The mitochondrion is a double-membrane organelle that includes an innermost aqueous compartment, the matrix. Within eukaryotic cells, mitochondria are linked together in highly dynamic networks. The morphology of these networks is regulated by a group of proteins residing in, or attached to, the inner and outer mitochondrial membranes. These proteins are closely related to mitochondrial function and implicated in pathologies associated with several disease conditions and aging (ref. 1). The current leading hypothesis is that the topology of the inner mitochondrial membrane (IMM), but not that of the entire mitochondria, is a regulated property of the mitochondrion. IMM invaginations, the cristae, greatly increase the surface area available for the oxidative phosphorylation system. Despite the importance of submitochondrial structures in mitochondrial function, very little is known about them and their dynamics in living cells, because the mitochondrial diameter is typically between 250 and 500 nm, close to the limit of resolution for light microscopy.
We recently used stimulated emission depletion (STED) super-resolution microscopy to investigate submitochondrial structure in living human cells (ref. 2). Our results provide intriguing details of the tetramethylrhodamine methyl ester (TMRM)-stained lamellar structure resembling the cristae. TMRM is a specific fluorescent dye for mitochondrial membrane potential. The 3D STED images revealed that TMRM-positive structures adopted a curtain or drapery formation closely resembling 3D electron microscopic images of cristae. Furthermore, time-lapse STED recordings documented quick morphological changes in TMRM-positive structures with coordinated mtDNA movements. Because changes in mitochondrial shape are accompanied by the formation or disintegration of cristae, we deduce that the highly dynamic behavior of the cristae is essential to the normal function of mitochondria.
1. Ohta S, Ohsawa I. Dysfunction of mitochondria and oxidative stress in the pathogenesis of Alzheimer's disease: On defects in the cytochrome c oxidase complex and aldehyde detoxification. J. Alzheimers Dis. 9:155-166, 2006.
2. Ishigaki M, Iketani M, Sugaya M, Takahashi M, Tanaka M, Hattori S, Ohsawa I. STED super-resolution imaging of mitochondria labeled with TMRM in living cells. Mitochondrion, 28:79-87, 2016.