Thiosulfate sulfurtransferase deficiency promotes oxidative distress and aberrant NRF2 function in the brain.

Thiosulfate sulfurtransferase (TST, EC 2.8.1.1) was discovered as an enzyme that detoxifies cyanide by conversion to thiocyanate (rhodanide) using thiosulfate as substrate; this rhodanese activity was subsequently identified to be almost exclusively located in mitochondria. More recently, the emphasis regarding its function has shifted to hydrogen sulfide metabolism, antioxidant defense, and mitochondrial function in the context of protective biological processes against oxidative distress. While TST has been described to play an important role in liver and colon, its function in the brain remains obscure. In the present study, we therefore sought to address its potential involvement in maintaining cerebral redox balance in a murine model of global TST deficiency (Tst-/- mice), primarily focusing on characterizing the biochemical phenotype of TST loss in relation to neuronal activity and sensitivity to oxidative stress under basal conditions. Here, we show that TST deficiency is associated with a perturbation of the reactive species interactome in the brain cortex secondary to altered ROS and RSS (specifically, polysulfide) generation as well as mitochondrial OXPHOS remodeling. These changes were accompanied by aberrant Nrf2-Keap1 expression and thiol-dependent antioxidant function. Upon challenging mice with the redox-active herbicide paraquat (25 mg/kg i.p. for 24 h), Tst-/- mice displayed a lower antioxidant capacity compared to wildtype controls (C57BL/6J mice). These results provide a first glimpse into the molecular and metabolic changes of TST deficiency in the brain and suggest that pathophysiological conditions associated with aberrant TST expression and/or activity renders neurons more susceptible to oxidative stress-related malfunction.

Increased NOS coupling by the metabolite tetrahydrobiopterin (BH4) reduces preeclampsia/IUGR consequences.

Preeclampsia (PE) is a high-prevalence pregnancy disease characterized by placental insufficiency, gestational hypertension, and proteinuria. Overexpression of the A isoform of the STOX1 transcription factor (STOX1A) recapitulates PE in mice, and STOX1A overexpressing trophoblasts recapitulate PE patients hallmarks in terms of gene expression and pathophysiology. STOX1 overexpression induces nitroso-redox imbalance and mitochondrial hyper-activation. Here, by a thorough analysis on cell models, we show that STOX1 overexpression in trophoblasts alters inducible nitric oxide synthase (iNOS), nitric oxide (NO) content, the nitroso-redox balance, the antioxidant defense, and mitochondrial function. This is accompanied by specific alterations of the Krebs cycle leading to reduced l-malate content. By increasing NOS coupling using the metabolite tetrahydrobiopterin (BH4) we restore this multi-step pathway in vitro. Moving in vivo on two different rodent models (STOX1 mice and RUPP rats, alike early onset and late onset preeclampsia, respectively), we show by transcriptomics that BH4 directly reverts STOX1-deregulated gene expression including glutathione metabolism, oxidative phosphorylation, cholesterol metabolism, inflammation, lipoprotein metabolism and platelet activation, successfully treating placental hypotrophy, gestational hypertension, proteinuria and heart hypertrophy. In the RUPP rats we show that the major fetal issue of preeclampsia, Intra Uterine Growth Restriction (IUGR), is efficiently corrected. Our work posits on solid bases BH4 as a novel potential therapy for preeclampsia.

The Reactive Species Interactome in the Brain.

Significance: Redox pioneer Helmut Sies attempted to explain reactive species' challenges faced by organelles, cells, tissues, and organs via three complementary definitions: (i) oxidative stress, that is, the disturbance in the prooxidant-antioxidant defense balance in favor of the prooxidants; (ii) oxidative eustress, the low physiological exposure to prooxidants; and (iii) oxidative distress, the supraphysiological exposure to prooxidants. Recent Advances: Identification, concentration, and interactions are the most important elements to improve our understanding of reactive species in physiology and pathology. In this context, the reactive species interactome (RSI) is a new multilevel redox regulatory system that identifies reactive species families, reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species, and it integrates their interactions with their downstream biological targets. Critical Issues: We propose a united view to fully combine reactive species identification, oxidative eustress and distress, and the RSI system. In this view, we also propose including the forgotten reactive carbonyl species, an increasingly rediscovered reactive species family related to the other reactive families, and key enzymes within the RSI. We focus on brain physiology and pathology to demonstrate why this united view should be considered. Future Directions: More studies are needed for an improved understanding of the contributions of reactive species through their identification, concentration, and interactions, including in the brain. Appreciating the RSI in its entirety should unveil new molecular players and mechanisms in physiology and pathology in the brain and elsewhere.

mTRIP, an Imaging Tool to Investigate Mitochondrial DNA Dynamics in Physiology and Disease at the Single-Cell Resolution.

