Mechanisms Of Mitochondrial Fusion Research Articles

Abstract 692: Uncoupling protein 2 (Ucp2) loss of function enhances mitochondrial ROS and sensitizes PDAC to radiation therapy

Abstract Introduction: Pancreatic ductal adenocarcinoma (PDAC) is a lethal cancer with a dependence on highly active mitochondria to grow and metastasize. Normally mitochondria exist in long networks due to mitochondrial fusion but in PDAC mitochondria are fragmented due to excessive fission. We previously found that forcing PDAC cells to increase mitochondrial fusion suppressed tumor growth and metastasis, leading to improved survival in multiple preclinical models. The tumor suppressive mechanisms of mitochondrial fusion remain unclear, and thus we explored the role of redox homeostasis through uncoupling protein 2 (UCP2), a metabolic protein normally upregulated in PDAC that was downregulated upon forced mitochondrial fusion. We hypothesize that UCP2 mediates the oncogenic effects of mitochondrial fragmentation in PDAC. Methods: We explored if Ucp2 is necessary and sufficient for inducing tumor suppression upon mitochondrial fusion. The effects of Ucp2 loss or overexpression were characterized by metabolic flux assays, proliferation, mitochondrial ROS (mROS), membrane potential, and morphology measurements. We also examined if Ucp2 recapitulates mitochondrial fusion-induced tumor suppression via pharmacological inhibition and genetic ablation and assessed the impact on mitochondrial health and cell growth. We utilized the LSL-KRASG12D; Trp53R172H/+; Ptf1⍺ Cre (KPC) cell line as our parental line for all genetic manipulations, with a nontargeting (sgGFP) KPC cell line as a control. Results: Murine PDAC cells with enhanced mitochondrial fusion reduced Ucp2 transcript levels by 70% and was correlated with 3.5-fold increase in mROS, 50% reduction in OXPHOS via oxygen consumption rate assay, and a 4-fold increase in membrane potential compared to controls. Ucp2 re-expression did not rescue these phenotypes, suggesting that Ucp2 alone may not be sufficient for tumor suppression. However, inhibition of Ucp2 in PDAC cells reduced proliferation by 70%, while inducing a 3-fold increase in mROS levels, which was recapitulated with Ucp2 abrogation, thus Ucp2 may be necessary for tumor suppression. Enhanced mROS via Ucp2 inhibition significantly increased radiosensitivity (p-value < 0.0001), suggesting that although it does not recapitulate the mitochondrial fusion phenotype, UCP2 may be a novel target for radiosensitization in PDAC. Conclusions: Mitochondrial fusion may not suppress tumor growth by altering redox homeostasis via UCP2, but forcing mitochondria to fuse elucidated the impacts of UCP2 downregulation on cell metabolism. Thus, we uncovered that UCP2 may be a novel target for radiosensitization. Further insight into UCP2’s role in redox balance, and how it is disrupted with UCP2 modulation enables development of novel therapeutic combinations with UCP2 inhibition that complement and potentially synergize with existing therapeutic strategies such as radiation. Citation Format: Emily G. Caggiano, Meifang Yu, Ariana Acevedo-Diaz, Cullen M. Taniguchi. Uncoupling protein 2 (Ucp2) loss of function enhances mitochondrial ROS and sensitizes PDAC to radiation therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 692.

TNF-α activates RELA expression via TNFRSF1B to upregulate OPA1 expression and inhibit chondrogenic differentiation of human adipose stem cells

