Abstract
An increasing volume of data suggests that changes in cellular metabolism have a major impact on the health of tissues and organs, including in the auditory system where metabolic alterations are implicated in both age-related and noise-induced hearing loss. However, the difficulty of access and the complex cyto-architecture of the organ of Corti has made interrogating the individual metabolic states of the diverse cell types present a major challenge. Multiphoton fluorescence lifetime imaging microscopy (FLIM) allows label-free measurements of the biochemical status of the intrinsically fluorescent metabolic cofactors NADH and NADPH with subcellular spatial resolution. However, the interpretation of NAD(P)H FLIM measurements in terms of the metabolic state of the sample are not completely understood. We have used this technique to explore changes in metabolism associated with hearing onset and with acquired (age-related and noise-induced) hearing loss. We show that these conditions are associated with altered NAD(P)H fluorescence lifetimes, use a simple cell model to confirm an inverse relationship between τbound and oxidative stress, and propose such changes as a potential index of oxidative stress applicable to all mammalian cell types.
Highlights
An increasing volume of data suggests that changes in cellular metabolism have a major impact on the health of tissues and organs, including in the auditory system where metabolic alterations are implicated in both age-related and noise-induced hearing loss
We segmented the metabolic responses into the primary functional cells of the organ of Corti shown in Fig. 1A: sensory inner hair cells (IHCs), that convert the mechanical sound stimulus into a nerve signal, sensori-motor outer hair cells (OHCs), which play a role in amplifying this response, and the non-sensory inner and outer pillar cells (IPCs and OPCs) which play structural and trophic roles to support hair cell function[31,33]
In line with our previous work, we observed that values of τbound under control conditions were significantly (P = 10−19) longer in pillar cells than in hair cells, at 3.24(±0.07) ns in OPCs (57 cells across 4 repeats) and IPCs (52 cells) compared to 3.00(±0.10) ns and 3.12(±0.09) ns in OHCs (90 cells) and IHCs (33 cells) respectively
Summary
An increasing volume of data suggests that changes in cellular metabolism have a major impact on the health of tissues and organs, including in the auditory system where metabolic alterations are implicated in both age-related and noise-induced hearing loss. The interpretation of NAD(P)H FLIM measurements in terms of the metabolic state of the sample are not completely understood We have used this technique to explore changes in metabolism associated with hearing onset and with acquired (age-related and noise-induced) hearing loss. We have used fluorescence lifetime imaging microscopy (FLIM) to observe cell-specific metabolic responses in the reduced pool of the metabolic cofactor nicotinamide adenine dinucleotide (NADH) and its phosphorylated analogue NADPH in response to hearing development, maturation and its noise-induced and age-related loss. FLIM extends conventional laser scanning techniques to measure the rate of excited state depopulation at each pixel of an image This photophysical property, known as the fluorescence lifetime, is altered by changes to the nanoscale local environment of a molecule, making FLIM a highly sensitive probe of intracellular biochemistry when applied to the intrinsic fluorescence of living samples[12]. The present study is a direct extension of that work, applying NAD(P)H FLIM in the intact cochlea to investigate metabolic contributions to the pathophysiology of NIHL and ARHL
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