Mitochondrial physiology and metabolism are closely linked to replication and transcription of mitochondrial DNA (mtDNA). However, the characterization of mtDNA processing is poorly defined at the single-cell level. We developed mTRIP (mitochondrial Transcription and Replication Imaging Protocol), an imaging approach based on modified fluorescence in situ hybridization (FISH), which simultaneously reveals mitochondrial structures committed to mtDNA initiation of replication as well as the mitochondrial RNA (mtRNA) content at the single-cell level in human cells. Also specific RNA regions, rather than global RNA, can be tracked with mTRIP. In addition, mTRIP can be coupled to immunofluorescence for in situ protein tracking, or to MitoTracker, thereby allowing for simultaneous labeling of mtDNA, mtRNA, and proteins or mitochondria, respectively. Altogether, qualitative and quantitative alterations of the dynamics of mtDNA processing are detected by mTRIP in human cells undergoing physiological changes, as well as stress and dysfunction. mTRIP helped elucidating mtDNA processing alterations in cancer cells, and has a potential for diagnostic of mitochondrial diseases.

Multimodal evaluation of hypoxia in brain metastases of lung cancer and interest of hypoxia image-guided radiotherapy.

Lung cancer patients frequently develop brain metastases (BM). Despite aggressive treatment including neurosurgery and external-radiotherapy, overall survival remains poor. There is a pressing need to further characterize factors in the microenvironment of BM that may confer resistance to radiotherapy (RT), such as hypoxia. Here, hypoxia was first evaluated in 28 biopsies from patients with non­small cell lung cancer (NSCLC) BM, using CA-IX immunostaining. Hypoxia characterization (pimonidazole, CA-IX and HIF-1α) was also performed in different preclinical NSCLC BM models induced either by intracerebral injection of tumor cells (H2030-Br3M, H1915) into the cortex and striatum, or intracardial injection of tumor cells (H2030-Br3M). Additionally, [18F]-FMISO-PET and oxygen-saturation-mapping-MRI (SatO2-MRI) were carried out in the intracerebral BM models to further characterize tumor hypoxia and evaluate the potential of Hypoxia-image-guided-RT (HIGRT). The effect of RT on proliferation of BM ([18F]-FLT-PET), tumor volume and overall survival was determined. We showed that hypoxia is a major yet heterogeneous feature of BM from lung cancer both preclinically and clinically. HIGRT, based on hypoxia heterogeneity observed between cortical and striatal metastases in the intracerebrally induced models, showed significant potential for tumor control and animal survival. These results collectively highlight hypoxia as a hallmark of BM from lung cancer and the value of HIGRT in better controlling tumor growth.

Reactivation of Dihydroorotate Dehydrogenase-Driven Pyrimidine Biosynthesis Restores Tumor Growth of Respiration-Deficient Cancer Cells.

Cancer cells without mitochondrial DNA (mtDNA) do not form tumors unless they reconstitute oxidative phosphorylation (OXPHOS) by mitochondria acquired from host stroma. To understand why functional respiration is crucial for tumorigenesis, we used time-resolved analysis of tumor formation by mtDNA-depleted cells and genetic manipulations of OXPHOS. We show that pyrimidine biosynthesis dependent on respiration-linked dihydroorotate dehydrogenase (DHODH) is required to overcome cell-cycle arrest, while mitochondrial ATP generation is dispensable for tumorigenesis. Latent DHODH in mtDNA-deficient cells is fully activated with restoration of complex III/IV activity and coenzyme Q redox-cycling after mitochondrial transfer, or by introduction of an alternative oxidase. Further, deletion of DHODH interferes with tumor formation in cells with fully functional OXPHOS, while disruption of mitochondrial ATP synthase has little effect. Our results show that DHODH-driven pyrimidine biosynthesis is an essential pathway linking respiration to tumorigenesis, pointing to inhibitors of DHODH as potential anti-cancer agents.

Correction: Endonuclease G promotes mitochondrial genome cleavage and replication.

