Fluorescence Imaging of Extracellular Potassium Ion Using Potassium Sensing Oligonucleotide

Potassium-sensing oligonucleotide, PSO, a conjugate of a quadruplex structure-forming oligonucleotide with a peptide incorporating a Förster Resonance Energy Transfer (FRET) chromophore pair, has been developed for fluorescent detection of potassium ion (K⁺) in aqueous medium. PSO 1 could be introduced into cells for real-time imaging of cytoplasmic K⁺ concentrations. To perform fluorescent imaging of K⁺ on the cell surface, we synthesized twelve PSO derivatives with different types of peptide types and lengths, and oligonucleotide sequences including thrombin-binding aptamer (TBA) sequences with FAM and TAMRA as a FRET chromophore pair, and evaluated their performance. 1 was shown to respond selectively to K⁺, not to most ions present in vivo, and to show reciprocal fluorescence changes in response to K⁺ concentration. For the peptide chains and oligonucleotide sequences examined in this study, the PSO derivatives had K _d values for K⁺ in the range of 5-30 mM. All PSO derivatives showed high K⁺ selectivity even in the presence of excess Na⁺. The PSO derivatives were successfully localized to the cell surface by biotinylated concanavalin A (ConA) or sulfo-NHS-biotin via streptavidin (StAv). Fluorescence imaging of extracellular K⁺ upon addition of apoptosis inducers was successfully achieved by 1 localized to the cell surface.