| Summary: | Cells require adenosine triphosphate (ATP) to drive the myriad processes associated with growth, replication, and homeostasis. Eukaryotic cells rely on mitochondria to produce the vast majority of their ATP. Mitochondria consist of a relatively smooth outer mitochondrial membrane (OMM) and a highly complex inner mitochondrial membrane (IMM), containing numerous invaginations, called cristae, which house the molecular machinery of oxidative phosphorylation (OXPHOS). Although mitochondrial form and function are intimately connected, limitations in the resolution of live-cell imaging have hindered the ability to directly visualize the relationship between the architecture of the IMM and its associated bioenergetic properties. Using advanced imaging technologies, including Airyscan, stimulated emission depletion (STED), and structured illumination microscopy (SIM), we developed an approach to image the IMM in living cells. Staining mitochondria with various ΔΨm-dependent dyes, we found that the fluorescence pattern along the IMM was heterogeneous, with cristae possessing a significantly greater fluorescence intensity than the contiguous inner boundary membrane (IBM). Applying the Nernst equation, we determined that the ΔΨm of cristae is approximately 12 mV stronger than that of IBM, indicating that the electrochemical gradient that drives ATP synthesis is compartmentalized in cristae membranes. Notably, deletion of key components of the mitochondrial contact site and cristae organizing system (MICOS), as well as OPA1, which regulate crista junctions (CJs), decreased ΔΨm heterogeneity. Complementing our super-resolution imaging of cristae in living cells, we also developed a machine-learning protocol to quantify IMM architecture. Tracking real-time changes in cristae density, size, and shape, we determined that cristae dynamically remodel on a scale of seconds. Furthermore, we found that cristae move away from sites of mitochondrial fission, and, prior to mitochondrial fusion, the IMM forms finger-like protrusions bridging the membranes of the fusing organelles. Lastly, we investigated the role of the motor adaptor protein, Milton1/TRAK1, in mitochondrial dynamics. Patient-derived Milton1-null fibroblasts not only had impaired mitochondrial motility but exhibited fragmentation corresponding to a roughly 40% decrease in mitochondrial aspect ratio and a 17% increase in circularity, associated with increased DRP1 activity. Conversely, we found that overexpression of Milton1 led to mitochondrial hyperfusion, decreased DRP1 activity, and aberrant clustering of mtDNA. Overall, our studies directly demonstrate that maintaining mitochondrial architecture is essential for preserving the functionality of mitochondria, the hubs of eukaryotic metabolism.
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