Recent research has shown that mtDNA-deficient cancer cells (ρ0 cells) acquire mitochondria from tumor stromal cells to restore respiration, facilitating tumor formation. We investigated the role of Miro1, an adaptor protein involved in movement of mitochondria along microtubules, in this phenomenon. Inducible Miro1 knockout (Miro1KO) mice markedly delayed tumor formation after grafting ρ0 cancer cells. Miro1KO mice with fluorescently labeled mitochondria revealed that this delay was due to hindered mitochondrial transfer from the tumor stromal cells to grafted B16 ρ0 cells, which impeded recovery of mitochondrial respiration and tumor growth. Miro1KO led to the perinuclear accumulation of mitochondria and impaired mobility of the mitochondrial network. In vitro experiments revealed decreased association of mitochondria with microtubules, compromising mitochondrial transfer via tunneling nanotubes (TNTs) in mesenchymal stromal cells. Here we show the role of Miro1 in horizontal mitochondrial transfer in mouse melanoma models in vivo and its involvement with TNTs.

Intercellular mitochondria transfer is an evolutionarily conserved process in which one cell delivers some of their mitochondria to another cell in the absence of cell division. This process has diverse functions depending on the cell types involved and physiological or disease context. Although mitochondria transfer was first shown to provide metabolic support to acceptor cells, recent studies have revealed diverse functions of mitochondria transfer, including, but not limited to, the maintenance of mitochondria quality of the donor cell and the regulation of tissue homeostasis and remodelling. Many mitochondria-transfer mechanisms have been described using a variety of names, generating confusion about mitochondria transfer biology. Furthermore, several therapeutic approaches involving mitochondria-transfer biology have emerged, including mitochondria transplantation and cellular engineering using isolated mitochondria. In this Consensus Statement, we define relevant terminology and propose a nomenclature framework to describe mitochondria transfer and transplantation as a foundation for further development by the community as this dynamic field of research continues to evolve. This Consensus Statement provides a nomenclature framework and experimental recommendations for studying mitochondrial transfer and transplantation.

Recent research highlights horizontal mitochondrial transfer as a key biological phenomenon linked to cancer onset and progression. The transfer of mitochondria and their genomes between cancer and non-cancer cells shifts our understanding of intercellular gene trafficking, increasing the metabolic fitness of cancer cells and modulating antitumor immune responses. This process not only facilitates tumor progression but also presents potential therapeutic opportunities.

Mitochondrial networks in eukaryotic cells are dynamic, undergoing fusion, fission, biogenesis and transfer between cells to manage cellular requirements for energy production and biosynthetic components. Mitochondrial transfer has been observed in response to cellular injury and can happen by a variety of mechanisms. To examine whether mitochondria could be restored to injured astrocytes, we established a model of discrete astrocyte-specific, mitochondrial-specific injury in a mixed culture model. Immortalized rho0 astrocytes were generated, then put under lethal metabolic stress by withdrawal of pyruvate and uridine. When cells stressed in this way were grown in the presence of mesenchymal stromal cells, enhanced survival of the rho0 cells was seen, accompanied by the rapid acquisition of an entire mitochondrial networks. Gap-fill ligation and rolling circle amplification for cell-specific mitochondrial genome SNP confirmed that the mitochondrial networks were derived from the mesenchymal stromal donor cells, and both mitochondrial DNA and mitochondrial function were restored to the rho0 cells long-term. The presence of two nuclei in the rho0 cells in the early stages of mitochondrial transfer and recovery was observed. This demonstrated that in this instance, the mechanism of mitochondrial rescue was fusion of the rho0 astrocyte and the mesenchymal stromal cell. In support of this, other neural cells were similarly shown to fuse with stressed rho0 astrocytes, suggesting that whole organelles may be donated to restore critical mitochondrial function. ### Competing Interest Statement The authors have declared no competing interest.

