NARP mutation and mtDNA depletion trigger mitochondrial biogenesis which can be modulated by selenite supplementation

Variants in Human ATP Synthase Mitochondrial Genes: Biochemical Dysfunctions, Associated Diseases, and Therapies

Mitochondrial ATP synthase (Complex V) catalyzes the last step of oxidative phosphorylation and provides most of the energy (ATP) required by human cells. The mitochondrial genes MT-ATP6 and MT-ATP8 encode two subunits of the multi-subunit Complex V. Since the discovery of the first MT-ATP6 variant in the year 1990 as the cause of Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) syndrome, a large and continuously increasing number of inborn variants in the MT-ATP6 and MT-ATP8 genes have been identified as pathogenic. Variants in these genes correlate with various clinical phenotypes, which include several neurodegenerative and multisystemic disorders. In the present review, we report the pathogenic variants in mitochondrial ATP synthase genes and highlight the molecular mechanisms underlying ATP synthase deficiency that promote biochemical dysfunctions. We discuss the possible structural changes induced by the most common variants found in patients by considering the recent cryo-electron microscopy structure of human ATP synthase. Finally, we provide the state-of-the-art of all therapeutic proposals reported in the literature, including drug interventions targeting mitochondrial dysfunctions, allotopic gene expression- and nuclease-based strategies, and discuss their potential translation into clinical trials.

Modulating mitochondrial DNA mutations: factors shaping heteroplasmy in the germ line and somatic cells

Until recently it was thought that most humans only harbor one type of mitochondrial DNA (mtDNA), however, deep sequencing and single-cell analysis has shown the converse - that mixed populations of mtDNA (heteroplasmy) are the norm. This is important because heteroplasmy levels can change dramatically during transmission in the female germ line, leading to high levels causing severe mitochondrial diseases. There is also emerging evidence that low level mtDNA mutations contribute to common late onset diseases such as neurodegenerative disorders and cardiometabolic diseases because the inherited mutation levels can change within developing organs and non-dividing cells over time. Initial predictions suggested that the segregation of mtDNA heteroplasmy was largely stochastic, with an equal tendency for levels to increase or decrease. However, transgenic animal work and single-cell analysis have shown this not to be the case during germ-line transmission and in somatic tissues during life. Mutation levels in specific mtDNA regions can increase or decrease in different contexts and the underlying molecular mechanisms are starting to be unraveled. In this review we provide a synthesis of recent literature on the mechanisms of selection for and against mtDNA variants. We identify the most pertinent gaps in our understanding and suggest ways these could be addressed using state of the art techniques.

Rewiring cell signalling pathways in pathogenic mtDNA mutations

Mitochondria generate the energy to sustain cell viability and serve as a hub for cell signalling. Their own genome (mtDNA) encodes genes critical for oxidative phosphorylation. Mutations of mtDNA cause major disease and disability with a wide range of presentations and severity. We review here an emerging body of data suggesting that changes in cell metabolism and signalling pathways in response to the presence of mtDNA mutations play a key role in shaping disease presentation and progression. Understanding the impact of mtDNA mutations on cellular energy homeostasis and signalling pathways seems fundamental to identify novel therapeutic interventions with the potential to improve the prognosis for patients with primary mitochondrial disease.

Constitutive activation of the PI3K-Akt-mTORC1 pathway sustains the m.3243 A > G mtDNA mutation

Mutations of the mitochondrial genome (mtDNA) cause a range of profoundly debilitating clinical conditions for which treatment options are very limited. Most mtDNA diseases show heteroplasmy – tissues express both wild-type and mutant mtDNA. While the level of heteroplasmy broadly correlates with disease severity, the relationships between specific mtDNA mutations, heteroplasmy, disease phenotype and severity are poorly understood. We have carried out extensive bioenergetic, metabolomic and RNAseq studies on heteroplasmic patient-derived cells carrying the most prevalent disease related mtDNA mutation, the m.3243 A > G. These studies reveal that the mutation promotes changes in metabolites which are associated with the upregulation of the PI3K-Akt-mTORC1 axis in patient-derived cells and tissues. Remarkably, pharmacological inhibition of PI3K, Akt, or mTORC1 reduced mtDNA mutant load and partially rescued cellular bioenergetic function. The PI3K-Akt-mTORC1 axis thus represents a potential therapeutic target that may benefit people suffering from the consequences of the m.3243 A > G mutation.

