Live visualization of endoplasmic reticulum redox potential in zebrafish embryos reveals region-specific differences

Redox homeostasis is an integral part of many cellular processes, and its perturbation is associated with conditions such as diabetes, aging, and neurodegenerative disorders. Redox homeostasis or redox potential in organelles is maintained within a particular range to facilitate the organelle specific cellular redox reactions. Previous studies using yeast, cell systems, and nematodes have demonstrated that the Endoplasmic Reticulum (ER) has a more oxidizing environment while the cytosol exhibits a reducing redox potential. However, we know very little about how universal this phenomenon is. We created transgenic zebrafish (Danio rerio) lines with roGFP sensors targeted to the ER and cytosol for studying physiological redox potential at the systems level. In the process, we also characterized the ER-targeting signal sequence in D. rerio for the first time. Measurements of the redox state in live embryos found that the endoplasmic reticulum exhibits deviations from its expected oxidizing redox state in different regions of the developing embryos. The ER is far more reducing than expected in certain tissues of the embryo. Cytosol also exhibited unexpected redox states in some parts of the embryo. Notably, the brain showed regions with unexpected redox states in both the ER and the cytosol. Tissue-specific differences in ER-redox potential became even more evident in a transgenic line expressing the more sensitive roGFPiE variant. Thus, live monitoring of redox potential across the developing zebrafish embryos revealed unanticipated redox states of the ER that will require new biological definitions.