[This corrects the article DOI: 10.18632/oncotarget.24822.].

Distinct metabolic states govern skeletal muscle stem cell fates during prenatal and postnatal myogenesis.

During growth, homeostasis and regeneration, stem cells are exposed to different energy demands. Here, we characterise the metabolic pathways that mediate the commitment and differentiation of mouse skeletal muscle stem cells, and how their modulation can influence the cell state. We show that quiescent satellite stem cells have low energetic demands and perturbed oxidative phosphorylation during ageing, which is also the case for cells from post-mortem tissues. We show also that myogenic fetal cells have distinct metabolic requirements compared to those proliferating during regeneration, with the former displaying a low respiration demand relying mostly on glycolysis. Furthermore, we show distinct requirements for peroxisomal and mitochondrial fatty acid oxidation (FAO) in myogenic cells. Compromising peroxisomal but not mitochondrial FAO promotes early differentiation of myogenic cells. Acute muscle injury and pharmacological block of peroxisomal and mitochondrial FAO expose differential requirements for these organelles during muscle regeneration. Taken together, these observations indicate that changes in myogenic cell state lead to significant alterations in metabolic requirements. In addition, perturbing specific metabolic pathways impacts on myogenic cell fates and the regeneration process.

Endonuclease G promotes mitochondrial genome cleavage and replication.

Endonuclease G (EndoG) is a nuclear-encoded endonuclease, mostly localised in mitochondria. In the nucleus EndoG participates in site-specific cleavage during replication stress and genome-wide DNA degradation during apoptosis. However, the impact of EndoG on mitochondrial DNA (mtDNA) metabolism is poorly understood. Here, we investigated whether EndoG is involved in the regulation of mtDNA replication and removal of aberrant copies. We applied the single-cell mitochondrial Transcription and Replication Imaging Protocol (mTRIP) and PCR-based strategies on human cells after knockdown/knockout and re-expression of EndoG. Our analysis revealed that EndoG stimulates both mtDNA replication initiation and mtDNA depletion, the two events being interlinked and dependent on EndoG's nuclease activity. Stimulation of mtDNA replication by EndoG was independent of 7S DNA processing at the replication origin. Importantly, both mtDNA-directed activities of EndoG were promoted by oxidative stress. Inhibition of base excision repair (BER) that repairs oxidative stress-induced DNA damage unveiled a pronounced effect of EndoG on mtDNA removal, reminiscent of recently discovered links between EndoG and BER in the nucleus. Altogether with the downstream effects on mitochondrial transcription, protein expression, redox status and morphology, this study demonstrates that removal of damaged mtDNA by EndoG and compensatory replication play a critical role in mitochondria homeostasis.

Mitochondrial MDM2 Regulates Respiratory Complex I Activity Independently of p53.

Accumulating evidence indicates that the MDM2 oncoprotein promotes tumorigenesis beyond its canonical negative effects on the p53 tumor suppressor, but these p53-independent functions remain poorly understood. Here, we show that a fraction of endogenous MDM2 is actively imported in mitochondria to control respiration and mitochondrial dynamics independently of p53. Mitochondrial MDM2 represses the transcription of NADH-dehydrogenase 6 (MT-ND6) in vitro and in vivo, impinging on respiratory complex I activity and enhancing mitochondrial ROS production. Recruitment of MDM2 to mitochondria increases during oxidative stress and hypoxia. Accordingly, mice lacking MDM2 in skeletal muscles exhibit higher MT-ND6 levels, enhanced complex I activity, and increased muscular endurance in mild hypoxic conditions. Furthermore, increased mitochondrial MDM2 levels enhance the migratory and invasive properties of cancer cells. Collectively, these data uncover a previously unsuspected function of the MDM2 oncoprotein in mitochondria that play critical roles in skeletal muscle physiology and may contribute to tumor progression.

Helicobacter pylori targets mitochondrial import and components of mitochondrial DNA replication machinery through an alternative VacA-dependent and a VacA-independent mechanisms.

