Small-molecule inhibitors of human mitochondrial DNA transcription

Altered expression of mitochondrial DNA (mtDNA) occurs in ageing and a range of human pathologies (for example, inborn errors of metabolism, neurodegeneration and cancer). Here we describe first-in-class specific inhibitors of mitochondrial transcription (IMTs) that target the human mitochondrial RNA polymerase (POLRMT), which is essential for biogenesis of the oxidative phosphorylation (OXPHOS) system^1-6. The IMTs efficiently impair mtDNA transcription in a reconstituted recombinant system and cause a dose-dependent inhibition of mtDNA expression and OXPHOS in cell lines. To verify the cellular target, we performed exome sequencing of mutagenized cells and identified a cluster of amino acid substitutions in POLRMT that cause resistance to IMTs. We obtained a cryo-electron microscopy (cryo-EM) structure of POLRMT bound to an IMT, which further defined the allosteric binding site near the active centre cleft of POLRMT. The growth of cancer cells and the persistence of therapy-resistant cancer stem cells has previously been reported to depend on OXPHOS^7-17, and we therefore investigated whether IMTs have anti-tumour effects. Four weeks of oral treatment with an IMT is well-tolerated in mice and does not cause OXPHOS dysfunction or toxicity in normal tissues, despite inducing a strong anti-tumour response in xenografts of human cancer cells. In summary, IMTs provide a potent and specific chemical biology tool to study the role of mtDNA expression in physiology and disease.

Sphingosine 1-phosphate protects against radiation-induced ovarian injury in female rats-impact on mitochondrial-related genes

The toxic effects of ionizing radiation on the gonads have been widely recognized. Sphingosine 1-phosphate (S1P) has a protective effect on ovarian injury, and although it is known that mitochondria are involved in this process, the specific mechanism is not fully understood. The present study analysed the changes in the serum AMH and ovarian histology in Sprague-Dawley female rats exposed to X-ray radiation only or co-administered with S1P. The mRNA expression profile of ovarian tissue was further analysed via next-generation sequencing and bioinformatics approaches to screen out candidate mitochondria-related genes. Finally, differentially expressed target genes were verified by real-time PCR. The results showed that ionizing radiation could reduce the serum AMH level, destroy ovarian structure and decrease the number of follicles in rats, while S1P administration significantly attenuated the impairment of ovarian function. Gene ontology (GO) and KEGG pathway analysis revealed that a variety of genes related to mitochondrial function were differentially expressed, and the protective effect of S1P on mitochondria was more obvious in the acute phase 24 h after radiation. The differentially expressed mitochondrial function-related genes associated with the protective effect of S1P were UQCRH, MICU2 and GPX4, which were subsequently verified by RT-PCR. Therefore, ionizing radiation has a significant effect on ovarian function, and S1P has a protective effect on radiation-induced ovarian injury, in which mitochondria may play an important role. This study sheds new light on the mechanism of radiation-induced ovarian injury and helps develop a novel potential strategy to control it.

Acute oral administration of L-leucine upregulates slow-fiber- and mitochondria-related genes in skeletal muscle of rats

Branched-chain amino acids promote both protein and mRNA synthesis through mechanistic target of rapamycin (mTOR) signaling. A previous report demonstrated that chronic branched-chain amino acid supplementation increased mitochondrial biogenesis in the skeletal muscle of middle-aged mice through activation of mTOR signaling. In this study, we hypothesized that the acute oral administration of L-leucine alone has the ability to alter the gene expression related to fiber type and metabolism in skeletal muscle of young rats through the activation of mTOR signaling. Although the gene expression of representative glycolytic enzymes (Hk2 and Eno3) was not altered, L-leucine administration (135 mg/100 g body weight) upregulated the expression of slow-fiber-related genes (Myh7, Myl3, and Tnni1) and a mitochondrial biogenesis-related gene (Ppargc1a) in the soleus and extensor digitorum longus muscles compared with the control. In addition, L-leucine treatment also upregulated the slow-fiber genes and mitochondrial gene expression in cultured C2C12 myotubes, whereas rapamycin inhibited the effects of L-leucine. However, L-alanine, L-phenylalanine, and L-valine treatment did not alter the expression of the fiber type- and metabolism-related genes as observed in L-leucine. Our results suggest that L-leucine may have the ability to alter skeletal muscle fiber type toward slow fiber and oxidative metabolism by upregulation of gene expression through mTOR signaling.

