Sensitive imaging of Endoplasmic reticulum (ER) autophagy with an acidity-reporting ER-Tracker

Macroautophagic/autophagic turnover of endoplasmic reticulum (reticulophagy) is critical for cell health. Herein we reported a sensitive fluorescence-on imaging of reticulophagy using a small molecule probe (ER-proRed) comprised of green-emissive fluorinated rhodol for ER targeting and nonfluorescent rhodamine-lactam prone to lysosome-triggered red fluorescence. Partitioned in ER to exhibit green fluorescence, ER-proRed gives intense red fluorescence upon co-delivery with ER into acidic lysosomes. Serving as the signal of reticulophagy, the turning on of red fluorescence enables discernment of reticulophagy induced by starvation, varied levels of reticulophagic receptors, and chemical agents such as etoposide and sodium butyrate. These results show ER probes optically activatable in lysosomes, such as ER-proRed, offer a sensitive and simplified tool for studying reticulophagy in biology and diseases.