In vivo visualization of bioactive iron and oxygen using the Labile Iron and Oxygen Notifier (LiON)

Iron and oxygen are essential for many biological processes; however, real-time in vivo monitoring of bioactive intracellular iron and oxygen remains challenging because of the limitations of existing detection methods. Here, we introduce Labile Iron and Oxygen Notifier (LiON), a genetically encoded, ratiometric fluorescent reporter enabling single-cell resolution monitoring of intracellular iron and oxygen levels in vivo. In cultured cells, LiONs respond dynamically and specifically to physiologically relevant changes in iron and oxygen levels. Using LiON knock-in mice, we revealed heterogeneous iron distribution across tissues, cell types, and among cells of the same type. In the liver, zone 1 hepatocytes show elevated iron levels and increased susceptibility to oxidative stress. These fluorescent signals were sensitive enough to detect changes resulting from dietary iron intake and genetic disruption of iron-regulatory genes. Our findings establish LiON as a powerful tool for visualizing iron and oxygen dynamics, revealing their in vivo diversity at single-cell resolution. Thus, LiON offers a platform for studying iron and oxygen metabolism in living organisms, providing new insights into the cellular diversity of iron and oxygen utilization and their roles in physiology and disease.