Transcriptome-wide analyses indicate mitochondrial responses to particulate air pollution exposure

BACKGROUND

Due to their lack of repair capacity mitochondria are critical targets for environmental toxicants. We studied genes and pathways reflecting mitochondrial responses to short- and medium-term PM₁₀ exposure.

METHODS

Whole genome gene expression was measured in peripheral blood of 98 adults (49% women). We performed linear regression analyses stratified by sex and adjusted for individual and temporal characteristics to investigate alterations in gene expression induced by short-term (week before blood sampling) and medium-term (month before blood sampling) PM₁₀ exposure. Overrepresentation analyses (ConsensusPathDB) were performed to identify enriched mitochondrial associated pathways and gene ontology sets. Thirteen Human MitoCarta genes were measured by means of quantitative real-time polymerase chain reaction (qPCR) along with mitochondrial DNA (mtDNA) content in an independent validation cohort (n = 169, 55.6% women).

RESULTS

Overrepresentation analyses revealed significant pathways (p-value <0.05) related to mitochondrial genome maintenance and apoptosis for short-term exposure and to the electron transport chain (ETC) for medium-term exposure in women. For men, medium-term PM₁₀ exposure was associated with the Tri Carbonic Acid cycle. In an independent study population, we validated several ETC genes, including UQCRH and COX7C (q-value <0.05), and some genes crucial for the maintenance of the mitochondrial genome, including LONP1 (q-value: 0.07) and POLG (q-value: 0.04) in women.

CONCLUSIONS

In this exploratory study, we identified mitochondrial genes and pathways associated with particulate air pollution indicating upregulation of energy producing pathways as a potential mechanism to compensate for PM-induced mitochondrial damage.

STX, a Novel Membrane Estrogen Receptor Ligand, Protects Against Amyloid-β Toxicity

Because STX is a selective ligand for membrane estrogen receptors, it may be able to confer the beneficial effects of estrogen without eliciting the deleterious side effects associated with activation of the nuclear estrogen receptors. This study evaluates the neuroprotective properties of STX in the context of amyloid-β (Aβ) exposure. MC65 and SH-SY5Y neuroblastoma cell lines, as well as primary hippocampal neurons from wild type (WT) and Tg2576 mice, were used to investigate the ability of STX to attenuate cell death, mitochondrial dysfunction, dendritic simplification, and synaptic loss induced by Aβ. STX prevented Aβ-induced cell death in both neuroblastoma cell lines; it also normalized the decrease in ATP and mitochondrial gene expression caused by Aβ in these cells. Notably, STX also increased ATP content and mitochondrial gene expression in control neuroblastoma cells (in the absence of Aβ). Likewise in primary neurons, STX increased ATP levels and mitochondrial gene expression in both genotypes. In addition, STX treatment enhanced dendritic arborization and spine densities in WT neurons and prevented the diminished outgrowth of dendrites caused by Aβ exposure in Tg2576 neurons. These data suggest that STX can act as an effective neuroprotective agent in the context of Aβ toxicity, improving mitochondrial function as well as dendritic growth and synaptic differentiation. In addition, since STX also improved these endpoints in the absence of Aβ, this compound may have broader therapeutic value beyond Alzheimer's disease.

