The special issue of Journal of Cell Science (JCS) on the “Cell Biology of Mitochondria” looks to be one of the most comprehensive open series of papers, opinions, perspectives, interviews, reviews and posters on the subject of mitochondria biology, perhaps ever.
This incredibly in-depth examination of mitochondria covers the subject in research, interviews and commentary and does so in what can only be described as a community spirit from the Journal of Cell Science (JCS) a publication of the Company of Biologists, which is “dedicated to supporting and inspiring the biological community.”
A Good Place to Start [Editorial]
The Cell Biology of Mitochondria Special Issue is a collaboration led by Ana J. Garcia-Saez, Max Planck Institute of Biophysics and Heidi McBride, Department of Neurology and Neurosurgery McGill University, guided by Cell Biology Executive Editor Seema Grewal, who has a deep publishing and research biology background. McBride is also a member of the MitoWorld Scientific Advisory Board (SAB).
MitoWorld was so impressed with this collection, enough reading for a month, that the questions arose of how does such an issue come together and how does it stand to help the mitochondrial research world as the subject becomes more popular. To get a sense of this, Executive Editor Seema Grewal answered a few questions.
MitoWorld: What prompted the special issue?
Grewal: JCS has a long and rich history of publishing papers in the field of mitochondrial cell biology; indeed, some of the early papers examining the factors that contribute to mitochondrial fusion and fission were published in JCS. We wanted to remind the community about this, so what better way to do this than to coordinate a special issue on the topic.
MitoWorld: This was a huge undertaking, how did this get going and how long has it taken?
Grewal: We started discussing the idea at the journal’s annual gathering of editors back in February 2024. Heidi and Ana are on the journal’s Editorial Advisory Board and are experts in the field, so the team felt that they would be well-placed to serve as Guest Editors for the issue. After some discussion with them, we decided on the aim, scope and timeline for the special issue. We put out a call for papers in the Spring of 2024 and had an encouraging response from the community, resulting in lots of research articles being submitted for consideration in the special issue. In parallel, we also identified and invited experts from across the field to contribute review-type articles.
MitoWorld: Can you talk about the Journal and its mission and how you are able to put together projects at this scale.
Grewal: JCS prides itself in being a community journal that is published by a not-for-profit publisher (The Company of Biologists) that exists to benefit scientists not shareholders. Coordinating a special issue is hard work, but we’re fortunate to have an in-house team that can support Guest Editors in coordinating the issue, so that they can focus on the science. This team works closely with the Guest Editors to ensure the smooth and efficient handling of articles.
MitoWorld: What do you hope comes from such a publication? Who should be reading this issue?
Grewal: We’re hoping that the issue will inspire the community and stimulate debate and discussion. It contains a range of research and technical articles that showcase the advances in the field. It also contains some in-depth reviews that synthesize the latest findings. We’ve also included some perspective and opinion articles that challenge our view of mitochondria. Finally, we strongly believe that it’s important to highlight the scientists that actually do the work, so the issue includes some interviews with researchers at different career stages. We’re hoping there’s something in the issue for everyone. It’s also worth pointing out that, thanks to our ‘Forest of Biologists’ initiative, we planted 24 trees to represent the 24 peer-reviewed articles published in the issue, so that’s another really positive outcome!
MitoWorld: There is so much material, do you have suggestions on what busy people should concentrate on or how to best use the whole resource.
Grewal: The Opinion piece is a good place to start, as it provides an over-arching view of mitochondria. After that, we suggest that people just bookmark the full table of contents and dip in and out of any reviews or research articles that appeal to them
MitoWorld: What do you hope the take-aways are for the serious reader of this special issue?
Grewal: That we’ve made lots of progress, largely thanks to new technologies…but there’s still so much to learn!
Columbia University mitochondria researcher Martin Picard’s life changed in 2014 when he peered through a microscope in Douglas Wallace’s lab at the Children’s Hospital Philadelphia. He saw something that didn’t fit the textbook “powerhouse of the cell” picture. Mitochondria are tiny elongated and sometimes spherical energy-producing organelles. While many times thinner in diameter than a human hair, hundreds to thousands of mitochondria populate the interior of each human cell.
After 20 years of wonder, Picard’s personal and professional journey was picked up by Scientific American, which has just published his article, The Social Lives of Mitochondria: When These Energy-Giving Organelles Thrive, So Do We (Scientific American June 2025). The online version of the print article, viewed here, is entitled Mitochondria Are More Than Powerhouses—They’re the Motherboard of the Cell.
Picard writes, “Under the high-power microscope, mitochondria have many tiny generally horizontal “baffles”, called cristae, the site of ATP production, the cellular energy currency. Energy transformation within cristae involves the stripping of electrons from food and allows them to flow onto the oxygen we breathe”.
With hundreds to thousands of mitochondria bunched together, it is hard to know if they are acting in concert or as random lone operators. What Picard saw through the microscope, featured here and below was the alignment of the cristae between mitochondria. “The first physical evidence of non-molecular information exchange between mitochondria,” says Picard.
https://www.nature.com/articles/ncomms7259/figures/3
Since then, Picard and others have probed this mitochondrial behavior to the point that it appears mitochondria are operating communally or failing to do so. Different organs, researchers have found, have different types of mitochondria. “Mitochondria have a bacterial origin in evolution, and there is ample evidence from bacteria today that they do what is called “quorum sensing” where they signal and align to perform tasks a single bacterium or mitochondrion could not accomplish on its own,” Picard explains.
For more background and context, MitoWorld talked with Picard:
MitoWorld: Can you show us any microscopy or artistic renditions or video that shows the “social” nature of mitochondria?
Picard: The best video is this. Also this video showing cristae align between mitos changed my life. This picture shows mitochondria networking.
MitoWorld: What led to your thoughts about the social nature of mitochondria?
Picard: Everything in biology has somewhat of an interactive nature to it. And across the universe, everything is interconnected, from electrostatically attracted protons and electrons within atoms, to attracted social human beings, to planets attracted to each other by gravitational forces. Why would our biology be different? And could there be some kind of “social” behavior deep within our cells that led to our organs, bodies, and to our mind to becoming “social”. And it could have started with the endosymbiosis of mitochondria.
MitoWorld: Has this been on your mind for a while, how did you begin to verify the social conjecture?
