Inner Ear Mitochondria

Abstract Body : The purpose of this research is to study mitochondrial structure in inner ear vestibular hair cells (HCs) and fibers. The main hypothesis of the study is that mitochondria are structurally heterogeneous, depending on HC type and location of mitochondria in proximity to other organelles within a HC, which may affect their function. One such structural difference is polarization of mitochondrial crista junctions (CJs), which are openings of the inner folds of the mitochondria into the intermembrane space, that may be functionally related to ATP production or transport.

Inner ear vestibular tissue obtained from Long-Evans rats under an approved UIC IACUC procedure was used for electron microscope tomography. Serial tomograms were joined using etomo (IMOD) and data were processed with the IMOD (v.3.13.6) software package (Kremer et al., 1996). Segmentation was performed with the program 3dmod (IMOD) to obtain the final models. Reconstructions were also visualized using 3dmod; this program allows stepping through slices of the reconstruction in any orientation (SLICER option) and tracking or modeling features of interest in any of 3 dimensions. Several drawing tools (Sculpt, Join, Interpolator) and a proximity analysis feature (Mtk) were also used.

Quantitative analyses were performed to test our hypothesis, including counts of total mitochondria within the two HC types, efferent and afferent boutons and calyces, counts of CJs on either side of a mitochondria in relation to other important cellular organelles (e.g., cuticular plate, striated organelle, ribbon synapses, and postsynaptic densities), analyzing polarization ratios (side toward vs side away) re: distances, bioenergetic calculations of ATP production, and a proximity analysis test (Mtk) using CJ randomization as a control. Preliminary results support the hypothesis of structural heterogeneity of mitochondria as CJs are polarized toward specific organelles within the cell and appear to be non-randomly distributed. In addition, three sizes of mitochondria were found depending on cell and afferent or efferent bouton type.

Our initial results suggest functional relevance for these structural differences. CJ polarization toward organelles that use energy may enable more efficient ATP transport to that organelle, e.g., the cuticular plate is an organelle that helps return stereocilia rootlets back to their original position. Likewise, synaptic ribbons send signals to the brain, another energy-requiring process. Our results indicate a non-random polarization of mitochondrial CJs toward these organelles. Thus, structural differences in mitochondria may provide key information to better understand their function and to address mitochondrial deafness and dizziness disorders.