Abstract
Hydrogen peroxide (H(2) O(2) ) has key signaling roles at physiological levels, while causing molecular damage at elevated concentrations. H(2) O(2) production by mitochondria is implicated in regulating processes inside and outside these organelles. However, it remains unclear whether and how mitochondria in intact cells release H(2) O(2) . Here, we employed a genetically encoded high-affinity H(2) O(2) sensor, HyPer7, in mammalian tissue culture cells to investigate different modes of mitochondrial H(2) O(2) release. We found substantial heterogeneity of HyPer7 dynamics between individual cells. We further observed mitochondria-released H(2) O(2) directly at the surface of the organelle and in the bulk cytosol, but not in the nucleus or at the plasma membrane, pointing to steep gradients emanating from mitochondria. Gradient formation is controlled by cytosolic peroxiredoxins, which act redundantly and with a substantial reserve capacity. Dynamic adaptation of cytosolic thioredoxin reductase levels during metabolic changes results in improved H(2) O(2) handling and explains previously observed differences between cell types. Our data suggest that H(2) O(2) -mediated signaling is initiated only in close proximity to mitochondria and under specific metabolic conditions.