Altered outer hair cell mitochondrial and subsurface cisternae connectomics are candidate mechanisms for hearing-loss in mice

Organelle crosstalk is vital for cellular functions. The propinquity of mitochondria, endoplasmic reticulum (ER), and plasma membrane promote regulation of multiple functions, which include intracellular Ca2+ flux, and cellular biogenesis. Although the purposes of apposing mitochondria and ER have been described, an understanding of altered organelle connectomics related to disease states is emerging. Since inner ear outer hair cell (OHC) degeneration is a common trait of age-related hearing loss, the objective of this study was to investigate whether the structural and functional coupling of mitochondria with subsurface cisternae (SSC), was affected by aging. We applied functional and structural probes to equal numbers of male and female mice with a hearing phenotype akin to human aging. We discovered the polarization of cristae and crista junctions in mitochondria tethered to the SSC in OHCs. Aging was associated with SSC stress and decoupling of mitochondria with the SSC, mitochondrial fission/fusion imbalance, a remarkable reduction in mitochondrial and cytoplasmic Ca2+ levels, reduced K+-induced Ca2+ uptake, and marked plasticity of cristae membranes. A model of structure-based ATP production predicts profound energy stress in older OHCs. This report provides data suggesting that altered membrane organelle connectomics may result in progressive hearing loss. SIGNIFICANCE STATEMENT We address the question, “Do aged OHCs exhibit detectable changes in organelle connectomics that would help us better understand human hearing loss in a relevant mouse model?” Because of the close association of mitochondria SSC over much of the OHC inner surface, mitochondria-SSC connectomics appears to play a central role in hearing. In polarized cells such as OHCs, where there is functional segregation of apical versus basal regions, the relationship between altered organelle connectomics and hearing loss is unknown. We propose a mechanism of mitochondria-SSC dysregulation related to aging and OHC degeneration, showing distinct altered mitochondrial and cytoplasmic Ca2+ regulation, mitochondrial polarization, and fission/fusion imbalance, mitochondrial-SSC decoupling, and SSC and cellular energy stress.