the brain. Hearing and balance rely on mechanosensation, the fastest sensory signals transmitted to the brain. The mechanoelectrical transducer (MET) channel is the entryway for the sound-balance-brain interface, but the channel-complex composition is not entirely known. Here, we report that the mouse utilizes Piezo1 (Pz1) and Piezo2 (Pz2) isoforms as MET-complex components. The Pz channels, expressed in HC stereocilia, and cell lines are co-localized and co-assembled with MET complex partners. Mice expressing non-functional Pz1 and Pz2 at the ROSA26 locus have impaired auditory and vestibular traits that can only be explained if the Pzs are integral to the MET complex. We suggest that Pz subunits constitute part of the MET complex and that interactions with other MET complex components yield functional MET units to generate HC MET currents.

Role of Kir4.1 Channels in Aminoglycoside-Induced Ototoxicity of Hair Cells

The Kir4.1 channel, an inwardly rectifying potassium ion (K+) channel, is located in the hair cells of the organ of Corti as well as the intermediate cells of the stria vascularis. The Kir4.1 channel has a crucial role in the generation of endolymphatic potential and maintenance of the resting membrane potential. However, the role and functions of the Kir4.1 channel in the progenitor remain undescribed. To observe the role of Kir4.1 in the progenitor treated with the one-shot ototoxic drugs (kanamycin and furosemide), we set the proper condition in culturing Immortomouse-derived HEI-OC1 cells to express the potassium-related channels well. And also, that was reproduced in mice experiments to show the important role of Kir4.1 in the survival of hair cells after treating the ototoxicity drugs. In our results, when kanamycin and furosemide drugs were cotreated with HEI-OC1 cells, the Kir4.1 channel did not change, but the expression levels of the NKCC1 cotransporter and KCNQ4 channel are decreased. This shows that inward and outward channels were blocked by the two drugs (kanamycin and furosemide). However, noteworthy here is that the expression level of Kir4.1 channel increased when kanamycin was treated alone. This shows that Kir4.1, an inwardly rectifying potassium channel, acts as an outward channel in place of the corresponding channel when the KCNQ4 channel, an outward channel, is blocked. These results suggest that the Kir4.1 channel has a role in maintaining K+ homeostasis in supporting cells, with K+ concentration compensator when the NKCC1 cotransporter and Kv7.4 (KCNQ4) channels are deficient.

Transient Receptor Potential Channels and Auditory Functions

Significance: Transient receptor potential (TRP) channels are cation-gated channels that serve as detectors of various sensory modalities, such as pain, heat, cold, and taste. These channels are expressed in the inner ear, suggesting that they could also contribute to the perception of sound. This review provides more details on the different types of TRP channels that have been identified in the cochlea to date, focusing on their cochlear distribution, regulation, and potential contributions to auditory functions. Recent Advances: To date, the effect of TRP channels on normal cochlear physiology in mammals is still unclear. These channels contribute, to a limited extent, to normal cochlear physiology such as the hair cell mechanoelectrical transduction channel and strial functions. More detailed information on a number of these channels in the cochlea awaits future studies. Several laboratories focusing on TRPV1 channels have shown that they are responsive to cochlear stressors, such as ototoxic drugs and noise, and regulate cytoprotective and/or cell death pathways. TRPV1 expression in the cochlea is under control of oxidative stress (produced primarily by NOX3 NADPH oxidase) as well as STAT1 and STAT3 transcription factors, which differentially modulate inflammatory and apoptotic signals in the cochlea. Inhibition of oxidative stress or inflammation reduces the expression of TRPV1 channels and protects against cochlear damage and hearing loss. Critical Issues: TRPV1 channels are activated by both capsaicin and cisplatin, which produce differential effects on the inner ear. How these differential actions are produced is yet to be determined. It is clear that TRPV1 is an essential component of cisplatin ototoxicity as knockdown of these channels protects against hearing loss. In contrast, activation of TRPV1 by capsaicin protected against subsequent hearing loss induced by cisplatin. The cellular targets that are influenced by these two drugs to account for their differential profiles need to be fully elucidated. Furthermore, the potential involvement of different TRP channels present in the cochlea in regulating cisplatin ototoxicity needs to be determined. Future Directions: TRPV1 has been shown to mediate the entry of aminoglycosides into the hair cells. Thus, novel otoprotective strategies could involve designing drugs to inhibit entry of aminoglycosides and possibly other ototoxins into cochlear hair cells. TRP channels, including TRPV1, are expressed on circulating and resident immune cells. These receptors modulate immune cell functions. However, whether they are activated by cochlear stressors to initiate cochlear inflammation and ototoxicity needs to be determined. A better understanding of the function and regulation of these TRP channels in the cochlea could enable development of novel treatments for treating hearing loss. Antioxid. Redox Signal. 36, 1158-1170.

