Cristae Remodeling Research Articles

What We're Reading

Caspase3 in apoptotic tumor cells after oncoviral therapy (from Tysome et al., CCR 2012)Blockade of the PD-1/PD-L1 interaction as a monotherapy helps a fraction of melanoma patients. This phase 1b clinical trial of 21 patients tested the addition of a local injection of oncolytic virus carrying GM-CSF to systemic treatment with pembrolizumab. This increased the release of antigen, altered the intratumoral environment, and increased CD8+ T-cell infiltration. The percentage of responders in this trial increased to 62%, with 33% showing a complete response after 72 months.Ribas A, …, Long GV. Cell 2017 Sep 7;170:1109–19.e10.GSK3 inhibition improves antitumor effects (from Cichocki et al.)Expanding highly functional NK cells ex vivo has been challenging. Inhibition of glycogen synthase kinase 3 (GSK3) in NK-cell cultures expanded with IL-15 was found to increase NK-cell maturation, cytokine production, and ADCC against tumors. When ovarian cancer in a xenograft model was treated with these NK cells, antitumor efficacy was better than with NK cells expanded without the GSK3 inhibitor.Cichocki F, …, Miller JS. Cancer Res 2017 Aug 8. doi: 10.1158/0008-5472.CAN-17-0799.Pancreatic adenocarcinoma (by E. Uthman via Flickr)Pancreatic ductal adenocarcinomas (PDAC) exclude immune infiltrates with fibrotic capsules supported by an immunosuppressive environment. The tumor-associated macrophages (TAMs) maintaining this state are derived from two sources: circulating monocytes and tissue-resident macrophages originating from embryonic progenitors. The resident macrophages proliferated and expanded with PDAC progression. Whereas monocyte-derived TAMs sample tumor antigens, and thus may impact T-cell responses, the tissue-resident macrophages distinctly promoted fibrosis and directly contributed to tumor progression.Zhu Y, …, DeNardo DG. Immunity 2017 Aug 15;47:323–38.Lymphatics (by OpenStax College via Wikimedia Commons)VEGF-C, which promotes lymphangiogenesis, promotes formation of an immuno-suppressive tumor environment, so the a priori assumption was that blocking it would improve responses to immunotherapies like vaccines. However, VEGF-C was found to increase CCL21 production from lymphatic endothelial cells, which attracts naïve T cells into mouse melanomas that can then be activated. Human melanoma patients with high serum VEGF-C expression were found to have longer progression-free survival.Fankhauser M, …, Swartz MA. Sci Transl Med 2017 Sep 13;9:eaaI4712.Binding for protection from degradation (by Soncabral from imgfave)Two groups, Burr et al. and Mezzadra et al., sought proteins that regulate the expression or function of PD-L1, which inhibits T-cell activation when bound to PD-1 on T cells. Mezzadra did an unbiased haploid screen of IFNγ-induced HAP1 cells and identified CMTM6 and CMTM4. Burr used a CRISPR/Cas9 screen and also identified CMTM6. CMTM6 binds to PD-L1 in the endosome and prevents its ubiquitination and degradation, thereby allowing persistent PD-L1 expression.Burr ML, …, Dawson MA. Nature 2017 Sep 7;549:101–5.Mezzadra R, …, Schumacher, TNM. Nature 2017 Sept 7;549:106–10.Mitochondria (by BruceBlaus via Wikimedia Commons)CD28 costimulation of T cells is necessary for T-cell activation and generation of memory T cells, but the mechanisms behind memory formation are unclear. CD28 induced the fatty acid oxidation pathway necessary for the mitochondrial adaptations, such as elongation and cristae remodeling. These changes are needed to support the reserve energy-generating capacity required for future rapid and effective memory T-cell recall responses.Klein Geltink RI, …, Pearce EL. Cell 2017 Sep 14. doi: 10.1016/j.cell.2017.08.018.

T-Cell Intracellular Antigens and Hu Antigen R Antagonistically Modulate Mitochondrial Activity and Dynamics by Regulating Optic Atrophy 1 Gene Expression.

