Unveiling the neuroprotection effects of Volvalerenic acid A: Mitochondrial fusion induction via IDO1-mediated Stat3-Opa1 signaling pathway

BACKGROUND

Ischemic stroke is a leading cause of death and long-term disability worldwide. Studies have suggested that cerebral ischemia induces massive mitochondrial damage. Valerianic acid A (VaA) is the main active ingredient of valerianic acid with neuroprotective activity.

PURPOSE

This study aimed to investigate the neuroprotective effects of VaA with ischemic stroke and explore the underlying mechanisms.

METHOD

In this study, we established the oxygen-glucose deprivation and reperfusion (OGD/R) cell model and the middle cerebral artery occlusion and reperfusion (MCAO/R) animal model in vitro and in vivo. Neurological behavior score, 2, 3, 5-triphenyl tetrazolium chloride (TTC) staining and Hematoxylin and Eosin (HE) Staining were used to detect the neuroprotection of VaA in MCAO/R rats. Also, the levels of ROS, mitochondrial membrane potential (MMP), and activities of NAD+ were detected to reflect mitochondrial function. Mechanistically, gene knockout experiments, transfection experiments, immunofluorescence, DARTS, and molecular dynamics simulation experiments showed that VaA bound to IDO1 regulated the kynurenine pathway of tryptophan metabolism and prevented Stat3 dephosphorylation, promoting Stat3 activation and subsequent transcription of the mitochondrial fusion-related gene Opa1.

RESULTS

We showed that VaA decreased the infarct volume in a dose-dependent manner and exerted neuroprotective effects against reperfusion injury. Furthermore, VaA promoted Opa1-related mitochondrial fusion and reversed neuronal mitochondrial damage and loss after reperfusion injury. In SH-SY5Y cells, VaA (5, 10, 20 μM) exerted similar protective effects against OGD/R-induced injury. We then examined the expression of significant enzymes regulating the kynurenine (Kyn) pathway of the ipsilateral brain tissue of the ischemic stroke rat model, and these enzymes may play essential roles in ischemic stroke. Furthermore, we found that VaA can bind to the initial rate-limiting enzyme IDO1 in the Kyn pathway and prevent Stat3 phosphorylation, promoting Stat3 activation and subsequent transcription of the mitochondrial fusion-related gene Opa1. Using in vivo IDO1 knockdown and in vitro IDO1 overexpressing models, we demonstrated that the promoted mitochondrial fusion and neuroprotective effects of VaA were IDO1-dependent.

CONCLUSION

VaA administration improved neurological function by promoting mitochondrial fusion through the IDO1-mediated Stat3-Opa1 pathway, indicating its potential as a therapeutic drug for ischemic stroke.

Mitochondrial morphology dynamics and ROS regulate apical polarity and differentiation in Drosophila follicle cells

Mitochondrial morphology dynamics regulate signaling pathways during epithelial cell formation and differentiation. The mitochondrial fission protein Drp1 affects the appropriate activation of EGFR and Notch signaling-driven differentiation of posterior follicle cells in Drosophila oogenesis. The mechanisms by which Drp1 regulates epithelial polarity during differentiation are not known. In this study, we show that Drp1-depleted follicle cells are constricted in early stages and present in multiple layers at later stages with decreased levels of apical polarity protein aPKC. These defects are suppressed by additional depletion of mitochondrial fusion protein Opa1. Opa1 depletion leads to mitochondrial fragmentation and increased reactive oxygen species (ROS) in follicle cells. We find that increasing ROS by depleting the ROS scavengers, mitochondrial SOD2 and catalase also leads to mitochondrial fragmentation. Further, the loss of Opa1, SOD2 and catalase partially restores the defects in epithelial polarity and aPKC, along with EGFR and Notch signaling in Drp1-depleted follicle cells. Our results show a crucial interaction between mitochondrial morphology, ROS generation and epithelial cell polarity formation during the differentiation of follicle epithelial cells in Drosophila oogenesis.

The loss of OPA1 accelerates intervertebral disc degeneration and osteoarthritis in aged mice

NP cells of the intervertebral disc and articular chondrocytes reside in avascular and hypoxic tissue niches. As a consequence of these environmental constraints the cells are primarily glycolytic in nature and were long thought to have a minimal reliance on mitochondrial function. Recent studies have challenged this long-held view and highlighted the increasingly important role of mitochondria in the physiology of these tissues. However, the foundational understanding of mechanisms governing mitochondrial dynamics and function in these tissues is lacking. We investigated the role of mitochondrial fusion protein OPA1 in maintaining the spine and knee joint health in mice. OPA1 knockdown in NP cells altered mitochondrial size and cristae shape and increased the oxygen consumption rate without affecting ATP synthesis. OPA1 governed the morphology of multiple organelles, including peroxisomes, early endosomes and cis-Golgi and its loss resulted in the dysregulation of NP cell autophagy. Metabolic profiling and 13C-flux analyses revealed TCA cycle anaplerosis and altered metabolism in OPA1-deficient NP cells. Noteworthy, Opa1AcanCreERT2 mice with Opa1 deletion in disc and cartilage showed age-dependent disc degeneration, osteoarthritis, and vertebral osteopenia. Our findings underscore that OPA1 regulation of mitochondrial dynamics and multi-organelle interactions is critical in preserving metabolic homeostasis of disc and cartilage.

FAM3A Deficiency - Induced Mitochondrial Dysfunction Underlies Post-Infarct Mortality and Heart Failure

Mitochondrial protein sequence similarity 3 gene family member A (FAM3A) plays important roles in the electron transfer chain, while its functions in the heart are still unknown. This study aims to explore the roles and mechanisms of FAM3A after myocardial infarction (MI). FAM3A-deficient (Fam3a-/-) mice were implemented with MI injury and showed lower survival rates at 4 weeks as well as decreased cardiac systolic function. Isolated cardiomyocytes of Fam3a-/- mice showed reduced basal, ATP-linked respiration and respiratory reserve compared to that of wild-type mice. Transmission electron microscopy studies showed Fam3a-/- mice had a larger size and elevated density of mitochondria. FAM3A deficiency also induced elevated mitochondrial Ca2+, higher opening level of mPTP, lower mitochondrial membrane potential and elevated apoptotic rates. Further analyses demonstrated that mitochondrial dynamics protein Opa1 contributed to the effects of FAM3A in cardiomyocytes. Our study discloses the important roles of mitochondrial protein FAM3A in the heart.

Role of Mitochondrial Dynamics in Heart Diseases

Mitochondrial dynamics, including fission and fusion processes, are essential for heart health. Mitochondria, the powerhouses of cells, maintain their integrity through continuous cycles of biogenesis, fission, fusion, and degradation. Mitochondria are relatively immobile in the adult heart, but their morphological changes due to mitochondrial morphology factors are critical for cellular functions such as energy production, organelle integrity, and stress response. Mitochondrial fusion proteins, particularly Mfn1/2 and Opa1, play multiple roles beyond their pro-fusion effects, such as endoplasmic reticulum tethering, mitophagy, cristae remodeling, and apoptosis regulation. On the other hand, the fission process, regulated by proteins such as Drp1, Fis1, Mff and MiD49/51, is essential to eliminate damaged mitochondria via mitophagy and to ensure proper cell division. In the cardiac system, dysregulation of mitochondrial dynamics has been shown to cause cardiac hypertrophy, heart failure, ischemia/reperfusion injury, and various cardiac diseases, including metabolic and inherited cardiomyopathies. In addition, mitochondrial dysfunction associated with oxidative stress has been implicated in atherosclerosis, hypertension and pulmonary hypertension. Therefore, understanding and regulating mitochondrial dynamics is a promising therapeutic tool in cardiac diseases. This review summarizes the role of mitochondrial morphology in heart diseases for each mitochondrial morphology regulatory gene, and their potential as therapeutic targets to heart diseases.

Mitochondrial dynamics-related genes DRP1 and OPA1 contributes to early diagnosis of cognitive impairment in diabetes

BACKGROUND AND AIM

DRP1 and OPA1 play important roles in mitochondrial fusion and fission. However, the role of DRP1 and OPA1 amplification in mitochondrial cognitive impairment has not been reported. This study aimed to investigate the relationship between DRP1 and OPA1 and the risk of cognitive impairment.

METHODS

In this study, 45 elderly patients with diabetes admitted to the Lianyungang Second People's Hospital from September 2020 to January 2021 were included. The patients were divided into normal group, mild cognitive impairment group and dementia group by using MMSE score, and the clinical characteristics of the three groups were compared. The amplification multiples of the two genes' DNA were calculated by ΔΔCT and defined as 2- K. Spearman rank correlation was used to analyze the correlation between the DNA amplification multiples of patients' DRP1 and OPA1 and AD8 and MoCA scores. The sensitivity and specificity of DNA amplification multiples of DRP1 and OPA1 to predict clinical outcomes of diabetic cognitive impairment were evaluated using Receiver operator characteristic (ROC) curves. Multiple logistic regression was used to evaluate the relationship between DNA amplification factor of DRP1 and OPA1 and cognitive function.