Hair cell uptake of gentamicin in the developing mouse utricle

Intratympanic injection of gentamicin has proven to be an effective therapy for intractable vestibular dysfunction. However, most studies to date have focused on the cochlea, so little is known about the distribution and uptake of gentamicin by the counterpart of the auditory system, specifically vestibular hair cells (HCs). Here, with a combination of in vivo and in vitro approaches, we used a gentamicin-Texas Red (GTTR) conjugate to investigate the mechanisms of gentamicin vestibulotoxicity in the developing mammalian utricular HCs. In vivo, GTTR fluorescence was concentrated in the apical cytoplasm and the cellular membrane of neonatal utricular HCs, but scarce in the nucleus of HCs and supporting cells. Quantitative analysis showed the GTTR uptake by striolar HCs was significantly higher than that in the extrastriola. In addition, the GTTR fluorescence intensity in the striola was increased gradually from 1 to 8 days, peaking at 8-9 days postnatally. In vitro, utricle explants were incubated with GTTR and candidate uptake conduits, including mechanotransduction (MET) channels and endocytosis in the HC, were inhibited separately. GTTR uptake by HCs could be inhibited by quinine, a blocker of MET channels, under both normal and stressed conditions. Meanwhile, endocytic inhibition only reduced GTTR uptake in the CoCl₂ hypoxia model. In sum, the maturation of MET channels mediated uptake of GTTR into vestibular HCs. Under stressed conditions, MET channels play a pronounced role, manifested by channel-dependent stress enhanced GTTR permeation, while endocytosis participates in GTTR entry in a more selective manner.

Citicoline Protects Auditory Hair Cells Against Neomycin-Induced Damage

Aminoglycoside-induced hair cell (HC) loss is one of the most important causes of hearing loss. After entering the inner ear, aminoglycosides induce the production of high levels of reactive oxygen species (ROS) that subsequently activate apoptosis in HCs. Citicoline, a nucleoside derivative, plays a therapeutic role in central nervous system injury and in neurodegenerative disease models, including addictive disorders, stroke, head trauma, and cognitive impairment in the elderly, and has been widely used in the clinic as an FDA approved drug. However, its effect on auditory HCs remains unknown. Here, we used HC-like HEI-OC-1 cells and whole organ explant cultured mouse cochleae to explore the effect of citicoline on aminoglycoside-induced HC damage. Consistent with previous reports, both ROS levels and apoptosis were significantly increased in neomycin-induced cochlear HCs and HEI-OC-1 cells compared to undamaged controls. Interestingly, we found that co-treatment with citicoline significantly protected against neomycin-induced HC loss in both HEI-OC-1 cells and whole organ explant cultured cochleae. Furthermore, we demonstrated that citicoline could significantly reduce neomycin-induced mitochondrial dysfunction and inhibit neomycin-induced ROS accumulation and subsequent apoptosis. Thus, we conclude that citicoline can protect against neomycin-induced HC loss by inhibiting ROS aggregation and thus preventing apoptosis in HCs, and this suggests that citicoline might serve as a potential therapeutic drug in the clinic to protect HCs.

