Quantitative dSTORM super-resolution microscopy localizes Aurora kinase A/AURKA in the mitochondrial matrix

Revealing mitochondrial architecture and functions with single molecule localization microscopy

Understanding the spatiotemporal organization of components within living systems requires the highest resolution possible. Microscopy approaches that allow for a resolution below 250 nm include electron and super-resolution microscopy (SRM). The latter combines advanced imaging techniques and the optimization of image processing methods. Over the last two decades, various SRM-related approaches have been introduced, especially those relying on single molecule localization microscopy (SMLM). To develop and apply SMLM approaches, mitochondria are an ideal cellular compartment due to their size, which is below the standard diffraction limit. Furthermore, mitochondria are a dynamic yet narrow compartment, and a resolution below 250 nm is required to study their composition and multifaceted functions. To this end, several SMLM technologies have been used to reveal mitochondrial composition. However, there is still room for improvement in existing techniques to study protein-protein interactions and protein dynamics within this compartment. This review aims to offer an updated overview of the existing SMLM techniques and probes associated with mitochondria to enhance their resolution at the nanoscale. Last, it paves the way for future SMLM improvements to better resolve mitochondrial dynamics and functions.

Visualizing VDAC1 in live cells using a tetracysteine tag

The voltage-dependent anion channel 1 (VDAC1) is a crucial gatekeeper in the outer mitochondrial membrane, controlling metabolic and energy homeostasis. The available methodological approaches fell short of accurate visualization of VDAC1 in living cells. To permit precise VDAC1 imaging, we utilized the tetracysteine (TC)-tag and visualized VDAC1 dynamics in living cells. TC-tagged VDAC1 had a cluster-like distribution on mitochondria. The labeling of TC-tagged VDAC1 was validated with immunofluorescence. The majority of VDAC1-TC-clusters were localized at endoplasmic reticulum (ER)-mitochondria contact sites. Notably, VDAC1 colocalized with BCL-2 Antagonist/Killer (BAK)-clusters upon apoptotic stimulation. Using this new tool, we were able to observe VDAC1-TC at mitochondrial fission sites. These findings highlight the suitability of the TC-tag for live-cell imaging of VDAC1, shedding light on the roles of VDAC1 in cellular processes.

Cristae shaping and dynamics in mitochondrial function

Mitochondria are multifunctional organelles of key importance for cell homeostasis. The outer mitochondrial membrane (OMM) envelops the organelle, and the inner mitochondrial membrane (IMM) is folded into invaginations called cristae. As cristae composition and functions depend on the cell type and stress conditions, they recently started to be considered as a dynamic compartment. A number of proteins are known to play a role in cristae architecture, such as OPA1, MIC60, LETM1, the prohibitin (PHB) complex and the F1FO ATP synthase. Furthermore, phospholipids are involved in the maintenance of cristae ultrastructure and dynamics. The use of new technologies, including super-resolution microscopy to visualize cristae dynamics with superior spatiotemporal resolution, as well as high-content techniques and datasets have not only allowed the identification of new cristae proteins but also helped to explore cristae plasticity. However, a number of open questions remain in the field, such as whether cristae-resident proteins are capable of changing localization within mitochondria, or whether mitochondrial proteins can exit mitochondria through export. In this Review, we present the current view on cristae morphology, stability and composition, and address important outstanding issues that might pave the way to future discoveries.