Targeting mitochondria is a powerful strategy for pathogens to subvert cell physiology and establish infection. Helicobacter pylori is a bacterial pathogen associated with gastric cancer development that is known to target mitochondria directly and exclusively through its pro-apoptotic and vacuolating cytotoxin VacA. By in vitro infection of gastric epithelial cells with wild-type and VacA-deficient H. pylori strains, treatment of cells with purified VacA proteins and infection of a mouse model, we show that H. pylori deregulates mitochondria by two novel mechanisms, both rather associated with host cell survival. First, early upon infection VacA induces transient increase of mitochondrial translocases and a dramatic accumulation of the mitochondrial DNA replication and maintenance factors POLG and TFAM. These events occur when VacA is not detected intracellularly, therefore do not require the direct interaction of the cytotoxin with the organelle, and are independent of the toxin vacuolating activity. In vivo, these alterations coincide with the evolution of gastric lesions towards severity. Second, H. pylori also induces VacA-independent alteration of mitochondrial replication and import components, suggesting the involvement of additional H. pylori activities in mitochondria-mediated effects. These data unveil two novel mitochondrial effectors in H. pylori-host interaction with links on gastric pathogenesis.

A novel paradigm links mitochondrial dysfunction with muscle stem cell impairment in sepsis.

Sepsis is an acute systemic inflammatory response of the body to microbial infection and a life threatening condition associated with multiple organ failure. Survivors may display long-term disability with muscle weakness that remains poorly understood. Recent data suggest that long-term myopathy in sepsis survivors is due to failure of skeletal muscle stem cells (satellite cells) to regenerate the muscle. Satellite cells impairment in the acute phase of sepsis is linked to unusual mitochondrial dysfunctions, characterized by a dramatic reduction of the mitochondrial mass and hyperactivity of residual organelles. Survivors maintain the impairment of satellite cells, including alterations of the mitochondrial DNA (mtDNA), in the long-term. This condition can be rescued by treatment with mesenchymal stem cells (MSCs) that restore mtDNA alterations and mitochondrial function in satellite cells, and in fine their regenerative potential. Injection of MSCs in turn increases the force of isolated muscle fibers and of the whole animal, and improves the survival rate. These effects occur in the context of reduced inflammation markers that also raised during sepsis. Targeting muscle stem cells mitochondria, in a context of reduced inflammation, may represent a valuable strategy to reduce morbidity and long-term impairment of the muscle upon sepsis.

A Single-Cell Resolution Imaging Protocol of Mitochondrial DNA Dynamics in Physiopathology, mTRIP, Which Also Evaluates Sublethal Cytotoxicity.

Mitochondria autonomously replicate and transcribe their own genome, which is present in multiple copies in the organelle. Transcription and replication of the mitochondrial DNA (mtDNA), which are defined here as mtDNA processing, are essential for mitochondrial function. The extent, efficiency, and coordination of mtDNA processing are key parameters of the mitochondrial state in living cells. Recently, single-cell analysis of mtDNA processing revealed a large and dynamic heterogeneity of mitochondrial populations in single cells, which is linked to mitochondrial function and is altered during disease. This was achieved using mitochondrial Transcription and Replication Imaging Protocol (mTRIP), a modified fluorescence in situ hybridization (FISH) approach that simultaneously reveals the mitochondrial RNA content and mtDNA engaged in initiation of replication at the single-cell level. mTRIP can also be coupled to immunofluorescence or MitoTracker, resulting in the additional labeling of proteins or active mitochondria, respectively. Therefore, mTRIP detects quantitative and qualitative alterations of the dynamics of mtDNA processing in human cells that respond to physiological changes or result from diseases. In addition, we show here that mTRIP is a rather sensitive tool for detecting mitochondrial alterations that may lead to loss of cell viability, and is thereby a useful tool for monitoring sublethal cytotoxicity for instance during chronic drug treatment.

Reversal of mitochondrial defects with CSB-dependent serine protease inhibitors in patient cells of the progeroid Cockayne syndrome.

UV-sensitive syndrome (UV(S)S) and Cockayne syndrome (CS) are human disorders caused by CSA or CSB gene mutations; both conditions cause defective transcription-coupled repair and photosensitivity. Patients with CS also display neurological and developmental abnormalities and dramatic premature aging, and their cells are hypersensitive to oxidative stress. We report CSA/CSB-dependent depletion of the mitochondrial DNA polymerase-γ catalytic subunit (POLG1), due to HTRA3 serine protease accumulation in CS, but not in UV(s)S or control fibroblasts. Inhibition of serine proteases restored physiological POLG1 levels in either CS fibroblasts and in CSB-silenced cells. Moreover, patient-derived CS cells displayed greater nitroso-redox imbalance than UV(S)S cells. Scavengers of reactive oxygen species and peroxynitrite normalized HTRA3 and POLG1 levels in CS cells, and notably, increased mitochondrial oxidative phosphorylation, which was altered in CS cells. These data reveal critical deregulation of proteases potentially linked to progeroid phenotypes in CS, and our results suggest rescue strategies as a therapeutic option.