Role of Calcium-Sensing Receptor in Mechanotransducer-Channel-Mediated Ca2+ Influx in Hair Cells of Zebrafish Larvae

The calcium-sensing receptor (CaSR) is an extracellular Ca²⁺ sensor that plays a critical role in maintaining Ca²⁺ homeostasis in several organs, including the parathyroid gland and kidneys. In this study, through in situ hybridization, the expression of CaSR mRNA was found in the neuromasts of zebrafish larvae. Immunohistochemistry further demonstrated that the CaSR protein was present in neuromast hair cell stereocilia and basolateral membranes. Based on the expression and subcellular localization of the CaSR in hair cells, we hypothesized that the CaSR is expressed in zebrafish lateral-line hair cells to regulate mechanotransducer (MET)-channel-mediated Ca²⁺ entry. Using the scanning ion-selective electrode technique, MET-channel-mediated Ca²⁺ influx at the stereocilia of hair cells was measured in intact larvae. Ca²⁺ influx was suppressed after larvae were pretreated with a CaSR activator (R-568) or high-Ca²⁺ (HCa) medium. Gene knockdown by using morpholino oligonucleotides decreased CaSR expression in hair cells and eliminated the effects of R-568 and HCa on Ca²⁺ influx. In addition, we found that treatment with R-568 attenuated neomycin-induced hair cell death. This study is the first to demonstrate that the CaSR is involved in mechanotransduction in zebrafish hair cells.

Clinical and genetic associations for carboplatin-related ototoxicity in children treated for retinoblastoma: A retrospective noncomparative single-institute experience

BACKGROUND

Children with retinoblastoma treated with carboplatin chemotherapy risk moderate to severe, irreversible hearing loss. Based on published evidence, we hypothesized that ototoxicity risk is associated with clinical parameters and variants in candidate genes in drug metabolism pathways (methyltransferases [thiopurine S-methyltransferase, TPMT] and [catechol-O-methyltransferase, COMT], and drug transporter ABCC3).

PROCEDURE

We retrospectively reviewed clinical records of patients with retinoblastoma treated with carboplatin chemotherapy regarding age (at diagnosis and chemotherapy initiation), chemotherapy sessions (cycles number, drug doses, and cumulative carboplatin dose), and hearing loss (defined as ototoxicity ≥grade 2 by at least one classification system). Blood samples were genotyped for genetic variants in TPMT (rs12201199, rs1800460), COMT (rs4646316, rs9332377), and ABCC3 (rs1051640) by quantitative PCR and confirmed by allele-specific PCR. Univariate statistical tests, receiver-operating characteristic analysis, and Kaplan-Meier curves were used to examine the association between hearing loss, clinical factors, and variants in candidate genes.

RESULTS

Audiometric data and stored DNA were available for 71 patients with retinoblastoma (88% carried an RB1 pathogenic variant allele). Median carboplatin cumulative dose was 1,400 mg/m² (260-5,148 mg/m² ). Ototoxicity occurred in 18 patients (25%), strongly associated with age at diagnosis (P = 0.01) and age at chemotherapy initiation (OR = 4.99, P = 0.008). The highest likelihood ratio of hearing loss was associated with chemotherapy initiation <4.25 months of age. Ototoxicity was not associated with any tested genetic variants.

CONCLUSIONS

We observed a 25% prevalence of ototoxicity in patients with retinoblastoma treated with carboplatin, higher than previously published. Age at chemotherapy initiation was associated with carboplatin-induced ototoxicity, with children <4.25 months of age at highest risk.

[The analysis of the association of the polymorphic variants of the TPMT, COMT, and ABCC3 genes with the development of hearing disorders induced by the cisplatin treatment]

Cisplatin and its derivatives are widely used chemotherapeutic agents for the treatment of many cancers, including hepatoblastoma, brain tumors, and germ-cell tumors. This therapy contributed to the dramatic increase in the survival rate. However, its use is restricted by the high incidence of irreversible ototoxicity associated with cisplatin application (in more than 60% of the children receiving it). Some studies have reported that genetic variants of TPMT (rs 12201199), COMT (rs4646316), and ABCC3 (rs 1051640) are conferring increased risk of developing cisplatin-induced hearing loss. However, in other studies the results were not replicated. In the present study, we replicated the previous studies based on an independent cohort of Russian patients. SNP genotypes for rs 12201199, rs4646316 and rs 1051640 were determined in DNA samples obtained from 16 patients who developed hearing loss and a group of 34 patients whose hearing was retained. The association between TPMT (rs 12201199), COMT (rs4646316), and ABCC3 (rs 1051640) variants and the hearing loss was not observed in our cohort.

