Hiromi Sesaki

Direct Membrane Association Drives Mitochondrial Fission by the Parkinson Disease-associated Protein α-Synuclein

The protein α-synuclein has a central role in Parkinson disease, but the mechanism by which it contributes to neural degeneration remains unknown. We now show that the expression of α-synuclein in mammalian cells, including neurons in vitro and in vivo, causes the fragmentation of mitochondria. The effect is specific for synuclein, with more fragmentation by α- than β- or γ-isoforms, and it is not accompanied by changes in the morphology of other organelles or in mitochondrial membrane potential. However, mitochondrial fragmentation is eventually followed by a decline in respiration and neuronal death. The fragmentation does not require the mitochondrial fission protein Drp1 and involves a direct interaction of synuclein with mitochondrial membranes. In vitro, synuclein fragments artificial membranes containing the mitochondrial lipid cardiolipin, and this effect is specific for the small oligomeric forms of synuclein. α-Synuclein thus exerts a primary and direct effect on the morphology of an organelle long implicated in the pathogenesis of Parkinson disease.

The dynamin-related GTPase Drp1 is required for embryonic and brain development in mice

1. 1. Wakabayashi, 2. et al . 2009. J. Cell Biol. doi:10.1083/jcb.200903065 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1083%252Fjcb.200903065%26rft_id%253Dinfo%253Apmid%252F19752021%26rft.genre%253Darticle%26rft_val_fmt%

Mitochondrial dynamics in neurodegeneration

It has been only 15 years since studies began on the molecular mechanisms underlying mitochondrial fission and fusion using simple model organisms such as Drosophila, yeast, and Caenorhabditis elegans. Beyond the primary functions of mitochondrial fission and fusion in controlling organelle shape, size, and number, it became clear that these dynamic processes are also critical to regulating cell death, mitophagy, and organelle distribution. Now, studies suggest that prominent changes occur in mitochondrial dynamics in a broad array of neurodegenerative diseases, and there is substantial evidence suggesting a key role in disease pathogenesis because neurons are among the most energy-consuming cell types and have a highly developed cell shape. Here, we review the recent findings on mitochondrial dynamics in neurodegeneration.

A mitochondrial origin for frontotemporal dementia and amyotrophic lateral sclerosis through CHCHD10 involvement

Mitochondrial DNA instability disorders are responsible for a large clinical spectrum, among which amyotrophic lateral sclerosis-like symptoms and frontotemporal dementia are extremely rare. We report a large family with a late-onset phenotype including motor neuron disease, cognitive decline resembling frontotemporal dementia, cerebellar ataxia and myopathy. In all patients, muscle biopsy showed ragged-red and cytochrome c oxidase-negative fibres with combined respiratory chain deficiency and abnormal assembly of complex V. The multiple mitochondrial DNA deletions found in skeletal muscle revealed a mitochondrial DNA instability disorder. Patient fibroblasts present with respiratory chain deficiency, mitochondrial ultrastructural alterations and fragmentation of the mitochondrial network. Interestingly, expression of matrix-targeted photoactivatable GFP showed that mitochondrial fusion was not inhibited in patient fibroblasts. Using whole-exome sequencing we identified a missense mutation (c.176C>T; p.Ser59Leu) in the CHCHD10 gene that encodes a coiled-coil helix coiled-coil helix protein, whose function is unknown. We show that CHCHD10 is a mitochondrial protein located in the intermembrane space and enriched at cristae junctions. Overexpression of a CHCHD10 mutant allele in HeLa cells led to fragmentation of the mitochondrial network and ultrastructural major abnormalities including loss, disorganization and dilatation of cristae. The observation of a frontotemporal dementia-amyotrophic lateral sclerosis phenotype in a mitochondrial disease led us to analyse CHCHD10 in a cohort of 21 families with pathologically proven frontotemporal dementia-amyotrophic lateral sclerosis. We identified the same missense p.Ser59Leu mutation in one of these families. This work opens a novel field to explore the pathogenesis of the frontotemporal dementia-amyotrophic lateral sclerosis clinical spectrum by showing that mitochondrial disease may be at the origin of some of these phenotypes.

Ugo1p links the Fzo1p and Mgm1p GTPases for mitochondrial fusion.