BackgroundTumor necrosis factor-alpha (TNF-α), one of the pro-inflammatory cytokines mediating the local inflammatory process in joints, inhibits cartilage formation and has a detrimental effect on stem cell-based cartilage regeneration for the treatment of osteoarthritis (OA). However, the mechanisms behind this inhibitory effect are still poorly understood. Mitochondrial morphological changes mediated by mitochondrial fusion and fission are highly plastic, are quite sensitive to environmental stimuli and play a crucial role in maintaining cell structure and function. In our study, chondrogenic differentiated human adipose stem cells (hADSCs) were exposed to TNF-α and the effect of TNF-α on the ability of hADSCs to chondrogenic differentiate and on mitochondrial fusion and fission was observed and analyzed. The aim was to investigate the role and mechanisms of mitochondrial fusion and fission regulation in the chondrogenic differentiation of hADSCs under normal conditions and under exposure to TNF-α.MethodsWe used flow cytometry to identify hADSCs immunophenotypes CD29, CD44, CD34, CD45, and HLA-DR. Alcian blue staining and Sirius red staining were used to observe the formation of proteoglycans and collagen during the chondrogenic differentiation of hADSCs, respectively. The mRNA and protein expression levels of the cartilage formation marker SOX9, type II collagen (COL2A1), and Aggrecan were measured by real-time fluorescent quantitative PCR (RT-qPCR) and western blot, respectively. The fluorescent probes MitoTracker® Red CMXRos and JC-1 were used to visualize mitochondria morphology and detect mitochondrial membrane electricity (MMP). Affymetrix PrimeView™ chips were used for gene expression profiling.ResultsThe results showed that the chondrogenic differentiation of hADSCs was inhibited in the presence of TNF-α that optic atrophy 1 (OPA1) expression was significantly upregulated and mitochondria were prolonged and interconnected during this process. Gene microarray and RT-qPCR data showed that the presence of TNF-α led to increased expression of TNFα receptor 2 (TNFRSF1B) and RELA during chondrogenic differentiation of hADSCs.ConclusionsTNF-α inhibits chondrogenic differentiation of human adipose stem cells by activating RELA expression through TNFRSF1B upregulating OPA1 expression thereby increasing mitochondrial fusion.

Abstract 4830: Uncoupling protein 2 (UCP2) loss of function mediates PDAC tumor suppression by mitochondrial fusion

Abstract Background: Pancreatic ductal adenocarcinoma (PDAC) is a lethal cancer with a dependence on highly active mitochondria to grow and metastasize. We previously found that mitochondrial fusion suppressed PDAC growth and metastasis, leading to improved survival in a preclinical model. The tumor suppressive mechanisms of mitochondrial fusion remain unclear, and thus we explored the role of redox homeostasis through uncoupling protein 2 (UCP2). Methods/Results: Murine pancreatic cancer cells (KPC) with enhanced mitochondrial fusion (sgDrp1) exhibited higher levels of mitochondrial ROS compared to controls targeting GFP (sgGFP), suggesting altered redox homeostasis. We performed RNAseq on these two cell lines and found that sgDRP1 cells exhibited reduced levels of UCP2. The UCP2 protein is known to have indirect antioxidant effects by reducing mitochondrial reactive oxygen species via uncoupling of ATP production from the electron transport chain. We found that abrogating UCP2 in KPC cells reduced its capacity for OXPHOS. Moreover, rescue of UCP2 in sgDrp1 cells restored their mitochondrial activity. Conclusions: Mitochondrial fusion may suppress tumor growth and induce mitophagy by altering redox homeostasis. UCP2 may be a mediator of the tumor suppressive phenotype induced by mitochondrial fusion, thus linking mitochondrial dynamics and PDAC antioxidant strategies. Future Work: These results will be explored for their translation in vivo and the effects of pharmacological inhibition of UCP2, both in vitro and in vivo, via the specific inhibitor Genipin, will be explored. Citation Format: Ariana Carolina Acevedo-Diaz, Emily G. Caggiano, Meifang Yu, Cullen Taniguchi. Uncoupling protein 2 (UCP2) loss of function mediates PDAC tumor suppression by mitochondrial fusion. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4830.

MFN2 Deficiency Impairs Mitochondrial Functions and PPAR Pathway During Spermatogenesis and Meiosis in Mice.