Heteroplasmic mtDNA mutations cause disease in humans. Here, Chung et al find the PI3K-Akt-mTORC1 pathway constitutively activated in cells with the heteroplasmic m.3243 A > G mutation, and inhibition of the pathway cell autonomously reduces mutant mtDNA load and rescues mitochondrial bioenergetics.

Assessment of mitochondrial function following short- and long-term exposure of human bronchial epithelial cells to total particulate matter from a candidate modified-risk tobacco product and reference cigarettes

Mitochondrial dysfunction caused by cigarette smoke is involved in the oxidative stress-induced pathology of airway diseases. Reducing the levels of harmful and potentially harmful constituents by heating rather than combusting tobacco may reduce mitochondrial changes that contribute to oxidative stress and cell damage. We evaluated mitochondrial function and oxidative stress in human bronchial epithelial cells (BEAS 2B) following 1- and 12-week exposures to total particulate matter (TPM) from the aerosol of a candidate modified-risk tobacco product, the Tobacco Heating System 2.2 (THS2.2), in comparison with TPM from the 3R4F reference cigarette. After 1-week exposure, 3R4F TPM had a strong inhibitory effect on mitochondrial basal and maximal oxygen consumption rates compared to TPM from THS2.2. Alterations in oxidative phosphorylation were accompanied by increased mitochondrial superoxide levels and increased levels of oxidatively damaged proteins in cells exposed to 7.5 μg/mL of 3R4F TPM or 150 μg/mL of THS2.2 TPM, while cytosolic levels of reactive oxygen species were not affected. In contrast, the 12-week exposure indicated adaptation of BEAS-2B cells to long-term stress. Together, the findings indicate that 3R4F TPM had a stronger effect on oxidative phosphorylation, gene expression and proteins involved in oxidative stress than TPM from the candidate modified-risk tobacco product THS2.2.

Mitochondrial determinants of cancer health disparities

Mitochondria, which are multi-functional, have been implicated in cancer initiation, progression, and metastasis due to metabolic alterations in transformed cells. Mitochondria are involved in the generation of energy, cell growth and differentiation, cellular signaling, cell cycle control, and cell death. To date, the mitochondrial basis of cancer disparities is unknown. The goal of this review is to provide an understanding and a framework of mitochondrial determinants that may contribute to cancer disparities in racially different populations. Due to maternal inheritance and ethnic-based diversity, the mitochondrial genome (mtDNA) contributes to inherited racial disparities. In people of African ancestry, several germline, population-specific haplotype variants in mtDNA as well as depletion of mtDNA have been linked to cancer predisposition and cancer disparities. Indeed, depletion of mtDNA and mutations in mtDNA or nuclear genome (nDNA)-encoded mitochondrial proteins lead to mitochondrial dysfunction and promote resistance to apoptosis, the epithelial-to-mesenchymal transition, and metastatic disease, all of which can contribute to cancer disparity and tumor aggressiveness related to racial disparities. Ethnic differences at the level of expression or genetic variations in nDNA encoding the mitochondrial proteome, including mitochondria-localized mtDNA replication and repair proteins, miRNA, transcription factors, kinases and phosphatases, and tumor suppressors and oncogenes may underlie susceptibility to high-risk and aggressive cancers found in African population and other ethnicities. The mitochondrial retrograde signaling that alters the expression profile of nuclear genes in response to dysfunctional mitochondria is a mechanism for tumorigenesis. In ethnic populations, differences in mitochondrial function may alter the cross talk between mitochondria and the nucleus at epigenetic and genetic levels, which can also contribute to cancer health disparities. Targeting mitochondrial determinants and mitochondrial retrograde signaling could provide a promising strategy for the development of selective anticancer therapy for dealing with cancer disparities. Further, agents that restore mitochondrial function to optimal levels should permit sensitivity to anticancer agents for the treatment of aggressive tumors that occur in racially diverse populations and hence help in reducing racial disparities.