In yeast, mitochondrial fusion requires Ugo1p and two GTPases, Fzo1p and Mgm1p. Ugo1p is anchored in the mitochondrial outer membrane with its N terminus facing the cytosol and C terminus in the intermembrane space. Fzo1p is also an outer membrane protein, whereas Mgm1p is located in the intermembrane space. Recent studies suggest that these three proteins form protein complexes that mediate mitochondrial fusion. Here, we show that the cytoplasmic domain of Ugo1p directly interacts with Fzo1p, whereas its intermembrane space domain binds to Mgm1p. We identified the Ugo1p-binding site in Fzo1p and demonstrated that Ugo1p-Fzo1p interaction is essential for the formation of mitochondrial shape, maintenance of mitochondrial DNA, and fusion of mitochondria. Although the GTPase domains of Fzo1p and Mgm1p regulate mitochondrial fusion, they were not required for association with Ugo1p. Furthermore, we found that Ugo1p bridges the interaction between Fzo1p and Mgm1p in mitochondria. Our data indicate that distinct regions of Ugo1p bind directly to Fzo1p and Mgm1p and thereby link these two GTPases during mitochondrial fusion.

Parkin-independent mitophagy requires Drp1 and maintains the integrity of mammalian heart and brain.

Mitochondrial dynamics and mitophagy have been linked to cardiovascular and neurodegenerative diseases. Here, we demonstrate that the mitochondrial division dynamin Drp1 and the Parkinsons disease‐associated E3 ubiquitin ligase parkin synergistically maintain the integrity of mitochondrial structure and function in mouse heart and brain. Mice lacking cardiac Drp1 exhibited lethal heart defects. In Drp1KO cardiomyocytes, mitochondria increased their connectivity, accumulated ubiquitinated proteins, and decreased their respiration. In contrast to the current views of the role of parkin in ubiquitination of mitochondrial proteins, mitochondrial ubiquitination was independent of parkin in Drp1KO hearts, and simultaneous loss of Drp1 and parkin worsened cardiac defects. Drp1 and parkin also play synergistic roles in neuronal mitochondrial homeostasis and survival. Mitochondrial degradation was further decreased by combination of Drp1 and parkin deficiency, compared with their single loss. Thus, the physiological importance of parkin in mitochondrial homeostasis is revealed in the absence of mitochondrial division in mammals.

Defects in Mitochondrial Dynamics and Metabolomic Signatures of Evolving Energetic Stress in Mouse Models of Familial Alzheimer's Disease

Background The identification of early mechanisms underlying Alzheimers Disease (AD) and associated biomarkers could advance development of new therapies and improve monitoring and predicting of AD progression. Mitochondrial dysfunction has been suggested to underlie AD pathophysiology, however, no comprehensive study exists that evaluates the effect of different familial AD (FAD) mutations on mitochondrial function, dynamics, and brain energetics. Methods and Findings We characterized early mitochondrial dysfunction and metabolomic signatures of energetic stress in three commonly used transgenic mouse models of FAD. Assessment of mitochondrial motility, distribution, dynamics, morphology, and metabolomic profiling revealed the specific effect of each FAD mutation on the development of mitochondrial stress and dysfunction. Inhibition of mitochondrial trafficking was characteristic for embryonic neurons from mice expressing mutant human presenilin 1, PS1(M146L) and the double mutation of human amyloid precursor protein APP(Tg2576) and PS1(M146L) contributing to the increased susceptibility of neurons to excitotoxic cell death. Significant changes in mitochondrial morphology were detected in APP and APP/PS1 mice. All three FAD models demonstrated a loss of the integrity of synaptic mitochondria and energy production. Metabolomic profiling revealed mutation-specific changes in the levels of metabolites reflecting altered energy metabolism and mitochondrial dysfunction in brains of FAD mice. Metabolic biomarkers adequately reflected gender differences similar to that reported for AD patients and correlated well with the biomarkers currently used for diagnosis in humans. Conclusions Mutation-specific alterations in mitochondrial dynamics, morphology and function in FAD mice occurred prior to the onset of memory and neurological phenotype and before the formation of amyloid deposits. Metabolomic signatures of mitochondrial stress and altered energy metabolism indicated alterations in nucleotide, Krebs cycle, energy transfer, carbohydrate, neurotransmitter, and amino acid metabolic pathways. Mitochondrial dysfunction, therefore, is an underlying event in AD progression, and FAD mouse models provide valuable tools to study early molecular mechanisms implicated in AD.