Mitochondria undergo frequent morphological changes to control their function. We show here that T-cell intracellular antigens (TIA1b/TIARb) and Hu antigen R (HuR) have antagonistic roles in mitochondrial function by modulating the expression of mitochondrial shaping proteins. Expression of TIA1b/TIARb alters the mitochondrial dynamic network by enhancing fission and clustering, which is accompanied by a decrease in respiration. In contrast, HuR expression promotes fusion and cristae remodeling and increases respiratory activity. Mechanistically, TIA proteins downregulate the expression of optic atrophy 1 (OPA1) protein via switching of the splicing patterns of OPA1 to facilitate the production of OPA1 variant 5 (OPA1v5). Conversely, HuR enhances the expression of OPA1 mRNA isoforms through increasing steady-state levels and targeting translational efficiency at the 3' untranslated region. Knockdown of TIA1/TIAR or HuR partially reversed the expression profile of OPA1, whereas knockdown of OPA1 or overexpression of OPA1v5 provoked mitochondrial clustering. Middle-term expression of TIA1b/TIARb triggers reactive oxygen species production and mitochondrial DNA damage, which is accompanied by mitophagy, autophagy, and apoptosis. In contrast, HuR expression promotes mitochondrion-dependent cell proliferation. Collectively, these results provide molecular insights into the antagonistic functions of TIA1b/TIARb and HuR in mitochondrial activity dynamics and suggest that their balance might contribute to mitochondrial physiopathology.

Abstract 206: Mitofilin Knockdown Induces Cell Death by Apoptosis via an AIF-PARP-dependent Mechanism and Cell Cycle Arrest

Introduction: Mitochondrial activity plays an essential role in the efficient function of cardiomyocytes. Mitofilin is an inner membrane protein that has been defined as a mitochondria-shaping protein in controlling and maintaining mitochondrial cristae structure and remodeling. In this study, we determined the role of mitofilin in H9c2 and C2C12 myoblasts survival by investigating the mechanism underlying mitofilin knockdown-induced cell death by apoptosis. Methods: H9c2 and C2C12 cells were cultured and treated with mitofilin siRNA or scramble siRNA for 24 hours. Cell death (apoptosis), caspase 3 activity, and cell cycle phases were assessed by flow-cytometry, cytochrome C release and intracellular ATP production were measured by ELISA. Mitofilin, AIF and PARP expression was measured by Western blot analysis and calpain activity assessed using calpain kit. Mitochondria images were taken using electron microscopy. Results: We found that mitofilin knockdown increases cell death by apoptosis mainly via activation of AIF pathway leading to nuclear fragmentation and subsequent PARP1 activation in the nucleus that is correlated with S phase arrest of the cell cycle. Knockdown of mitofilin in H9c2 as well as C2C12 myoblasts with siRNA led to mitochondrial swelling and damage of mitochondrial cristae that is associated with the increase in ROS production, decrease in intracellular ATP production and a marked decrease in mitochondrial membrane potential. Moreover, cells treated with mitofilin siRNA displayed an increase in calpain activity versus scramble siRNA. Conclusion: Together, these results indicate that mitofilin knockdown by siRNA increases calpain activity that presumably leads to mitochondrial structural degradation resulting in a critical reduction of mitochondrial function that is responsible for the increase in cell death by apoptosis via AIF-PARP mechanism which is associated with nuclear fragmentation and S phase arrest of cell cycle.

Mitochondrial-Shaping Proteins in Cardiac Health and Disease \u2013 the Long and the Short of It!

Mitochondrial health is critically dependent on the ability of mitochondria to undergo changes in mitochondrial morphology, a process which is regulated by mitochondrial shaping proteins. Mitochondria undergo fission to generate fragmented discrete organelles, a process which is mediated by the mitochondrial fission proteins (Drp1, hFIS1, Mff and MiD49/51), and is required for cell division, and to remove damaged mitochondria by mitophagy. Mitochondria undergo fusion to form elongated interconnected networks, a process which is orchestrated by the mitochondrial fusion proteins (Mfn1, Mfn2 and OPA1), and which enables the replenishment of damaged mitochondrial DNA. In the adult heart, mitochondria are relatively static, are constrained in their movement, and are characteristically arranged into 3 distinct subpopulations based on their locality and function (subsarcolemmal, myofibrillar, and perinuclear). Although the mitochondria are arranged differently, emerging data supports a role for the mitochondrial shaping proteins in cardiac health and disease. Interestingly, in the adult heart, it appears that the pleiotropic effects of the mitochondrial fusion proteins, Mfn2 (endoplasmic reticulum-tethering, mitophagy) and OPA1 (cristae remodeling, regulation of apoptosis, and energy production) may play more important roles than their pro-fusion effects. In this review article, we provide an overview of the mitochondrial fusion and fission proteins in the adult heart, and highlight their roles as novel therapeutic targets for treating cardiac disease.

