Balance Of Mitochondrial Dynamics Research Articles

A threshold of transmembrane potential is required for mitochondrial dynamic balance mediated by DRP1 and OMA1

As an organellar network, mitochondria dynamically regulate their organization via opposing fusion and fission pathways to maintain bioenergetic homeostasis and contribute to key cellular pathways. This dynamic balance is directly linked to bioenergetic function: loss of transmembrane potential across the inner membrane (Δψm) disrupts mitochondrial fission/fusion balance, causing fragmentation of the network. However, the level of Δψm required for mitochondrial dynamic balance, as well as the relative contributions of fission and fusion pathways, have remained unclear. To explore this, mitochondrial morphology and Δψm were examined via confocal imaging and tetramethyl rhodamine ester (TMRE) flow cytometry, respectively, in cultured 143B osteosarcoma cells. When normalized to the TMRE value of untreated 143B cells as 100%, both genetic (mtDNA-depleted ρ0) and pharmacological [carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-treated] cell models below 34% TMRE fluorescence were unable to maintain mitochondrial interconnection, correlating with loss of fusion-active long OPA1 isoforms (L-OPA1). Mechanistically, this threshold is maintained by mechanistic coordination of DRP1-mediated fission and OPA1-mediated fusion: cells lacking either DRP1 or the OMA1 metalloprotease were insensitive to loss of Δψm, instead maintaining an obligately fused morphology. Collectively, these findings demonstrate a mitochondrial ‘tipping point’ threshold mediated by the interaction of Δψm with both DRP1 and OMA1; moreover, DRP1 appears to be required for effective OPA1 maintenance and processing, consistent with growing evidence for direct interaction of fission and fusion pathways. These results suggest that Δψm below threshold coordinately activates both DRP1-mediated fission and OMA1 cleavage of OPA1, collapsing mitochondrial dynamic balance, with major implications for a range of signaling pathways and cellular life/death events.

Relationship Between β-Amyloid and Mitochondrial Dynamics.

Mitochondria as dynamic organelles undergo morphological changes through the processes of fission and fusion which are major factors regulating their functions. A disruption in the balance of mitochondrial dynamics induces functional disorders in mitochondria such as failed energy production and the generation of reactive oxygen species, which are closely related to pathophysiological changes associated with Alzheimer's disease (AD). Recent studies have demonstrated a relationship between abnormalities in mitochondrial dynamics and impaired mitochondrial function, clarifying the effects of morphofunctional aberrations which promote neuronal cell death in AD. Several possible signaling pathways have been suggested for a better understanding of the mechanism behind the key molecules regulating mitochondrial morphologies. However, the exact machinery involved in mitochondrial dynamics still has yet to be elucidated. This paper reviews the current knowledge on signaling mechanisms involved in mitochondrial dynamics and the significance of mitochondrial dynamics in controlling associated functions in neurodegenerative diseases, particularly in AD.

Nutrient Starvation Sensitizes Human Ovarian Cancer SKOV3 Cells to BH3 Mimetic via Modulation of Mitochondrial Dynamics.

The aberrant proliferation of tumor cells necessitates compensatory changes in tumor metabolic processes. Previous studies on tumor growth and metabolism have established a relationship between nutrient stress and Bcl-2 anti-apoptotic proteins, although the mechanisms connecting these processes remain unclear. We induced nutrient deprivation in human ovarian cancer SKOV3 cells by culturing cells in Earle's balanced salt solution (EBSS) as a starvation model. We used EBSS treatment with the BH3 domain of Bcl-2 family proteins (BH3) mimetic ABT737, which targets Bcl-2/Bcl-xL, to examine mitochondrial dynamics and the interactive regulatory mechanisms between nutrition and Bcl-2 proteins. We found that EBSS combined with ABT737 can promote SKOV3 cells to undergo apoptosis and convert tubular mitochondria into small, fragmented morphologies. Bcl-2 family proteins participated in the regulation of mitochondrial fusion and fission through apoptosis, and the decrease of Mcl-1 expression was the key to ABT737 sensitization. Our findings showed that nutrient stress could sensitize SKOV3 cells to ABT737 via regulation of the mitochondrial dynamic balance and interaction of Bcl-2 family proteins. Our data suggest that nutrient starvation combined with the BH3 mimetic ABT737 could reduce the required effective dose of ABT737, and that inhibition of Bcl-2 and Mcl-1 together with nutrient starvation could serve as an effective strategy for the treatment of human ovarian cancer. Anat Rec, 300:326-339, 2017. © 2016 Wiley Periodicals, Inc.