Neuropathological Correlates of Hyperglycemia During Prolonged Polymicrobial Sepsis in Mice.

Glucose toxicity may play a crucial role in evoking neurologic complications of critical illness. We studied whether the neuropathological alterations in fatal human critical illness observed under hyperglycemia are present and can be attenuated by maintaining normoglycemia in a mouse model of prolonged sepsis induced by cecal ligation and puncture. Mice were randomized to moderate hyperglycemia (>8.3 mmol/L, n = 8) or normoglycemia (4.4-6.7 mmol/L, n = 8). After 5 days, hippocampus and frontal cortex from septic mice were compared with those from healthy controls (n = 8). Blood glucose was 7.8 ± 1.3 mmol/L in hyperglycemic and 6.1 ± 0.7 mmol/L in normoglycemic critically ill mice (P = 0.007). The percentage of damaged neurons was twofold higher in frontal cortex (P = 0.01) and hippocampus (P = 0.06) of hyperglycemic ill mice than that of healthy mice. In frontal cortex, neuronal damage was attenuated under normoglycemia (P = 0.04). Critical illness reduced astrocyte density and activation status fourfold in hippocampus (P ≤ 0.02), but not in frontal cortex, irrespective of glycemic control. Microglia were twofold to fourfold more abundant in both brain areas of hyperglycemic critically ill mice (P ≤ 0.002), but only in frontal cortex were they reduced in number with normoglycemia (P = 0.0008). The density of apoptotic cells and abundance of carbonylated proteins were significantly higher than normal in frontal cortex of hyperglycemic ill mice only (P = 0.05). In a mouse model of prolonged polymicrobial sepsis, remarkable neuropathological changes develop with neuronal damage, impaired astrocyte activation, increased microglia, apoptosis, and accumulation of carbonylated proteins. These changes were partially prevented or attenuated when hyperglycemia was prevented with insulin. Frontal cortex appeared more vulnerable to hyperglycemic insults than hippocampus.

mTRIP: an imaging tool to investigate mitochondrial DNA dynamics in physiology and disease at the single-cell resolution.

Mitochondrial physiology and metabolism are closely linked to replication and transcription of the genome of the organelle, the mitochondrial DNA (mtDNA). However, the characterization of mtDNA processing is poorly defined at the single-cell level. Here, we describe mTRIP (mitochondrial transcription and replication imaging protocol), an imaging approach based on modified fluorescence in situ hybridization (FISH), which simultaneously reveals mitochondrial structures engaged in mtDNA initiation of replication and global mitochondrial RNA (mtRNA) content at the single-cell level in human cells. In addition, mTRIP can be coupled to immunofluorescence for in situ protein tracking, or to MitoTracker, thereby allowing simultaneous labelling of mtDNA, mtRNA, and proteins or mitochondria, respectively. Altogether, qualitative and quantitative alterations of the dynamics of mtDNA processing are detected by mTRIP in human cells undergoing physiological changes, as well as stress and dysfunction, with a potential for diagnostic of mitochondrial diseases.

Are mitochondria the Achilles' heel of the Kingdom Fungi?

A founding event in the origin of eukaryotes is the acquisition of an extraordinary organelle, the mitochondrion, which contains its own genome. Being linked to energy metabolism, oxidative stress, cell signalling, and cell death, the mitochondrion to a certain extent controls life and death in eukaryotic cells. The large metabolic diversity and living strategies of the Kingdom Fungi make their mitochondria of particular evolutionary interest. The review focuses first on the characteristics of mitochondria in the Kingdom Fungi, then on their implications in the organism survival, pathogenicity and resistance, and finally on proposing unconventional strategies to investigate the biology of fungal mitochondria, unveiling the possibility that mitochondria play as the Achilles' heel of this kingdom.

Nitroso-redox balance and mitochondrial homeostasis are regulated by STOX1, a pre-eclampsia-associated gene.