Picard: Early work by David Chan on mitochondrial fusion. In 2012 I wrote a piece called “Mitochondria: Starving to Reach Quorum” that touched on their “social” nature, like bacteria that talk to each other to do “quorum sensing” and increase their virulence. Then in 2014, I saw cristae alignment between mitochondria. Since then, many labs have observed that, if you prevent mito-mito interactions (disrupt their social interactions), they go bad, as do the cells that house them too. My neuroscientist colleague Carmen Sandi and I detailed this in a paper in 2021.
MitoWorld: Help readers understand how important mitochondria are to the life of cells.
Picard: Without mitochondria, we would not exist. When they appeared in evolution, the result of a merger. This was the click—the beginning of a new phase of life. Somehow their presence allowed a type of multicellular life that wasn’t possible before. My hunch is that mitochondria provided the ability to process information: they made cells smarter and elevated their “social” behavior to a next level. With this, cells could come together into larger collectives, hold larger goals, and grow organisms that behave, think, and feel. This may all have been possible because the Mitochondrial Information Processing System (MIPS) became the “brain” of the cell.
MitoWorld: What are the behaviors of mitochondria that are “social”?
Picard: Mitochondria 1) communicate with each other and other organelles, 2) exhibit group formation, 3) are interdependent, 4) synchronize their behaviors, and 5) functionally specialize to accomplish specific functions.
MitoWorld: What are the health, medical and research avenues that open as a result of the mounting evidence of the social nature of mitochondria?
Picard: I think it’s time to see ourselves energetically. We are not just molecular machines. That mechanical, somewhat static view has been propagated for too long without seeing the wider energetic context. Understanding the “social” layer of biological organization makes it clear that there are biological processes and forces, including “goals” that cells and organisms have, that aren’t just the product of cogwheels. For example, the healing process is a completely untapped area of medicine and science that needs attention. I would suggest that, alongside our mitochondrial research, we need Healing Science, a new area of science that will map out how we manage to heal, every day. Charting this new territory of health and healing science has to be grounded in first principles. The interconnectedness of our biology, together with our fundamental energetic nature, are those first principles. Realizing that mitochondria are “social” is a step towards a more accurate view of how life works, and of what keeps us healthy day after day.
Next-generation sequencing has become the method of choice diagnosing diseases and risks, but the size difference between the nuclear and mitochondrial genomes complicates its value for mitochondrial diseases. A new study led by Rita Horváth at the University of Cambridge offers new hope for this technology.
Mitochondrial diseases affect about 1 in 4,300 people. Unfortunately, they have different manifestations at different times in development. They also overlap with other common diseases. Finally, there are only a limited number of biomarkers for blood samples.
The Horváth laboratory developed the MitoPhen database that includes genotype-phenotype information and mitochondria (mt)DNA variant levels in blood and tissues from published reports. They test MitoPhen for its ability to determine phenotype similarity scores for patients with mitochondrial diseases and then in a large European Solve-RD rare disease cohort. They then used those data to develop a workflow to identify mtDNA variants using MToolBox and annotated them with the MITOMAP database.
Using these data-management tools, the team was able to identify additional patients with mtDNA mutations. Other methods leave significant uncertainty. The workflow developed here allows analysis of DNA samples for possible mtDNA-based diseases to reveal rare mitochondrial diseases in patients not suspected of those diseases. Thus, the workflow and analysis provide another tool for diagnosing these rare disorders.
Reference
Ratnaike T, Paramonov I, Olimpio C, Hoischen A, Beltran S, Matalonga L, Solve-RD Consortium, Horváth R (2025) Mitochondrial DNA disease discovery through evaluation of genotype and phenotype data: The Solve-RD experience. Am J Hum Genet https://www.cell.com/ajhg/fulltext/S0002-9297(25)00144-2?rss=yes.
A discussion with Dr. Horváth:
What are the likely next steps in your research?
Thiloka: It will be really interesting to expand this phenotype-based prioritisation techniques to other primary mitochondrial diseases, and we have indeed compiled a large manually curated updated version of MitoPhen for this purpose. We are now looking at nuclear genes that cause mitochondrial diseases to see whether we can highlight patients with phenotypes that are suggestive of mitochondrial diseases, to prioritise known or novel variants for further evaluation.
You found a deceptively small additional number (0.4%) of patients where no disease was expected. Of course, that number spread across a large population would become a big number. Do you think it will expand interest in mitochondrial diseases?
Thiloka: Absolutely! Mitochondrial diseases are known to affect in 1 in 4300 individuals, so this group of conditions is one of the largest in the field of inherited metabolic conditions. Being able to confidently diagnose individuals affected by these conditions mean that our pool of families to invite for potential therapeutic strategies or clinical trials will grow, enabling advancements in this challenging field.
As you noted, there is overlap with some diseases (e.g., diabetes). Will your technique help to parse the different diseases?
Thiloka: Great point. We believe this technique can help understand contribution of the variant to the clinical features. For example, in our study, we diagnosed individuals with sensorineural hearing loss with mitochondrial DNA variants that cause this presentation, however, there was one individual where the mitochondrial DNA variant didn’t fully explain the phenotype which consisted of several more features than just sensorineural hearing impairment. That is important to know as well because we are increasingly finding dual genetic diagnoses in this era of genetic testing. We can only try to achieve this level of phenotypic certainty by adding to existing genotype-phenotype databases with curated data at the individual level.
Can your workflow be “translated” so that it can be transitioned into the clinic? What would that take?
Thiloka: I am aiming to a tool which could be used in the clinical setting to understand the probability of a person having a primary mitochondrial disease based on their clinical features. A tool such as this could be helpful in utilizing resources effectively to prioritise advanced genetic testing for individuals with a high likelihood of this diagnosis. However, to get to this stage we would need to have a comprehensive resource that has compiled individual level data on primary mitochondrial diseases and have been tested in the setting of individuals with other genetic conditions (non-mitochondrial). We are trying to achieve this currently with our updated MitoPhen database that now contains data on 117 genotypes of primary mitochondrial diseases, and we are testing the utility of this dataset in large datasets including the 100,000 Genomes Project and RD-Connect. The unexplored situation is that of ‘real world’ clinical data extracted from electronic health records, which is likely to contain more ‘noise’ in the sense of phenotypic features which may not be relevant to mitochondrial disease, but is very much the next major step to take in this research.
How did you first become interested in mitochondria?