Protein Synthesis Inhibition and Activation of the c-Jun N-Terminal Kinase Are Potential Contributors to Cisplatin Ototoxicity

Cisplatin has been regarded as an effective and versatile chemotherapeutic agent for nearly 40 years. Though the associated dose-dependent ototoxicity is known, the cellular mechanisms by which cochleovestibular hair cell death occur are not well understood. We have previously shown that aminoglycoside ototoxicity is mediated in part by cytosolic protein synthesis inhibition. Despite a lack of molecular similarity, aminoglycosides were shown to elicit similar stress pathways to cisplatin. We therefore reasoned that there may be some role of protein synthesis inhibition in cisplatin ototoxicity. Employing a modification of the bioorthogonal noncanonical amino acid tagging (BONCAT) method, we evaluated the effects of cisplatin on cellular protein synthesis. We show that cisplatin inhibits cellular protein synthesis in organ of Corti explant cultures. Similar to what was found after gentamicin exposure, cisplatin activates both the c-Jun N-terminal kinase (JNK) and mammalian target of rapamycin (mTOR) pathways. In contrast to aminoglycosides, cisplatin also inhibits protein synthesis in all cochlear cell types. We further demonstrate that the multikinase inhibitor sorafenib completely prevents JNK activation, while providing only moderate hair cell protection. Simultaneous stimulation of cellular protein synthesis by insulin, however, significantly improved hair cell survival in culture. The presented data provides evidence for a potential role of protein synthesis inhibition in cisplatin-mediated ototoxicity.

Towards the Prevention of Aminoglycoside-Related Hearing Loss

Aminoglycosides are potent antibiotics deployed worldwide despite their known side-effect of sensorineural hearing loss. The main etiology of this sensory deficit is death of inner ear sensory hair cells selectively triggered by aminoglycosides. For decades, research has sought to unravel the molecular events mediating sensory cell demise, emphasizing the roles of reactive oxygen species and their potentials as therapeutic targets. Studies in recent years have revealed candidate transport pathways including the mechanotransducer channel for drug entry into sensory cells. Once inside sensory cells, intracellular targets of aminoglycosides, such as the mitochondrial ribosomes, are beginning to be elucidated. Based on these results, less ototoxic aminoglycoside analogs are being generated and may serve as alternate antimicrobial agents. In this article, we review the latest findings on mechanisms of aminoglycoside entry into hair cells, their intracellular actions and potential therapeutic targets for preventing aminoglycoside ototoxicity.

It is 2015: What are the best diagnostic and treatment options for Ménière’s disease?

Ménière’s disease (MD) is a common cause of recurrent vertigo. Its pathophysiology is still unclear and controversial. The most common histological finding in postmortem temporal bone studies of patients is endolymphatic hydrops (EH). However, not all cases of hydrops are associated with MD and it may represent the end point of various etiologies. The diagnostic criteria for MD have undergone changes during the past few decades. A recent collaboration among specialty societies in United States, Europe and Japan has given rise to a new set of guidelines for the diagnosis and classification of MD. The aim is to develop international consensus criteria for MD that would help improve the quality of data collected from patients. The diagnosis of MD can be difficult in some cases as there is no gold standard for testing. Previous use of audiometric data and electrocochleography are poorly sensitive as screening tools. Recently magnetic resonance imaging as a diagnostic tool for identifying EH has gained popularity in Asia and Europe. Vestibular evoked myogenic potentials are also used but lack specificity. Finally, the treatment for MD has improved with the introduction of intratympanic treatments with steroids and gentamicin as well as less invasive treatment with the Meniett device.

Establishment of a model of cochlear lesions in rats to study potential gene therapy for sensorineural hearing loss

INTRODUCTION

Sensorineural hearing loss seriously influences a patient's daily life, and no effective treatments exist to date. Gene therapy is a potential treatment for regenerating hair cells to restore hearing.

METHODS

In this study, we established a cochlear lesions model to study hair cell regeneration by co-administration of kanamycin and furosemide. After the injections, we assessed the survival of outer hair cells (OHC), inner hair cells (IHC), supporting cells (SC), spiral ganglion neurons (SGN) and peripheral axons. Moreover, we used two viral vectors to detect the transgene distribution.