Horizontal transfer of mitochondria from the tumour microenvironment to cancer cells to support proliferation and enhance tumour progression has been shown for various types of cancer in recent years. Glioblastoma, the most aggressive adult brain tumour, has proven to be no exception when it comes to dynamic intercellular mitochondrial movement, as shown in this study using an orthotopic tumour model of respiration-deficient glioblastoma cells. Although confirmed mitochondrial transfer was shown to facilitate tumour progression in glioblastoma, we decided to investigate whether the related electron transport chain recovery is necessary for tumour formation in the brain. Based on experiments using time-resolved analysis of tumour formation by glioblastoma cells depleted of their mitochondrial DNA, we conclude that functional mitochondrial respiration is essential for glioblastoma growth in vivo, because it is needed to support coenzyme Q redox cycling for de novo pyrimidine biosynthesis controlled by respiration-linked dihydroorotate dehydrogenase enzyme activity. We also demonstrate here that astrocytes are key mitochondrial donors in this model.

BackgroundFast adaptation of glycolytic and mitochondrial energy pathways to changes in the tumour microenvironment is a hallmark of cancer. Purely glycolytic ρ0 tumour cells do not form primary tumours unless they acquire healthy mitochondria from their micro-environment. Here we explored the effects of severely compromised respiration on the metastatic capability of 4T1 mouse breast cancer cells.Methods4T1 cell lines with different levels of respiratory capacity were generated; the Seahorse extracellular flux analyser was used to evaluate oxygen consumption rates, fluorescent confocal microscopy to assess the number of SYBR gold-stained mitochondrial DNA nucleoids, and the presence of the ATP5B protein in the cytoplasm and fluorescent in situ nuclear hybridization was used to establish ploidy. MinION nanopore RNA sequence analysis was used to compare mitochondrial DNA transcription between cell lines. Orthotopic injection was used to determine the ability of cells to metastasize to the lungs of female Balb/c mice.ResultsOXPHOS-deficient ATP5B-KO3.1 cells did not generate primary tumours. Severely OXPHOS compromised ρ0D5 cells generated both primary tumours and lung metastases. Cells generated from lung metastasis of both OXPHOS-competent and OXPHOS-compromised cells formed primary tumours but no metastases when re-injected into mice. OXPHOS-compromised cells significantly increased their mtDNA content, but this did not result in increased OXPHOS capacity, which was not due to decreased mtDNA transcription. Gene set enrichment analysis suggests that certain cells derived from lung metastases downregulate their epithelial-to-mesenchymal related pathways.ConclusionIn summary, OXPHOS is required for tumorigenesis in this orthotopic mouse breast cancer model but even very low levels of OXPHOS are sufficient to generate both primary tumours and lung metastases.

Mitochondria are vital organelles with their own DNA (mtDNA). mtDNA is circular and composed of heavy and light chains that are structurally more accessible than nuclear DNA (nDNA). While nDNA is typically diploid, the number of mtDNA copies per cell is higher and varies considerably during development and between tissues. Compared with nDNA, mtDNA is more prone to damage that is positively linked to many diseases, including cancer. Similar to nDNA, mtDNA undergoes repair processes, although these mechanisms are less well understood. In this review, we discuss the various forms of mtDNA damage and repair and their association with cancer initiation and progression. We also propose horizontal mitochondrial transfer as a novel mechanism for replacing damaged mtDNA.

Mammalian genes were long thought to be constrained within somatic cells in most cell types. This concept was challenged recently when cellular organelles including mitochondria were shown to move between mammalian cells in culture via cytoplasmic bridges. Recent research in animals indicates transfer of mitochondria in cancer and during lung injury in vivo, with considerable functional consequences. Since these pioneering discoveries, many studies have confirmed horizontal mitochondrial transfer (HMT) in vivo, and its functional characteristics and consequences have been described. Additional support for this phenomenon has come from phylogenetic studies. Apparently, mitochondrial trafficking between cells occurs more frequently than previously thought and contributes to diverse processes including bioenergetic crosstalk and homeostasis, disease treatment and recovery, and development of resistance to cancer therapy. Here we highlight current knowledge of HMT between cells, focusing primarily on in vivo systems, and contend that this process is not only (patho)physiologically relevant, but also can be exploited for the design of novel therapeutic approaches.