AIMS: Storkhead box 1 (STOX1) is a winged-helix transcription factor that is implicated in the genetic forms of a high-prevalence human gestational disease, pre-eclampsia. STOX1 overexpression confers pre-eclampsia-like transcriptomic features to trophoblastic cell lines and pre-eclampsia symptoms to pregnant mice. The aim of this work was to evaluate the impact of STOX1 on free radical equilibrium and mitochondrial function, both in vitro and in vivo. RESULTS: Transcriptome analysis of STOX1-transgenic versus nontransgenic placentas at 16.5 days of gestation revealed alterations of mitochondria-related pathways. Placentas overexpressing STOX1 displayed altered mitochondrial mass and were severely biased toward protein nitration, indicating nitroso-redox imbalance in vivo. Trophoblast cells overexpressing STOX1 displayed an increased mitochondrial activity at 20% O2 and in hypoxia, despite reduction of the mitochondrial mass in the former. STOX1 overexpression is, therefore, associated with hyperactive mitochondria, resulting in increased free radical production. Moreover, nitric oxide (NO) production pathways were activated, resulting in peroxynitrite formation. At low oxygen pressure, STOX1 overexpression switched the free radical balance from reactive oxygen species (ROS) to reactive nitrogen species (RNS) in the placenta as well as in a trophoblast cell line. INNOVATION: In pre-eclamptic placentas, NO interacts with ROS and generates peroxynitrite and nitrated proteins as end products. This process will deprive the maternal organism of NO, a crucial vasodilator molecule. CONCLUSION: Our data posit STOX1 as a genetic switch in the ROS/RNS balance and suggest an explanation for elevated blood pressure in pre-eclampsia.

Prevalent coordination of mitochondrial DNA transcription and initiation of replication with the cell cycle.

Nuclear (nDNA) and mitochondrial DNA (mtDNA) communication is essential for cell function, but it remains unclear whether the replication of these genomes is linked. We inspected human cells with a novel fluorescence in situ hybridization protocol (mitochondrial Transcription and Replication Imaging Protocol) that identifies mitochondrial structures engaged in initiation of mtDNA replication and unique transcript profiles, and reconstruct the temporal series of mitochondrial and nuclear events in single cells during the cell cycle. We show that mtDNA transcription and initiation of replication are prevalently coordinated with the cell cycle, preceding nuclear DNA synthesis, and being reactivated towards the end of S-phase. This coordination is achieved by modulating the fraction of mitochondrial structures that intiate mtDNA synthesis and/or contain transcript at a given time. Thus, although replication of the mitochondrial genome is active through the entire cell cycle, but in a limited fraction of mitochondrial structures, peaks of these activities are synchronized with nDNA synthesis. After release from blockage of mtDNA replication with either nocodazole or double thymidine treatment, prevalent mtDNA and nDNA synthesis occurred simultaneously, indicating that mitochondrial coordination with the nuclear phase can be adjusted in response to physiological alterations. These findings will help redefine other nuclear-mitochondrial links in cell function.

Large heterogeneity of mitochondrial DNA transcription and initiation of replication exposed by single-cell imaging.

Mitochondrial DNA (mtDNA) replication and transcription are crucial for cell function, but these processes are poorly understood at the single-cell level. We describe a novel fluorescence in situ hybridization protocol, called mTRIP (mitochondrial transcription and replication imaging protocol), that reveals simultaneously mtDNA and RNA, and that can also be coupled to immunofluorescence for in situ protein examination. mTRIP reveals mitochondrial structures engaged in initiation of DNA replication by identification of a specific sequence in the regulatory D-loop, as well as unique transcription profiles in single human cells. We observe and quantify at least three classes of mitochondrial structures: (i) replication initiation active and transcript-positive (Ia-Tp); (ii) replication initiation silent and transcript-positive (Is-Tp); and (iii) replication initiation silent and transcript-negative (Is-Tn). Thus, individual mitochondria are dramatically heterogeneous within the same cell. Moreover, mTRIP exposes a mosaic of distinct nucleic acid patterns in the D-loop, including H-strand versus L-strand transcripts, and uncoupled rRNA transcription and mtDNA initiation of replication, which might have functional consequences in the regulation of the mtDNA. Finally, mTRIP identifies altered mtDNA processing in cells with unbalanced mtDNA content and function, including in human mitochondrial disorders. Thus, mTRIP reveals qualitative and quantitative alterations that provide additional tools for elucidating the dynamics of mtDNA processing in single cells and mitochondrial dysfunction in diseases.