RESULTS

Our results showed at 12h post-treatment, numerous OHC were missing in the basal turn. At 24h post-treatment, all OHCs in basal half of the cochlea were lost, and by 48h, OHC loss had spread to the apical coil. Four days after the injections, all OHCs were absent. At 1mo post-treatment, the organ of Corti had collapsed. In contrast, most of the SC remained 4d after the injections. The loss of SGN and peripheral axons was consistent with this time course post-treatment. The results of transgene distribution suggested the correlative gene can be transferred into the organ of Corti using adenoviruses (AdV) vectors and lentiviruses (LV) vectors in our cochlear lesion model.

COMPARISON WITH EXISTING METHOD(S)

We assessed the details of HC death at more time point and chosen the time point for gene transfer in this model.

CONCLUSIONS

We conclude that this cochlear lesion model would be suitable for the study of hair cell regeneration.

Evaluation of adverse drug reactions in HIV positive patients in a tertiary care hospital

CONTEXT

The advancement and development of new drugs and treatment strategies increase the risk of unusual Adverse Events (AEs) in HIV patients.

AIMS

The objective of our study was to assess the incidence, types and nature of AEs in HIV positive subjects.

SETTINGS AND DESIGN

Patients with WHO stage IV disease irrespective of the CD4 cell count, or WHO stage III disease with a CD4 cell count <350 cell/cu. Mm, or, WHO stage I or II disease with a CD4 cell count of <200 cells/cu. mm, and on prior anti-retroviral therapy for not more than six months preceding the observation date, were included in the study. After initiation of therapy, the patients were examined for the occurrence any adverse events including the type and severity, or any other abnormal laboratory findings. Causality assessment of the adverse events was done using the Naranjo's scale.

RESULTS

Out of 327 patients studied prospectively, 43 patients developed AEs. Out of these, 23 (53.5%) were males and 20 (46.5%) were females. A total of 53 (16.21%) AEs were reported. Antitubercular drugs caused the maximum AEs (28.3%) followed by zidovudine (20.7%), nevirapine (15.0%) and efavirenz (5.6%). Stavudine, ethambutol, sulfamethoxazole and trimethoprim, and atazanavir were also responsible for 3.7% of AEs individually. Causality assessment done according to the Naranjo's scale revealed that 66.04% AEs were 'probable' and 33.96% were 'possible'.

CONCLUSIONS

Anemia, hepatitis and dermatological adverse effects are the most common AEs. Antitubercular drugs contributed significantly for the incidence of AEs in these patients. Frequency of AEs was slightly more in males compared to females.

Functional and structural changes throughout the auditory system following congenital and early-onset deafness: implications for hearing restoration

The absence of auditory input, particularly during development, causes widespread changes in the structure and function of the auditory system, extending from peripheral structures into auditory cortex. In humans, the consequences of these changes are far-reaching and often include detriments to language acquisition, and associated psychosocial issues. Much of what is currently known about the nature of deafness-related changes to auditory structures comes from studies of congenitally deaf or early-deafened animal models. Fortunately, the mammalian auditory system shows a high degree of preservation among species, allowing for generalization from these models to the human auditory system. This review begins with a comparison of common methods used to obtain deaf animal models, highlighting the specific advantages and anatomical consequences of each. Some consideration is also given to the effectiveness of methods used to measure hearing loss during and following deafening procedures. The structural and functional consequences of congenital and early-onset deafness have been examined across a variety of mammals. This review attempts to summarize these changes, which often involve alteration of hair cells and supporting cells in the cochleae, and anatomical and physiological changes that extend through subcortical structures and into cortex. The nature of these changes is discussed, and the impacts to neural processing are addressed. Finally, long-term changes in cortical structures are discussed, with a focus on the presence or absence of cross-modal plasticity. In addition to being of interest to our understanding of multisensory processing, these changes also have important implications for the use of assistive devices such as cochlear implants.

