Separate fusion of outer and inner mitochondrial membranes

Tagging and tracking individual networks within a complex mitochondrial web with photoactivatable GFP

Assembly of mitochondria into networks supports fuel metabolism and calcium transport and is involved in the cellular response to apoptotic stimuli. A mitochondrial network is defined as a continuous matrix lumen whose boundaries limit molecular diffusion. Observation of individual networks has proven challenging in live cells that possess dense populations of mitochondria. Investigation into the electrical and morphological properties of mitochondrial networks has therefore not yielded consistent conclusions. In this study we used matrix-targeted, photoactivatable green fluorescent protein to tag single mitochondrial networks. This approach, coupled with real-time monitoring of mitochondrial membrane potential, permitted the examination of matrix lumen continuity and fusion and fission events over time. We found that adjacent and intertwined mitochondrial structures often represent a collection of distinct networks. We additionally found that all areas of a single network are invariably equipotential, suggesting that a heterogeneous pattern of membrane potential within a cell's mitochondria represents differences between discrete networks. Interestingly, fission events frequently occurred without any gross morphological changes and particularly without fragmentation. These events, which are invisible under standard confocal microscopy, redefine the mitochondrial network boundaries and result in electrically disconnected daughter units.

High resolution imaging of live mitochondria

Classically, mitochondria have been studied by biochemical, genetic and electron microscopic approaches. In the last two decades, it became evident that mitochondria are highly dynamic organelles that are frequently dividing and fusing, changing size and shape and traveling long distances throughout the life of a cell. The study of the complex structural changes of mitochondria in vivo became possible with the advent of fluorescent labeling techniques in combination with live cell imaging microscopy. This review aims to provide an overview on novel fluorescent markers that are used in combination with mitochondrial fusion assays and various live cell microscopy techniques to study mitochondrial dynamics. In particular, approaches to study the movement of mitochondrial proteins and novel imaging techniques (FRET imaging-, 4Pi- and STED-microscopy) that provide high spatial resolution are considered.

Mitochondrial dynamics and aging: Mitochondrial interaction preventing individuals from expression of respiratory deficiency caused by mutant mtDNA

In mammalian cells, there is an extensive and continuous exchange of mitochondrial DNA (mtDNA) and its products between mitochondria. This mitochondrial complementation prevents individuals from expression of respiration deficiency caused by mutant mtDNAs. Thus, the presence of mitochondrial complementation does not support the generally accepted mitochondrial theory of aging, which proposes that accumulation of somatic mutations in mtDNA is responsible for age-associated mitochondrial dysfunction. Moreover, the presence of mitochondrial complementation enables gene therapy for mitochondrial diseases using nuclear transplantation of zygotes.

Mitochondrial membrane dynamics, cristae remodelling and apoptosis

Mitochondria form a highly dynamic reticular network in living cells, and undergo continuous fusion/fission events and changes in ultrastructural architecture. Although significant progress has been made in elucidating the molecular events underlying these processes, their relevance to normal cell function remains largely unexplored. Emerging evidence, however, suggests an important role for mitochondrial dynamics in cellular apoptosis. The mitochondria is at the core of the intrinsic apoptosis pathway, and provides a reservoir for protein factors that induce caspase activation and chromosome fragmentation. Additionally, mitochondria modulate Ca2+ homeostasis and are a source of various metabolites, including reactive oxygen species, that have the potential to function as second messengers in response to apoptotic stimuli. One of the mitochondrial factors required for activation of caspases in most intrinsic apoptotic pathways, cytochrome c, is largely sequestered within the intracristae compartment, and must migrate into the boundary intermembrane space in order to allow passage across the outer membrane to the cytosol. Recent evidence argues that inner mitochondrial membrane dynamics regulate this process. Here, we review the contribution of mitochondrial dynamics to the intrinsic apoptosis pathway, with emphasis on the inner membrane.

Molecular mechanism of mitochondrial membrane fusion

Mitochondrial fusion requires coordinated fusion of the outer and inner membranes. This process leads to exchange of contents, controls the shape of mitochondria, and is important for mitochondrial function. Two types of mitochondrial GTPases are essential for mitochondrial fusion. On the outer membrane, the fuzzy onions/mitofusin proteins form complexes in trans that mediate homotypic physical interactions between adjacent mitochondria and are likely directly involved in outer membrane fusion. Associated with the inner membrane, the OPA1 dynamin-family GTPase maintains membrane structure and is a good candidate for mediating inner membrane fusion. In yeast, Ugo1p binds to both of these GTPases to form a fusion complex, although a related protein has yet to be found in mammals. An understanding of the molecular mechanism of fusion may have implications for Charcot-Marie-Tooth subtype 2A and autosomal dominant optic atrophy, neurodegenerative diseases caused by mutations in Mfn2 and OPA1.