“What makes some rats live so long?” The mitochondrial contribution to longevity through balance of mitochondrial dynamics and mtDNA content

Extremely interesting for aging research are those individuals able to reach older ages still with functions similar to those of younger counterparts. We examined liver samples from ad libitum-fed old (28-month-old, AL-28) and ad libitum-fed very old (32-month-old, AL-32) rats for a number of markers, relevant for mitochondrial functionality and mitochondrial DNA (mtDNA) content. As for the mtDNA content and the protein amounts of the citrate synthase and the antioxidant peroxiredoxin III there were no significant changes in the AL-32 animals. No significant longevity-related change was found for TFAM amount, but a 50% reduction in the amount of the Lon protease, responsible for turnover of TFAM inside mitochondria, characterized the AL-32 rats. No longevity-related change was observed also for the amounts of the mtDNA repair enzymes OGG1 and APE1, whereas the intra-mitochondrial amount of the cytochrome c protein showed a 50% increase in the AL-32 rats, indicating a likely reduced initiation of the intrinsic apoptotic pathway. Totally unexpected was the doubling of two proteins, very relevant for mitochondrial dynamics, namely MFN2 and DRP1, in the AL-32 rats. This prompted us to the calculation of all individual fusion indexes that grouped together in the AL-32 rats, while in the AL-28 animals were very different. We found a strong positive correlation between the fusion indexes and the respective mtDNA contents in two AL-28 and four AL-32 rats. This supports the idea that the limited prevalence of fusion above a still active fission should have ensured a functional mitochondrial network and should have led to a quite narrow range of high mtDNA contents, likely the best-suitable for extended longevity. Our findings strongly suggest that, among the multiple causes leading to the longevity of the AL-32 rats, the maintenance of an adult-like balance of mitochondrial dynamics seems to be very relevant for the regulation of mtDNA content and functionality.

Mitochondrial dynamics in neurodegeneration: from cell death to energetic states

From Parkinson's disease to an ischemic stroke, a consistently reoccurring theme in the context of neuronal degeneration is the dysfunction of mitochondria as the underlying factor. Insight into the mechanistic basis for mitochondrial dysfunction in neurodegenerative disorders has allowed the theme of mitochondrial dynamics to be highlighted as a central player. The precise balance of mitochondrial dynamics is among the most critical features for the juxtaposed processes of cell death and survival. More recently, studies have allowed mitochondrial shape to emerge as a key regulator of respiratory efficiency that can enforce the bioenergetic status of cells and thereby determine cell fate. Here we review the most current advances that provide an explanation for the long-standing question of how mitochondrial shape can impact cellular metabolism. Furthermore, we discuss the implications of an imbalance in mitochondrial dynamics in neurodegenerative disorders.

Editorial (Thematic Issue: Mitochondrial Biogenesis: Pharmacological Approaches)

Editorial (Thematic Issue: Mitochondrial Biogenesis: Pharmacological Approaches)

Hepatitis B Virus Disrupts Mitochondrial Dynamics: Induces Fission and Mitophagy to Attenuate Apoptosis