Mitochondrial transcription termination factor 2 binds to entire mitochondrial DNA and negatively regulates mitochondrial gene expression

Mitochondrial transcription termination factor 2 (mTERF2) is a mitochondrial matrix protein that binds to the mitochondrial DNA. Previous studies have shown that overexpression of mTERF2 can inhibit cell proliferation, but the mechanism has not been well defined so far. This study aimed to present the binding pattern of mTERF2 to the mitochondrial DNA (mtDNA) in vivo, and investigated the biological function of mTERF2 on the replication of mtDNA, mRNA transcription, and protein translation. The mTERF2 binding to entire mtDNA was identified via the chromatin immunoprecipitation analysis. The mtDNA replication efficiency and expression levels of mitochondria genes were significantly inhibited when the mTERF2 was overexpressed in HeLa cells. The inhibition level of mtDNA content was the same with the decreased levels of mRNA and mitochondrial protein expression. Overall, the mTERF2 might be a cell growth inhibitor based on its negative effect on mtDNA replication, which eventually down-regulated all of the oxidative phosphorylation components in the mitochondria that were essential for the cell's energy metabolism.

[Mitochondrial genetic apparatus functioning in mice spleen cells under radiation-induced apoptosis]

An important role of mitochondria in the process of programmed cell death is widely accepted now. There is a set of nuclear-encoded mitochondrial proteins involved in this process. Apart from this, a mitochondrion contains its own genetic apparatus comprising mtDNA and replication, transcription, and translation systems. However, a mechanism of mitochondria genetic information realization under apoptosis-inducing conditions has been understood poorly. Here, using the real-time PCR technique the number of mitochondrial genes and their transcripts in mouse spleen cells after whole-body X-ray irradiation at the dosage of 10 Gy has been evaluated. During 5 h after the irradiation a nuclear DNA was subjected to fragmentation, whereas mtDNA remained intact. Moreover, in the course of time after irradiation the number of mtDNA copies increased threefold. A process of mtDNA transcription was more susceptible to the irradiation: in 1 h after exposure the number of ND2, ND4 and CYTB gene transcripts were sharply decreased. In 24 and 72 h after the irradiation the amount of ND2 and ND4 transcripts was restored to the control values, while the CYTB one remained low; the number of ATP6 transcripts was compared with the control within the whole period of observation. The difference in levels of mRNAs for the genes transcribed under the control of the same promoter for mice to be grown both under normal conditions and after x-ray irradiation allows us to propose the existing a posttranscriptional mechanism which regulates expression of mitochondrial genes and provides different recovery rates for different mitochondrial transcripts at the development of apoptosis.

Effects of TCDD on the expression of nuclear encoded mitochondrial genes

Generation of mitochondrial reactive oxygen species (ROS) can be perturbed following exposure to environmental chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Reports indicate that the aryl hydrocarbon receptor (AhR) mediates TCDD-induced sustained hepatic oxidative stress by decreasing hepatic ATP levels and through hyperpolarization of the inner mitochondrial membrane. To further elucidate the effects of TCDD on the mitochondria, high-throughput quantitative real-time PCR (HTP-QRTPCR) was used to evaluate the expression of 90 nuclear genes encoding mitochondrial proteins involved in electron transport, oxidative phosphorylation, uncoupling, and associated chaperones. HTP-QRTPCR analysis of time course (30 microg/kg TCDD at 2, 4, 8, 12, 18, 24, 72, and 168 h) liver samples obtained from orally gavaged immature, ovariectomized C57BL/6 mice identified 54 differentially expressed genes (/fold change/ > 1.5 and P-value < 0.1). Of these, 8 exhibited a sigmoidal or exponential dose-response profile (0.03 to 300 microg/kg TCDD) at 4, 24 or 72 h. Dose-responsive genes encoded proteins associated with electron transport chain (ETC) complexes I (NADH dehydrogenase), III (cytochrome c reductase), IV (cytochrome c oxidase), and V (ATP synthase) and could be generally categorized as having proton gradient, ATP synthesis, and chaperone activities. In contrast, transcript levels of ETC complex II, succinate dehydrogenase, remained unchanged. Putative dioxin response elements were computationally found in the promoter regions of all 8 dose-responsive genes. This high-throughput approach suggests that TCDD alters the expression of genes associated with mitochondrial function which may contribute to TCDD-elicited mitochondrial toxicity.