Control of Mitochondrial Remodeling by the ATPase Inhibitory Factor 1 Unveils a Pro-survival Relay via OPA1

The ubiquitously expressed ATPase inhibitory factor 1 (IF1) is a mitochondrial protein that blocks the reversal of the F1Fo-ATPsynthase, preventing dissipation of cellular ATP and ischemic damage. IF1 suppresses programmed cell death, enhancing tumor invasion and chemoresistance, and is expressed in various types of human cancers. In this study, we examined its effect on mitochondrial redox balanceand apoptotic cristae remodeling, finding that, by maintaining ATP levels, IF1 reduces glutathione (GSH) consumption and inactivation of peroxiredoxin 3 (Prx3) during apoptosis. This correlates with inhibition of metallopeptidase OMA1-mediated processing of the pro-fusion dynamin-related protein optic atrophy 1 (OPA1). Stabilization of OPA1 impedes cristae remodeling and completion of apoptosis. Taken together, these data suggest that IF1 acts on both mitochondrial bioenergetics and structure, is involved in mitochondrial signaling in tumor cells, and may underlie their proliferative capacity.

Control of Mitochondrial Structure and Antioxidant Response by the ATPase Inhibitory Factor 1 Define a Novel Potential Oncogenic Mechanism

The ATPase inhibitory factor 1 (IF1) is an ubiquitously expressed mitochondrial protein that blocks the reversal of the F1Fo-ATPsynthase, preventing dissipation of cellular ATP and ischaemic damage. Many human cancers express high levels of IF1, which suppress cell death and enhance tumour cell invasion and chemoresistance. In this study, we assessed the effect of IF1 over-expression on mitochondrial redox balance and apoptotic cristae remodelling. We found that IF1 maintains ATP levels under apoptosis and reduces glutathione (GSH) loss and inactivation of peroxiredoxin 3 (Prx3). This correlates with inhibition of the metallopeptidase OMA1-mediated processing of the pro-fusion dynamin related protein optic atrophy 1 (OPA1), impeding cristae remodelling and apoptosis completion. IF1 therefore has a pivotal antioxidant activity that hinders the OMA1/OPA1-dependent deconstruction of mitochondrial morphology and cellular demise. The data presented here highlight a dual regulatory activity of IF1 on both mitochondrial bioenergetics and structure, resulting in increased proliferative capacity of tumour cells and significantly contributing to the molecular signalling of mitochondria in cancer.

Optic Atrophy 1 Is Epistatic to the Core MICOS Component MIC60 in Mitochondrial Cristae Shape Control

SummaryThe mitochondrial contact site and cristae organizing system (MICOS) and Optic atrophy 1 (OPA1) control cristae shape, thus affecting mitochondrial function and apoptosis. Whether and how they physically and functionally interact is unclear. Here, we provide evidence that OPA1 is epistatic to MICOS in the regulation of cristae shape. Proteomic analysis identifies multiple MICOS components in native OPA1-containing high molecular weight complexes disrupted during cristae remodeling. MIC60, a core MICOS protein, physically interacts with OPA1, and together, they control cristae junction number and stability, OPA1 being epistatic to MIC60. OPA1 defines cristae width and junction diameter independently of MIC60. Our combination of proteomics, biochemistry, genetics, and electron tomography provides a unifying model for mammalian cristae biogenesis by OPA1 and MICOS.

Cristae remodeling causes acidification detected by integrated graphene sensor during mitochondrial outer membrane permeabilization.

The intrinsic apoptotic pathway and the resultant mitochondrial outer membrane permeabilization (MOMP) via BAK and BAX oligomerization, cytochrome c (cytc) release, and caspase activation are well studied, but their effect on cytosolic pH is poorly understood. Using isolated mitochondria, we show that MOMP results in acidification of the surrounding medium. BAK conformational changes associated with MOMP activate the OMA1 protease to cleave OPA1 resulting in remodeling of the cristae and release of the highly concentrated protons within the cristae invaginations. This was revealed by utilizing a nanomaterial graphene as an optically clear and ultrasensitive pH sensor that can measure ionic changes induced by tethered mitochondria. With this platform, we have found that activation of mitochondrial apoptosis is accompanied by a gradual drop in extra-mitochondrial pH and a decline in membrane potential, both of which can be rescued by adding exogenous cytc. These findings have importance for potential pharmacological manipulation of apoptosis, in the treatment of cancer.