RESULTS

DRP1(2- K) and OPA1(2- K) significantly increased and decreased in dementia and MCI groups compared with the normal group (P ≤ 0.001). The DNA amplification factor of DRP1 was positively correlated with AD8 score and negatively correlated with MoCA score (P < 0.001). The DNA amplification factor of OPA1 was positively correlated with the MoCA score (P = 0.0002). Analysis of ROCs showed that the DNA amplification factor of OPA1 had a higher predictive value for dementia (P < 0.0001), and that it had a higher predictive value when used in combination with DRP1. Multiple logistic regression results showed that increased DNA amplification in DRP1 was associated with increased risk of dementia (OR 1.149;95%CI,1.035-1.275), and increased DNA amplification in OPA1 was associated with decreased risk of MCI (OR 0.004;95%CI,0.000-0.251) and dementia (OR 0.000;95%CI,0.000-0.134).

CONCLUSION

DNA amplification multiples of DRP1 and OPA1 are associated with the risk of dementia in elderly patients and may serve as potential biomarkers.

A novel group of dynamin-related proteins shared by eukaryotes and giant viruses is able to remodel mitochondria from within the matrix

The diverse GTPases of the dynamin superfamily play various roles in the cell, as exemplified by the dynamin-related proteins (DRPs) Mgm1 and Opa1, which remodel the mitochondrial inner membrane in fungi and metazoans, respectively. Via an exhaustive search of genomic and metagenomic databases we found previously unknown DRP types occurring in diverse eukaryotes and giant viruses (phylum Nucleocytoviricota). One novel DRP clade, termed MidX, combined hitherto uncharacterized proteins from giant viruses and six distantly related eukaryote taxa (Stramenopiles, Telonemia, Picozoa, Amoebozoa, Apusomonadida, and Choanoflagellata). MidX stood out because it was not only predicted to be mitochondria-targeted, but also to assume a tertiary structure not observed in other DRPs before. To understand how MidX affects mitochondria, we exogenously expressed MidX from Hyperionvirus in the kinetoplastid Trypanosoma brucei, which lacks Mgm1 or Opa1 orthologs. MidX massively affected mitochondrial morphology from inside the matrix, where it closely associates with the inner membrane. This unprecedented mode of action is contrasts to those of Mgm1 and Opa1, which mediate inner membrane remodeling in the intermembrane space. We speculate that MidX was acquired in Nucleocytoviricota evolution by horizontal gene transfer from eukaryotes and is used by giant viruses to remodel host mitochondria during infection. MidX's unique structure may be an adaptation for reshaping mitochondria from the inside. Finally, Mgm1 forms a sister to MidX and not Opa1 in our phylogenetic analysis, throwing into question the long-presumed homology of these DRPs with similar roles in sister lineages.

In Vivo Efficacy and Safety Evaluations of Therapeutic Splicing Correction Using U1 snRNA in the Mouse Retina

Efficacy and safety considerations constitute essential steps during development of in vivo gene therapies. Herein, we evaluated efficacy and safety of splice factor-based treatments to correct mutation-induced splice defects in an Opa1 mutant mouse line. We applied adeno-associated viruses to the retina. The viruses transduced retinal cells with an engineered U1 snRNA splice factor designed to correct the Opa1 splice defect. We found the treatment to be efficient in increasing wild-type Opa1 transcripts. Correspondingly, Opa1 protein levels increased significantly in treated eyes. Measurements of retinal morphology and function did not reveal therapy-related side-effects supporting the short-term safety of the treatment. Alterations of potential off-target genes were not detected. Our data suggest that treatments of splice defects applying engineered U1 snRNAs represent a promising in vivo therapeutic approach. The therapy increased wild-type Opa1 transcripts and protein levels without detectable morphological, functional or genetic side-effects in the mouse eye. The U1 snRNA-based therapy can be tailored to specific disease gene mutations, hence, raising the possibility of a wider applicability of this promising technology towards treatment of different inherited retinal diseases.

Opa1 and Drp1 reciprocally regulate cristae morphology, ETC function, and NAD+ regeneration in KRas-mutant lung adenocarcinoma

Oncogenic KRas activates mitochondrial fission through Erk-mediated phosphorylation of the mitochondrial fission GTPase Drp1. Drp1 deletion inhibits tumorigenesis of KRas-driven pancreatic cancer, but the role of mitochondrial dynamics in other Ras-driven malignancies is poorly defined. Here we show that in vitro and in vivo growth of KRas-driven lung adenocarcinoma is unaffected by deletion of Drp1 but is inhibited by deletion of Opa1, the GTPase that regulates inner membrane fusion and proper cristae morphology. Mechanistically, Opa1 knockout disrupts cristae morphology and inhibits electron transport chain (ETC) assembly and activity, which inhibits tumor cell proliferation through loss of NAD⁺ regeneration. Simultaneous inactivation of Drp1 and Opa1 restores cristae morphology, ETC activity, and cell proliferation indicating that mitochondrial fission activity drives ETC dysfunction induced by Opa1 knockout. Our results support a model in which mitochondrial fission events disrupt cristae structure, and tumor cells with hyperactive fission activity require Opa1 activity to maintain ETC function.

Berberine mitigates hepatic insulin resistance by enhancing mitochondrial architecture via the SIRT1/Opa1 signalling pathway

The aberrant changes of fussion/fission-related proteins can trigger mitochondrial dynamics imbalance, which cause mitochondrial dysfunctions and result insulin resistance (IR). However, the relationship between the inner mitochondrial membrane fusion protein optic atrophy 1 (Opa1) and hepatic IR as well as the specific molecular mechanisms of signal transduction has not been fully elucidated. In this study, we explore whether abnormalities in the Opa1 cause hepatic IR and whether berberine (BBR) can prevent hepatic IR through the SIRT1/Opa1 signalling pathway. High-fat diet (HFD)-fed mice and db/db mice are used as animal models to study hepatic IR in vivo. IR, morphological changes, and mitochondrial injury of the liver are examined to explore the effects of BBR. SIRT1/Opa1 protein expression is determined to confirm whether the signalling pathway is damaged in the model animals and is involved in BBR treatment-mediated mitigation of hepatic IR. A palmitate (PA)-induced hepatocyte IR model is established in HepG2 cells in vitro. Opa1 silencing and SIRT1 overexpression are induced to verify whether Opa1 deficiency causes hepatocyte IR and whether SIRT1 improves this dysfunction. BBR treatment and SIRT1 silencing are employed to confirm that BBR can prevent hepatic IR by activating the SIRT1/Opa1 signalling pathway. Western blot analysis and JC-1 fluorescent staining results show that Opa1 deficiency causes an imbalance in mitochondrial fusion/fission and impairs insulin signalling in HepG2 cells. SIRT1 and BBR overexpression ameliorates PA-induced IR, increases Opa1, and improves mitochondrial function. SIRT1 silencing partly reverses the effects of BBR on HepG2 cells. SIRT1 and Opa1 expressions are downregulated in the animal models. BBR attenuates hepatic IR and enhances SIRT1/Opa1 signalling in db/db mice. In summary, Opa1 silencing-mediated mitochondrial fusion/fission imbalance could lead to hepatocyte IR. BBR may improve hepatic IR by regulating the SIRT1/Opa1 signalling pathway, and thus, it may be used to treat type-2 diabetes.

Mitochondrial hyperfusion via metabolic sensing of regulatory amino acids

The relationship between nutrient starvation and mitochondrial dynamics is poorly understood. We find that cells facing amino acid starvation display clear mitochondrial fusion as a means to evade mitophagy. Surprisingly, further supplementation of glutamine (Q), leucine (L), and arginine (R) did not reverse, but produced stronger mitochondrial hyperfusion. Interestingly, the hyperfusion response to Q + L + R was dependent upon mitochondrial fusion proteins Mfn1 and Opa1 but was independent of MTORC1. Metabolite profiling indicates that Q + L + R addback replenishes amino acid and nucleotide pools. Inhibition of fumarate hydratase, glutaminolysis, or inosine monophosphate dehydrogenase all block Q + L + R-dependent mitochondrial hyperfusion, which suggests critical roles for the tricarboxylic acid (TCA) cycle and purine biosynthesis in this response. Metabolic tracer analyses further support the idea that supplemented Q promotes purine biosynthesis by serving as a donor of amine groups. We thus describe a metabolic mechanism for direct sensing of cellular amino acids to control mitochondrial fusion and cell fate.