Early cerebellar deficits in mitochondrial biogenesis and respiratory chain complexes in the KIKO mouse model of Friedreich ataxia

Friedreich ataxia (FRDA), the most common recessive inherited ataxia, results from deficiency of frataxin, a small mitochondrial protein crucial for iron-sulphur cluster formation and ATP production. Frataxin deficiency is associated with mitochondrial dysfunction in FRDA patients and animal models; however, early mitochondrial pathology in FRDA cerebellum remains elusive. Using frataxin knock-in/knockout (KIKO) mice and KIKO mice carrying the mitoDendra transgene, we show early cerebellar deficits in mitochondrial biogenesis and respiratory chain complexes in this FRDA model. At asymptomatic stages, the levels of PGC-1α (PPARGC1A), the mitochondrial biogenesis master regulator, are significantly decreased in cerebellar homogenates of KIKO mice compared with age-matched controls. Similarly, the levels of the PGC-1α downstream effectors, NRF1 and Tfam, are significantly decreased, suggesting early impaired cerebellar mitochondrial biogenesis pathways. Early mitochondrial deficiency is further supported by significant reduction of the mitochondrial markers GRP75 (HSPA9) and mitofusin-1 in the cerebellar cortex. Moreover, the numbers of Dendra-labeled mitochondria are significantly decreased in cerebellar cortex, confirming asymptomatic cerebellar mitochondrial biogenesis deficits. Functionally, complex I and II enzyme activities are significantly reduced in isolated mitochondria and tissue homogenates from asymptomatic KIKO cerebella. Structurally, levels of the complex I core subunit NUDFB8 and complex II subunits SDHA and SDHB are significantly lower than those in age-matched controls. These results demonstrate complex I and II deficiency in KIKO cerebellum, consistent with defects identified in FRDA patient tissues. Thus, our findings identify early cerebellar mitochondrial biogenesis deficits as a potential mediator of cerebellar dysfunction and ataxia, thereby providing a potential therapeutic target for early intervention of FRDA.

Implications of mitochondrial network organization in mitochondrial stress signalling in NARP cybrid and Rho0 cells

Mitochondrial dysfunctions lead to the generation of signalling mediators that influence the fate of that organelle. Mitochondrial dynamics and their positioning within the cell are important elements of mitochondria-nucleus communication. The aim of this project was to examine whether mitochondrial shape, distribution and fusion/fission proteins are involved in the mitochondrial stress response in a cellular model subjected to specifically designed chronic mitochondrial stress: WT human osteosarcoma cells as controls, NARP cybrid cells as mild chronic stress and Rho0 as severe chronic stress. We characterized mitochondrial distribution in these cells using confocal microscopy and evaluated the level of proteins directly involved in the mitochondrial dynamics and their regulation. We found that the organization of mitochondria within the cell is correlated with changes in the levels of proteins involved in mitochondrial dynamics and proteins responsible for regulation of this process. Induction of the autophagy/mitophagy process, which is crucial for cellular homeostasis under stress conditions was also shown. It seems that mitochondrial shape and organization within the cell are implicated in retrograde signalling in chronic mitochondrial stress.

Selenium suppresses glutamate-induced cell death and prevents mitochondrial morphological dynamic alterations in hippocampal HT22 neuronal cells

Background

Previous studies have indicated that selenium supplementation may be beneficial in neuroprotection against glutamate-induced cell damage, in which mitochondrial dysfunction is considered a major pathogenic feature. However, the exact mechanisms by which selenium protects against glutamate-provoked mitochondrial perturbation remain ambiguous. In this study glutamate exposed murine hippocampal neuronal HT22 cell was used as a model to investigate the underlying mechanisms of selenium-dependent protection against mitochondria damage.

Results

We find that glutamate-induced cytotoxicity was associated with enhancement of superoxide production, activation of caspase-9 and -3, increases of mitochondrial fission marker and mitochondrial morphological changes. Selenium significantly resolved the glutamate-induced mitochondria structural damage, alleviated oxidative stress, decreased Apaf-1, caspases-9 and -3 contents, and altered the autophagy process as observed by a decline in the ratio of the autophagy markers LC3-I and LC3-II.