Thiloka: I became interested in mitochondria because I wanted to understand the processes behind what caused my cousin’s fatal degenerative condition, known as Kearns-Sayre Syndrome (a primary mitochondrial disease). I undertook my PhD at Newcastle University where I worked on understanding what we could learn from muscle biopsy findings from patients, to explain disease progression in different primary mitochondrial diseases, but also how muscle mitochondrial function changes with exercise. Since the PhD, I have remained committed to trying to streamline the diagnostic process for families because I realized we could better use the clinical record to inform their disease profile. It has been challenging trying to juggle this with clinical and family commitments, while I train to become a Paediatric Neurologist, but keeping in close contact with the amazing Lily Foundation maintains my motivation and desire to help add to this scientific domain!
The laboratory of Dr. Jennifer Trowbridge at The Jackson Laboratory published a paper that describes how a genetic mutation that alters the function of mitochondria may provide a target for slowing aging.
Blood cells routinely wear out and must be replaced. Hematopoietic stem and progenitor cells (HSPCs) are critical to maintaining an adequate supply of mature blood cells. During aging, these long-lived stem and progenitor cells tend to accrue somatic mutations that give them a selective advantage. This condition is clonal hematopoiesis (CH). Most humans develop detectable CH by the age of 60, and this contributes to serious health conditions that occur during aging.
The Trowbridge team examined a mutation in the gene for DNA methyltransferase (DNMT3A) that is commonly associated with CH in humans. The team tested a mouse model that they engineered to carry the Dnmt3a mutation. They found that HSPCs from mice with the mutation had higher levels of mitochondrial activity than the same cells in wild-type mice. This was a surprising result since Dnmt3a had never been associated with mitochondrial function.
They found that molecules that disrupt the enhanced mitochondrial function in mutant cells, such as MitoQ and d-TPP, reduced CH in mice and in human cells. This was also true when using metformin, which impacts mitochondrial metabolism and is commonly used to treat type 2 diabetes. This work enhances knowledge of how blood stem cells contribute to aging and may provide a new therapeutic strategy for treating age-associated diseases, such as cancer, diabetes, and heart disease.
Reference
Young KA, Hosseini M, Mistry JJ, et al. (2025) Elevated mitochondrial membrane potential is a therapeutic vulnerability in Dnmt3a-mutant clonal hematopoiesis. Nat Commun 16: 3306. https://doi.org/10.1038/s41467-025-57238-2
Mito-World talks to Dr. Trowbridge
The metformin result is especially interesting since it is already widely used. Do you think it will become a treatment?
Indeed, the results with metformin were very exciting and provocative. While we have shown this to be effective over the short term in mouse models and in human cells in a dish, more studies are needed in human patients to determine whether there is long-term benefit to metformin. If metformin turns out not to be the best option, the good news is that our work has identified other molecules and drugs that target mitochondrial metabolism that could also be tested for efficacy.
Can you speculate on the mechanism behind this connection? You suggest involvement of DNA hypomethylation and increased gene expression of the electron transport chain and respiratory supercomplex formation.
It appears that hematopoietic stem and progenitor cells that carry somatic mutations that provide them with a selective advantage with aging do so, at least in part, through enhancing mitochondrial metabolism. In our study, we observed a connection between DNA hypomethylation and production of the molecules that drive mitochondrial metabolism. However, other somatic mutations that cause CH also enhance mitochondrial metabolism through what are likely different mechanisms. This may represent an example of ‘convergent evolution’ where the mechanisms may differ but the end result—enhanced mitochondrial function—is the same.
Do you plan to follow up the Phase II clinical studies with the various compounds? Of course, metformin, tamoxifen, and diclofenac are already safe in humans.
We are actively collaborating with clinicians that are doing prospective controlled studies to assess CH in individuals receiving metformin over time. In addition, we are pursuing collaborations to assess CH in ongoing placebo-controlled trials where aged individuals are receiving MitoQ for other indications.
The connection to hematopoiesis is clear, but a connection to other diseases (e.g., cancer, diabetes) is less so. Any thoughts?
The connection between CH and risk of diseases, such as cancer and heart disease, is very well-established. In the field, we have made an educated guess that reducing or controlling the development of CH will reduce the incidence and risk of these other diseases; however, long-term prospective studies in humans are needed to definitively prove this.
Extrapolating from mice to humans has not always been successful. Do you have any plans to look at human cells or tissues?
Our paper does include examination of primary human hematopoietic stem and progenitor cells and shows that DNMT3A-mutant human cells have the same phenotypes as Dnmt3a-mutant mouse cells with respect to enhanced mitochondrial metabolism. We also show that MitoQ is effective in suppressing the competitive advantage and growth of human DNMT3A-mutant cells. What we have found, in my opinion, is a fairly rare example where the phenotypes are remarkably similar in mice and in humans. Moving forward to test these interventions in long-term mouse and long-term human studies are the clear next steps.
Mitophagy in Aging Brains
The brain uses a great deal of energy, and thus, mitochondria are critical to brain health. Damaged or worn-out mitochondria are removed by mitophagy, and impaired mitophagy has been associated with Alzheimer’s and Parkinson’s diseases. The laboratory of Thomas McWilliams at the University of Helsinki showed that mitophagy levels increased differently in different areas of aging mouse brains. They increased in areas responsible for movement, but decreased in those for memory. These results provide insights that may be valuable for healthy brain aging.
Rappe A, Vihinen HA, Suomi F, Hassinen AJ, Ehsan H, Jokitalo ES, McWilliams TG (2024) Longitudinal autophagy profiling of the mammalian brain reveals sustained mitophagy throughout healthy aging. The EMBO Journal 43(23): 6199-6231.
Impaired Mitochondria Affect Skeletal Aging
Patients with impaired mitochondrial respiratory chain capacity often have show reduced levels of skeletal growth that are similar to premature degeneration of cartilage and aging. A research team led by Bent Brachvogel at the University of Cologne in Germany examined cartilage cells in mice with impaired cellular respiration. They found that those cells tended to lose the ability to regenerate and died to accelerate aging. In this way, they identified basic processes that might be therapeutic targets.
Bubb K, Etich J, Probst K, Parashar T, Schuetter M, Dethloff F, Reincke S, Nolte JL, Krüger M, Schlötzer-Schrehard U, Nüchel J (2025) Metabolic rewiring caused by mitochondrial dysfunction promotes mTORC1-dependent skeletal aging. Science Advances 11(16): eads1842.