Altered Mitochondrial Anatomy of Trigeminal Ganglia Neurons in Diabetes.

Neurons from sensory ganglia are exposed to oxidative attack in diabetes. Altered mitochondrial morphologies are due to impaired dynamics (fusion, fission) and to cristae remodeling. This study aimed to evaluate using transmission electron microscopy mitochondrial changes in diabetic trigeminal ganglia suggestive for ignition of apoptosis, in absence of "classical" morphological signs of apoptosis. We used samples of trigeminal ganglia (from six type 2 diabetes human donors and five streptozotocin (STZ)-induced diabetic rats). In human diabetic samples we found three main distributions of mitochondria: (a) small "dark" normal mitochondria, seemingly resulted from fission processes; (b) small "dark" damaged mitochondria, with side-vesiculations (single- and double-coated), large matrix vesicles and cytosolic leakage of reactive species, mixed with larger "light" mitochondria, swollen, and with crystolysis; (c) prevailing "light" mitochondria. In STZ-treated rats a type (c) distribution prevailed, except for nociceptive neurons where we found a different distribution: large and giant mitochondria, suggestive for impaired mitochondrial fission, mitochondrial fenestrations, matrix vesicles interconnected by lamellar cristae, and mitochondrial leakage into the cytosol. Thus, the ultrastructural pattern of mitochondria damage in diabetic samples of sensory neurons may provide clues on the initiation of intrinsic apoptosis, even if the classical morphological signs of apoptosis are not present. Further studies, combining use of biochemical and ultrastructural techniques, may allow a better quantification of the degree in which mitochondrial damage, with membrane alterations and cytosolic leaks, may be used as morphological signs suggesting the point-of-no return for apoptosis. Anat Rec, 299:1561-1570, 2016. © 2016 Wiley Periodicals, Inc.

In vitro and in vivo study of the role of the mitochondria - shaping protein opa1 in cancer

In vitro and in vivo study of the role of the mitochondria - shaping protein opa1 in cancer

DRP1-dependent apoptotic mitochondrial fission occurs independently of BAX, BAK and APAF1 to amplify cell death by BID and oxidative stress

During apoptosis mitochondria undergo cristae remodeling and fragmentation, but how the latter relates to outer membrane permeabilization and downstream caspase activation is unclear. Here we show that the mitochondrial fission protein Dynamin Related Protein (Drp) 1 participates in cytochrome c release by selected intrinsic death stimuli. While Bax, Bak double deficient (DKO) and Apaf1−/− mouse embryonic fibroblasts (MEFs) were less susceptible to apoptosis by Bcl-2 family member BID, H2O2, staurosporine and thapsigargin, Drp1−/− MEFs were protected only from BID and H2O2. Resistance to cell death of Drp1−/− and DKO MEFs correlated with blunted cytochrome c release, whereas mitochondrial fragmentation occurred in all cell lines in response to all tested stimuli, indicating that other mechanisms accounted for the reduced cytochrome c release. Indeed, cristae remodeling was reduced in Drp1−/− cells, potentially explaining their resistance to apoptosis. Our results indicate that caspase-independent mitochondrial fission and Drp1-dependent cristae remodeling amplify apoptosis. This article is part of a Special Issue entitled ‘EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2–6, 2016’, edited by Prof. Paolo Bernardi.

Drp1-dependent mitochondrial fission via MiD49/51 is essential for apoptotic cristae remodeling

Drp1-dependent mitochondrial fission via MiD49/51 is essential for apoptotic cristae remodeling

Drp1-dependent mitochondrial fission via MiD49/51 is essential for apoptotic cristae remodeling.

Mitochondrial fission facilitates cytochrome c release from the intracristae space into the cytoplasm during intrinsic apoptosis, although how the mitochondrial fission factor Drp1 and its mitochondrial receptors Mff, MiD49, and MiD51 are involved in this reaction remains elusive. Here, we analyzed the functional division of these receptors with their knockout (KO) cell lines. In marked contrast to Mff-KO cells, MiD49/MiD51-KO and Drp1-KO cells completely resisted cristae remodeling and cytochrome c release during apoptosis. This phenotype in MiD49/51-KO cells, but not Drp1-KO cells, was completely abolished by treatments disrupting cristae structure such as OPA1 depletion. Unexpectedly, OPA1 oligomers generally thought to resist cytochrome c release by stabilizing the cristae structure were similarly disassembled in Drp1-KO and MiD49/51-KO cells, indicating that disassembly of OPA1 oligomers is not directly linked to cristae remodeling for cytochrome c release. Together, these results indicate that Drp1-dependent mitochondrial fission through MiD49/MiD51 regulates cristae remodeling during intrinsic apoptosis.