Paeonol promotes Opa1-mediated mitochondrial fusion via activating the CK2α-Stat3 pathway in diabetic cardiomyopathy

Diabetes disrupts mitochondrial function and often results in diabetic cardiomyopathy (DCM). Paeonol is a bioactive compound that has been reported to have pharmacological potential for cardiac and mitochondrial protection. This study aims to explore the effects of paeonol on mitochondrial disorderes in DCM and the underlying mechanisms. We showed that paeonol promoted Opa1-mediated mitochondrial fusion, inhibited mitochondrial oxidative stress, and preserved mitochondrial respiratory capacity and cardiac performance in DCM in vivo and in vitro. Knockdown of Opa1 blunted the above protective effects of paeonol in both diabetic hearts and high glucose-treated cardiomyocytes. Mechanistically, inhibitor screening, siRNA knockdown and chromatin immunoprecipitation experiments showed that paeonol-promoted Opa1-mediated mitochondrial fusion required the activation of Stat3, which directly bound to the promoter of Opa1 to upregulate its transcriptional expression. Moreover, pharmmapper screening and molecular docking studies revealed that CK2α served as a direct target of paeonol that interacted with Jak2 and induced the phosphorylation and activation of Jak2-Stat3. Knockdown of CK2α blunted the promoting effect of paeonol on Jak2-Stat3 phosphorylation and Opa1-mediated mitochondrial fusion. Collectively, we have demonstrated for the first time that paeonol is a novel mitochondrial fusion promoter in protecting against hyperglycemia-induced mitochondrial oxidative injury and DCM at least partially via an Opa1-mediated mechanism, a process in which paeonol interacts with CK2α and restores its kinase activity that subsequently increasing Jak2-Stat3 phosphorylation and enhancing the transcriptional level of Opa1. These findings suggest that paeonol or the promotion of mitochondrial fusion might be a promising strategy for the treatment of DCM.

Punicalagin Protects Against Diabetic Cardiomyopathy by Promoting Opa1-Mediated Mitochondrial Fusion via Regulating PTP1B-Stat3 Pathway

Aims: This study aims to explore the efficacy of punicalagin (PG) on diabetic cardiomyopathy (DCM), with a specific focus on the mechanisms underlying the effects of PG on mitochondrial fusion/fission dynamics. Results: Cardiac structural and functional abnormalities were ameliorated in diabetic rats receiving PG administration as evidenced by increased ejection fraction, and attenuated myocardial fibrosis and hypertrophy. PG enhanced mitochondrial function and inhibited mitochondria-derived oxidative stress by promoting Opa1-mediated mitochondrial fusion. The benefits of PG could be abrogated by knockdown of Opa1 in vivo and in vitro. Inhibitor screening and chromatin immunoprecipitation analysis showed that Stat3 directly regulated the transcriptional expression of Opa1 by binding to its promoter and was responsible for PG-induced Opa1-mediated mitochondrial fusion. Moreover, pharmmapper screening and molecular docking studies revealed that PG embedded into the activity pocket of PTP1B and inhibited the activity of PTP1B. Overexpression of PTP1B blocked the promoting effect of PG on Stat3 phosphorylation and Opa1-mediated mitochondrial fusion, whereas knockdown of PTP1B mimicked the benefits of PG in high-glucose-treated cardiomyocytes. Innovation: Our study is the first to identify PG as a novel mitochondrial fusion promoter against hyperglycemia-induced mitochondrial oxidative injury and cardiomyopathy by upregulating Opa1 via regulating PTP1B-Stat3 pathway. Conclusion: PG protects against DCM by promoting Opa1-mediated mitochondrial fusion, a process in which PG interacts with PTP1B and inhibits its activity, which in turn increases Stat3 phosphorylation and then enhances the transcriptional expression of Opa1. These results suggest that PG might be a promising new therapeutic approach against diabetic cardiac complication. Antioxid. Redox Signal. 35, 618-641.

MICOS and the mitochondrial inner membrane morphology - when things get out of shape

Mitochondria play a key role in cellular signalling, metabolism and energetics. Proper architecture and remodelling of the inner mitochondrial membrane are essential for efficient respiration, apoptosis and quality control in the cell. Several protein complexes including mitochondrial contact site and cristae organizing system (MICOS), F₁ F_O -ATP synthase, and Optic Atrophy 1 (OPA1), facilitate formation, maintenance and stability of cristae membranes. MICOS, the F₁ F_O -ATP synthase, OPA1 and inner membrane phospholipids such as cardiolipin and phosphatidylethanolamine interact with each other to organize the inner membrane ultra-structure and remodel cristae in response to the cell's demands. Functional alterations in these proteins or in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine result in an aberrant inner membrane architecture and impair mitochondrial function. Mitochondrial dysfunction and abnormalities hallmark several human conditions and diseases including neurodegeneration, cardiomyopathies and diabetes mellitus. Yet, they have long been regarded as secondary pathological effects. This review discusses emerging evidence of a direct relationship between protein- and lipid-dependent regulation of the inner mitochondrial membrane morphology and diseases such as fatal encephalopathy, Leigh syndrome, Parkinson's disease, and cancer.

Protective role of the mitochondrial fusion protein OPA1 in hypertension

Hypertension is associated with excessive reactive oxygen species (ROS) production in vascular cells. Mitochondria undergo fusion and fission, a process playing a role in mitochondrial function. OPA1 is essential for mitochondrial fusion. Loss of OPA1 is associated with ROS production and cell dysfunction. We hypothesized that mitochondria fusion could reduce oxidative stress that defect in fusion would exacerbate hypertension. Using (a) Opa1 haploinsufficiency in isolated resistance arteries from Opa1+/- mice, (b) primary vascular cells from Opa1+/- mice, and (c) RNA interference experiments with siRNA against Opa1 in vascular cells, we investigated the role of mitochondria fusion in hypertension. In hypertension, Opa1 haploinsufficiency induced altered mitochondrial cristae structure both in vascular smooth muscle and endothelial cells but did not modify protein level of long and short forms of OPA1. In addition, we demonstrated an increase of mitochondrial ROS production, associated with a decrease of superoxide dismutase 1 protein expression. We also observed an increase of apoptosis in vascular cells and a decreased VSMCs proliferation. Blood pressure, vascular contractility, as well as endothelium-dependent and -independent relaxation were similar in Opa1+/- , WT, L-NAME-treated Opa1+/- and WT mice. Nevertheless, chronic NO-synthase inhibition with L-NAME induced a greater hypertension in Opa1+/- than in WT mice without compensatory arterial wall hypertrophy. This was associated with a stronger reduction in endothelium-dependent relaxation due to excessive ROS production. Our results highlight the protective role of mitochondria fusion in the vasculature during hypertension by limiting mitochondria ROS production.

Opa1 Deficiency Promotes Development of Retinal Vascular Lesions in Diabetic Retinopathy

This study investigates whether reduced optic atrophy 1 (Opa1) level promotes apoptosis and retinal vascular lesions associated with diabetic retinopathy (DR). Four groups of mice: wild type (WT) control mice, streptozotocin (STZ)-induced diabetic mice, Opa1^+/- mice, and diabetic Opa1^+/- mice were used in this study. 16 weeks after diabetes onset, retinas were assessed for Opa1 and Bax levels by Western blot analysis, and retinal networks were examined for acellular capillaries (AC) and pericyte loss (PL). Apoptotic cells were detected in retinal capillaries using TUNEL assay, and caspase-3 activity was assessed using fluorometric analysis. Opa1 expression was significantly downregulated in retinas of diabetic and Opa1^+/- mice compared with those of WT mice. Inducing diabetes further decreased Opa1 expression in retinas of Opa1^+/- mice. Increased cytochrome c release concomitant with increased level of pro-apoptotic Bax and elevated caspase-3 activity were observed in retinas of diabetic and Opa1^+/- mice; the number of TUNEL-positive cells and AC/PL was also significantly increased. An additional decrease in the Opa1 level in retinas of diabetic Opa1^+/- mice exacerbated the development of apoptotic cells and AC/PL compared with those of diabetic mice. Diabetes-induced Opa1 downregulation contributes, at least in part, to the development of retinal vascular lesions characteristic of DR.