Mitochondria are highly dynamic organelles and their activity is known to be regulated by changes in morphology via fusion and fission events. However, the role of mitochondrial dynamics on cellular differentiation remains largely unknown. Here, we explored the molecular mechanism of mitochondrial fusion during spermatogenesis by generating an Mfn2 (mitofusin 2) conditional knock-out (cKO) mouse model. We found that depletion of MFN2 in male germ cells led to disrupted spermatogenesis and meiosis during which the majority of Mfn2 cKO spermatocytes did not develop to the pachytene stage. We showed that in these Mfn2 cKO spermatocytes, oxidative phosphorylation in the mitochondria was affected. In addition, RNA-Seq analysis showed that there was a significantly altered transcriptome profile in the Mfn2 deficient pachytene (or pachytene-like) spermatocytes, with a total of 262 genes up-regulated and 728 genes down-regulated, compared with wild-type (control) mice. Pathway enrichment analysis indicated that the peroxisome proliferator-activated receptor (PPAR) pathway was altered, and subsequent more detailed analysis showed that the expression of PPAR α and PPAR γ was up-regulated and down-regulated, respectively, in the MFN2 deficient pachytene (or pachytene-like) spermatocytes. We also demonstrated that there were more lipid droplets in the Mfn2 cKO cells than in the control cells. In conclusion, our study demonstrates a novel finding that MFN2 deficiency negatively affects mitochondrial functions and alters PPAR pathway together with lipid metabolism during spermatogenesis and meiosis.

Insights Into Mitochondrial Dynamics in Chlamydial Infection

Mitochondria are intracellular organelles that are instrumental in the creation of energy, metabolism, apoptosis, and intrinsic immunity. Mitochondria exhibit an extraordinarily high degree of flexibility, and are constantly undergoing dynamic fusion and fission changes. Chlamydia is an intracellular bacterium that causes serious health problems in both humans and animals. Due to a deficiency of multiple metabolic enzymes, these pathogenic bacteria are highly dependent on their eukaryotic host cells, resulting in a close link between Chlamydia infection and host cell mitochondria. Indeed, Chlamydia increase mitochondrial fusion by inhibiting the activation of dynein-related protein 1 (DRP1), which can regulate host cell metabolism for extra energy. Additionally, Chlamydia can inhibit mitochondrial fission by blocking DRP1 oligomerization, preventing host cell apoptosis. These mechanisms are critical for maintaining a favorable environment for reproduction and growth of Chlamydia. This review discusses the molecular mechanisms of mitochondrial fusion and fission, as well as the mechanisms by which Chlamydia infection alters the mitochondrial dynamics and the prospects of limiting chlamydial development by altering mitochondrial dynamics.

Molecular Machinery and Pathophysiology of Mitochondrial Dynamics.

Mitochondria are double-membraned organelles that exhibit fluidity. They are the main site of cellular aerobic respiration, providing energy for cell proliferation, migration, and survival; hence, they are called “powerhouses.” Mitochondria play an important role in biological processes such as cell death, cell senescence, autophagy, lipid synthesis, calcium homeostasis, and iron balance. Fission and fusion are active processes that require many specialized proteins, including mechanical enzymes that physically alter mitochondrial membranes, and interface proteins that regulate the interaction of these mechanical proteins with organelles. This review discusses the molecular mechanisms of mitochondrial fusion, fission, and physiopathology, emphasizing the biological significance of mitochondrial morphology and dynamics. In particular, the regulatory mechanisms of mitochondria-related genes and proteins in animal cells are discussed, as well as research trends in mitochondrial dynamics, providing a theoretical reference for future mitochondrial research.

The role of mitochondrial fusion and fission in the process of cardiac oxidative stress.

Mitochondria are the energy suppliers in the cell and undergo constant fusion and fission to meet metabolic demand during the cell life cycle. Well-balanced mitochondrial dynamics are extremely important and necessary for cell survival as well as for tissue homeostasis. Cardiomyocytes contain large numbers of mitochondria to satisfy the high energy demand. It has been established that deregulated processes of mitochondrial dynamics play a major role in myocardial cell death. Currently, cardiac mitochondrial cell death pathways attract great attention in the cell biology and regenerative medicine fields. Importantly, mitochondrial dynamics are tightly linked to oxidative stress-induced cardiac damage. This review summarizes molecular mechanisms of mitochondrial fusion and fission processes and their potential roles in myocardial cell death triggered by oxidative stress. Advances in understanding the effect of both normal and abnormal mitochondrial dynamics on heart protection will lead to significant improvement of therapeutic discoveries.

