A new pH-sensitive fluorescent probe for visualization of endoplasmic reticulum acidification during stress

A novel pH-sensitive fluorescent probe termed ER-H for visualization of ER acidification during ER stress in live cells and in vivo was presented. ER-H consists of a piperazine-linked 1,8-naphthalimide as a fluorescence turn-on signaling unit, a 4-methyl benzenesulphonamide moiety for ER anchoring, and a thiol reactive benzyl chloride subunit for biomacromolecules fixation at the ER. ER-H displays sensitive fluorescence enhancement under acidic condition, thus enabling it to monitor ER acidification. Meanwhile, ER-H shows negligible response to common species, high photostability and low cytotoxicity. In biological experiments, the probe ER-H can solely accumulate into ER and track the acidification during ER stress. By combining ER-H with calcium ion sensor Fluo-3 AM, we achieved the simultaneous fluorescence imaging of decrease of ER pH and increase of cytoplasmic calcium ion. Additionally, the probe was also utilized for fluorescence imaging of the acidification in zebrafish and mice model successfully. We thus believe that ER-H has great potential as a practical tool for real-time monitoring of the acidification of ER, contributing to revealing the pathogenesis of ER stress associated diseases.