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 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. was shown to respond selectively to K, not to most ions present , 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 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 streptavidin (StAv). Fluorescence imaging of extracellular K upon addition of apoptosis inducers was successfully achieved by localized to the cell surface.