Periodic Reporting for period 4 - MITOvTOXO (Understanding how mitochondria compete with Toxoplasma for nutrients to defend the host cell)
Reporting period: 2024-12-01 to 2025-05-31
As the powerhouses of our cells, mitochondria play an essential role in cellular health. During infection however, mitochondria have historically been viewed as passive victims hijacked by pathogens, despite that these organelles also use and consume many nutrients that invading pathogens rely on. Although mitochondria number in the hundreds to up to the thousands in mammalian cells—in contrast to few invading pathogens—it remains unclear whether host cells leverage mitochondria to restrict pathogen access to essential metabolites.
With the support of the ERC Starting Grant, my lab tackled this question and made 3 key discoveries. The first, that host cells rewire mitochondrial one-carbon metabolism to sequester the essential vitamin B folate from an invading pathogen, thereby restricting its growth. The second, that to counterattack nutrient competition, pathogens induce mitochondria to shed their membrane, revealing a host-pathogen arms race rooted in metabolic conflict. These findings reveal noncanonical functions and stress responses of mitochondria and shed light on how we can harness metabolism to develop anti-microbial therapies. For example, by boosting mitochondrial metabolism to increase uptake of fatty acids, we may be able to limit pathogen proliferation and resulting disease. Importantly, they represent a paradigm-shift in how we view mitochondria: not just as cellular powerhouses but as cellular infantry. This perspective raises exciting questions about how mitochondria and other organelles execute defensive functions and positions my lab to make transformative discoveries at the interaction of cell biology, metabolism, and infectious disease.
Our third major discovery is derived from an approach we developed to study mitochondria-Toxoplasma interactions. In brief, we generated a sensor to monitor these interactions in live cell imaging. We realized that this approach could by applied to studying a question that has been an enigma for more than 70 years: how does Toxoplasma interact with another major organelle in the cell, the endoplasmic reticulum. We reasoned that, analogous to our mitochondrial work, studying this relationship might reveal novel cellular defenses and pathways to target for anti-microbials. We succeeded in identifying the molecules that enable Toxoplasma and host endoplasmic reticulum to interact.
With the support of the ERC Starting Grant, my lab tackled this question and made 3 key discoveries. The first, that host cells rewire mitochondrial one-carbon metabolism to sequester the essential vitamin B folate from an invading pathogen, thereby restricting its growth. The second, that to counterattack nutrient competition, pathogens induce mitochondria to shed their membrane, revealing a host-pathogen arms race rooted in metabolic conflict. These findings reveal noncanonical functions and stress responses of mitochondria and shed light on how we can harness metabolism to develop anti-microbial therapies. For example, by boosting mitochondrial metabolism to increase uptake of fatty acids, we may be able to limit pathogen proliferation and resulting disease. Importantly, they represent a paradigm-shift in how we view mitochondria: not just as cellular powerhouses but as cellular infantry. This perspective raises exciting questions about how mitochondria and other organelles execute defensive functions and positions my lab to make transformative discoveries at the interaction of cell biology, metabolism, and infectious disease.
Our third major discovery is derived from an approach we developed to study mitochondria-Toxoplasma interactions. In brief, we generated a sensor to monitor these interactions in live cell imaging. We realized that this approach could by applied to studying a question that has been an enigma for more than 70 years: how does Toxoplasma interact with another major organelle in the cell, the endoplasmic reticulum. We reasoned that, analogous to our mitochondrial work, studying this relationship might reveal novel cellular defenses and pathways to target for anti-microbials. We succeeded in identifying the molecules that enable Toxoplasma and host endoplasmic reticulum to interact.
The aims of MITOvTOXO are to: 1) define the mechanism by which mitochondria enhance fatty acid oxidation to defend the cell against fatty acid siphoning by microbes; 2) determine whether mitochondrial fatty acid oxidation can be exploited to restrict microbial growth in vivo; and 3) determine whether mitochondria play a broader role in cellular defence by sequestering other essential metabolites from microbes. In addition, we seek to ask whether the mechanisms we identify also regulate cellular metabolic homeostasis independently of infection. In this project we have made significant progress in aim 1, 2, and 3. Regarding aim 1, we have: a) defined a potential mechanism by which mitochondria enhance mitochondrial FAO—through the tethering of lipid droplets by MFN1 and MFN2 (unpublished data); b) shown that MFN1 and MFN2 restrict parasite use of host fatty acids for phospholipids important for membrane biogenesis (unpublished data); and c) discovered that mitochondria shed their outer membrane in response to parasite-induced and infection-independent import stress (Li et al, Science, 2022). In addition, by applying the tools we developed to the study of another key organelle in lipid metabolism, we identified the molecules that enable Toxoplasma and host endoplasmic reticulum to interact (Mehra et al., in press, Nature Metabolism). Regarding progress in aim 2: given that fatty acid oxidation is altered during a cellular aging program termed senescence, and more than 50% of the population over 60 in Germany is estimated to be infected with Toxoplasma, we addressed the following question: what is the relationship between cellular aging and Toxoplasma infection? In preliminary work that I am continuing in my lab, we have found that Toxoplasma induces cellular senescence in infected cells. We are currently testing the hypothesis that Toxoplasma and cellular senescence cooperate to prolong the life of the cellular niche of Toxoplasma. Regarding aim 3, to directly address our aim, we turned to what we hypothesized to be the battle between Toxoplasma and host mitochondria — the only organelle in mammalian cells to have their own DNA — for nutrients required for DNA synthesis. We found that, following the detection of Toxoplasma, host cells increased mitochondrial DNA levels and mitochondrial metabolism. This restricted the growth of Toxoplasma by promoting mitochondrial use of the essential B vitamin folate, thereby limiting Toxoplasma's access to the nutrients it requires for its own DNA synthesis (Medeiros, Science, 2025).
