Local mechanisms for loud sound-enhanced aminoglycoside entry into outer hair cells.

Hongzhe Li,David N Furness,Allan Kachelmeier,Peter S Steyger

doi:10.3389/fncel.2015.00130

Hongzhe Li, David N Furness + Show 2 more

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https://doi.org/10.3389/fncel.2015.00130

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Abstract

Loud sound exposure exacerbates aminoglycoside ototoxicity, increasing the risk of permanent hearing loss and degrading the quality of life in affected individuals. We previously reported that loud sound exposure induces temporary threshold shifts (TTS) and enhances uptake of aminoglycosides, like gentamicin, by cochlear outer hair cells (OHCs). Here, we explore mechanisms by which loud sound exposure and TTS could increase aminoglycoside uptake by OHCs that may underlie this form of ototoxic synergy. Mice were exposed to loud sound levels to induce TTS, and received fluorescently-tagged gentamicin (GTTR) for 30 min prior to fixation. The degree of TTS was assessed by comparing auditory brainstem responses (ABRs) before and after loud sound exposure. The number of tip links, which gate the GTTR-permeant mechanoelectrical transducer (MET) channels, was determined in OHC bundles, with or without exposure to loud sound, using scanning electron microscopy. We found wide-band noise (WBN) levels that induce TTS also enhance OHC uptake of GTTR compared to OHCs in control cochleae. In cochlear regions with TTS, the increase in OHC uptake of GTTR was significantly greater than in adjacent pillar cells. In control mice, we identified stereociliary tip links at ~50% of potential positions in OHC bundles. However, the number of OHC tip links was significantly reduced in mice that received WBN at levels capable of inducing TTS. These data suggest that GTTR uptake by OHCs during TTS occurs by increased permeation of surviving, mechanically-gated MET channels, and/or non-MET aminoglycoside-permeant channels activated following loud sound exposure. Loss of tip links would hyperpolarize hair cells and potentially increase drug uptake via aminoglycoside-permeant channels expressed by hair cells. The effect of TTS on aminoglycoside-permeant channel kinetics will shed new light on the mechanisms of loud sound-enhanced aminoglycoside uptake, and consequently on ototoxic synergy.

Highlights

Aminoglycoside antibiotics, like gentamicin and tobramycin, are clinically-essential antibiotics for treating life-threatening Gram-negative bacterial infections in premature babies or patients with cystic fibrosis, Gram-positive infections like tuberculosis, and protozoalLoud sound-enhanced aminoglycoside uptake infections (Durante-Mangoni et al, 2009; Zimmerman and Lahav, 2013)
Enhanced GTTR Uptake by outer hair cells (OHCs) in Cochlear Regions with threshold shifts (TTS) We systematically measured the degree of TTS induced by 3 different sound levels and assessed auditory brainstem responses (ABRs) threshold at multiple time points after sound exposure
Since serial ABR measurements are required to determine the degree of TTS, an independent set of mice was examined to determine GTTR uptake for each sound protocol

Summary

Introduction

Loud sound-enhanced aminoglycoside uptake infections (Durante-Mangoni et al, 2009; Zimmerman and Lahav, 2013) Despite their wide use, broad-spectrum bactericidal efficacy, and low cost, clinical dosing with aminoglycosides is limited by the risk of acute kidney damage and life-long hearing loss, with significant ramifications for quality of life (Forge and Schacht, 2000; Zimmerman and Lahav, 2013). Auditory sensory cells, outer hair cells (OHCs), are more susceptible to aminoglycosideinduced cytotoxicity than other cochlear cells, at the base of the cochlea most sensitive to high frequency sound. Once these sensory cells are lost, they cannot be regenerated, leading to life-long hearing loss and deafness. Other mechanisms by which aminoglycosides can enter hair cells include endocytosis (Hashino and Shero, 1995), and permeation through other aminoglycosidepermeant cation channels expressed by hair cells, such as TRPV4 on apical membranes (Karasawa et al, 2008), or TRPA1 on basolateral membranes, of OHCs (Stepanyan et al, 2011)

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