Extracellular Ca2+ and Mg2+ modulate aminoglycoside blockade of mechanotransducer channel-mediated Ca2+ entry in zebrafish hair cells: an in vivo study with the SIET

Zebrafish lateral-line hair cells are an in vivo model for studying hair cell development, function, and ototoxicity. However, the molecular identification and properties of the mechanotransducer (MET) channel in hair cells are still controversial. In this study, a noninvasive electrophysiological method, the scanning ion-electrode technique (SIET), was applied for the first time to investigate properties of MET channels in intact zebrafish embryos. With the use of a Ca2+-selective microelectrode to deflect hair bundles and simultaneously record the Ca2+ flux, the inward Ca2+ flux was detected at stereocilia of hair cells in 2- to ∼4-day postfertilization embryos. Ca2+ influx was blocked by MET channel blockers (BAPTA, La3+, Gd3+, and curare). In addition, 10 μM aminoglycoside antibiotics (neomycin and gentamicin) were found to effectively block Ca2+ influx within 10 min. Elevating the external Ca2+ level (0.2–2 mM) neutralized the effects of neomycin and gentamicin. However, elevating the Mg2+ level up to 5 mM neutralized blockade by gentamicin but not by neomycin. This study demonstrated MET channel-mediated Ca2+ entry at hair cells and showed that the SIET to be a sensitive approach for functionally assaying MET channels in zebrafish.

Replication of TPMT and ABCC3 genetic variants highly associated with cisplatin-induced hearing loss in children

Cisplatin is a widely used chemotherapeutic agent for the treatment of solid tumors. A serious complication of cisplatin treatment is permanent hearing loss. The aim of this study was to replicate previous genetic findings in an independent cohort of 155 pediatric patients. Associations were replicated for genetic variants in TPMT (rs12201199, P = 0.0013, odds ratio (OR) 6.1) and ABCC3 (rs1051640, P = 0.036, OR 1.8). A predictive model combining variants in TPMT, ABCC3, and COMT with clinical variables (patient age, vincristine treatment, germ-cell tumor, and cranial irradiation) significantly improved the prediction of hearing-loss development as compared with using clinical risk factors alone (area under the curve (AUC) 0.786 vs. 0.708, P = 0.00048). The novel combination of genetic and clinical factors predicted the risk of hearing loss with a sensitivity of 50.3% and a specificity of 92.7%. These findings provide evidence to support the importance of TPMT, COMT, and ABCC3 in the prediction of cisplatin-induced hearing loss in children.

Mutations in ap1b1 cause mistargeting of the Na(+)/K(+)-ATPase pump in sensory hair cells

The hair cells of the inner ear are polarized epithelial cells with a specialized structure at the apical surface, the mechanosensitive hair bundle. Mechanotransduction occurs within the hair bundle, whereas synaptic transmission takes place at the basolateral membrane. The molecular basis of the development and maintenance of the apical and basal compartments in sensory hair cells is poorly understood. Here we describe auditory/vestibular mutants isolated from forward genetic screens in zebrafish with lesions in the adaptor protein 1 beta subunit 1 (ap1b1) gene. Ap1b1 is a subunit of the adaptor complex AP-1, which has been implicated in the targeting of basolateral membrane proteins. In ap1b1 mutants we observed that although the overall development of the inner ear and lateral-line organ appeared normal, the sensory epithelium showed progressive signs of degeneration. Mechanically-evoked calcium transients were reduced in mutant hair cells, indicating that mechanotransduction was also compromised. To gain insight into the cellular and molecular defects in ap1b1 mutants, we examined the localization of basolateral membrane proteins in hair cells. We observed that the Na(+)/K(+)-ATPase pump (NKA) was less abundant in the basolateral membrane and was mislocalized to apical bundles in ap1b1 mutant hair cells. Accordingly, intracellular Na(+) levels were increased in ap1b1 mutant hair cells. Our results suggest that Ap1b1 is essential for maintaining integrity and ion homeostasis in hair cells.

Mechanisms of aminoglycoside ototoxicity and targets of hair cell protection

Aminoglycosides are commonly prescribed antibiotics with deleterious side effects to the inner ear. Due to their popular application as a result of their potent antimicrobial activities, many efforts have been undertaken to prevent aminoglycoside ototoxicity. Over the years, understanding of the antimicrobial as well as ototoxic mechanisms of aminoglycosides has increased. These mechanisms are reviewed in regard to established and potential future targets of hair cell protection.

