Excessive activation of ionotropic glutamate receptors induces apoptotic hair-cell death independent of afferent and efferent innervation

Lavinia Sheets

doi:10.1038/srep41102

Abstract

Accumulation of excess glutamate plays a central role in eliciting the pathological events that follow intensely loud noise exposures and ischemia-reperfusion injury. Glutamate excitotoxicity has been characterized in cochlear nerve terminals, but much less is known about whether excess glutamate signaling also contributes to pathological changes in sensory hair cells. I therefore examined whether glutamate excitotoxicity damages hair cells in zebrafish larvae exposed to drugs that mimic excitotoxic trauma. Exposure to ionotropic glutamate receptor (iGluR) agonists, kainic acid (KA) or N-methyl-D-aspartate (NMDA), contributed to significant, progressive hair cell loss in zebrafish lateral-line organs. To examine whether hair-cell loss was a secondary effect of excitotoxic damage to innervating neurons, I exposed neurog1a morphants—fish whose hair-cell organs are devoid of afferent and efferent innervation—to KA or NMDA. Significant, dose-dependent hair-cell loss occurred in neurog1a morphants exposed to either agonist, and the loss was comparable to wild-type siblings. A survey of iGluR gene expression revealed AMPA-, Kainate-, and NMDA-type subunits are expressed in zebrafish hair cells. Finally, hair cells exposed to KA or NMDA appear to undergo apoptotic cell death. Cumulatively, these data reveal that excess glutamate signaling through iGluRs induces hair-cell death independent of damage to postsynaptic terminals.

Highlights

Glutamate excitotoxicity has been characterized in cochlear nerve terminals, but much less is known about whether excess glutamate signaling contributes to pathological changes in sensory hair cells
Analysis of ionotropic glutamate receptor (iGluR) expression in isolated hair cells populations subsequently revealed that, similar to what has been previously reported in mammalian systems, amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), Kainate and NMDA-type receptor subunits are expressed in zebrafish hair cells
There is an abundance of evidence that cochlear nerve fibers are damaged by exposure to iGluR agonists: previous studies have reported excitotoxic damage to cochlear nerve fibers akin to that brought about by noise overexposure in cochleae briefly treated with the agonist α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)[8,22] or the more potent excitotoxic agonist KA23–25

Summary

Introduction

Hair cells exposed to KA or NMDA appear to undergo apoptotic cell death These data reveal that excess glutamate signaling through iGluRs induces hair-cell death independent of damage to postsynaptic terminals. Whether excessive activation of iGluRs contributes to hair-cell damage has not been directly studied in a mammalian model system because it is difficult to discern whether hair-cell death in iGluR-agonist exposed cochleae is the result of damage to the hair cells themselves or collateral damage from injured postsynaptic nerve terminals[15]. Exposure to the iGluR agonists kainic acid (KA) or N-methyl-D-aspartate (NMDA) contributed to significant, progressive hair-cell loss is both wild-type larvae and in neurogenin1a morphants—fish that have morphologically mature hair cells devoid of afferent and efferent innervation. These data indicate that excessive signaling through iGluRs induces apoptotic hair-cell death, and suggests cell death may be instigated through iGluRs on the hair cells themselves

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Conclusion