Mitochondrial fusion exploits a therapeutic vulnerability of pancreatic cancer.

Pancreatic ductal adenocarcinoma (PDAC) requires mitochondrial oxidative phosphorylation (OXPHOS) to fuel its growth, however, broadly inhibiting this pathway might also disrupt essential mitochondrial functions in normal tissues. PDAC cells exhibit abnormally fragmented mitochondria that are essential to its oncogenicity, but it was unclear if this mitochondrial feature was a valid therapeutic target. Here, we present evidence that normalizing the fragmented mitochondria of pancreatic cancer via the process of mitochondrial fusion reduces OXPHOS, which correlates with suppressed tumor growth and improved survival in preclinical models. Mitochondrial fusion was achieved by genetic or pharmacologic inhibition of dynamin related protein-1 (Drp1) or through overexpression of mitofusin-2 (Mfn2). Notably, we found that oral leflunomide, an FDA-approved arthritis drug, promoted a two-fold increase in Mfn2 expression in tumors and was repurposed as a chemotherapeutic agent, improving the median survival of mice with spontaneous tumors by 50% compared to vehicle. We found that the chief tumor suppressive mechanism of mitochondrial fusion was enhanced mitophagy, which proportionally reduced mitochondrial mass and ATP production. These data suggest that mitochondrial fusion is a specific and druggable regulator of pancreatic cancer growth that could be rapidly translated to the clinic.

Association between Mitofusin 2 Gene Polymorphisms and Late-Onset Alzheimer's Disease in the Korean Population.

ObjectiveMitochondrial dysfunction is a prominent and early feature of Alzheimer's disease (AD). The morphologic changes observed in the AD brain could be caused by a failure of mitochondrial fusion mechanisms. The aim of this study was to investigate whether genetic polymorphisms of two genes involved in mitochondrial fusion mechanisms, optic atrophy 1 (OPA1) and mitofusin 2 (MFN2), were associated with AD in the Korean population by analyzing genotypes and allele frequencies.MethodsOne coding single nucleotide polymorphism (SNP) in the MFN2, rs1042837, and two coding SNPs in the OPA1, rs7624750 and rs9851685, were compared between 165 patients with AD (83 men and 82 women, mean age 72.3±4.41) and 186 healthy control subjects (82 men and 104 women, mean age 76.5±5.98).ResultsAmong these three SNPs, rs1042837 showed statistically significant differences in allele frequency, and genotype frequency in the co-dominant 1 model and in the dominant model.ConclusionThese results suggest that the rs1042837 polymorphism in MFN2 may be involved in the pathogenesis of AD.

Mitochondrial Dynamics Controlled by Mitofusins Regulate Agrp Neuronal Activity and Diet-Induced Obesity

Mitochondria are key organelles in the maintenance of cellular energy metabolism and integrity. Here, we show that mitochondria number decrease but their size increase in orexigenic agouti-related protein (Agrp) neurons during the transition from fasted to fed to overfed state. These fusion-like dynamic changes were cell-type specific, as they occurred in the opposite direction in anorexigenic pro-opiomelanocortin (POMC) neurons. Interfering with mitochondrial fusion mechanisms in Agrp neurons by cell-selectively knocking down mitofusin 1 (Mfn1) or mitofusin 2 (Mfn2) resulted in altered mitochondria size and density in these cells. Deficiency in mitofusins impaired the electric activity of Agrp neurons during high-fat diet (HFD), an event reversed by cell-selective administration of ATP. Agrp-specific Mfn1 or Mfn2 knockout mice gained less weight when fed a HFD due to decreased fat mass. Overall, our data unmask an important role for mitochondrial dynamics governed by Mfn1 and Mfn2 in Agrp neurons in central regulation of whole-body energy metabolism.