Medical and noninvasive therapy for Meniere's disease

Nonoperative therapy continues to be the mainstay of treatment of patients suffering from Meniere disease. Despite extensive research, the exact pathogenesis of Meniere disease remains elusive. The poorly understood nature of this condition has made it nearly impossible to develop treatments that are curative. Most modern treatments are aimed at controlling symptoms. This article reviews the various nonoperative treatments that have been used to treat Meniere disease historically as well as outlining the authors' clinical treatment paradigm.

Rapid hair cell loss: a mouse model for cochlear lesions

In comparison to other mammals, mice have proved extremely resistant to aminoglycoside-induced hair cell ablation in vivo. In this paper we examine the pattern and extent of cochlear lesions rapidly induced with a combination of a single dose of aminoglycoside (kanamycin) followed by a loop diuretic (bumetanide). With this protocol, the vestibular system was unaffected, but in the cochlea, there was extensive loss of outer hair cells (OHC) that commenced in the basal coil and progressed apically so that, by 48 h, OHC loss was almost complete. TUNEL-positive nuclei and activated caspase-3 labeling demonstrated that most OHC died via a classical apoptotic pathway. However, scattered debris within the OHC region suggested that many apoptotic cells ruptured prior to completion of apoptosis. Following lesion repair, supporting cells retained characteristics of differentiated cells but positional shift occurred. In comparison to OHC loss, inner hair cell (IHC) death was delayed and only observed in 50% of all cochleae examined even after extensive reorganization of the tissue. The coadmininstration of diuretic with FM1-43, used as a tracer for aminoglycoside uptake, indicated entry into IHC as readily as OHC, suggesting that the differential response to aminoglycoside was not due to differential uptake. Where IHC death was ongoing, there were indications of different modes of cell death: cells with morphological features of autophagy, necrosis, and apoptosis were apparent. In addition to damage to the organ of Corti, there was a significant and progressive decrease in strial thickness beginning as early as 7 days posttreatment. This was due predominantly to degeneration of marginal cells. The strial pathology resembled that reported after noise damage and with aging. This in vivo protocol provides a robust model in which to obtain extensive OHC loss in the mature cochleae of mice and is a means with which to examine different aspects of cochlear pathology in transgenic or mutant strains.

Synthesis of paromomycin derivatives modified at C(5″) to selectively target bacterial rRNA

The furanosyl moiety (ring III) of C(6')-deoxyparomomycin and paromomycin was modified in search of aminoglycoside antibiotics with altered selectivity. The key intermediates were the N-Boc-protected derivative of C(6')-deoxyparomomycin and the benzylidene-protected paromomycin. Their H(2)C(5'')-OH group was oxidised with trichlorocyanuric acid or [bis(acetoxy)iodo]benzene in the presence of catalytic amounts of TEMPO to yield the corresponding aldehydes and acids, which were transformed into the protected alkoxy imines, amides and the amine. Standard deprotection gave the title compounds derived from C(6')-deoxyparomomycin and derived from paromomycin that proved less active than paromomycin and its C(6')-deoxy analogue.

Hair cell mechanotransduction: the dynamic interplay between structure and function

Hair cells are capable of detecting mechanical vibrations of molecular dimensions at frequencies in the 10s to 100s of kHz. This remarkable feat is accomplished by the interplay of mechanically gated ion channels located near the top of a complex and dynamic sensory hair bundle. The hair bundle is composed of a series of actin-filled stereocilia that has both active and passive mechanical components as well as a highly active turnover process, whereby the components of the hair bundle are rapidly and continually recycled. Hair bundle mechanical properties have significant impact on the gating of the mechanically activated channels, and delineating between attributes intrinsic to the ion channel and those imposed by the channel's microenvironment is often difficult. This chapter describes what is known and accepted regarding hair-cell mechanotransduction and what remains to be explored, particularly, in relation to the interplay between hair bundle properties and mechanotransducer channel response. The interplay between hair bundle dynamics and mechanotransduction are discussed.