A Fluorescent RNA-Based Genetically Encoded Mg2+ Sensor Allows Robust Detection of Mg2+ in Live Cells

Magnesium plays crucial roles in many biological processes and the stabilization of biomolecules, including DNA, RNA, and proteins. Despite significant progress, however, our understanding of how cells regulate Mg2+ homeostasis and transport remains incomplete. One of the goals is to develop approaches for detecting Mg2+ dynamics with high spatial and temporal resolution. Herein, we describe the development of MagFR, a ratiometric Mg2+ sensor based on Pepper and Clivia fluorescent RNAs. In this sensor, Pepper fluorescence is highly Mg2+ dependent, and Clivia fluorescence acts as the normalizer. MagFR has favourable properties, including a large dynamic range, pH insensitivity, and high selectivity, allowing robust detection of Mg2+ in both live bacterial and mammalian cells. By targeting MagFR to distinct subcellular compartments in mammalian cells, we observed that the free cytoplasmic Mg2+ concentration is slightly higher than that in the nucleoplasm. We demonstrated that mammalian cells maintain Mg2+ homeostasis even under conditions of elevated extracellular Mg2+, revealing the robustness of intracellular regulatory mechanisms governing Mg2+ balance. MagFR also enabled real-time detection of cellular Mg2+ dynamics following ATP depletion. Overall, this study offers a robust and versatile tool for imaging Mg2+ dynamics in cells, which will be useful for elucidating the functionality and mechanism of Mg2+ homeostasis underlying diverse cellular processes.