Below, I discuss our progress beyond the state of the art and our results for this project.
A novel mitochondrial stress response.
The outer mitochondrial membrane (OMM), the gateway between mitochondria and the rest of the cell. Thus, preserving the integrity of the OMM is essential for cellular homeostasis. Little was known of the mechanisms by which mammalian cells respond to naturally occurring stresses of the OMM. We used infection as a model to study cellular responses to mitochondrial membrane stress, and discovered that mitochondria shed their OMM into large micron-sized structures we termed SPOTs (structures positive for outer mitochondrial membrane). We defined the proteins required for SPOT formation, and showed that they form during infection-independent stress.
A mitochondrial defense against parasitic infection.
To generate energy, mitochondria consume nutrients that invading microbes depend on. This competing interest predicts an inverse relationship between mitochondrial health and microbial fitness. Although several pathogens disrupt host mitochondrial function, it was not known if mitochondria act to impede pathogen replication. We have shown that host mitochondria counteract microbes by acting as nutrient competitors for folate. In the process, we discovered a novel regulator of mitochondrial DNA levels in infection-dependent and independent stress. Our finding paves the way for future studies exploring noncanonical defense strategies mediated by mitochondria, and how we can harness metabolism to develop anti-microbial therapies.
Identification of factors underlying host organelle-pathogen interactions.
Host endoplasmic reticulum (ER) and mitochondria surround and interact with the vacuoles in which certain intracellular bacteria and parasites reside. Since the discovery of this phenomenon in Toxoplasma- infected cells in 1950s, the mediating factors and biological consequences of the phenomenon have been a mystery. From the time that I was a PhD student, a major focus of my research has been focused on determining the mechanisms by which host ER and mitochondria form contact sites with the parasite vacuole. We have now identified these factors, and in future work will address the biological significance of host organelle-parasite contact sites during infection. These factors remain the only known single molecule mediators of trans-kingdom contacts sites and are thus an invaluable tool to probe their biological significance.
A novel mitochondrial stress response.
The outer mitochondrial membrane (OMM), the gateway between mitochondria and the rest of the cell. Thus, preserving the integrity of the OMM is essential for cellular homeostasis. Little was known of the mechanisms by which mammalian cells respond to naturally occurring stresses of the OMM. We used infection as a model to study cellular responses to mitochondrial membrane stress, and discovered that mitochondria shed their OMM into large micron-sized structures we termed SPOTs (structures positive for outer mitochondrial membrane). We defined the proteins required for SPOT formation, and showed that they form during infection-independent stress.
A mitochondrial defense against parasitic infection.
To generate energy, mitochondria consume nutrients that invading microbes depend on. This competing interest predicts an inverse relationship between mitochondrial health and microbial fitness. Although several pathogens disrupt host mitochondrial function, it was not known if mitochondria act to impede pathogen replication. We have shown that host mitochondria counteract microbes by acting as nutrient competitors for folate. In the process, we discovered a novel regulator of mitochondrial DNA levels in infection-dependent and independent stress. Our finding paves the way for future studies exploring noncanonical defense strategies mediated by mitochondria, and how we can harness metabolism to develop anti-microbial therapies.
Identification of factors underlying host organelle-pathogen interactions.
Host endoplasmic reticulum (ER) and mitochondria surround and interact with the vacuoles in which certain intracellular bacteria and parasites reside. Since the discovery of this phenomenon in Toxoplasma- infected cells in 1950s, the mediating factors and biological consequences of the phenomenon have been a mystery. From the time that I was a PhD student, a major focus of my research has been focused on determining the mechanisms by which host ER and mitochondria form contact sites with the parasite vacuole. We have now identified these factors, and in future work will address the biological significance of host organelle-parasite contact sites during infection. These factors remain the only known single molecule mediators of trans-kingdom contacts sites and are thus an invaluable tool to probe their biological significance.