Cellular respiration is highly regulated, changes dynamically in response to the microenvironment of individual cells and during differentiation and differs between cell and tissue types. Too little cell respiration can cause an accumulation of reductants, leading to reductive stress, while inefficient respiration, that causes a build-up of reactive oxygen species (ROS), can result in oxidative stress. Most of the discussion of this central redox dichotomy has centred around oxidative stress because the damaging effects of cellular oxidants on DNA, lipids and proteins are well-established, and have been shown to contribute to health issues including, mitochondrial and cardiovascular diseases, tumorigenesis, and to the effects of ageing. Much less attention has been paid to cellular reductive stress. Nevertheless, excessive levels of key cellular reductants including NADH, NADPH and glutathione, as well as an imbalance in protein thiols, and insufficient levels of ROS to maintain cell signalling pathways, can be harmful to cells and result in poor health outcomes. Recently, cellular mechanisms that sense and regulate cellular reductive stress associated with low ROS levels have been identified. In addition, plasma membrane electron transport has been shown to be a key player in cellular redox homeostasis involving NAD(P)H/NAD(P)+ ratios. It is now well-established that the plasma membrane contains coenzyme Q-mediated electron transport pathways capable of oxidizing intracellular NAD(P)H and reducing extracellular electron acceptors such as molecular oxygen. A better understanding of the origins, cellular and subcellular compartmentalization and regulation of cellular reductants could lead to the development of new anticancer strategies.

The ability of cancer cells to adjust their metabolism in response to environmental changes is a well-recognized hallmark of cancer. Diverse cancer and non-cancer cells within tumors compete for metabolic resources. Metabolic demands change frequently during tumor initiation, progression and metastasis, challenging our quest to better understand tumor biology and develop novel therapeutics. Vascularization, physical constraints, immune responses and genetic instability promote tumor evolution resulting in immune evasion, opportunities to breach basement membrane barriers and spread through the circulation and lymphatics. In addition, the unfolded protein response linked to the ubiquitin proteasome system is a key player in addressing stoichiometric imbalances between nuclear and mitochondrially-encoded protein subunits of respiratory complexes, and nuclear-encoded mitochondrial ribosomal protein subunits. While progressive genetic changes, some of which affect metabolic adaptability, contribute to tumorigenesis and metastasis through clonal expansion, epigenetic changes are also important and more dynamic in nature. Understanding the role of stromal and immune cells in the tumor microenvironment in remodeling cancer cell energy metabolism has become an increasingly important area of research. In this perspective, we discuss the adaptations made by cancer cells to balance mitochondrial and glycolytic energy metabolism. We discuss how hypoxia and nutrient limitations affect reductive and oxidative stress through changes in mitochondrial electron transport activity. We propose that integrated responses to cellular stress in cancer cells are central to metabolic flexibility in general and bioenergetic adaptability in particular and are paramount in tumor progression and metastasis.

Pancreatic cancer is one of the deadliest forms of cancer, which is attributed to lack of effective treatment options and drug resistance. Mitochondrial inhibitors have emerged as a promising class of anticancer drugs, and several inhibitors of the electron transport chain (ETC) are being clinically evaluated. We hypothesized that resistance to ETC inhibitors from the biguanide class could be induced by inactivation of SMAD4, an important tumor suppressor involved in transforming growth factor β (TGFβ) signaling, and associated with altered mitochondrial activity. Here we show that, paradoxically, both TGFβ-treatment and the loss of SMAD4, a downstream member of TGFβ signaling cascade, induce resistance to biguanides, decrease mitochondrial respiration, and fragment the mitochondrial network. Mechanistically, the resistance of SMAD4-deficient cells is mediated by increased mitophagic flux driven by MAPK/ERK signaling, whereas TGFβ-induced resistance is autophagy-independent and linked to epithelial-to-mesenchymal transition (EMT). Interestingly, mitochondria-targeted tamoxifen, a complex I inhibitor under clinical trial, overcomes resistance mediated by SMAD4-deficiency or TGFβ signaling. Our data point to differential mechanisms underlying the resistance to treatment in PDAC arising from TGFβ signaling and SMAD4 loss, respectively. The findings will help the development of mitochondria-targeted therapy for pancreatic cancer patients with SMAD4 as a plausible predictive marker.

