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Browsing by Subject "calcium homeostasis"

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  • Leinonen, Saija (2023)
    The sense of hearing is dependent on the sensory cells of the cochlea: inner and outer hair cells. The critical functional structure of these cells is the stereocilia bundle, the mechanotransduction (MET) organelle. The outer hair cells (OHCs) are highly vulnerable to environmental assaults, the effects of aging, and gene mutations. This vulnerability is thought to be mediated by challenges in maintaining intracellular calcium homeostasis. Endoplasmic reticulum (ER) stress is a form of cellular stress that was previously shown to cause hair cell impairment. A possibility is that this impairment is mediated by perturbations in calcium homeostasis. In this thesis, the aim was to find out how the OHC calcium homeostasis is affected by specific ER stress-inducing mutations and age in mouse models exhibiting progressive hearing loss. I studied OHC calcium homeostasis in Manf conditional knock-out (cKO) mice under C57BL/6 (B6) strain in which ER homeostasis-promoting protein MANF (Mesencephalic Astrocyte-derived Neurotrophic Factor) is depleted in cochlear hair cells. Previous studies have shown that these mice develop progressive hearing loss that includes OHC loss and prominent stereocilia pathology, stereocilia fusion. By itself, the B6 mouse strain is a model of age-related hearing loss due to a Cdh23 missense mutation that is known to cause progressive hearing loss and, according to earlier evidence, may be a source of ER stress. I studied B6 mice at 6–9 months of age and Manf cKO mice at 2 months of age to comparatively examine changes to OHC calcium homeostasis that may correlate with the changes in the stereocilia bundle morphology and with hearing loss. I studied hearing function by auditory brainstem recordings in vivo. I estimated the functionality of MET channels in OHCs by FM1-43 uptake. I used immunofluorescence microscopy to study subcellular expression of key calcium-buffering and calcium-extrusion proteins in OHCs. I used a novel super-resolution imaging technique, expansion microscopy (ExM), to study stereocilia bundle morphology. OHCs of Manf cKO mice showed changes in calcium homeostasis in all the studied aspects: (1) FM1-43 uptake through MET channels was reduced, (2) the expression of the calcium extrusion pump PMCA2 and its obligate partner, the cell adhesion protein neuroplastin, was decreased, (3) and the expression of calcium-buffering protein oncomodulin was increased. All this data indicated OHC calcium dyshomeostasis. These molecular changes were consistent with the hair bundle pathology—stereocilia fusion—revealed by phalloidin staining of the actin-rich core of the stereocilia followed by ExM imaging. OHCs of 6–9-month-old B6 mice exhibited reduced FM1-43 uptake, yet not to the extent seen in cKO mice, and there were no changes in PMCA2 and neuroplastin expression and no prominent stereocilia fusion. Together, I show in this study that OHC hair bundle dysmorphology is linked with changes in calcium homeostasis in the mouse model of ER stress-induced hearing loss. This is consistent with the fact that calcium dyshomeostasis is an integral part of cellular ER stress. An intriguing, yet unanswered question is whether these changes in stereocilia bundle physiology could actually be the trigger for the death of these sensory cells.