Human hepatitis B virus (HBV) causes chronic hepatitis and is associated with the development of hepatocellular carcinoma. HBV infection alters mitochondrial metabolism. The selective removal of damaged mitochondria is essential for the maintenance of mitochondrial and cellular homeostasis. Here, we report that HBV shifts the balance of mitochondrial dynamics toward fission and mitophagy to attenuate the virus-induced apoptosis. HBV induced perinuclear clustering of mitochondria and triggered mitochondrial translocation of the dynamin-related protein (Drp1) by stimulating its phosphorylation at Ser616, leading to mitochondrial fission. HBV also stimulated the gene expression of Parkin, PINK1, and LC3B and induced Parkin recruitment to the mitochondria. Upon translocation to mitochondria, Parkin, an E3 ubiquitin ligase, underwent self-ubiquitination and facilitated the ubiquitination and degradation of its substrate Mitofusin 2 (Mfn2), a mediator of mitochondrial fusion. In addition to conventional immunofluorescence, a sensitive dual fluorescence reporter expressing mito-mRFP-EGFP fused in-frame to a mitochondrial targeting sequence was employed to observe the completion of the mitophagic process by delivery of the engulfed mitochondria to lysosomes for degradation. Furthermore, we demonstrate that viral HBx protein plays a central role in promoting aberrant mitochondrial dynamics either when expressed alone or in the context of viral genome. Perturbing mitophagy by silencing Parkin led to enhanced apoptotic signaling, suggesting that HBV-induced mitochondrial fission and mitophagy promote cell survival and possibly viral persistence. Altered mitochondrial dynamics associated with HBV infection may contribute to mitochondrial injury and liver disease pathogenesis.

Deconstructing mitochondrial dysfunction in Alzheimer disease.

There is mounting evidence showing that mitochondrial damage plays an important role in Alzheimer disease. Increased oxygen species generation and deficient mitochondrial dynamic balance have been suggested to be the reason as well as the consequence of Alzheimer-related pathology. Mitochondrial damage has been related to amyloid-beta or tau pathology or to the presence of specific presenilin-1 mutations. The contribution of these factors to mitochondrial dysfunction is reviewed in this paper. Due to the relevance of mitochondrial alterations in Alzheimer disease, recent works have suggested the therapeutic potential of mitochondrial-targeted antioxidant. On the other hand, autophagy has been demonstrated to play a fundamental role in Alzheimer-related protein stress, and increasing data shows that this pathway is altered in the disease. Moreover, mitochondrial alterations have been related to an insufficient clearance of dysfunctional mitochondria by autophagy. Consequently, different approaches for the removal of damaged mitochondria or to decrease the related oxidative stress in Alzheimer disease have been described. To understand the role of mitochondrial function in Alzheimer disease it is necessary to generate human cellular models which involve living neurons. We have summarized the novel protocols for the generation of neurons by reprogramming or direct transdifferentiation, which offer useful tools to achieve this result.

A Small Molecule Promotes Mitochondrial Fusion in Mammalian Cells

Mitochondrial dynamics: An image-based screen identified a small molecule, M1, that specifically promotes the fusion of fragmented mitochondria and protects cells from mitochondrial-fragmentation-associated cell death. Mechanistic studies revealed that M1 shifts the mitochondrial dynamic balance towards fusion (see picture).

Mitochondrial Fission Contributes to Mitochondrial Dysfunction and Insulin Resistance in Skeletal Muscle

Mitochondrial dysfunction in skeletal muscle has been implicated in the development of insulin resistance and type 2 diabetes. Considering the importance of mitochondrial dynamics in mitochondrial and cellular functions, we hypothesized that obesity and excess energy intake shift the balance of mitochondrial dynamics, further contributing to mitochondrial dysfunction and metabolic deterioration in skeletal muscle. First, we revealed that excess palmitate (PA), but not hyperglycemia, hyperinsulinemia, or elevated tumor necrosis factor alpha, induced mitochondrial fragmentation and increased mitochondrion-associated Drp1 and Fis1 in differentiated C2C12 muscle cells. This fragmentation was associated with increased oxidative stress, mitochondrial depolarization, loss of ATP production, and reduced insulin-stimulated glucose uptake. Both genetic and pharmacological inhibition of Drp1 attenuated PA-induced mitochondrial fragmentation, mitochondrial depolarization, and insulin resistance in C2C12 cells. Furthermore, we found smaller and shorter mitochondria and increased mitochondrial fission machinery in the skeletal muscle of mice with genetic obesity and those with diet-induced obesity. Inhibition of mitochondrial fission improved the muscle insulin signaling and systemic insulin sensitivity of obese mice. Our findings indicated that aberrant mitochondrial fission is causally associated with mitochondrial dysfunction and insulin resistance in skeletal muscle. Thus, disruption of mitochondrial dynamics may underlie the pathogenesis of muscle insulin resistance in obesity and type 2 diabetes.