Tumor cells without mitochondrial (mt) DNA (ρ0 cells) are auxotrophic for uridine, and their growth is supported by pyruvate. While ATP synthesis in ρ0 cells relies on glycolysis, they fail to form tumors unless they acquire mitochondria from stromal cells. Mitochondrial acquisition restores respiration that is essential for de novo pyrimidine biosynthesis and for mitochondrial ATP production. The physiological processes that underpin intercellular mitochondrial transfer to tumor cells lacking mtDNA and the metabolic remodeling and restored tumorigenic properties of cells that acquire mitochondria are not well understood. Here, we investigated the changes in mitochondrial and nuclear gene expression that accompany mtDNA deletion and acquisition in metastatic murine 4T1 breast cancer cells. Loss of mitochondrial gene expression in 4T1ρ0 cells was restored in cells recovered from subcutaneous tumors that grew from 4T1ρ0 cells following acquisition of mtDNA from host cells. In contrast, the expression of most nuclear genes that encode respiratory complex subunits and mitochondrial ribosomal subunits was not greatly affected by loss of mtDNA, indicating ineffective mitochondria-to-nucleus communication systems for these nuclear genes. Further, analysis of nuclear genes whose expression was compromised in 4T1ρ0 cells showed that immune- and stress-related genes were the most highly differentially expressed, representing over 70% of those with greater than 16-fold higher expression in 4T1 compared with 4T1ρ0 cells. The monocyte recruiting chemokine, Ccl2, and Psmb8, a subunit of the immunoproteasome that generates MHCI-binding peptides, were the most highly differentially expressed. Early monocyte/macrophage recruitment into the tumor mass was compromised in 4T1ρ0 cells but recovered before mtDNA could be detected. Taken together, our results show that mitochondrial acquisition by tumor cells without mtDNA results in bioenergetic remodeling and re-expression of genes involved in immune function and stress adaptation.

The mammalian mitochondrial genome has evolved into an essential circular mini-chromosome present in multiple copies inside mitochondria of all nucleated cells in the body. Consistent with their microbial origin, mitochondria can no longer be considered to be cell-constrained as a growing body of evidence shows that they not only can, but do move between cells demonstrating that horizontal transfer of mitochondrial genes is a normal (patho)physiological process in animal models of disease and in humans. While the molecular mechanisms driving intercellular mitochondrial transfer remain uncertain, functional consequences include respiration recovery and bioenergetic remodeling. Mitochondrial uptake by damaged cells has been demonstrated in vivo, and therapeutic applications of mitochondrial therapy following cardiac damage, brain ischemia-reperfusion injury, and lung inflammation have been described supporting the concept of physiologically important mitochondrial trafficking between cells.

standards for evaluation of respiratory states and rates will ultimately contribute to reproducibility between laboratories and thus support the development of databases of mitochondrial respiratory function in species, tissues, and cells. Clarity of concept and consistency of nomenclature facilitate effective transdisciplinary communication, education, and ultimately further discovery.

