Functionally and morphologically damaged mitochondria observed in auditory cells under senescence-inducing stress

Abstract

We aimed at determining the mitochondrial function in premature senescence model of auditory cells. Short exposure to H ₂O ₂ (1 h, 0.1 mM) induced premature cellular senescence in House Ear Institute-Organ of Corti 1 auditory cells. The transmission electron microscopy analysis revealed that damaged mitochondria and autophagosomes containing dense organelles appeared in the auditory cells after short exposure to H ₂O ₂. The branch and junction parameters of the skeletonized image of the mitochondria were found to decrease significantly in H ₂O ₂-treated cells. A branched reticulum of tubules was poorly formed, featuring coexistence of numerous tiny clusters along with few relatively large entities in the H ₂O ₂-treated cells. In terms of bioenergetics, H ₂O ₂-treatment led to the dose-dependent decrease in mitochondrial membrane potential in the auditory cells. The fragmented mitochondria (fusion < fission) were in a low potential. In addition, the potential of hyperfused mitochondria (fusion > fission) was slightly lower than the control cells. The short-time exposure of live auditory cells to H ₂O ₂ damaged the mitochondrial respiratory capacity without any effect on the baseline ATP production rates. The vulnerability of the mitochondrial membrane potential to the uncoupling reagent was increased after H ₂O ₂ treatment. Our findings indicated that the mitochondrial dysfunction due to the decline in the O ₂ consumption rate should be the first event of premature senescence process in the auditory cells, resulting in the imbalance of mitochondrial fusion/fission and the collapse of the mitochondrial network.

Auditory system senescence: Mitochondrial dynamics and respiration

The mitochondrial morphology and physiology could influence the process of age-related hearing loss. Prof. Tatsuya Yamasoba’s research group at the University of Tokyo has examined the functional changes of mitochondria in terms of its respiratory function, membrane potential and morphology under premature senescence induced by oxidative stress in an auditory cell line. The morphological and functional mitochondrial damage were observed as the respiratory capacity deficiency and the fluctuation of the fusion/fission balance. Their results provide evidence of the fundamental interdependence between mitochondrial metabolic activity and its network structure in premature senescence process of auditory cells. This is a pioneer study to indicate the influence of mitochondrial dynamics and respiratory system on the premature senescence process of auditory cells. Further studies into inter cellular communication including cytoskeleton and nucleus can help us understand the etiology underlying age-related hearing loss.