Where in the tissues of Danio rerio is more H2O2 produced during acute hypoxia?

The lack of oxygen (O2) causes changes in the cell functioning. Modeling hypoxic conditions in vitro is challenging given that different cell types exhibit different sensitivities to tissue O2 levels. We present an effective in vivo platform for assessing various tissue and organ parameters in Danio rerio larvae under acute hypoxic conditions. Our system allows simultaneous positioning of multiple individuals within a chamber where O2 level in the water can be precisely and promptly regulated, all while conducting microscopy. We applied this approach in combination with a genetically encoded pH-biosensor SypHer3s and a highly H2O2-sensitive Hyper7 biosensor. Hypoxia causes H2O2 production in areas of brain, heart and skeletal muscles, exclusively in the mitochondrial matrix; it is noteworthy that H2O2 does not penetrate into the cytosol and is neutralized in the matrix upon reoxygenation. Hypoxia causes pronounced tissue acidosis, expressed by a decrease in pH by 0.4-0.6 units everywhere. Using imaging photoplethysmography, we measured in D.rerio fry real-time heart rate decrease under conditions of hypoxia and subsequent reoxygenation. Our observations in this experimental system lead to the hypothesis that mitochondria are the only source of H2O2 in cells of D.rerio under hypoxia.