Opa1 relies on cristae preservation and ATP synthase to curtail reactive oxygen species accumulation in mitochondria

Reactive oxygen species (ROS) are a common product of active mitochondrial respiration carried in mitochondrial cristae, but whether cristae shape influences ROS levels is unclear. Here we report that the mitochondrial fusion and cristae shape protein Opa1 requires mitochondrial ATP synthase oligomers to reduce ROS accumulation. In cells fueled with galactose to force ATP production by mitochondria, cristae are enlarged, ATP synthase oligomers destabilized, and ROS accumulate. Opa1 prevents both cristae remodeling and ROS generation, without impinging on levels of mitochondrial antioxidant defense enzymes that are unaffected by Opa1 overexpression. Genetic and pharmacologic experiments indicate that Opa1 requires ATP synthase oligomerization and activity to reduce ROS levels upon a blockage of the electron transport chain. Our results indicate that the converging effect of Opa1 and mitochondrial ATP synthase on mitochondrial ultrastructure regulate ROS abundance to sustain cell viability.

Cell-Free Analysis of Mitochondrial Fusion by Fluorescence Microscopy

Dynamin-related proteins on both the mitochondrial outer and inner membranes mediate membrane fusion. Mitochondrial fusion is regulated in many different physiological contexts including cell cycle progression, differentiation pathways, stress responses, and cell death. Mitochondrial fusion is opposed by mitochondrial division and requires movement of mitochondria on microtubules. We developed a cell-free reconstituted mitochondrial fusion assay to circumvent the complexity of the pathways impinging on the activity of the mitochondrial fusion machinery in vivo. This allows for quantification of mitochondrial fusion in defined conditions and in the absence of other processes such as mitochondrial division or transport. The impact of proteins or small molecules on mitochondria fusion can also be assessed. Here we describe the cell-free mitochondrial fusion assay using mitochondria isolated from mouse embryonic fibroblasts.

Cells Lacking PA200 Adapt to Mitochondrial Dysfunction by Enhancing Glycolysis via Distinct Opa1 Processing

The conserved Blm10/PA200 proteins are proteasome activators. Previously, we identified PA200-enriched regions in the genome of SH-SY5Y neuroblastoma cells by chromatin immunoprecipitation (ChIP) and ChIP-seq analysis. We also found that selective mitochondrial inhibitors induced PA200 redistribution in the genome. Collectively, our data indicated that PA200 regulates cellular homeostasis at the transcriptional level. In the present study, our aim is to investigate the impact of stable PA200 depletion (shPA200) on the overall transcriptome of SH-SY5Y cells. RNA-seq data analysis reveals that the genetic ablation of PA200 leads to overall changes in the transcriptional landscape of SH-SY5Y neuroblastoma cells. PA200 activates and represses genes regulating metabolic processes, such as the glycolysis and mitochondrial function. Using metabolic assays in live cells, we showed that stable knockdown of PA200 does not change basal respiration. Spare respiratory capacity and proton leak however are slightly, yet significantly, reduced in PA200-deficient cells by 99.834% and 84.147%, respectively, compared to control. Glycolysis and glycolytic capacity show a 42.186% and 26.104% increase in shPA200 cells, respectively, compared to control. These data suggest a shift from oxidative phosphorylation to glycolysis especially when cells are exposed to oligomycin-induced stress. Furthermore, we observed a preserved long and compact tubular mitochondrial morphology after inhibition of ATP synthase by oligomycin, which might be associated with the glycolytic change of shPA200 cells. The present study also demonstrates that the proteolytic cleavage of Opa1 is affected, and that the level of OMA1 is significantly reduced in shPA200 cells upon oligomycin-induced mitochondrial insult. Together, these findings suggest a role for PA200 in the regulation of metabolic changes in response to selective inhibition of ATP synthase in an in vitro cellular model.

Inhibition of the mitochondrial ATPase function by IF1 changes the spatiotemporal organization of ATP synthase

• Mitochondrial F1FO ATP synthase is the key enzyme for mitochondrial bioenergetics. Dimeric F1FO-ATP synthase, is preferentially located at the edges of the cristae and its oligomerization state determines mitochondrial ultrastructure. The ATP synthase inhibitor protein IF1 modulates not only ATP synthase activity but also regulates both the structure and function of mitochondria. In order to understand this in more detail, we have investigated the effect of IF1 on the spatiotemporal organization of the ATP synthase. Stable cell lines were generated that overexpressed IF1 and constitutively active IF1-H49K. The expression of IF1-H49K induced a change in the localization and mobility of the ATP synthase as analyzed by single molecule tracking and localization microscopy (TALM). In addition, the ultrastructure and function of mitochondria in cells with higher levels of active IF1 displayed a gradual alteration. In state III, cristae structures were significantly altered. The inhibition of the hydrolase activity of the F1FO-ATP synthase by IF1 together with altered inner mitochondrial membrane caused re-localization and altered mobility of the enzyme.

Mitochondrial Safeguard: a stress response that offsets extreme fusion and protects respiratory function via flickering-induced Oma1 activation

The connectivity of mitochondria is regulated by a balance between fusion and division. Many human diseases are associated with excessive mitochondrial connectivity due to impaired Drp1, a dynamin-related GTPase that mediates division. Here, we report a mitochondrial stress response, named mitochondrial safeguard, that adjusts the balance of fusion and division in response to increased mitochondrial connectivity. In cells lacking Drp1, mitochondria undergo hyperfusion. However, hyperfusion does not completely connect mitochondria because Opa1 and mitofusin 1, two other dynamin-related GTPases that mediate fusion, become proteolytically inactivated. Pharmacological and genetic experiments show that the activity of Oma1, a metalloprotease that cleaves Opa1, is regulated by short pulses of the membrane depolarization without affecting the overall membrane potential in Drp1-knockout cells. Re-activation of Opa1 and Mitofusin 1 in Drp1-knockout cells further connects mitochondria beyond hyperfusion, termed extreme fusion, leading to bioenergetic deficits. These findings reveal an unforeseen safeguard mechanism that prevents extreme fusion of mitochondria, thereby maintaining mitochondrial function when the balance is shifted to excessive connectivity.

Melatonin Improves Mitochondrial Dynamics and Function in the Kidney of Zücker Diabetic Fatty Rats

Obesity and associated diabetes (diabesity) impair kidney mitochondrial dynamics by augmenting fission and diminishing fusion, which results in mitochondrial and renal dysfunction. Based on available evidence, the antioxidant activities of melatonin may improve impaired renal mitochondrial function in obese diabetic animals by restoring the imbalanced dynamics through inhibiting fission and promoting fusion. Male Zücker diabetic fatty (ZDF) rats and lean littermates (ZL) were orally treated either with melatonin (10 mg/kg BW/day) (M-ZDF and M-ZL) or vehicle (C-ZDF and C-ZL) for 17 weeks. Kidney function was evaluated by measurement of total urine volume, proteinuria, creatinine clearance, and assessment of kidney mitochondrial dynamics and function. C-ZDF exhibited impaired dynamics and function of kidney mitochondria in comparison to C-ZL. Melatonin improved nephropathy of ZDF rats and modulated their mitochondrial dynamics by reducing expression of Drp1 fission marker and increasing that of fusion markers, Mfn2 and Opa1. Furthermore, melatonin ameliorated mitochondrial dysfunction by increasing respiratory control index and electron transfer chain complex IV activity. In addition, it lowered mitochondrial oxidative status. Our findings show that melatonin supplementation improves nephropathy likely via modulation of the mitochondrial fission/fusion balance and function in ZDF rats.

Opa1 Reduces Hypoxia-Induced Cardiomyocyte Death by Improving Mitochondrial Quality Control

Mitochondrial dysfunction contributes to cardiovascular disorders, especially post-infarction cardiac injury, through incompletely characterized mechanisms. Among the latter, increasing evidence points to alterations in mitochondrial quality control, a range of adaptive responses regulating mitochondrial morphology and function. Optic atrophy 1 (Opa1) is a mitochondrial inner membrane GTPase known to promote mitochondrial fusion. In this study, hypoxia-mediated cardiomyocyte damage was induced to mimic post-infarction cardiac injury in vitro. Loss- and gain-of-function assays were then performed to evaluate the impact of Opa1 expression on mitochondrial quality control and cardiomyocyte survival and function. Hypoxic stress reduced cardiomyocyte viability, impaired contractile/relaxation functions, and augmented the synthesis of pro-inflammatory mediators. These effects were exacerbated by Opa1 knockdown, and significantly attenuated by Opa1 overexpression. Mitochondrial quality control was disturbed by hypoxia, as reflected by multiple mitochondrial deficits; i.e., increased fission, defective fusion, impaired mitophagy, decreased biogenesis, increased oxidative stress, and blunted respiration. By contrast, overexpression of Opa1 normalized mitochondrial quality control and sustained cardiomyocyte function. We also found that ERK, AMPK, and YAP signaling can regulate Opa1 expression. These results identify Opa1 as a novel regulator of mitochondrial quality control and highlight a key role for Opa1 in protecting cardiomyocytes against post-infarction cardiac injury.