Mitochondrial fission is crucial for cristae remodeling

![Figure][1] During the early stages of apoptosis, mitochondrial cristae swell up in a wild-type cell (left), but mostly remain flat in a cell lacking MiD49 and MiD51 (right). [Otera et al.][2] reveal that the mitochondrial fission factor Drp1 and its receptors MiD49 and MiD51 promote

Mito-Morphosis: Mitochondrial Fusion, Fission, and Cristae Remodeling as Key Mediators of Cellular Function.

Permanent residency in the eukaryotic cell pressured the prokaryotic mitochondrial ancestor to strategize for intracellular living. Mitochondria are able to autonomously integrate and respond to cellular cues and demands by remodeling their morphology. These processes define mitochondrial dynamics and inextricably link the fate of the mitochondrion and that of the host eukaryote, as exemplified by the human diseases that result from mutations in mitochondrial dynamics proteins. In this review, we delineate the architecture of mitochondria and define the mechanisms by which they modify their shape. Key players in these mechanisms are discussed, along with their role in manipulating mitochondrial morphology during cellular action and development. Throughout, we highlight the evolutionary context in which mitochondrial dynamics emerged and consider unanswered questions whose dissection might lead to mitochondrial morphology-based therapies.

MAPL SUMOylation of Drp1 Stabilizes an ER/Mitochondrial Platform Required for Cell Death

There has been evidence that mitochondrial fragmentation is required for apoptosis, but the molecular links between the machinery regulating dynamics and cell death have been controversial. Indeed, activated BAX and BAK can form functional channels inliposomes, bringing into question the contribution of mitochondrial dynamics in apoptosis. We now demonstrate that the activation of apoptosis triggers MAPL/MUL1-dependent SUMOylation of the fission GTPase Drp1, a process requisite for cytochrome c release. SUMOylated Drp1 functionally stabilizes ER/mitochondrial contact sites that act as hotspots for mitochondrial constriction, calcium flux, cristae remodeling, and cytochrome c release. The loss of MAPL does not alter the activation and assembly ofBAX/BAK oligomers, indicating that MAPL is activated downstream of BAX/BAK. This work demonstrates how interorganellar contacts are dynamically regulated through active SUMOylation during apoptosis, creating a stabilized platform that signals cytochrome c release.

The Opa1-Dependent Mitochondrial Cristae Remodeling Pathway Controls Atrophic, Apoptotic, and Ischemic Tissue Damage

SummaryMitochondrial morphological and ultrastructural changes occur during apoptosis and autophagy, but whether they are relevant in vivo for tissue response to damage is unclear. Here we investigate the role of the optic atrophy 1 (OPA1)-dependent cristae remodeling pathway in vivo and provide evidence that it regulates the response of multiple tissues to apoptotic, necrotic, and atrophic stimuli. Genetic inhibition of the cristae remodeling pathway in vivo does not affect development, but protects mice from denervation-induced muscular atrophy, ischemic heart and brain damage, as well as hepatocellular apoptosis. Mechanistically, OPA1-dependent mitochondrial cristae stabilization increases mitochondrial respiratory efficiency and blunts mitochondrial dysfunction, cytochrome c release, and reactive oxygen species production. Our results indicate that the OPA1-dependent cristae remodeling pathway is a fundamental, targetable determinant of tissue damage in vivo.