Balancing Mitochondrial Transcription and Replication
The small mitochondrial genome serves as a template for replicating the DNA genome and for transcribing RNAs that encode proteins. The balance between these two functions is critical for mitochondrial health. However, the mechanism for this balancing is unknown. Recently, a team led by Takehiro Yasukawa at Kyushu University in Japan studied the role of mitochondrial transcription elongation factor (TEFM) in the process. They report that knockout of the TEFM gene resulted in reduced ability to make the transition from transcription to replication. This study provides additional insights into the biology of mitochondria.
Matsuda S, Nakayama M, Do Y, Ishiuchi T, Yagi M, Wanrooij S, Nakada K, Wei FY, Ichiyanagi K, Sasaki H, Kang D (2025) TEFM facilitates transition from RNA synthesis to DNA synthesis at H-strand replication origin of mtDNA. Communications Biology 8(1): 202.
Mitochondria at the Synapse
Mitochondria are important in synaptic transmission and plasticity, but how they do this is unknown. Sannon Farris and her team at Virginia Technical University looked at the mitochondrial calcium uniporter (MCU) that couples neuronal activity to ATP production. There is more MCU in hippocampal CA2 distal than proximal dendrites, and distal dendrites have more plasticity. However, the mechanism is unknown. Mice with a CA2-specific MCU knockout had fragmented mitochondria that might explain their functional deficits a synapses. These differences in MCU expression might be the mechanism used by different cell types to modulate mitochondrial function to different needs.
Pannoni KE, Fischer QS, Tarannum R, Cawley ML, Alsalman MM, Acosta N, Ezigbo C, Gil DV, Campbell LA, Farris S (2025) MCU expression in hippocampal CA2 neurons modulates dendritic mitochondrial morphology and synaptic plasticity. Scientific Reports 15(1): 4540.
Recycling Mitochondria
Cells with damaged mitochondrial DNA have been implicated in numerous diseases and aging. A team led by David Pla-Martín at Heinrich-Heine University Düsseldorf in Germany discovered a mechanism that deals with those mitochondria with damaged DNA. That mechanism depends on a protein complex called the retromer and lysosomes, which contain digestive enzymes. Together they recycle the mitochondrial components and eliminate the damaged mitochondria. The authors speculate that dysfunction in this recycling mechanism might be involved in disease pathogenesis.
Kakanj P, Bonse M, Kshirsagar A, Gökmen A, Gaedke F, Sen A, Mollá B, Vogelsang E, Schauss A, Wodarz A, Pla-Martín D (2025) Retromer promotes the lysosomal turnover of mtDNA. Science Advances 11(14): eadr6415.
How Mitochondria Produce Energy
Mitochondria have long been known to be the organelle that produces energy from sugar in the food we eat. However, the precise machinery that accomplishes this activity is unknown. Now, using cryo-electron microscopy, a team from the laboratory of Edmund Kunji at the MRC Mitochondrial Biology Unit, Cambridge determined the atomic structure of the mitochondrial pyruvate carrier that transports the key molecule pyruvate. This mechanism allows pyruvate to cross the normally impermeable inner mitochondrial membrane. Understanding this machine will allow scientists to develop drugs that interact with it and serve as therapies for a variety of diseases.
Sichrovsky M, Lacabanne D, Ruprecht JJ, Rana JJ, Stanik K, Dionysopoulou M, Sowton AP, King MS, Jones SA, Cooper L, Hardwick SW. (2025) Molecular basis of pyruvate transport and inhibition of the human mitochondrial pyruvate carrier. Science Advances 11(16): eadw1489.
Mitochondria have come to be more appreciated for their roles in health and disease. Mitochondrial dysfunction has been implicated in a direct role in more than 50 diseases. In others, mitochondrial dysfunction is an important contributing factor, including neurodegenerative diseases (e.g., Alzheimer’s disease, Parkinson’s disease), cancer, and metabolic diseases.
Mitochondria have increasingly become the focus of new biotech companies. One of those is Pretzel Therapeutics. Pretzel seeks to harness cellular energetics to treat a wide range of conditions now recognized as having a mitochondrial connection. Mitochondria are the powerhouses of the cell, and each cell contains hundreds of mitochondria. Interestingly, mitochondria have their own small genome that encodes enzymes that control mitochondrial activities. Several of the projects at Pretzel focus on controlling cellular energetics by modulating the cellular copy number of mitochondrial DNA (mtDNA).
The Pretzel researchers hope to restore cellular energetics to treat neurodegenerative and rare diseases associated with low mtDNA levels or to modify bioenergetics to treat obesity. In one program, they hope to restore energetics by activating the mitochondrial DNA polymerase. This program was disclosed in a recent Nature report (Valenzuela et al., 2025). In that publication, the researchers show that a small-molecule activator restored function to the most common mutants of the mtDNA polymerase, PolG. A lead therapeutic in this program was recently entered into Phase 1 clinical study. In another program, they hope to modulate the activity of the mitochondrial RNA polymerase. A third program employs gene editing to reduce mutated mtDNA and increase levels of healthy mtDNA.
Pretzel launched in 2022 with financing of $72.5 million. The founders include Gabriel Martinez, Claes Gustafsson, Maria Falkenberg, Michal Minczuk, Nils-Göran Larsson, and the late Paul Thurk. Their facilities are located in Waltham, Massachusetts, and Mölndal, Sweden.
For more information on Pretzel Therapeutics, visit their website (https://www.pretzeltx.com/).
Reference
Valenzuela S, Zhu X, Macao B, et al. (2025) Small molecules restore mutant mitochondrial DNA polymerase activity. Nature, doi.org/10.1038/s41586-025-08856-9
Questions for Pretzel Therapeutics
Pretzel researchers and others recently published an excellent paper in Nature that supports your therapeutic strategy. Congratulations on that publication.
Mitochondrial dysfunction has been implicated, directly or indirectly, in many diseases. What diseases do you see as the “low-hanging fruit” for treatments?
The lack of treatments for mitochondrial dysfunction and disease indicates to us that there really isn’t any “low-hanging fruit” in this area. That said, diseases with a clear and direct genetic cause, such as POLG disease, offer the clearest mechanistic rationale to drug discoverers.
Are you considering testing approved drugs for potential off-label uses for treating mitochondrial diseases?
We are focused on our own, novel targets and drug candidates. We do not work in re-purposing projects.
Neurodegenerative diseases are projected to increase rapidly with our aging population. Do you have any thoughts about how your therapies might fit into preventing or treating those diseases?