Molecular cloning and characterization of a mitochondrial dicarboxylate/tricarboxylate transporter gene in Citrus junos response to aluminum stress

A mitochondrial dicarboxylate/tricarboxylate carrier gene, CjDTC, was isolated from Citrus junos by the rapid amplification of cDNA ends and the Y-shaped adaptor-dependent extension methods. It consisted of a 472-base pair (bp) upstream regulatory region and an 897-bp open reading frame encoding a protein of 299 amino acids. Homologous analysis revealed that CjDTC protein might be a plant dicarboxylate/tricarboxylate carrier protein involved in the mitochondria dicarboxylate/tricarboxylate transport. The putative light responsiveness and salicylic acid responsiveness regulatory elements were identified in the upstream regulatory region of CjDTC. Southern blot analysis demonstrated the presence of a single CjDTC gene located on the genome of citrus. As shown by northern hybridization, CjDTC was expressed in all plant tissues examined and the highest transcript level was observed in roots with significantly lower transcript amounts in leaves and stems. Moreover, real-time polymerase chain reaction analysis demonstrated that CjDTC expression was induced by aluminum treatment, suggesting that CjDTC protein might be involved in the excretion of organic acids and rhizotoxic aluminum tolerance.

Oestrogen influences on mitochondrial gene expression and respiratory chain activity in cortical and mesencephalic astrocytes

The regulation of mitochondrial energy metabolism plays an essential role in the central nervous system (CNS). Abnormalities of the mitochondrial respiratory chain often accompany neurodegenerative diseases. This makes mitochondria a perfect target for strategies of cellular protection against toxic compounds and pathological conditions. Steroid hormones, such as oestrogen, are well-known to fulfil a protective role in the brain during ischaemic and degenerative processes. Because astrocytes function as the major energy supplier in the CNS, we have analysed oestrogen effects on the mitochondrial respiratory chain of this cell type. In our studies, we applied semi- and quantitative polymerase chain reaction analysis of gene expression and polarographic measurements of the respiratory chain activity of mitochondria. We observed that structural and functional properties were regulated dependent on the oestrogen exposure time and the brain region, but independent of the nuclear oestrogen receptors. We could demonstrate that long-term oestrogen exposure increases the subunit gene expression of respiratory chain complexes and the mitochondrial DNA content, thereby indicating an up-regulation of the amount of mitochondria per cell together with an increase of mitochondrial energy production. This could represent an important indirect mechanism by which long-term oestrogen exposure protects neurones from cell death under neurotoxic conditions. On the other hand, we observed short-term effects of oestrogen on the activity of mitochondrial, proton-pumping respiratory chain complexes. In astrocytes from the cortex, respiratory chain activity was decreased, whereas it was increased in astrocytes from the mesencephalon. An increased production of reactive oxygen species would be the consequence of an increased respiratory chain activity in mesencephalic astrocytes. This could explain the different efficiencies of oestrogen-mediated short-term protection in distinct brain regions, but also indicates the limitations for a therapeutic short-term application of oestrogen.

[Impact of benzo [a] pyrene the expression of mitochondrion-encoded genes in the earthworm Eisenia fetida]

The earthworm Eisenia fetida's benzo [a] pyrene (BaP) exposure experiments were carried out in artificial soil according to ISO 11268-1:1993. And then the upregulated and downregulated subtractive cDNA libraries were constructed by Clontech PCR-Select cDNA Subtration Kit. From the BaP exposure upregulated subtractive cDNA library, several cDNA segments matched mitochondrion-encoded genes were found, including cytochrome c oxidase subunit I (CO I), subunit II (CO II), subunit Ill (CO III), NADH dehydrogenase subunit 1 (NDH1), and ATP synthase subunit 6. The result indicated BaP and the subsequent oxidative stress disturbed the expression of mitochondrion-encoded genes, and this was potential biomarker for oxidative stress following xenobiotic exposure.