Calcium Regulation in Cochlear Cells - PROJECT SUMMARY Acquired hearing loss (AHL) is the third most common health condition affecting the aging population, behind heart disease and arthritis. There is no cure for AHL, and our knowledge of the underlying mechanisms of AHL is not sufficient to develop robust treatment strategies to mitigate it. Cochlear hair cells are rich in mitochondria and are known to be prone to irreversible damage and cell death in AHL. Predictably, evidence implicates that mitochondrial dysfunction is a lead cause of several forms of AHL. Like in many other cell types, mitochondria in the cochlear hair cells are responsible for vital cellular functions, including energy production, apoptosis, cell signaling, and Ca2+ storage. These processes are dependent on the ability of mitochondria to modulate Ca2+ levels. Particularly, Ca2+ uptake via mitochondrial calcium uniporter (MCU) is critical for rapidly buffering significant increases in intracellular Ca2+ loads. Ca2+ signaling and handling by mitochondria are essential, especially in cochlear hair cells. For instance, outer hair cells are prone to Ca2+ overload, particularly in the high-frequency basal cochlear turn. We hypothesize that acoustic overexposure leads to mitochondrial Ca2+ overload in cochlear hair cells, contributing to their vulnerability. In Aim 1, we will determine initial changes in function and morphology of cochlear hair cells following overstimulation and compare the results to ones obtained from hearing-impaired Micu1–/– mice, a model for mitochondrial Ca2+ overload. Micu1–/– mice are prone to Ca2+ overload and develop high-frequency hearing loss starting at 3 months of age. Using deep learning-based analysis (DLA), we will assess cochlear hair cell morphology and cellular structures. We will quantify and closely evaluate the morphology of mitochondria at the serial electron microscopy level using FIB SEM and 3D reconstruction. We will also measure distortion product otoacoustic emissions, endocochlear potential, synaptic ribbon counts, and spiral ganglion neuron (SGN) counts on histological sections (using DLA). Using similar methods, within Aim 2, we will determine whether preventing mitochondrial Ca2+ overload protects against acoustic overstimulation and rescues hearing deficit in mice. We will assess whether EMRE-deficient mice, which lack rapid mitochondrial calcium uptake, are less susceptible to cochlear insults, such as noise exposure and ototoxicity. Similarly, we will test Mcufl/fl; Gfi1Cre+ and Mcufl/fl; NeuroD1Cre+ mice, which lack MCU in hair cells and spiral ganglion neurons correspondingly. In addition, we will evaluate whether reducing EMRE expression in Micu1–/– mice, a model of mitochondrial Ca2+ overload, alleviates hearing loss by assessing Micu1–/–; Emre+/– double mutant mice. As it was reported, deleting one allele of Emre normalizes mitochondrial Ca2+ uptake in Micu1–/– mice. We hypothesize that Micu1–/–; Emre+/– mice will exhibit improved hearing preservation in comparison to Micu1–/– mice. Linking the damaging effects of acoustic overstimulation to mitochondrial Ca2+ homeostasis may provide a mechanistic understanding of the process and identify novel clinical strategies to ameliorate noise-induced hearing loss.
