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.