Opa1 Overexpression Protects from Early-Onset Mpv17-/--Related Mouse Kidney Disease

Moderate overexpression of Opa1, the master regulator of mitochondrial cristae morphology, significantly improved mitochondrial damage induced by drugs, surgical denervation, or oxidative phosphorylation (OXPHOS) defects due to specific impairment of a single mitochondrial respiratory chain complex. Here, we investigated the effectiveness of this approach in the Mpv17-/- mouse, characterized by profound, multisystem mitochondrial DNA (mtDNA) depletion. After the crossing with Opa1tg mice, we found a surprising anticipation of the severe, progressive focal segmental glomerulosclerosis, previously described in Mpv17-/- animals as a late-onset clinical feature (after 12-18 months of life). In contrast, Mpv17-/- animals from this new "mixed" strain died at 8-9 weeks after birth because of severe kidney failure However, Mpv17-/-::Opa1tg mice lived much longer than Mpv17-/- littermates and developed the kidney dysfunction much later. mtDNA content and OXPHOS activities were significantly higher in Mpv17-/-::Opa1tg than in Mpv17-/- kidneys and similar to those for wild-type (WT) littermates. Mitochondrial network and cristae ultrastructure were largely preserved in Mpv17-/-::Opa1tg versus Mpv17-/- kidney and isolated podocytes. Mechanistically, the protective effect of Opa1 overexpression in this model was mediated by a block in apoptosis due to the stabilization of the mitochondrial cristae. These results demonstrate that strategies aiming at increasing Opa1 expression or activity can be effective against mtDNA depletion syndromes.

Altered MICOS Morphology and Mitochondrial Ion Homeostasis Contribute to Poly(GR) Toxicity Associated with C9-ALS/FTD

Amyotrophic lateral sclerosis (ALS) manifests pathological changes in motor neurons and various other cell types. Compared to motor neurons, the contribution of the other cell types to the ALS phenotypes is understudied. G4C2 repeat expansion in C9ORF72 is the most common genetic cause of ALS along with frontotemporal dementia (C9-ALS/FTD), with increasing evidence supporting repeat-encoded poly(GR) in disease pathogenesis. Here, we show in Drosophila muscle that poly(GR) enters mitochondria and interacts with components of the Mitochondrial Contact Site and Cristae Organizing System (MICOS), altering MICOS dynamics and intra-subunit interactions. This impairs mitochondrial inner membrane structure, ion homeostasis, mitochondrial metabolism, and muscle integrity. Similar mitochondrial defects are observed in patient fibroblasts. Genetic manipulation of MICOS components or pharmacological restoration of ion homeostasis with nigericin effectively rescue the mitochondrial pathology and disease phenotypes in both systems. These results implicate MICOS-regulated ion homeostasis in C9-ALS pathogenesis and suggest potential new therapeutic strategies.

Developmental and Tumor Angiogenesis Requires the Mitochondria-Shaping Protein Opa1

While endothelial cell (EC) function is influenced by mitochondrial metabolism, the role of mitochondrial dynamics in angiogenesis, the formation of new blood vessels from existing vasculature, is unknown. Here we show that the inner mitochondrial membrane mitochondrial fusion protein optic atrophy 1 (OPA1) is required for angiogenesis. In response to angiogenic stimuli, OPA1 levels rapidly increase to limit nuclear factor kappa-light-chain-enhancer of activated B cell (NFκB) signaling, ultimately allowing angiogenic genes expression and angiogenesis. Endothelial Opa1 is indeed required in an NFκB-dependent pathway essential for developmental and tumor angiogenesis, impacting tumor growth and metastatization. A first-in-class small molecule-specific OPA1 inhibitor confirms that EC Opa1 can be pharmacologically targeted to curtail tumor growth. Our data identify Opa1 as a crucial component of physiological and tumor angiogenesis.

Mitochondrial fusion promoter restores mitochondrial dynamics balance and ameliorates diabetic cardiomyopathy in an optic atrophy 1-dependent way

AIM

Imbalanced mitochondrial dynamics including suppressed mitochondrial fusion has been observed in diabetic hearts. However, it is still unknown whether mitochondrial fusion promoter is an effective protection to diabetic hearts. This study was designed to explore the efficacy of mitochondrial fusion promoter on diabetic cardiomyopathy (DCM).

METHODS

Male Sprague-Dawley rats were injected with streptozotocin (STZ, 65 mg/kg/d) intraperitoneally to induce diabetes. Seven weeks after vehicle or STZ injection, control or diabetic rats were treated with the vehicle or a mitochondrial fusion promoter-M1 (2 mg/kg/d) intraperitoneally for 6 weeks. Moreover, M1 was administrated to the primary cardiomyocytes cultured in normal glucose medium (NG, 5.5 mmol/L) or high glucose (HG, 33 mnol/L).

RESULTS

Administration of M1 significantly promoted mitochondrial fusion and attenuated the reduction in optic atrophy 1 (Opa1) expression in diabetic hearts. Importantly, M1 treatment attenuated oxidative stress, improved mitochondrial function and alleviated DCM in diabetic rats. In HG-treated cardiomyocytes, M1 treatment consistently increased the expression of Opa1, promoted mitochondrial fusion, enhanced mitochondrial respiratory capacity and reduced mitochondria-derived superoxide production, all of which were blunted by Opa1 siRNA knockdown. In addition, selective upregulation of Opa1 alone can also promote mitochondrial fusion, improve mitochondrial function and inhibit mitochondria-derived superoxide production in HG-cultured cardiomyocytes.

CONCLUSION

Our findings show for the first time that mitochondrial fusion promoter M1 effectively balances mitochondrial dynamics and protects against diabetic cardiomyopathy (DCM) via an Opa1-dependent way, suggesting that promoting mitochondrial fusion might be a potential therapeutic strategy for DCM.

Irisin activates Opa1-induced mitophagy to protect cardiomyocytes against apoptosis following myocardial infarction

Myocardial infarction is characterized by sudden ischemia and cardiomyocyte death. Mitochondria have critical roles in regulating cardiomyocyte viability and can sustain damage under ischemic conditions. Mitophagy is a mechanism by which damaged mitochondria are removed by autophagy to maintain mitochondrial structure and function. We investigated the role of the dynamin-like GTPase optic atrophy 1 (Opa1) in mitophagy following myocardial infarction. Opa1 expression was downregulated in infarcted hearts in vivo and in hypoxia-treated cardiomyocytes in vitro. We found that Opa1 overexpression protected cardiomyocytes against hypoxia-induced damage and enhanced cell viability by inducing mitophagy. Opa1-induced mitophagy was activated by treatment with irisin, which protected cardiomyocytes from further damage following myocardial infarction. Opa1 knockdown abolished the cardioprotective effects of irisin resulting in an enhanced inflammatory response, increased oxidative stress, and mitochondrial dysfunction in cardiomyocytes. Our data indicate that Opa1 plays an important role in maintaining cardiomyocyte viability and mitochondrial function following myocardial infarction by inducing mitophagy. Irisin can activate Opa1-induced mitophagy and protect against cardiomyocyte injury following myocardial infarction.

Individual cristae within the same mitochondrion display different membrane potentials and are functionally independent

The mitochondrial membrane potential (ΔΨm ) is the main driver of oxidative phosphorylation (OXPHOS). The inner mitochondrial membrane (IMM), consisting of cristae and inner boundary membranes (IBM), is considered to carry a uniform ΔΨm . However, sequestration of OXPHOS components in cristae membranes necessitates a re-examination of the equipotential representation of the IMM. We developed an approach to monitor ΔΨm at the resolution of individual cristae. We found that the IMM was divided into segments with distinct ΔΨm , corresponding to cristae and IBM. ΔΨm was higher at cristae compared to IBM. Treatment with oligomycin increased, whereas FCCP decreased, ΔΨm heterogeneity along the IMM. Impairment of cristae structure through deletion of MICOS-complex components or Opa1 diminished this intramitochondrial heterogeneity of ΔΨm . Lastly, we determined that different cristae within the individual mitochondrion can have disparate membrane potentials and that interventions causing acute depolarization may affect some cristae while sparing others. Altogether, our data support a new model in which cristae within the same mitochondrion behave as independent bioenergetic units, preventing the failure of specific cristae from spreading dysfunction to the rest.