Conclusion

These findings suggest that the protection of selenium against glutamate stimulated cell damage of HT22 cells is associated with amelioration of mitochondrial dynamic imbalance.

Protective Effects of Selol Against Sodium Nitroprusside-Induced Cell Death and Oxidative Stress in PC12 Cells

Selol is an organic selenitetriglyceride formulation containing selenium at +4 oxidation level that can be effectively incorporated into catalytic sites of of Se-dependent antioxidants. In the present study, the potential antioxidative and cytoprotective effects of Selol against sodium nitroprusside (SNP)-evoked oxidative/nitrosative stress were investigated in PC12 cells and the underlying mechanisms analyzed. Spectrophoto- and spectrofluorimetic methods as well as fluorescence microscopy were used in this study; mRNA expression was quantified by real-time PCR. Selol dose-dependently improved the survival and decreased the percentage of apoptosis in PC12 cells exposed to SNP. To determine the mechanism of this protective action, the effect of Selol on free radical generation and on antioxidative potential was evaluated. Selol offered significant protection against the elevation of reactive oxidative species (ROS) evoked by SNP. Moreover, this compound restored glutathione homeostasis by ameliorating the SNP-evoked disturbance of GSH/GSSG ratio. The protective effect exerted by Selol was associated with the prevention of SNP-mediated down-regulation of antioxidative enzymes: glutathione peroxidase (Se-GPx), glutathione reductase (GR), and thioredoxin reductase (TrxR). Finally, GPx inhibition significantly abolished the cytoprotective effect of Selol. In conclusion, these results suggest that Selol effectively protected PC12 cells against SNP-induced oxidative damage and death by adjusting free radical levels and antioxidant system, and suppressing apoptosis. Selol could be successfully used in the treatments of diseases that involve oxidative stress and resulting apoptosis.

Screen for mitochondrial DNA copy number maintenance genes reveals essential role for ATP synthase

The machinery of mitochondrial DNA (mtDNA) maintenance is only partially characterized and is of wide interest due to its involvement in disease. To identify novel components of this machinery, plus other cellular pathways required for mtDNA viability, we implemented a genome-wide RNAi screen in Drosophila S2 cells, assaying for loss of fluorescence of mtDNA nucleoids stained with the DNA-intercalating agent PicoGreen. In addition to previously characterized components of the mtDNA replication and transcription machineries, positives included many proteins of the cytosolic proteasome and ribosome (but not the mitoribosome), three proteins involved in vesicle transport, some other factors involved in mitochondrial biogenesis or nuclear gene expression, > 30 mainly uncharacterized proteins and most subunits of ATP synthase (but no other OXPHOS complex). ATP synthase knockdown precipitated a burst of mitochondrial ROS production, followed by copy number depletion involving increased mitochondrial turnover, not dependent on the canonical autophagy machinery. Our findings will inform future studies of the apparatus and regulation of mtDNA maintenance, and the role of mitochondrial bioenergetics and signaling in modulating mtDNA copy number.

Mitochondrial retrograde signaling at the crossroads of tumor bioenergetics, genetics and epigenetics

Mitochondria play a central role not only in energy production but also in the integration of metabolic pathways as well as signals for apoptosis and autophagy. It is becoming increasingly apparent that mitochondria in mammalian cells play critical roles in the initiation and propagation of various signaling cascades. In particular, mitochondrial metabolic and respiratory states and status on mitochondrial genetic instability are communicated to the nucleus as an adaptive response through retrograde signaling. Each mammalian cell contains multiple copies of the mitochondrial genome (mtDNA). A reduction in mtDNA copy number has been reported in various human pathological conditions such as diabetes, obesity, neurodegenerative disorders, aging and cancer. Reduction in mtDNA copy number disrupts mitochondrial membrane potential (Δψm) resulting in dysfunctional mitochondria. Dysfunctional mitochondria trigger retrograde signaling and communicate their changing metabolic and functional state to the nucleus as an adaptive response resulting in an altered nuclear gene expression profile and altered cell physiology and morphology. In this review, we provide an overview of the various modes of mitochondrial retrograde signaling focusing particularly on the Ca(2+)/Calcineurin mediated retrograde signaling. We discuss the contribution of the key factors of the pathway such as Calcineurin, IGF1 receptor, Akt kinase and HnRNPA2 in the propagation of signaling and their role in modulating genetic and epigenetic changes favoring cellular reprogramming towards tumorigenesis.