There has been substantial published research on the links between degenerative conditions of aging and declining mitochondrial function, far more than we can summarize here. We are of course interested in investigating how our novel mechanisms may influence these conditions. To this end, we have been awarded a grant from Parkinson’s-UK, a leading charity supporting research, to study our mechanisms in Parkinson’s disease.
Cytotoxic T cells are vital for eliminating cancer cells and thus for restraining tumour growth. However, many types of cancer cell blunt this immune attack by issuing signals that somehow cripple the attacking cell. Like almost everything in immunity, this undoubtedly has multiple dimensions. However, one crucial aspect seems to be centred on mitochondria. By rendering the mitochondria of the cytotoxic cell incapable of energizing the process(es) that kill the target cell, cancer cells survive immune attack. This concept has received support from studies showing that T cells that invade the tumour micro-environment exhibit signs of mitochondrial energetic dysfunction. However, the cellular and molecular mechanisms behind this phenomenon have, until recently, remained obscure.
This phenomenon could have serious consequences. Cancer cells frequently harbour mtDNA mutations that impair their own capacity for oxidative ATP generation (OXPHOS). Surprisingly, mitochondria from T cells use ‘tunnelling nanotubes,’ to transfer into cancer cells [2-4] and promote tumour survival. Even if only a small fraction of tumour cells are ‘empowered’ in this manner, it should be sufficient to protect them from cytotoxic drugs that target cells dependent on non-mitochondrial pathways for their energy supply, DNA repair and cell division. Thus, a pool of chemoresistant cells would survive to reconstitute the tumour in an even more aggressive form that is refractory to future treatments. The phenomenon of intercellular transfer of mitochondria remains somewhat controversial. However, it has been independently documented by multiple reputable laboratories and must now be considered physiological. Nevertheless, it could still be an enhanced property of tumour cells or other cells with metabolic impairment.
A recent paper from the lab of Yosuke Togashi in Japan shed light on one important part of this puzzle [1]. They report that mitochondrial transfer also occurs in the opposite direction, via tunnelling nanotubes and or/extracellular vesicles. OXPHOS-incompetent mitochondria from the tumour, bearing deleterious mutations in mtDNA, were detected in T cells that had invaded the tumour micro-environment. The origin of these mitochondria was confirmed by co-culture experiments with tumour cells expressing a mitochondrially targeted fluorescent reporter protein. Prolonged co-culture resulted in the almost complete replacement of endogenous T-cell mitochondria by those derived from the tumour cells. This replacement reflected the fact that tumour-derived mitochondria were resistant to mitophagy, whereas the original T-cell mitochondria were not.
T cells whose mitochondria had suffered this replacement showed OXPHOS impairment, increased dependence on non-mitochondrial ATP generation, downregulation of T-cell markers associated with cytotoxic functions, and signs of cellular senescence. Importantly, the acquisition of tumour- derived mitochondria impaired their activation and, hence, their ability to kill cancer cells, as shown by an in vivo model.
The work breaks new ground by showing that the interaction of immune cells with cancer cells can facilitate a bidirectional mitochondrial exchange that simultaneously favours cancer cell viability and immune evasion. However, this obviously raises many questions. The authors showed that the exchange can operate in at least three cancer-cell types (melanoma, breast and lung cancer), but does it work for all cancer types? Does the phenomenon depend on which surface markers are expressed by a given cancer cell, or by which antigen-receptors are expressed on the cytotoxic cell with which it interacts? The immune cell repertoire includes many different cell types, each with a role in anti-cancer immunity. Does mitochondrial exchange operate only in cytotoxic T cells, or is it seen in other immune cell-types, such as Treg and other CD4+ cells, dendritic cells, NK (natural killer) cells, phagocytes or B lymphocytes? If so, how is their function modified? Are those other immune cell functions secondarily deranged by impaired cytokine secretion from OXPHOS- deficient T cells? Does tumour mtDNA migrate to other cells in the body than immune cells, and if so, is their function also compromised?
The discovery of the mitochondrial fusion/fission cycle forced us to rethink the mitochondrial content of each cell as being akin to a single entity. The pioneering work of the Togashi lab and others now obliges us to revise another fundamental tenet of mitochondrial biology, recognizing that mitochondria and their DNA are not simply the chattels of single cells, but belong to populations of many cells and to tissues, organs and even the body as a whole.
References
- Ikeda H, et al. (2025) Immune evasion through mitochondrial transfer in the tumour microenvironment. Nature 638: 225–236. doi: 10.1038/s41586-024-08439-0.
- Tan AS, et al. (2015). Mitochondrial genome acquisition restores respiratory function and tumorigenic potential of cancer cells without mitochondrial DNA. Cell Metab. 21: 81-94. doi: 1016/j.cmet.2014.12.003.
- Saha T, et al. (2022) Intercellular nanotubes mediate mitochondrial trafficking between cancer and immune cells. Nat Nanotechnol. 17: 98–106. doi: 10.1038/s41565-021-01000-4.
- Zhang H, et al (2023) Systematic investigation of mitochondrial transfer between cancer cells and T cells at single-cell resolution. Cancer Cell 41: 1788–1802.e10. doi: 10.1016/j.ccell.2023.09.003.
In a new paper in Nature, the laboratory of Martin Picard, PhD, mapped the location and activity of the many mitochondria in a human brain.1, 2 Dr. Picard is associate professor of behavioral medicine at Columbia University Irving Medical Center and directs the Mitochondrial Psychobiology Group. This project was conducted in collaboration with Michel Thiebaut de Shottten, PhD, at Director of research the CNRS in Bordeaux,
When most of us think about burning energy, we think of exercising our muscles. However, small as it is, the brain consumes an outsized proportion of our energy. It needs that energy to perform its myriad functions, and to provide all that energy, each brain cell contains thousands of mitochondria, the organelles that produce energy.
Dr. Picard wondered how the many mitochondria are located in the brain and how they relate to energy production and usage in different areas of the brain. How are the cellular components associated with brain anatomical structures? To explore these and other questions, they developed MitoBrainMap v1.0.3
They began by developing a method to physically divide or “voxelize” a frozen human brain into small cubes or voxels (3x3x3mm). Interestingly, magnetic resonance imaging (MRI) images small cubes of brain, virtually “voxelizing” the brain, but the voxelization of the brain into physical cubes had to await the development of new methods. This was spearheaded by Eugene Mosharov, PhD, first author on the paper.