Differential mitochondrial morphology in ventral striatal projection neuron subtypes

The two striatal projection neuron subtypes (medium spiny neurons- MSNs), those enriched in dopamine receptor 1 versus 2 (D1-MSNs and D2-MSNs), display dichotomous properties at the level of the transcriptome, projections, morphology, and electrophysiology. Recent work illustrates dichotomous mitochondrial length in NAc MSN subtype dendrites after cocaine self-administration, with a shift toward smaller mitochondria, due to enhanced fission, occurring in D1-MSN dendrites and a shift toward larger mitochondria in D2-MSN dendrites. However, to date there has been no comparison of mitochondrial morphological properties between MSN subtypes. In this study, we examine mitochondrial morphology in NAc D1-MSNs versus D2-MSNs. We observe an increase in the frequency of smaller length mitochondria in D2-MSN dendrites relative to D1-MSN dendrites, while D1-MSN dendrites display an increase in larger length mitochondria. The differences in mitochondrial length occur in both NAc core and shell, although to a greater extent in NAc core. Finally, we demonstrate that the mitochondrial fusion molecule, Opa1, is differentially expressed in NAc MSN subtypes, with D1-MSNs displaying higher expression of Opa1 ribosome-associated mRNA. The difference in Opa1 levels may account for the bias toward enhanced smaller mitochondria in D2-MSNs and enhanced larger mitochondria in D1-MSNs. Collectively, our study demonstrates differential mitochondrial size and a potential molecular mediator of these mitochondrial differences in NAc MSN subtypes.

Defective Mitochondrial tRNA Taurine Modification Activates Global Proteostress and Leads to Mitochondrial Disease

A subset of mitochondrial tRNAs (mt-tRNAs) contains taurine-derived modifications at 34U of the anticodon. Loss of taurine modification has been linked to the development of mitochondrial diseases, but the molecular mechanism is still unclear. Here, we showed that taurine modification is catalyzed by mitochondrial optimization 1 (Mto1) in mammals. Mto1 deficiency severely impaired mitochondrial translation and respiratory activity. Moreover, Mto1-deficient cells exhibited abnormal mitochondrial morphology owing to aberrant trafficking of nuclear DNA-encoded mitochondrial proteins, including Opa1. The mistargeted proteins were aggregated and misfolded in the cytoplasm, which induced cytotoxic unfolded protein response. Importantly, application of chemical chaperones successfully suppressed cytotoxicity by reducing protein misfolding and increasing functional mitochondrial proteins in Mto1-deficient cells and mice. Thus, our results demonstrate the essential role of taurine modification in mitochondrial translation and reveal an intrinsic protein homeostasis network between the mitochondria and cytosol, which has therapeutic potential for mitochondrial diseases.

Functional wiring of proteostatic and mitostatic modules ensures transient organismal survival during imbalanced mitochondrial dynamics

Being an assembly of protein machines, cells depend on adequate supply of energetic molecules for retaining their homeodynamics. Consequently, mitochondria functionality is ensured by quality control systems and mitochondrial dynamics (fusion/fission). Similarly, proteome stability is maintained by the machineries of the proteostasis network. We report here that reduced mitochondrial fusion rates in Drosophila caused developmental lethality or if induced in the adult accelerated aging. Imbalanced mitochondrial dynamics were tolerable for various periods in young flies, where they caused oxidative stress and proteome instability that mobilized Nrf2 and foxo to upregulate cytoprotective antioxidant/proteostatic modules. Consistently, proteasome inhibition or Nrf2, foxo knock down in young flies exaggerated perturbed mitochondrial dynamics toxicity. Neither Nrf2 overexpression (with concomitant proteasome activation) nor Atg8a upregulation suppressed the deregulated mitochondrial dynamics toxicity, which was mildly mitigated by antioxidants. Thus, despite extensive functional wiring of mitostatic and antioxidant/proteostatic modules, sustained loss-of mitostasis exhausts adaptation responses triggering premature aging.

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Reduced mitochondrial fusion rates cause severe organismal toxicity and progeria.

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Perturbed mitostasis activates cytoprotective antioxidant and proteostatic modules.

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Nrf2 or Foxo KD exaggerates the imbalanced mitochondrial dynamics induced toxicity.

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Antioxidants mildly alleviate loss-of mitochondrial dynamics-mediated progeria.

Mitochondrial Stasis Reveals p62-Mediated Ubiquitination in Parkin-Independent Mitophagy and Mitigates Nonalcoholic Fatty Liver Disease

It is unknown what occurs if both mitochondrial division and fusion are completely blocked. Here, we introduced mitochondrial stasis by deleting two dynamin-related GTPases for division (Drp1) and fusion (Opa1) in livers. Mitochondrial stasis rescues liver damage and hypotrophy caused by the single knockout (KO). At the cellular level, mitochondrial stasis re-establishes mitochondrial size and rescues mitophagy defects caused by division deficiency. Using Drp1KO livers, we found that the autophagy adaptor protein p62/sequestosome-1-which is thought to function downstream of ubiquitination-promotes mitochondrial ubiquitination. p62 recruits two subunits of a cullin-RING ubiquitin E3 ligase complex, Keap1 and Rbx1, to mitochondria. Resembling Drp1KO, diet-induced nonalcoholic fatty livers enlarge mitochondria and accumulate mitophagy intermediates. Resembling Drp1Opa1KO, Opa1KO rescues liver damage in this disease model. Our data provide a new concept that mitochondrial stasis leads the spatial dimension of mitochondria to a stationary equilibrium and a new mechanism for mitochondrial ubiquitination in mitophagy.

Rapidly purified ganglion cells from neonatal mouse retinas allow studies of mitochondrial morphology and autophagy

Retinal explants and mixed primary cultures are currently used to investigate retinal ganglion cells (RGCs) pathophysiology and pharmacology, but information on yield, quality and quantity of contaminant cells for the available RGCs enrichment techniques is lacking. Here we compare two methods of mouse primary RGCs purification and show that mitochondrial and autophagy parameters can be measured in rapidly purified RGCs. RGCs were purified from P0 mouse eyes using two methods based on the surface antigen Thy1. In a two-step immunopanning purification, a subtraction plate bound macrophage antiserum removed contaminant macrophages and endothelial cells; unbound RGCs were then affinity selected using a plate-bound antiThy1 antibody. In an immunopanning-magnetic separation, macrophage-antiserum bound cells were first subtracted and then RGCs were positively selected using an antiThy1 antibody bound to a magnetic column. The two-steps immunopanning yielded low amounts of 90% pure RGCs, whereas RGCs represented 30% of the 6-fold more cells collected by immunopanning-magnetic separation. RGCs purified with both methods could be microelectroporated to image expressed mitochondria and autophagosomes fluorescent probes and to show that expression of pathogenic Optic atrophy 1 mutants causes mitochondrial fragmentation. Thus, these two methods purify primary mouse RGCs amenable to studies of cell morphology, mitochondrial biology and autophagy.

Mitochondrial and Lipid Droplet Dynamics Regulate Intra- and Intercellular Fatty Acid Trafficking

Imaging of fatty acid (FA) trafficking revealed that FAs stored in lipid droplets were delivered to mitochondria when the cells were starved. This delivery required cytoplasmic lipases and mitochondrial fusion activity, whereas lipid droplets were replenished with FAs supplied by autophagy. These findings have important implications for cancer.

Age-Associated Loss of OPA1 in Muscle Impacts Muscle Mass, Metabolic Homeostasis, Systemic Inflammation, and Epithelial Senescence

Mitochondrial dysfunction occurs during aging, but its impact on tissue senescence is unknown. Here, we find that sedentary but not active humans display an age-related decline in the mitochondrial protein, optic atrophy 1 (OPA1), that is associated with muscle loss. In adult mice, acute, muscle-specific deletion of Opa1 induces a precocious senescence phenotype and premature death. Conditional and inducible Opa1 deletion alters mitochondrial morphology and function but not DNA content. Mechanistically, the ablation of Opa1 leads to ER stress, which signals via the unfolded protein response (UPR) and FoxOs, inducing a catabolic program of muscle loss and systemic aging. Pharmacological inhibition of ER stress or muscle-specific deletion of FGF21 compensates for the loss of Opa1, restoring a normal metabolic state and preventing muscle atrophy and premature death. Thus, mitochondrial dysfunction in the muscle can trigger a cascade of signaling initiated at the ER that systemically affects general metabolism and aging.

Mitochondrial Dynamics as a Therapeutic Target for Treating Cardiac Diseases

Mitochondria are dynamic in nature and are able to shift their morphology between elongated interconnected mitochondrial networks and a fragmented disconnected arrangement by the processes of mitochondrial fusion and fission, respectively. Changes in mitochondrial morphology are regulated by the mitochondrial fusion proteins - mitofusins 1 and 2 (Mfn1 and 2), and optic atrophy 1 (Opa1) as well as the mitochondrial fission proteins - dynamin-related peptide 1 (Drp1) and fission protein 1 (Fis1). Despite having a unique spatial arrangement, cardiac mitochondria have been implicated in a variety of disorders including ischemia-reperfusion injury (IRI), heart failure, diabetes, and pulmonary hypertension. In this chapter, we review the influence of mitochondrial dynamics in these cardiac disorders as well as their potential as therapeutic targets in tackling cardiovascular disease.