Selenite stimulates mitochondrial biogenesis signaling and enhances mitochondrial functional performance in murine hippocampal neuronal cells

Supplementation of selenium has been shown to protect cells against free radical mediated cell damage. The objectives of this study are to examine whether supplementation of selenium stimulates mitochondrial biogenesis signaling pathways and whether selenium enhances mitochondrial functional performance. Murine hippocampal neuronal HT22 cells were treated with sodium selenite for 24 hours. Mitochondrial biogenesis markers, mitochondrial respiratory rate and activities of mitochondrial electron transport chain complexes were measured and compared to non-treated cells. The results revealed that treatment of selenium to the HT22 cells elevated the levels of nuclear mitochondrial biogenesis regulators PGC-1α and NRF1, as well as mitochondrial proteins cytochrome c and cytochrome c oxidase IV (COX IV). These effects are associated with phosphorylation of Akt and cAMP response element-binding (CREB). Supplementation of selenium significantly increased mitochondrial respiration and improved the activities of mitochondrial respiratory complexes. We conclude that selenium activates mitochondrial biogenesis signaling pathway and improves mitochondrial function. These effects may be associated with modulation of AKT-CREB pathway.

Contractile activity-induced mitochondrial biogenesis and mTORC1

In response to exercise training, or chronic contractile activity, mitochondrial content is known to be enriched within skeletal muscle. However, the molecular mechanisms that mediate this adaptation are incompletely defined. Recently, the protein complex, mammalian target of rapamycin complex 1 (mTORC1), has been identified to facilitate the expression of nuclear genes encoding mitochondrial proteins (NUGEMPs) in resting muscle cells via the interaction of the mTORC1 components, mTOR and raptor, the transcription factor Yin Yang 1, and peroxisome proliferator-activated receptor-γ coactivator-1α. It is currently unknown if this mechanism is operative during the increase in mitochondrial content that occurs within skeletal muscle with chronic contractile activity (CCA). Thus we employed a cell culture model of murine skeletal muscle and subjected the myotubes to CCA for 3 h per day for 4 consecutive days in the presence or absence of the mTORC1 inhibitor rapamycin. CCA produced increases in the mitochondrial markers cytochrome oxidase (COX) IV (2.5-fold), Tfam (1.5-fold), and COX activity (1.6-fold). Rapamycin-mediated inhibition of mTORC1 did not suppress these CCA-induced increases in mitochondrial proteins and organelle content. mTORC1 inhibition alone produced a selective upregulation of mitochondrial proteins (COX IV, Tfam), but diminished organelle state 3 respiration. CCA restored this impairment to normal. Our results suggest that mTORC1 activity is not integral for the increase in mitochondrial content elicited by CCA, but is required to maintain mitochondrial function and homeostasis in resting muscle.

Selenium preserves mitochondrial function, stimulates mitochondrial biogenesis, and reduces infarct volume after focal cerebral ischemia

Background

Mitochondrial dysfunction is one of the major events responsible for activation of neuronal cell death pathways during cerebral ischemia. Trace element selenium has been shown to protect neurons in various diseases conditions. Present study is conducted to demonstrate that selenium preserves mitochondrial functional performance, activates mitochondrial biogenesis and prevents hypoxic/ischemic cell damage.

Results

The study conducted on HT22 cells exposed to glutamate or hypoxia and mice subjected to 60-min focal cerebral ischemia revealed that selenium (100 nM) pretreatment (24 h) significantly attenuated cell death induced by either glutamate toxicity or hypoxia. The protective effects were associated with reduction of glutamate and hypoxia-induced ROS production and alleviation of hypoxia-induced suppression of mitochondrial respiratory complex activities. The animal studies demonstrated that selenite pretreatment (0.2 mg/kg i.p. once a day for 7 days) ameliorated cerebral infarct volume and reduced DNA oxidation. Furthermore, selenite increased protein levels of peroxisome proliferator-activated receptor-γ coactivator 1alpha (PGC-1α) and nuclear respiratory factor 1 (NRF1), two key nuclear factors that regulate mitochondrial biogenesis. Finally, selenite normalized the ischemia-induced activation of Beclin 1 and microtubule-associated protein 1 light chain 3-II (LC3-II), markers for autophagy.