Purpose: Cancer cells rapidly adjust their balance between glycolytic and mitochondrial ATP production in response to changes in their microenvironment and to treatments like radiation and chemotherapy. Reliable, simple, high throughput assays that measure the levels of mitochondrial energy metabolism in cells are useful determinants of treatment effects. Mitochondrial metabolism is routinely determined by measuring the rate of oxygen consumption (OCR). We have previously shown that indirect inhibition of plasma membrane electron transport (PMET) by the mitochondrial uncoupler, FCCP, may also be a reliable measure of mitochondrial energy metabolism. Here, we aimed to validate these earlier findings by exploring the relationship between stimulation of oxygen consumption by FCCP and inhibition of PMET. Methods: We measured PMET by reduction of the cell impermeable tetrazolium salt WST-1/PMS. We characterised the effect of different growth conditions on the extent of PMET inhibition by FCCP. Next, we compared FCCP-mediated PMET inhibition with FCCP-mediated stimulation of OCR using the Seahorse XF96e flux analyser, in a panel of cancer cell lines. Results: We found a strong inverse correlation between stimulation of OCR and PMET inhibition by FCCP. PMET and OCR were much more severely affected by FCCP in cells that rely on mitochondrial energy production than in cells with a more glycolytic phenotype. Conclusion: Indirect inhibition of PMET by FCCP is a reliable, simple and inexpensive high throughput assay to determine the level of mitochondrial energy metabolism in cancer cells.

Small regulatory peptides encoded within the mitochondrial genome (mitochondrial-derived peptides) have been shown to have cellular cyto- and metabolo-protective roles that parallel those of exercise. Here we provide evidence that humanin and SHLP6 are exercise-sensitive mitochondrial derived peptides. Studies to determine whether mitochondrial-derived peptides play a role in regulating exercise-induced adaptations are warranted.

Executive summary – Box 1: In brief: Mitochondria and bioblasts .......................................... 2 1. Introduction ............................................................................................................................................................................. 8 2. Coupling states and rates in mitochondrial preparations ................................................................................... 8 2.1. Cellular and mitochondrial respiration ................................................................................................................. 8 2.1.1. Aerobic and anaerobic catabolism and ATP turnover Consortium Communication 2 of 44 Gnaiger E et al ― MitoEAGLE Task Group (2020) Bioenerg Commun 2020.1 2.1.2. Specification of biochemical dose and exposure 2.2. Mitochondrial preparations .................................................................................................................................... 10 2.3. Electron transfer pathways ..................................................................................................................................... 11 2.4. Respiratory coupling control .................................................................................................................................. 12 2.4.1. Coupling 2.4.2. Phosphorylation P» and P»/O2 ratio 2.4.3. Uncoupling 2.5. Coupling states and respiratory rates ................................................................................................................. 13 2.5.1. LEAK state 2.5.2. OXPHOS state 2.5.3. Electron transfer state 2.5.4. ROX state 2.5.5. Quantitative relations 2.5.6. The steady state 2.6. Classical terminology for isolated mitochondria ............................................................................................ 19 2.6.1. – 2.6.5. State 1 – State 5 2.7. Control and regulation .............................................................................................................................................. 21 3. What is a rate? – Box 2: Metabolic flows and fluxes: vectoral, vectorial, and scalar ............................. 21 4. Normalization of rate per sample................................................................................................................................. 23 4.1. Flow: per object ............................................................................................................................................................ 23 4.1.1. Count concentration 4.1.2. Flow per single object 4.2. Size-specific flux: per sample size .......................................................................................................................... 25 4.2.1. Mass concentration 4.2.2. Size-specific flux 4.3. Marker-specific flux: per mitochondrial content ............................................................................................. 26 4.3.1. Mitochondrial concentration and mitochondrial density 4.3.2. mt-Marker-specific flux 5. Normalization of rate per system ................................................................................................................................ 28 5.1. Flow: per chamber ...................................................................................................................................................... 28 5.2. Flux: per chamber volume ....................................................................................................................................... 28 5.2.1. System-specific flux 5.2.2. Advancement per volume 6. Conversion of units ............................................................................................................................................................ 30 7. Conclusions – Box 3: Recommendations for studies with mitochondrial preparations ...................... 31 References ............................................................................................................................................................................. 36 Authors (MitoEAGLE Task Group) – Author contributions .............................................................................. 41 Acknowledgements – Competing financial interests – Correspondence