A Water-soluble, Cell-permeable Mn(II) Sensor Enables Visualization of Manganese Dynamics in Live Mammalian Cells

Central roles of Mn2+ ions in immunity, brain function, and photosynthesis, necessitate probes for tracking this essential metal ion in living systems. However, developing a cell-permeable, fluorescent sensor for selective imaging of Mn2+ ions in the aqueous cellular milieu has remained a challenge. This is because Mn2+ is a weak binder to ligand scaffolds when compared to other physiological metal ions, in water. Further, Mn2+ ions quench fluorescent dyes leading to turn-off sensors that are not applicable for in vivo imaging. Hence, the few existing small molecule-based sensors are not exclusively Mn2+ ion selective and suffer from low aqueous solubility. The only reported protein and DNA-based probes are cell-impermeable and show either sub-optimal selectivity or low sensitivity. Here we report a novel, completely water-soluble, reversible, fluorescent turn-on, Mn2+ selective sensor with a Kd of 1.4 µM for Mn2+ ions. The probe enters cells within 15 min of direct incubation and was applied to image Mn2+ ions in living mammalian cells in both confocal fluorescence intensity and lifetime-based set-ups. In a key experiment, we show that the probe was able to visualize Mn2+ dynamics in live cells revealing differential Mn2+ localization and uptake dynamics in disease model cells replicating conditions of Mn-transporter defect induced Parkinsonism when compared to normal cells. The computationally-designed sensor with a 620 nM limit-of-detection for Mn2+ in water, distinctly overcomes selectivity, sensitivity, and cell-permeability issues associated with the few reported Mn2+ probes, thus opening floodgates to elucidate the hitherto intractable mammalian Mn2+ homeostasis.