Conclusions

These results suggest that selenium protects neurons against hypoxic/ischemic damage by reducing oxidative stress, restoring mitochondrial functional activities and stimulating mitochondrial biogenesis.

TNFα affects energy metabolism and stimulates biogenesis of mitochondria in EA.hy926 endothelial cells

Mitochondrial response of EA.hy926 endothelial cells to tumour necrosis factor alpha (TNFα) was investigated. It was confirmed that TNFα stimulates reactive oxygen species (ROS) generation and increases intercellular adhesion molecule-1 (ICAM-1) level. These changes were paralleled by elevated oxygen consumption, slightly raised total mitochondrial mass and increased manganese superoxide dismutase (Mn-SOD) and uncoupling protein 2 (UCP2) content. They also correlated with a rise of mitochondrial transcription factor 1 (TFAM), nuclear respiratory factor-1 (NRF-1) and peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α, which are involved in regulation of mitochondrial biogenesis and an elevated level of selected respiratory chain proteins. Thus, the apparent stimulatory effect of TNFα on mitochondrial metabolism probably reflects an increased amount of mitochondria rather than activation of biochemical processes per se, although the latter cannot be excluded definitely. These observations are similar to those described for cardiac muscle cells challenged with bacterial lipopolysaccharide (LPS), in which mitochondrial biogenesis was postulated. Stimulation of mitochondrial biogenesis could be a mechanism activated to prevent TNFα-induced cell death.

PGC-1 family coactivators and cell fate: roles in cancer, neurodegeneration, cardiovascular disease and retrograde mitochondria-nucleus signalling

Over the past two decades, a complex nuclear transcriptional machinery controlling mitochondrial biogenesis and function has been described. Central to this network are the PGC-1 family coactivators, characterised as master regulators of mitochondrial biogenesis. Recent literature has identified a broader role for PGC-1 coactivators in both cell death and cellular adaptation under conditions of stress, here reviewed in the context of the pathology associated with cancer, neurodegeneration and cardiovascular disease. Moreover, we propose that these studies also imply a novel conceptual framework on the general role of mitochondrial dysfunction in disease. It is now well established that the complex nuclear transcriptional control of mitochondrial biogenesis allows for adaptation of mitochondrial mass and function to environmental conditions. On the other hand, it has also been suggested that mitochondria alter their function according to prevailing cellular energetic requirements and thus function as sensors that generate signals to adjust fundamental cellular processes through a retrograde mitochondria-nucleus signalling pathway. Therefore, altered mitochondrial function can affect cell fate not only directly by modifying cellular energy levels or redox state, but also indirectly, by altering nuclear transcriptional patterns. The current literature on such retrograde signalling in both yeast and mammalian cells is thus reviewed, with an outlook on its potential contribution to disease through the regulation of PGC-1 family coactivators. We propose that further investigation of these pathways will lead to the identification of novel pharmacological targets and treatment strategies to combat disease.

Effect of selenite on basic mitochondrial function in human osteosarcoma cells with chronic mitochondrial stress

Mitochondrial chronic stress that originates from defective mitochondria is implicated in a growing list of human diseases. To enhance understanding of pathophysiology of chronic mitochondrial dysfunction we investigated human osteosarcoma cells with 2 types of chronic stress: corresponding to the mutation in ATP synthase subunit 6 encoded by mtDNA (NARP syndrome-mild stress) and to a total lack of mtDNA (Rho0 cells-heavy stress). We previously found that selenium influenced mitochondrial stress response and lowered ROS production. Therefore, in this study effect of selenite on other mitochondrial parameters was investigated. We showed that presence of selenium improved survival of starved cells, modified organization of mitochondrial network in NARP cybrids and decreased cytosolic calcium level in NARP and Rho0 cells. Selenium did not affect mitochondrial membrane potential, ATP level, activity of ATP synthase and activity of complex II of the respiratory chain.