Next the team had to define the different mitochondrial types across the brain. They found that the different regions of the gray and white matter had quite different mitochondrial densities and activities (measured as oxidative phosphorylation activity). Intriguingly, there was an intriguing correlation: regions with higher activity were those that developed later in evolution.
The final challenge was to create a computational model to assemble the information from a single slice to the whole brain. This was accomplished by Michel Thiebaut de Shotten, PhD, a neuroanatomist from the CNRS in Bordeaux. The team then expanded that model into a prediction model that allowed them to assign predictions to other regions of the brain, based on date from MRIs.
The map is only the beginning. The team can now use that map to further explore the relationship of mitochondria in different regions of the brain with psychosocial and cognitive factors; exploring the mind-mitochondria connection. This new tool will enable them to study how brain energy relates to anatomy, development, behavior, and neurodegeneration, and the nature of the mind and consciousness.
“This is the kind of project that is nearly impossible to do” said Picard, “we had to find an exceptionally high-quality brain, align engineering, computational, molecular biology, and neuroscience expertise, and find the right students courageous enough to undertake this massive effort! I am so grateful for this amazing team that came together to create MitoBrainMap v1.0.”
A Conversation with Martin Picard
How did the idea come about to create the mitochondria brain map?
One November evening in the lab, after discovering the diversity of mitochondria in the mouse brain (Nature Communications 2023) and imaging the brain of our patients with rare mitochondrial diseases. I dreamed of a way to systematically map mitochondria across the whole brain. Chatting with my engineer-at-heart neuroscientist colleague Eugene Mosharov, we came up with the technical approach to turn the brain into cubes to create the MitoBrainMap v1.0.
What were the steps needed to get the data?
1. Find a brain, 2. Develop a hardware/software approach to physically partition, or voxelize the brain, 3. Perform >18,000 laboratory assays, 4. Visualize the massive multi-modal dataset in the brain space, 5. Register that data to the standard human brain space used in neuroscience with Michel Thiebaut de Shotten.
Were there many debates and questions about how to get the right sample sizes and make sure they were representative?
We initially wanted to do the whole brain, but estimated that this would cost >5 million dollars and take multiple years. We abandoned that ambition, and focused on a single section of a single brain hemisphere.
What problems did you have to overcome?
The major challenge was one of scale. This was by far our largest mitochondrial phenotyping project. We developed a higher throughput lab platform and a robust data processing pipeline to overcome this challenge. The other challenge was the working across disciplines, combining molecular and single-cell RNA sequencing, mitochondrial biochemistry, neuroscience, and neuroanatomy.
How did the team work together?
Cohesive teamwork was critical. Students generated a ton of data. Eugene handled and integrated the unique dataset. Orian Shirihai’s team validated and enhanced our mitochondrial assays. And Michel Thiebaut de Shotten mapped this data onto the brain, developing an algorithm to extend to the whole brain.
How long did it take from inception to final paper?
November 2020 to March 2024 – 3.5 years! Plus a year to revise the paper.
What are the next steps on your research?
Cynthia Liu in our lab is now measuring mitochondrial content and OxPhos enzyme activities in 5,000 human samples. This will validate and extend this line of work.
Was there any correlation between mitochondria and regions of known brain functions (e.g., speech, vision, hearing)?
Great question. This is something to explore in future research.
The gap between molecular processes and macro-imaging often contains fascinating information. Your work bridged that gap. Might there be other organelle targets of your work?
Mitochondria are pretty special. They have their own genome, two membranes, and without them, mammalian cells cannot live at all. All cellular and molecular processes converge on their requirements for energy, and the human genome is actively regulated by signals from the Mitochondrial Information Processing System (MIPS) (Cell Metabolism 2022). That being said, one could certainly profile other organelles across the brain.
Can your work be correlated to mitochondrial brain work on neurodegenerative diseases, such as Parkinson’s or Alzheimer’s diseases?
Yes, this is a future direction some of our colleagues are interested in. But when we study diseased brains, it’s like trying to understand the causes of a car crash by looking at the totaled, permanently damaged vehicle. Without looking at the driver, we likely will never know why the vehicle/brain crashed in the first place. In the brain and living organisms, the driver is energy. And energy fluxes through our mitochondria.
You suggest that your method correlates with other imaging methods. Do you think these might have potential to eventually be used in diagnostic methods?
Yes, our hope is that we can quantify mitochondrial content and quality or health with standard neuroimaging methods. In the living human brain, non-invasively.
How did you come to be interested in mitochondria in the first place?
Mitochondria are the dynamic energetic portal between the physical processes that convert food and oxygen into subjective experiences, and consciousness. In graduate school, I became increasingly convinced that mitochondria were going to teach us something meaningful about mind-body processes. Now we study the mind-mitochondria connection, in and outside the brain.
References
1Mosharov EV, Rosenberg AM, Monzel AS, Osto CA, Stiles L, Rosoklija GB, Dwork AJ, Bindra S, Junker A, Zhang Y, Fujita M, Mariani MB, Bakalian M, Sulzer D, De Jager PL, Menon V, Shirihai OS, Mann JJ, Underwood M, Boldrini M, Thiebaut de Schotten M, Picard M (2025) A human brain map of mitochondrial respiratory capacity and diversity. Nature https://doi.org/10.1038/s41586-025-08740-6.
2Thiebaut de Schotten M, Picard M (2025) A map of mitochondrial biology reveals the energy
landscape of the human brain. Nature https://doi.org/10.1038/d41586-025-00872-z.
3Human MitoBrain Map (2025) Michel Thiebaut de Schotten. Retrieved from: http://humanmitobrainmap.bcblab.com/. April 1, 2025.
For more links and videos, visit http://humanmitobrainmap.bcblab.com/
Prediction of whole-brain mitochondrial density visualized along a red (high) yellow (low) gradient projected onto the lateral view of the brain. The map is overlaid on a tractographic representation of the brain, illustrating the connectional organization of white matter pathways.
Prediction of whole-brain mitochondrial density visualized along a standard heat gradient projected onto an axial slice. Warmer colors (e.g., red/yellow) indicate higher predicted mitochondrial density, whereas cooler colors (e.g., green/blue) denote lower density. The map is overlaid on a tractographic representation of the brain, illustrating the connectional organization of white matter pathways.
Since the days of van Leeuwenhoek and his microscope, new technologies have allowed researchers to look at life, quite literally, in new ways. As technologies have grown in power, they also increased in cost and complexity, often putting them beyond the means of individual researchers. Many research organizations solved this challenge by developing core facilities in which an expensive instrument is purchased for multiple researchers. Knowledgeable individuals are hired to operate it.