Mitofusin-2 protects against cold stress-induced cell injury in HEK293 cells

Mitochondrial impairment is hypothesized to contribute to cell injury during cold stress. Mitochondria fission and fusion are closely related in the function of the mitochondria, but the precise mechanisms whereby these processes regulate cell injury during cold stress remain to be determined. HEK293 cells were cultured in a cold environment (4.0±0.1°C) for 2, 4, 8, or 12h. Western blot analyses showed that these cells expressed decreased fission-related protein Drp1 and increased fusion-related protein Mfn2 at 4h; meanwhile, electron microscopy analysis revealed large and long mitochondrial morphology within these cells, indicating increased mitochondrial fusion. With silencing of Mfn2 but not of Mfn1 by siRNA promoted cold-stress-induced cell death with decreased ATP production in HEK293 cells. Our results show that increased expression of Mfn2 and mitochondrial fusion are important for mitochondrial function as well as cell survival during cold stress. These findings have important implications for understanding the mechanisms of mitochondrial fusion and fission in cold-stress-induced cell injury.

Mechanisms of mitochondrial fusion and fission

Mechanisms of mitochondrial fusion and fission

OPA1 requires mitofusin 1 to promote mitochondrial fusion.

The regulated equilibrium between mitochondrial fusion and fission is essential to maintain integrity of the organelle. Mechanisms of mitochondrial fusion are largely uncharacterized in mammalian cells. It is unclear whether OPA1, a dynamin-related protein of the inner membrane mutated in autosomal dominant optic atrophy, participates in fusion or fission. OPA1 promoted the formation of a branched network of elongated mitochondria, requiring the integrity of both its GTPase and C-terminal coiled-coil domain. Stable reduction of OPA1 levels by RNA interference resulted in small, fragmented, and scattered mitochondria. Levels of OPA1 did not affect mitochondrial docking, but they correlated with the extent of fusion as measured by polyethylene glycol mitochondrial fusion assays. A genetic analysis proved that OPA1 was unable to tubulate and fuse mitochondria lacking the outer membrane mitofusin 1 but not mitofusin 2. Our data show that OPA1 functionally requires mitofusin 1 to regulate mitochondrial fusion and reveal a specific functional difference between mitofusin 1 and 2.

Mitochondrial fusion intermediates revealed in vitro.

The events that occur during the fusion of double-membraned mitochondria are unknown. As an essential step toward determining the mechanism of mitochondrial fusion, we have captured this event in vitro. Mitochondrial outer and inner membrane fusion events were separable and mechanistically distinct, but both required guanosine 5'-triphosphate hydrolysis. Homotypic trans interactions of the ancient outer transmembrane guanosine triphosphatase, Fzo1, were required to promote the fusion of mitochondrial outer membranes, whereas electrical potential was also required for fusion of inner membranes. Our conclusions provide fundamental insights into the molecular events driving mitochondrial fusion and advance our understanding of the evolution of mitochondrial fusion in eukaryotic cells.

Mitochondrial changes in flight muscles of normal and flightless Drosophila melanogaster with age.

Fine structural changes in mitochondrial morphology pertaining to size, number and growth were examined in flight muscles of normal and experimentally dewinged male Drosophila melanogaster ranging up to 26 days of age. In the normal winged flies, the number of mitochondria decreases during the first week of adult life whereas the size of individual mitochondrial profile increases significantly. Changes in mitochondrial size and number are due to the fusion of mitochondria. Fused mitochondria are are extremely large in size and irregular in shape. In 26-day old normal flies, the number of mitochondria increases while the mitochondrial size is recuced indicating mitochondrial division. In comparison to the normal flies, dewinged flies exhibit a similar degree of mitochondrial fusion and growth during the first week of life. However, the extent of mitochondrial fission in 26-day old dewinged flies is greater than in the normal flies of this age. Structural mechanisms of mitochondrial fusion and fission are described. The objective of this study was to examine the relative effects of age and flight activity on the mitochondria.