Mitochondria, the dynamic organelles that power cells, are constantly changing shape in response to the cell’s changing needs. They can cluster to form long interconnected networks or split apart into scattered fragments. For decades, scientists believed that mitochondrial shape was simply a reflection of their activity. However, a new study by researchers at the Institute for Stem Cell Science and Regenerative Medicine (inStem), Bengaluru, shows that mitochondrial form is governed by the cell’s redox environment. The work reveals a rapid biochemical switch that fragments or reconnects mitochondrial networks independent of how much energy they produce.

Mitochondria undergo remodelling by fusion, where organelles join together to form a network, or fission, where they split apart into fragments. These processes, which are largely conserved across eukaryotic cells, allow mitochondria to adapt to changes in nutrients, environmental conditions, and cellular stress. However, the mechanisms that drive mitochondrial remodelling have remained unclear.

To investigate how cells control mitochondrial architecture, the researchers examined budding yeast grown on different carbon sources that alter cellular metabolism. Yeast cells typically repress mitochondrial respiration when growing on glucose, but switch it up when metabolising alternative carbon sources like lactate, ethanol, and glycerol. Leveraging this metabolic flexibility, the researchers compared the activity and organisation of mitochondrial networks under specific nutritional conditions.

They observed that cells grown in glucose showed lower levels of mitochondrial activity and oxygen consumption compared to cells grown in ethanol, lactate, or glycerol. Notably, the different carbon sources induced distinct mitochondrial architectures. Cells grown on glucose had fewer mitochondria but with well-connected tubules. In contrast, cells grown on glycerol and lactate showed completely fragmented mitochondria with short and round tubules, whereas those grown on ethanol had well-networked mitochondria with long and interconnected tubules. Cells grown on glycerol-ethanol exhibited an intermediate morphology, with long connected tubules interspersed with shorter tubules. Together, these findings suggest that mitochondrial organisation does not necessarily correlate with mitochondrial activity under different nutrient conditions.

Since mitochondrial activity did not explain the differences in network structure, the researchers turned to the cells’ metabolic environment. Different nutrients alter the flow of electrons through metabolic pathways, which can shift the balance between oxidised and reduced states and alter the levels of reactive oxygen species (ROS). Using a fluorescent dye that lights up when oxidised, the researchers measured shifts in intracellular redox states. While cells with fragmented mitochondria consistently showed higher levels of ROS, and a more oxidized intracellular environment, those with well-networked mitochondria showed lower levels of ROS and a more reduced environment. These findings suggest that mitochondrial architecture might be linked to shifts in cellular redox states, independent of activity.

Strikingly, in presence of an antioxidant, cells with fragmented mitochondria transitioned to a fully networked morphology within 30 minutes, accompanied by a shift towards a more reduced intracellular environment. Conversely, the mitochondrial networks rapidly fragmented in the presence of an oxidant. These rapid changes did not alter mitochondrial activity, suggesting that mitochondrial network organisation can respond independently and dynamically to redox states.

Next, the researchers examined how different metabolic pathways shape the cell’s redox state. They found that the path by which electrons entered the mitochondrial electron transport chain (ETC) determined ROS generation. In presence of glucose, electrons flowed through the tricarboxylic acid (Krebs) cycle, maintaining tubular mitochondrial networks, whereas in lactate or glycerol, electrons were fed directly into the ETC, producing higher ROS and fragmented mitochondria. Consistent with these findings, when electron transport was inhibited, ROS levels increased, causing mitochondrial fragmentation.

Finally, the team identified Dnm1, a conserved protein known to mediate mitochondrial fission in yeast, as a key player that connects redox signals to mitochondrial shape. Under oxidising conditions with high ROS levels, Dnm1 assembled on mitochondrial membranes, driving fragmentation. Conversely, reducing the redox state, either by switching carbon sources or adding antioxidants, suppressed Dnm1 activity, allowing mitochondria to reconnect into tubular networks. These experiments revealed that Dnm1 acts as a molecular bridge between cellular metabolism and mitochondrial architecture.

Overall, the research highlights a fundamental mechanism by which cells integrate metabolic signals with the structural organisation of organelles, allowing mitochondria to maintain flexibility under stress and optimize cellular respiration. By linking ROS and redox states to Dnm1-mediated fission, the researchers show how cells can rapidly fragment or reconnect mitochondria in response to changes in nutrients and metabolism. Beyond offering mechanistic insights, these findings may also help explain mitochondrial dysfunction, a hallmark feature of various diseases, including neurodegeneration, metabolic disorders, and cancer.