Author: Erin A. Greguske

Publisher:

Published: 2019

Total Pages: 151

ISBN-13:

"Progressive ototoxicity of the inner ear is prevalent in patients administered aminoglycoside antibiotics with little understanding of how this damage occurs and to what extent it can be recovered. Numerous in vitro and in vivo studies using acute methods have been completed to demonstrate various types of damage in vestibular and auditory tissue, including hair cell damage that results in apoptosis or necrosis, excitotoxic damage, and/or degeneration of their afferents. However, progressive damage has only just recently been studied utilizing a sub-chronic exposure rat model; this model takes into account the progressive exposure mirrored in aminoglycoside administration that is not implied in acute experimentation. With this in mind, the sub-chronic exposure model was adapted for a new mouse model to characterize the progressive damage taking place in vestibular sensory epithelia and ganglia, along with a preliminary characterization in cochlear sensory epithelia. Mice were exposed to 30 mM IDPN (3,3'- iminodipropionitrile) in regular drinking water for 8 weeks, and monitored for vestibular deficits using an established test battery; auditory deficits were recorded using auditory brainstem response (ABR) measurements. Various techniques for identifying functional, histological (scanning/transmission electron microscopy; immunoconfocal), and molecular (mRNA; protein) data were utilized to study alterations in the vestibular and auditory tissues after sub-chronic intoxication. In the vestibular tissue, SEM/TEM imaging demonstrated progressive damage with the loss of calyceal junctions between type I hair cells and their calyx afferents, the fragmentation and retraction of the afferents, stereociliary bundle coalescence, and the unique mechanism of hair cell extrusion, where the cell is ejected from the epithelia into the endolymphatic cavity. Immunoconfocal and qRT-PCR data demonstrated a loss of caspr1 and tenascin-c in the calyceal junctions of type I hair cells and their afferents. A loss of active synapses between hair cells and their afferents was also noted, where active synapses were defined by the pre-synaptic ribeye of the hair cells and the post-synaptic GluA2 receptor of the afferents. Synaptic scaffolding protein expression was upregulated (PSD95, Homer1), which translated into an increase in the protein level (PSD95), likely for hair cell-afferent synapse stabilization and compensation. Progressive damage was noted to be at least partially or completely recoverable up until stereocilia coalescence of the hair cells. Finally, the expression of numerous scaffolding and signaling proteins were shown to be downregulated (qRT-PCR; RNAseq) during the exposure in the vestibular epithelium and ganglion, leading to the hypothesis of a depression in cell-cell adhesion between hair cells and their afferents and a depression in afferent signaling, resulting in an overall depressed system. In the cochlea, profound hearing loss was observed in a tonotopic pattern during the exposure; higher frequencies were affected first with longer exposure times affecting lower frequencies. Outer hair cells were lost tonotopically due to prolonged exposure, followed by active synapse loss of the inner hair cells. Those intoxicated for the first two weeks demonstrated a capacity for recovery before any outer hair cell or active synapse losses were seen. A sub-chronic ototoxic IDPN model demonstrates the progressive damage of the inner ear, allowing for the study of this damage and its potential for recoverability, gaining a clearer understanding of the mechanisms affecting the tissues." -- TDX.