Thioredoxin reductase controls the capacity of peroxiredoxins to limit mitochondrial H2O2 release

H2O2 performs central roles in signaling at physiological levels, while at elevated levels it causes molecular damage. Mitochondria are major producers of H2O2, which has been implied in regulating diverse processes inside and outside the organelle. However, it still remains unclear whether and how mitochondria in intact cells release H2O2. Here we employed the genetically encoded high-affinity H2O2 sensor HyPer7 in mammalian tissue culture cells to investigate different modes of mitochondrial H2O2 release. We found substantial heterogeneity of HyPer7 dynamics between individual cells, and observed H2O2 released from mitochondria directly at the surface of the organelle and in the bulk cytosol, but not in the nucleus nor on the plasma membrane, pointing to steep gradients emanating from mitochondria. These gradients are controlled by cytosolic peroxiredoxins that act redundantly and are present with a substantial reserve capacity. Furthermore, dynamic adaptation of cytosolic thioredoxin reductase levels during metabolic changes results in improved H2O2 handling and explains previously observed cell-to-cell differences. Thus, our data indicate that H2O2-mediated signaling likely occurs close to mitochondria during specific metabolic conditions. HIGHLIGHTSO_LIMitochondrial H2O2 can be detected in the cytosol in intact human cells C_LIO_LIMitochondrial H2O2 gradients are steep and controlled by peroxiredoxins 1 and 2 C_LIO_LIPeroxiredoxins 1 and 2 complement for each other C_LIO_LIPeroxiredoxins 1 and 2 are present with a substantial reserve capacity C_LIO_LIMetabolism-induced changes of reducing processes control peroxiredoxin activity C_LI