Yeast Mitochondrial Dynamics: Fusion, Division, Segregation, and Shape

Mitochondria are essential organelles found in virtually all eukaryotic cells that play key roles in a variety of cellular processes. Mitochondria show a striking heterogeneity in their number, location, and shape in many different cell types. Although the dynamic nature of mitochondria has been known for decades, the molecules and mechanisms that mediate these processes are largely unknown. Recently, several laboratories have isolated and analyzed mutants in the yeast Saccharomyces cerevisiae defective in mitochondrial fusion and division, in the segregation of mitochondria to daughter cells, and in the establishment and maintenance of mitochondrial shape. These studies have identified several proteins that appear to mediate different aspects of mitochondrial morphogenesis. Although it is clear that many additional components have yet to be identified, some of the newly discovered proteins raise intriguing possibilities for how the processes of mitochondrial division, fusion, and segregation occur. Below we summarize our current understanding of the molecules known to be required for yeast mitochondrial dynamics. Microsc. Res. Tech. 51:573–583, 2000.

Cells lacking Pcp1p/Ugo2p, a rhomboid-like protease required for Mgm1p processing, lose mtDNA and mitochondrial structure in a Dnm1p-dependent manner, but remain competent for mitochondrial fusion.

The dynamin-related GTPase, Mgm1p, is critical for the fusion of the mitochondrial outer membrane, maintenance of mitochondrial DNA (mtDNA), formation of normal inner membrane structures, and inheritance of mitochondria. Although there are two forms of Mgm1p, 100 and 90 kDa, their respective functions and the mechanism by which these two forms are produced are not clear. We previously isolated ugo2 mutants in a genetic screen to identify components involved in mitochondrial fusion [J. Cell Biol. 152 (2001) 1123]. In this paper, we show that ugo2 mutants are defective in PCP1, a gene encoding a rhomboid-related serine protease. Cells lacking Pcp1p are defective in the processing of Mgm1p and produce only the larger (100 kDa) form of Mgm1p. Similar to mgm1delta cells, pcp1delta cells contain partially fragmented mitochondria, instead of the long tubular branched mitochondria of wild-type cells. In addition, pcp1delta cells, like mgm1delta cells, lack mtDNA and therefore are unable to grow on nonfermentable medium. Mutations in the catalytic domain lead to complete loss of Pcp1p function. Similar to mgm1delta cells, the fragmentation of mitochondria and loss of mtDNA of pcp1delta cells were rescued when mitochondrial division was blocked by inactivating Dnm1p, a dynamin-related GTPase. Surprisingly, in contrast to mgm1delta cells, which are completely defective in mitochondrial fusion, pcp1delta cells can fuse their mitochondria after yeast cell mating. Our study demonstrates that Pcp1p is required for the processing of Mgm1p and controls normal mitochondrial shape and mtDNA maintenance by producing the 90 kDa form of Mgm1p. However, the processing of Mgm1p is not strictly required for mitochondrial fusion, indicating that the 100 kDa form is sufficient to promote fusion.

Ups1p and Ups2p antagonistically regulate cardiolipin metabolism in mitochondria.

Cardiolipin, a unique phospholipid composed of four fatty acid chains, is located mainly in the mitochondrial inner membrane (IM). Cardiolipin is required for the integrity of several protein complexes in the IM, including the TIM23 translocase, a dynamic complex which mediates protein import into the mitochondria through interactions with the import motor presequence translocase–associated motor (PAM). In this study, we report that two homologous intermembrane space proteins, Ups1p and Ups2p, control cardiolipin metabolism and affect the assembly state of TIM23 and its association with PAM in an opposing manner. In ups1Δ mitochondria, cardiolipin levels were decreased, and the TIM23 translocase showed altered conformation and decreased association with PAM, leading to defects in mitochondrial protein import. Strikingly, loss of Ups2p restored normal cardiolipin levels and rescued TIM23 defects in ups1Δ mitochondria. Furthermore, we observed synthetic growth defects in ups mutants in combination with loss of Pam17p, which controls the integrity of PAM. Our findings provide a novel molecular mechanism for the regulation of cardiolipin metabolism.