Suppressors of Superoxide-H2O2 Production at Site IQ of Mitochondrial Complex I Protect against Stem Cell Hyperplasia and Ischemia-Reperfusion Injury.

Using high-throughput screening we identified small molecules that suppress superoxide and/or H2O2 production during reverse electron transport through mitochondrial respiratory complex I (site IQ) without affecting oxidative phosphorylation (suppressors of site IQ electron leak, "S1QELs"). S1QELs diminished endogenous oxidative damage in primary astrocytes cultured at ambient or low oxygen tension, showing that site IQ is a normal contributor to mitochondrial superoxide-H2O2 production in cells. They diminished stem cell hyperplasia in Drosophila intestine in vivo and caspase activation in a cardiomyocyte cell model driven by endoplasmic reticulum stress, showing that superoxide-H2O2 production by site IQ is involved in cellular stress signaling. They protected against ischemia-reperfusion injury in perfused mouse heart, showing directly that superoxide-H2O2 production by site IQ is a major contributor to this pathology. S1QELs are tools for assessing the contribution of site IQ to cell physiology and pathology and have great potential as therapeutic leads.

Suppressors of superoxide production from mitochondrial complex III.

Mitochondrial electron transport drives ATP synthesis but also generates reactive oxygen species, which are both cellular signals and damaging oxidants. Superoxide production by respiratory complex III is implicated in diverse signaling events and pathologies, but its role remains controversial. Using high-throughput screening, we identified compounds that selectively eliminate superoxide production by complex III without altering oxidative phosphorylation; they modulate retrograde signaling including cellular responses to hypoxic and oxidative stress.

Mitochondrial fusion in vitro.

The field of mitochondrial dynamics has received a great deal of attention as a result of a number of studies linking mitochondrial fission and fusion machinery to apoptosis. Specifically, elevated levels of mitochondrial fission or compromised mitochondrial fusion can sensitize cells to apoptotic stimuli. Conversely, stimulation of mitochondrial fusion can render cells resistant to apoptotic stimuli. In addition, the machinery involved in fission and fusion has been spatially linked to Bax, a pro-apoptotic protein. However, the mechanistic implications of interactions between the machinery of mitochondrial fission and fusion and apoptotic effectors are largely unknown. Our understanding of the pathways of mitochondrial fission and fusion have come from genetic studies coupled with direct observation of both fission and fusion components and mitochondrial organelle morphology and behavior in vivo in Saccharomyces cerevisiae. These approaches have identified the key players in both mitochondrial fission and fusion and have generated good models for their roles in mitochondrial dynamics. However, the lack of in vitro systems for studying these processes has impeded a deeper investigation of the mechanism. We have recapitulated the process of mitochondrial fusion in vitro (5). Using this in vitro fusion assay, we have separated outer mitochondrial membrane fusion from inner and identified the mechanistic requirements for each step.

How mitochondria fuse.

Mitochondrial fusion is unique; no paradigm exists to explain how two sets of compositionally distinct membranes become coordinately fused. Genetic approaches coupled with in vivo observations of mitochondrial dynamics and morphology have identified the machinery involved in mitochondrial fusion but these approaches alone yield limited mechanistic insight. The recent recapitulation of mitochondrial fusion in vitro has allowed the fusion process to be dissected into two mechanistically distinct, resolvable steps: outer membrane fusion and inner membrane fusion. Outer membrane fusion requires homotypic trans interactions of the ancient dynamin-related GTPase Fzo1, the proton-gradient component of the inner membrane electrical potential, and low levels of GTP hydrolysis. Fusion of inner membranes requires the electrical component (Deltapsi) of the inner membrane electrical potential and elevated levels of GTP hydrolysis. Regulation of mitochondrial fusion is likely to involve transcript processing in mammalian cells as well as variation in the level of fusion proteins in a given cell; slight changes in the electrical potential of the inner membrane may also serve to fine-tune fusion rates. Mitochondrial fusion components also serve to protect cells against apoptosis through mechanisms that are largely unknown. Resolving the mechanism of mitochondrial fusion will provide insight into the role of fusion components in apoptosis.

Staying in aerobic shape: how the structural integrity of mitochondria and mitochondrial DNA is maintained.

The structure and integrity of the mitochondrial compartment are features essential for it to function efficiently. The maintenance of mitochondrial structure in cells ranging from yeast to humans has been shown to require both ongoing fission and fusion. Recent characterization of many of the molecular components that direct mitochondrial fission and fusion events have led to a more complete understanding of how these processes take place. Further, mitochondrial fragmentation observed when cells undergo apoptosis requires mitochondrial fission, underlying the importance of mitochondrial dynamics in cellular homeostasis. Mitochondrial structure also impacts mitochondrial DNA inheritance. Recent studies suggest that faithful transmission of mitochondrial DNA to daughter cells might require a mitochondrial membrane tethering apparatus.