While many research organizations have core facilities, they typically focus on one instrument type (e.g., imaging, bioinformatics, HPLC). At UCLA, Orian Shirihai (professor of medicine and molecular and medical pharmacology) took this concept to the next level by combining advanced technologies of different modalities into a single integrative core. His Mitochondria and Metabolism Core (Core) brings together capabilities in bioenergetics, imaging, and biochemistry to facilitate the study of mitochondria metabolism. The Core may be unique in this comprehensive approach to research on mitochondria.
“Our mission is to empower scientists across academia, biotechnology, and pharmaceutical industries to design, execute, and interpret experiments related to mitochondria and metabolism. By doing so, we aim to accelerate scientific discovery and the development of novel therapies and diagnostics,” said Dr. Shirihai.
The Core comprises three sections. The bioenergetics section, led by Linsey Stiles, features several Seahorse instruments that measure oxygen consumption and extracellular acidification providing readouts of mitochondrial respiratory function, glycolysis, ATP production rates, and other metabolic measurements. The imaging section, led by Cristiane Beninca, has a broad selection of microscopes (brightfield, widefield, confocal, FLIM, and super-resolution microscopes as well as a transmission electron microscope) and techniques to study mitochondrial structure and function (e.g., membrane potential, reactive oxygen species, and NADH measurements, mitophagy flux, mitochondrial and cristae dynamics, organelle interactions) in live and fixed samples. A biochemistry section, led by Lucia Fernandez-del-Rio complements the other sections with enzyme assays, western blots, non-denaturing gels to examine mitochondria supercomplexes, and mtDNA-based assays.
The Core provides an exceptional resource for basic, clinical, and industry researchers who are interested in mitochondria research. It helps in all phases of mitochondrial research from study design to data interpretation. The Core operates on a fee-for-service recharge system, offering a range of options from full-service support to specialized training for autonomous use of advanced equipment, and everything in between.
In recent years, physicians and researchers have come to appreciate the intimate involvement of mitochondrial dysfunction in human disease. The Core provides a unique resource to accelerate research in this critical area. Its value is documented by its broad spectrum of users, including researchers at UCLA and many other organizations, such as CalTech, UC Santa Barbara, and USC.
David Shackelford, PhD (professor of medicine, UCLA), said, “The mitochondrial metabolism and the imaging cores at UCLA have been an incredible resource and been transformative with respect to our research.” Amy Wang, PhD (CEO, Enspire Bio Inc.), added, “The UCLA mitochondria cores are indispensable collaborators and have transformed the direction and focus of our research and were important for informing our scientific strategy.” Clearly, the Core is having an impact on research into metabolism and mitochondria.
For more information about the Core, please visit their website: https://medschool.ucla.edu/research/themed-areas/metabolism-research/metabolism-core.
The Core directors respond to our questions
The UCLA Core seems to be a unique resource for researchers. Are you aware of anyone else who has combined so many powerful technologies to the study of mitochondria?
Providing services as we do, no. We are the only place that can offer services and access to cutting-edge equipment to internal and external scientists in need of the techniques provided. There are certainly places with multiple researchers working in mitochondria research and sharing expertise and access to equipment, but the only way for other researchers to have access to their techniques is through collaborations. In the Core, we offer a fee-by-service system, so anybody can “hire” us to work for them.
Do you plan to add any new resources to your collection (e.g., mass spec, HPLC, new imaging methods)?
At UCLA, additional Core facilities (e.g., Lipidomics, Metabolomics, or Proteomics) are available to clients interested in a multi-omic approach, so we are not considering an expansion in that direction. In the Biochemistry Core, we are working to incorporate mitochondrial DNA (mtDNA)-based assays, such as mtDNA copy number, mutation frequency, or mtDNA nanopore sequencing, into our services. But our portfolio is always expanding as new techniques are constantly in development, and in certain cases, we even work with the clients to develop or improve the techniques they need. Particularly, in the Imaging Core, new dyes, imaging, and analysis techniques are always in development. And in the Bioenergetics Core, new ways to measure mitochondrial function or optimization of different samples are always happening. Once techniques are optimized, they can be added to our portfolio as a service to clients.
Can you describe the general idea of a couple of the studies that the Core has participated in where each of the three sections were involved?
One example is the research paper https://doi.org/10.15252/embj.2022111699. In this project, we started with a Sponsored Research Agreement with a Company that provided us with a compound to be tested. The story progressed into a scientific discovery that was published and even a new technique was developed 10.26508/lsa.202201628
The three sections work very closely together, and we also have researchers with Grants in need of the expertise of all three Cores, and we can work together with them almost as if we were part of their laboratory personnel too.
What would you estimate as the breakdown of your time spent in training, planning experiments and evaluating data, and actual experimenting?
Our top priority is to address the needs of our clients in the best way possible. So, we work together to find the best approach to do so. There are times when we have clients with more knowledge of the techniques needed, and they can be more independent on planning and even running the experiments, so training is just needed in the beginning. Other times, samples are shipped to us, and we are responsible for the whole process until the delivery of results.
How did each of your become interested in the study of mitochondria?
Cristiane Beninca: For me, it was during my PhD that my mentor discovered a new protein localizing at the mitochondria, and since that, everything became mitochondria related.
Lucia Fernandez del Rio: Back in my undergrad days, I joined the Cell Biology Department and started working in a lab that focused on oxidative stress. Since this phenomenon is deeply interconnected to mitochondria, that is where my mitochondria journey started.
Linsey Stiles: I did a rotation focused on the role of mitochondrial fusion and fission in erythropoiesis and knew immediately that I wanted to continue to study mitochondria for my PhD project.
In a paper published in Cell, a research group led by Agnel Sfeir at Memorial Sloan Kettering Cancer Center showed that gene editing can be used to engineer specific mtDNA mutations.
Mitochondria are unusual cell organelles in that they contain their own mitochondrial (mt)DNA, and like other DNA, mtDNA can develop mutations that result in diseases. Some mutations involve the deletion of parts of the coding sequences. Fortunately, cells typically have more than 1,000 mitochondria, and in many cases, a minimum number of healthy mitochondria are sufficient to avoid disease. Nevertheless, mitochondrial diseases can be highly debilitating.