Aberrant Alterations of Mitochondrial Factors Drp1 and Opa1 in the Brains of Scrapie Experiment Rodents

The abnormal mitochondrial dynamics has been reported in the brains of some neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), but limitedly described in prion disease. Dynamin-related protein 1 (Drpl) and optic atrophy protein 1 (Opa1) are two essential elements for mitochondria fission and fusion. To evaluate possible changes of mitochondria dynamics during prion infection, the situations of brain Drp1 and Opa1 of scrapie strains 139A, ME7, and S15 mice, as well as 263K-infected hamsters, were analyzed. Significant decreases of brain Drp1 were observed in scrapie-infected rodents at terminal stage by Western blots and immunohistochemical assays, while the levels of Opa1 also showed declined tendency in the brains of scrapie-infected rodents. Immunofluorescent assays illustrated well localization of Drp1 or Opa1 within NeuN-positive cells. Moreover, the S-nitrosylated forms of Drp1significantly increased in the brain tissues of 139A- and ME7-infected mice at terminal stage. Dynamic analysis of Drp1 and SNO-Dpr1 in the brains collected at different time points within the incubation period of 139A-infected mice demonstrated that the whole Drp1 decreased at all tested samples, whereas the SNO-Drp1 remarkably increased in the sample of 90-day post-infection (dpi), reached to the peak in that of 120 dpi and dropped down but still maintained at higher level at the end of disease. The levels of apoptotic factors cleaved caspase 9, caspase 3, and Bax were also markedly increased in the brain tissues of the mice infected with agents 139A and ME7. Our data indicate a disorder of mitochondria dynamics in the brains of prion infection, largely depending on the abnormal alteration of brain Drp1.

Mitochondrial dynamics following global cerebral ischemia

Global brain ischemia/reperfusion induces neuronal damage in vulnerable brain regions, leading to mitochondrial dysfunction and subsequent neuronal death. Induction of neuronal death is mediated by release of cytochrome c (cyt c) from the mitochondria though a well-characterized increase in outer mitochondrial membrane permeability. However, for cyt c to be released it is first necessary for cyt c to be liberated from the cristae junctions which are gated by Opa1 oligomers. Opa1 has two known functions: maintenance of the cristae junction and mitochondrial fusion. These roles suggest that Opa1 could play a central role in both controlling cyt c release and mitochondrial fusion/fission processes during ischemia/reperfusion. To investigate this concept, we first utilized in vitro real-time imaging to visualize dynamic changes in mitochondria. Oxygen-glucose deprivation (OGD) of neurons grown in culture induced a dual-phase mitochondrial fragmentation profile: (i) fragmentation during OGD with no apoptosis activation, followed by fusion of mitochondrial networks after reoxygenation and a (ii) subsequent extensive fragmentation and apoptosis activation that preceded cell death. We next evaluated changes in mitochondrial dynamic state during reperfusion in a rat model of global brain ischemia. Evaluation of mitochondrial morphology with confocal and electron microscopy revealed a similar induction of fragmentation following global brain ischemia. Mitochondrial fragmentation aligned temporally with specific apoptotic events, including cyt c release, caspase 3/7 activation, and interestingly, release of the fusion protein Opa1. Moreover, we uncovered evidence of loss of Opa1 complexes during the progression of reperfusion, and electron microscopy micrographs revealed a loss of cristae architecture following global brain ischemia. These data provide novel evidence implicating a temporal connection between Opa1 alterations and dysfunctional mitochondrial dynamics following global brain ischemia.

Restoration of Opa1-long isoform inhibits retinal injury-induced neurodegeneration

Optic atrophy 1 (Opa1) is a critical factor that regulates fusion and other important functions of mitochondria. In mitochondrion, the N-terminal mitochondrial targeting sequence of Opa1 precursors is removed to generate Opa1 long isoforms (L-Opa1), which are further cleaved into short isoforms (S-Opa1). In the present study, we found that retinal ischemia-reperfusion (I/R) injury and intravitreal injection of carbonylcyanide m-chlorophenyl hydrazone (CCCP) both dramatically induced Opa1 cleavage and caused loss of L-Opa1. In cultured neuronal cells under hypoxia-reoxygenation (H/R) injury, similar changes for Opa1 were also observed. In contrast, restoration of L-Opa1 level by overexpression of S1 cleavage site deletion Opa1 splice 1 (Opa1-ΔS1) not only normalized the H/R-induced mitochondrial morphology changes, but also inhibited the H/R-induced apoptosis, necrosis, and the intracellular ATP loss. Furthermore, recovering L-Opa1 level in the I/R-injured retina by intravitreal injection of genipin or overexpression of Opa1-ΔS1 inhibited apoptosis, necrosis, cell loss in the ganglion cell layer and retinal thickness reduction. Together, our data demonstrated the loss of L-Opa1 is involved in the development of retinal I/R injury, indicating restoring L-Opa1 level may be considered as a therapeutic target for I/R injury-related diseases, at least for the retina. Key messages: Retinal ischemia-reperfusion (I/R) or hypoxia-reoxygenation (H/R) injury induces L-Opa1 loss. Opa1-ΔS1 overexpression inhibits H/R-induced L-Opa1 loss. Opa1-ΔS1 overexpression inhibits H/R-induced mitochondria morphology change. Opa1-ΔS1 and genipin inhibit retinal I/R injury-induced necroptosis. Opa1-ΔS1 and genipin inhibit retinal I/R injury-induced neurodegeneration.

Effect of electrical stimulation-induced resistance exercise on mitochondrial fission and fusion proteins in rat skeletal muscle

It is well known that resistance exercise increases muscle protein synthesis and muscle strength. However, little is known about the effect of resistance exercise on mitochondrial dynamics, which is coupled with mitochondrial function. In skeletal muscle, mitochondria exist as dynamic networks that are continuously remodeling through fusion and fission. The purpose of this study was to investigate the effect of acute and chronic resistance exercise, which induces muscle hypertrophy, on the expression of proteins related to mitochondrial dynamics in rat skeletal muscle. Resistance exercise consisted of maximum isometric contraction, which was induced by percutaneous electrical stimulation of the gastrocnemius muscle. Our results revealed no change in levels of proteins that regulate mitochondrial fission (Fis1 and Drp1) or fusion (Opa1, Mfn1, and Mfn2) over the 24-h period following acute resistance exercise. Phosphorylation of Drp1 at Ser616 was increased immediately after exercise (P < 0.01). Four weeks of resistance training (3 times/week) increased Mfn1 (P < 0.01), Mfn2 (P < 0.05), and Opa1 (P < 0.01) protein levels without altering mitochondrial oxidative phosphorylation proteins. These observations suggest that resistance exercise has little effect on mitochondrial biogenesis but alters the expression of proteins involved in mitochondrial fusion and fission, which may contribute to mitochondrial quality control and improved mitochondrial function.

In vivo time-lapse imaging of mitochondria in healthy and diseased peripheral myelin sheath

The myelin sheath that covers a large amount of neurons is critical for their homeostasis, and myelinating glia mitochondria have recently been shown to be essential for neuron survival. However morphological and physiological properties of these organelles remain elusive. Here we report a method to analyze mitochondrial dynamics and morphology in myelinating Schwann cells of living mice using viral transduction and time-lapse multiphoton microscopy. We describe the distribution, shape, size and dynamics of mitochondria in live cells. We also report mitochondrial alterations in Opa1(delTTAG) mutant mice cells at presymptomatic stages, suggesting that mitochondrial defects in myelin contribute to OPA1 related neuropathy and represent a biomarker for the disease.

