UC Irvine

Analysis of mitochondrial structure and function is increasingly being recognized as central to understanding human health and disease. Mitochondria ultrastructure has been extensively characterized with transmission electron microscopy (TEM) and CryoTEM in fixed cells. Yet mitochondria within tissue cells can have markedly different structures and functions. Only through imaging functional, intact mitochondria can one ascertain information about the electrophysiology of the organelle. Hence, there is a critical need to be able to characterize the structure and function of the individual isolated mitochondrion.Our work aims to delve into procedures for characterizing the mitochondrial structure and function of isolated, functional mitochondria using super-resolution microscopy. This revealed that we can use super-resolution quantification of mitochondrial membrane potential using lipophilic cationic dye fluorescence to characterize the respiratory function of individual isolated, functional mitochondria in a way not possible in whole cells, potentially permitting elucidation of differences between individual mitochondria. Finally, by careful analysis of structure dyes and voltage dyes (lipophilic cations), we demonstrate that most of the fluorescent signal seen from voltage dyes is due to membrane-bound dyes, and develop a model for the membrane potential dependence of the fluorescence contrast for the case of super-resolution imaging, and how it relates to membrane potential. This quantitative voltage-dependent membrane binding model explains why super-resolution images of lipophilic cationic dyes show strong intensity near the cristae and not in the matrix. This model enables quantitative imaging of super-resolution voltages inside functional, intact mitochondria with super-resolution.