Advances in genome engineering and editing (e.g., TALENS, CRISPR, zinc finger nucleases) have raised the possibility of reconstituting point mutations repairing defective mtDNA to curing the diseases. However, mtDNA deletions in human cells have been challenging. Recently, a research team at Memorial Sloan Kettering Cancer Center, led by Agnel Sfeir, succeeded in engineering mtDNA deletions with an innovative approach by expressing bacterial DNA end joining enzymes in human cells.
The Sfeir team developed a group of human cell lines with deletions to the mtDNA of varying extents (Fu et al., 2025). Cells with deletions in more than 75% of the mitochondria had affected mitochondrial function. With these cells, the team found two patterns of effects on nuclear genes. One was activated at the deletion threshold. The other was in response to increasing levels of mutations.
More importantly, this combination of tools facilitates modeling of disease-associated mtDNA deletions in different cell types. In addition, the panel of cell lines with the different levels of deletions will greatly benefit the research community and help in developing therapeutics.
A Discussion with Agnel Sfeir
What are the next steps in your research?
Our research focuses on elucidating the molecular pathways that govern the propagation versus elimination of mtDNA deletions. Understanding these mechanisms is crucial for identifying factors influencing mtDNA integrity and stability.
Additionally, we aim to investigate how cells, particularly neurons and muscle cells, respond to mtDNA deletions. These cell types are highly dependent on mitochondrial function and are disproportionately affected by mtDNA mutations, leading to severe pathologies. By studying their adaptive or maladaptive responses, we hope to uncover potential therapeutic targets that could mitigate the impact of mtDNA deletions in mitochondrial diseases.
You note that you can use this system to examine stem cells. It sounds like that is a great way to look at looks of cell types, especially muscle and brain. Maybe even iPS cells could be used.
Yes, that’s exactly what we are in the process of doing. IPS cells provide a powerful strategy for studying a wide range of cell types, including muscle and neuronal cells, which are particularly affected by mitochondrial dysfunction. By differentiating iPS cells into specific lineages, we can systematically investigate how mtDNA deletions impact various cell types and really model disease-relevant phenotypes. This approach also allows for high-throughput screening of potential therapeutic interventions in a controlled and physiologically relevant system.
The ability to switch the carbon source between galactose and glucose is a real advantage to your system.
Absolutely. The ability to switch between glucose and galactose as carbon sources is a key advantage of our system. Heteroplasmy refers to the proportion of mutated mtDNA within a cell, and when it surpasses 75%, cells typically exhibit severe respiratory defects and abnormal morphology. However, the RPE1 cells we used primarily rely on glycolysis, allowing them to proliferate normally in glucose-containing media—even with heteroplasmy levels exceeding 75%. The defect became evident only when glucose was replaced with galactose, forcing the cells to rely on oxidative phosphorylation. This metabolic shift was crucial in unmasking the mtDNA deletion-associated defects in galactose-containing media, whereas culturing cells in glucose allowed us to obtain cells with ~100% mtDNA deletion. In fact, to date, no cellular models of mtDNA deletions have been able to achieve near-homoplasmy.
You discuss the application of your method for the study of disease, but I assume it might also be used to explore mitochondrial regulation as well.
While our method is highly applicable to studying disease, it also provides a powerful tool for exploring mitochondrial regulation. By analyzing the dynamic behavior of intact and deleted mtDNA, we can gain valuable insights into key mitochondrial quality control mechanisms, such as mitophagy and selective replication. Specifically, our cellular system allows us to examine how cells manage mutant mtDNA through preferential degradation of dysfunctional mitochondria via mitophagy, or the selective replication of intact mitochondrial genomes. This, in turn, enables us to understand the mechanisms that maintain mitochondrial homeostasis under normal conditions and how they are altered in the presence of mtDNA deletions.
Moreover, our approach provides a window into the biochemical and metabolic adaptations that occur in response to mitochondrial stress. By assessing how cells adjust their energy metabolism, stress signaling pathways, and mitochondrial dynamics, we can uncover the requirements necessary for cellular survival and adaptation. This is particularly crucial for understanding the threshold at which mitochondrial dysfunction becomes pathological and identifying potential targets for therapeutic intervention.
Your work and that of others attempts to correct mutations in mtDNA. Are there any genetic diseases of mitochondria that involve genes in the host genome that are specific to mitochondria?
Nuclear-encoded mitochondrial genes play a crucial role in mitochondrial function, and their mutations can lead to severe disorders. Several mitochondrial diseases arise from mutations in nuclear-encoded genes that regulate mitochondrial DNA replication and transcription. For example, POLG mutations impair mtDNA replication, leading to disorders such as Alpers syndrome, and defects in TFAM reduce mtDNA copy number and transcription.
How did you get interested in mitochondria?
I am a chromosome biologist with a deep fascination for how cells repair DNA breaks to maintain genome integrity. During my training and early career, I focused on nuclear DNA repair pathways and how their deregulation contributes to cancer progression. However, after starting my lab, I felt the need for a new intellectual challenge—an opportunity to step outside my comfort zone and explore an entirely different aspect of biology (something I believe all scientists should do at least once in their careers).
mtDNA had always intrigued me, and I saw a unique opportunity to apply my expertise in nuclear DNA repair to studying this independent genome within a distinct organelle. To immerse myself in the field, I dedicated two months solely to reading the mtDNA literature, identifying key questions, outlining a research plan, and recruiting a student and a post-doc to pursue these challenges. Despite its unique regulatory mechanisms, mtDNA still adheres to fundamental principles of genome maintenance, making it an exciting system to study. Today, half of my lab is dedicated to studying mtDNA biology, and with every discovery we make our curiosity deepens. The same curiosity that led me to work on mtDNA in the first place continues to drive our research forward.
Reference
Fu Y, Land M, Cui R, Kavlashvili T, Kim M, Lieber T, Ryu KW, DeBitetto E, Masilionis I, Saha R, Takizawa M, Baker D, Tigano M, Lareau CA, Reznik E, Sharma R, Chaligne R, Thompson CB, Pe’er D, Sfeir A (2025) Engineering mtDNA deletions by reconstituting end-joining in human mitochondria. Cell
https://www.sciencedirect.com/science/article/pii/S0092867425001941?dgcid=author
Fu et al. (2025) used nucleases and an end-joining procedure to produce mutations in mitochondrial (mt) DNA. The mitochondria were then introduced into cells, and cell lines were produced with different numbers of mutated mitochondria. The resulting cell lines provide excellent models for testing the effects of the mutated mtDNA in diseases.