A Mitofusin-2-dependent inactivating cleavage of Opa1 links changes in mitochondria cristae and ER contacts in the postprandial liver

Hepatic metabolism requires mitochondria to adapt their bioenergetic and biosynthetic output to accompany the ever-changing anabolic/catabolic state of the liver cell, but the wiring of this process is still largely unknown. Using a postprandial mouse liver model and quantitative cryo-EM analysis, we show that when the hepatic mammalian target of rapamycin complex 1 (mTORC1) signaling pathway disengages, the mitochondria network fragments, cristae density drops by 30%, and mitochondrial respiratory capacity decreases by 20%. Instead, mitochondria-ER contacts (MERCs), which mediate calcium and phospholipid fluxes between these organelles, double in length. These events are associated with the transient expression of two previously unidentified C-terminal fragments (CTFs) of Optic atrophy 1 (Opa1), a mitochondrial GTPase that regulates cristae biogenesis and mitochondria dynamics. Expression of Opa1 CTFs in the intermembrane space has no effect on mitochondria morphology, supporting a model in which they are intermediates of an Opa1 degradation program. Using an in vitro assay, we show that these CTFs indeed originate from the cleavage of Opa1 at two evolutionarily conserved consensus sites that map within critical folds of the GTPase. This processing of Opa1, termed C-cleavage, is mediated by the activity of a cysteine protease whose activity is independent from that of Oma1 and presenilin-associated rhomboid-like (PARL), two known Opa1 regulators. However, C-cleavage requires Mitofusin-2 (Mfn2), a key factor in mitochondria-ER tethering, thereby linking cristae remodeling to MERC assembly. Thus, in vivo, mitochondria adapt to metabolic shifts through the parallel remodeling of the cristae and of the MERCs via a mechanism that degrades Opa1 in an Mfn2-dependent pathway.

Mitochondrial fusion but not fission regulates larval growth and synaptic development through steroid hormone production

Mitochondrial fusion and fission affect the distribution and quality control of mitochondria. We show that Marf (Mitochondrial associated regulatory factor), is required for mitochondrial fusion and transport in long axons. Moreover, loss of Marf leads to a severe depletion of mitochondria in neuromuscular junctions (NMJs). Marf mutants also fail to maintain proper synaptic transmission at NMJs upon repetitive stimulation, similar to Drp1 fission mutants. However, unlike Drp1, loss of Marf leads to NMJ morphology defects and extended larval lifespan. Marf is required to form contacts between the endoplasmic reticulum and/or lipid droplets (LDs) and for proper storage of cholesterol and ecdysone synthesis in ring glands. Interestingly, human Mitofusin-2 rescues the loss of LD but both Mitofusin-1 and Mitofusin-2 are required for steroid-hormone synthesis. Our data show that Marf and Mitofusins share an evolutionarily conserved role in mitochondrial transport, cholesterol ester storage and steroid-hormone synthesis.

DOI:http://dx.doi.org/10.7554/eLife.03558.001

Mitochondria are the main source of energy for cells. These vital and highly dynamic organelles continually change shape by fusing with each other and splitting apart to create new mitochondria, repairing and replacing those damaged by cell stress.

For nerve impulses to be transmitted across the gaps (called synapses) between nerve cells, mitochondria need to supply the very ends of the nerve fibers with energy. To do this, the mitochondria must be transported from the main body of the nerve cell to the tips of the nerve fibers. This may not happen if mitochondria are the wrong shape, size or damaged.

While searching for genetic mutations that disrupt nerve function in the fruit fly Drosophila, Sandoval et al. spotted mutations in a gene called Marf. Further investigations revealed that flies with mutant versions of Marf have small, round mitochondria, and their nerves cannot transmit signals to muscles when they are highly stimulated. This is because the mutant mitochondria are not easily transported along nerve fibers, and so not enough energy is supplied to the synapses. The synapses of the Marf mutants are also abnormally shaped. Sandoval et al. found that this is not because Marf is lost in the neurons themselves, but because it is lost from a hormone-producing tissue called the ring gland.

Another problem found in flies with mutated Marf genes is that they stop developing while in their larval stage. Sandoval et al. established that this could also be related to the loss of Marf from the ring gland. The Marf protein has two different functions in the ring gland: forming and storing droplets of fatty molecules used in hormone production, and synthesising a hormone that controls when a fly larva matures into the adult fly. This suggests that the lower levels of this hormone produced by Marf mutant flies underlies their prolonged larval stages and synapse defects.

Vertebrates (animals with backbones, such as humans) have two genes that are related to the fly's Marf gene. When the human forms of these genes were introduced into mutant flies that lack a working copy of Marf, hormone production was only restored if both genes were introduced together. This indicates that these genes have separate roles in vertebrates, but that these roles are both performed by the single fly gene.

The role of Marf in tethering mitochondria in the ring gland may allow us to better understand how this process affects hormone production and how the different parts of the cell communicate.

DOI:http://dx.doi.org/10.7554/eLife.03558.002

Mitochondrial dynamics regulate melanogenesis through proteasomal degradation of MITF via ROS-ERK activation

Mitochondrial dynamics control mitochondrial functions as well as their morphology. However, the role of mitochondrial dynamics in melanogenesis is largely unknown. Here, we show that mitochondrial dynamics regulate melanogenesis by modulating the ROS-ERK signaling pathway. Genetic and chemical inhibition of Drp1, a mitochondrial fission protein, increased melanin production and mitochondrial elongation in melanocytes and melanoma cells. In contrast, down-regulation of OPA1, a mitochondria fusion regulator, suppressed melanogensis but induced massive mitochondrial fragmentation in hyperpigmented cells. Consistently, treatment with CCCP, a mitochondrial fission chemical inducer, also efficiently repressed melanogenesis. Furthermore, we found that ROS production and ERK phosphorylation were increased in cells with fragmented mitochondria. And inhibition of ROS or ERK suppressed the antimelanogenic effect of mitochondrial fission in α-MSH-treated cells. In addition, the activation of ROS-ERK pathway by mitochondrial fission induced phosphorylation of serine73 on MITF accelerating its proteasomal degradation. In conclusion, mitochondrial dynamics may regulate melanogenesis by modulating ROS-ERK signaling pathway.

p53 is required for cisplatin-induced processing of the mitochondrial fusion protein L-Opa1 that is mediated by the mitochondrial metallopeptidase Oma1 in gynecologic cancers

Mitochondria are highly dynamic organelles, and mitochondrial fission is a crucial step of apoptosis. Although Oma1 is believed to be responsible for long form Opa1 (L-Opa1) processing during mitochondrial fragmentation, whether and how Oma1 is involved in L-Opa1 processing and participates in the regulation of chemoresistance is unknown. Chemosensitive and chemoresistant ovarian (OVCA) and cervical (CECA) cancer cells were treated with cisplatin (CDDP). Mitochondrial dynamics and protein contents were assessed by immunofluorescence and Western blot, respectively. The requirements of Oma1 and p53 for CDDP-induced L-Opa1 processing, mitochondrial fragmentation, and apoptosis were examined by siRNA or cDNA. CDDP induces L-Opa1 processing and mitochondrial fragmentation in chemosensitive but not in chemoresistant cells. CDDP induced Oma1 40-kDa form increases in OV2008 cells, not in C13* cells. Oma1 knockdown inhibited L-Opa1 processing, mitochondrial fragmentation, and apoptosis. Silencing p53 expression attenuated the effects of CDDP in Oma1 (40 kDa) increase, L-Opa1 processing, mitochondrial fragmentation, and apoptosis in chemosensitive OVCA cells, whereas reconstitution of p53 in p53 mutant or null chemoresistant OVCA cells induced Oma1 (40 kDa) increase, L-Opa1 processing, mitochondrial fragmentation, and apoptosis irrespective of the presence of CDDP. Prohibitin 1 (Phb1) dissociates from Opa1-Phb1 complex and binds phosphorylated p53 (serine 15) in response to CDDP in chemosensitive but not chemoresistant CECA cells. These findings demonstrate that (a) p53 and Oma1 mediate L-Opa1 processing, (b) mitochondrial fragmentation is involved in CDDP-induced apoptosis in OVCA and CECA cells, and (c) dysregulated mitochondrial dynamics may in part be involved in the pathophysiology of CDDP resistance.

Mitochondrial fragmentation impairs insulin-dependent glucose uptake by modulating Akt activity through mitochondrial Ca2+ uptake

Insulin is a major regulator of glucose metabolism, stimulating its mitochondrial oxidation in skeletal muscle cells. Mitochondria are dynamic organelles that can undergo structural remodeling in order to cope with these ever-changing metabolic demands. However, the process by which mitochondrial morphology impacts insulin signaling in the skeletal muscle cells remains uncertain. To address this question, we silenced the mitochondrial fusion proteins Mfn2 and Opa1 and assessed insulin-dependent responses in L6 rat skeletal muscle cells. We found that mitochondrial fragmentation attenuates insulin-stimulated Akt phosphorylation, glucose uptake and cell respiratory rate. Importantly, we found that insulin induces a transient rise in mitochondrial Ca(2+) uptake, which was attenuated by silencing Opa1 or Mfn2. Moreover, treatment with Ruthenium red, an inhibitor of mitochondrial Ca(2+) uptake, impairs Akt signaling without affecting mitochondrial dynamics. All together, these results suggest that control of mitochondrial Ca(2+) uptake by mitochondrial morphology is a key event for insulin-induced glucose uptake.