Ratiometric fluorescence imaging of cell surface pH by poly(ethylene glycol)-phospholipid conjugated with fluorescein isothiocyanate

Various physiological and pathological processes are accompanied with the alteration of pH at extracellular juxtamembrane region. Accordingly, the methods to analyze the cell surface pH have been demanded in biological and medical sciences. In this study, we have established a novel methodology for cell surface pH imaging using poly(ethylene glycol)-phospholipid (PEG-lipid) as a core structure of ratiometric fluorescent probes. PEG-lipid is a synthetic amphiphilic polymer originally developed for the cell surface modification in transplantation therapy. Via its hydrophobic alkyl chains of the phospholipid moiety, PEG-lipid is, when applied extracellularly, spontaneously inserted into the plasma membrane and retained at the surface of the cells. We have demonstrated that the PEG-lipid conjugated with fluorescein isothiocyanate (FITC-PEG-lipid) can be used as a sensitive and reversible cell-surface-anchored pH probe between weakly alkaline and acidic pH with an excellent spatiotemporal resolution. The remarkably simple procedure for cell-surface labeling with FITC-PEG-lipid would also be advantageous when considering its application to high-throughput in vitro assay. This study further indicates that various probes useful for the investigation of juxtamembrane environments could also be developed by using PEG-lipid as the core structure for bio-membrane anchoring.