Analysis of opa1 isoforms expression and apoptosis regulation in autosomal dominant optic atrophy (ADOA) patients with mutations in the opa1 gene

Autosomal dominant optic atrophy (ADOA) is a hereditary optic neuropathy characterized by bilateral symmetrical visual loss, decrease in retinal ganglion cells and a loss of myelin within the optic nerve. ADOA is associated to mutations in Optic atrophy 1 gene (OPA1), which encodes a mitochondrial protein involved in cristae remodeling, maintenance of mitochondrial membrane integrity, mitochondrial fusion and apoptosis regulation. We thus evaluated the rate of apoptosis and the expression levels of OPA1 isoforms in ADOA and control cells.Peripheral blood lymphocytes from eight patients with OPA1 mutation and age matched controls were cultivated both in basal conditions or with 2-deoxy-D-ribose, a reducing sugar that induces apoptosis through oxidative stress. Apoptosis was analyzed by flow cytometry, phosphatidylserine translocation, mitochondrial membrane depolarization and caspase 3 activation. We also analyzed the expression levels of OPA1 isoforms in ADOA and control cells cultured with and without 2-deoxy-D-ribose.We showed an increased percentage of apoptotic cells in ADOA patients compared to controls, both in basal culture conditions and after 2-deoxy-D-ribose treatment. This suggested a great susceptibility of ADOA cells to oxidative stress and a strong correlation between OPA1 protein dysfunctions and morphological–functional alterations to mitochondria. Moreover OPA1 protein expression was significantly decreased in lymphocytes from the ADOA patients after 2-deoxy-D-ribose treatment, implying a great sensitivity of the mutated protein to free radical damage.Concluding, we could confirm that oxidative stress-induced apoptosis may play a key role in the pathophysiological process bringing to retinal ganglion cells degeneration in ADOA.

Neural-Specific Deletion of Htra2 Causes Cerebellar Neurodegeneration and Defective Processing of Mitochondrial OPA1

HTRA2, a serine protease in the intermembrane space, has important functions in mitochondrial stress signaling while its abnormal activity may contribute to the development of Parkinson’s disease. Mice with a missense or null mutation of Htra2 fail to thrive, suffer striatal neuronal loss, and a parkinsonian phenotype that leads to death at 30–40 days of age. While informative, these mouse models cannot separate neural contributions from systemic effects due to the complex phenotypes of HTRA2 deficiency. Hence, we developed mice carrying a Htra2-floxed allele to query the consequences of tissue-specific HTRA2 deficiency. We found that mice with neural-specific deletion of Htra2 exhibited atrophy of the thymus and spleen, cessation to gain weight past postnatal (P) day 18, neurological symptoms including ataxia and complete penetrance of premature death by P40. Histologically, increased apoptosis was detected in the cerebellum, and to a lesser degree in the striatum and the entorhinal cortex, from P25. Even earlier at P20, mitochondria in the cerebella already exhibited abnormal morphology, including swelling, vesiculation, and fragmentation of the cristae. Furthermore, the onset of these structural anomalies was accompanied by defective processing of OPA1, a key molecule for mitochondrial fusion and cristae remodeling, leading to depletion of the L-isoform. Together, these findings suggest that HTRA2 is essential for maintenance of the mitochondrial integrity in neurons. Without functional HTRA2, a lifespan as short as 40 days accumulates a large quantity of dysfunctional mitochondria that contributes to the demise of mutant mice.

Activation of mitochondrial protease OMA1 by Bax and Bak promotes cytochrome c release during apoptosis.

Intrinsic apoptotic stimuli initiate mammalian cells' apoptotic program by first activating the proteins that have only Bcl-2 homology domain 3 (BH3), such as Bcl-2 interacting mediator of cell death (Bim) and truncated BH3 interacting death domain agonist (tBid), which in turn trigger conformational changes in BCL2-associated X (Bax) and BCL2-antagonist/killer (Bak) proteins that enable oligomer formation on the mitochondria, causing cytochrome c and other apoptogenic proteins in the intermembrane space to leak out. Leaked cytochrome c then initiates apoptotic caspase activation through a well-defined biochemical pathway. However, how oligomerized Bax and Bak cause cytochrome c release from mitochondria remains unknown. We report here the establishment of cell lines in which Bim or tBid can be inducibly expressed to initiate apoptosis in a controlled, quantitative manner. We used these cell lines to examine apoptotic events after Bax and Bak oligomerization but before cytochrome c release. The mitochondrial metalloprotease OMA1 was activated in this system in a Bax- and Bak-dependent fashion. Activated OMA1 cleaved the dynamin-like GTPase, optical nerve atrophy 1, an event that is critical for remodeling of mitochondrial cristae. Knockdown or knockout of OMA1 in these cells attenuated cytochrome c release. Thus it is clear that oligomerized Bax and Bak trigger apoptosis by causing both the permeabilization of the mitochondrial outer membrane and activation OMA1.