A small-molecule probe to decipher stress-induced ER microenvironments and ER-Golgi communication.

Cellular stress is a crucial factor in regulating and maintaining both organismal and microenvironmental homeostasis. It induces a response that also affects the micropolarity of specific cellular compartments, which is essential for early disease diagnosis. In this contribution, we present a quantitative study of micropolarity changes inside the endoplasmic reticulum (ER) during the G1/S and G2/M phases, using a biocompatible small-molecule fluorophore called ER-Oct. This probe is selectively driven to the ER by its hydrophobicity, and it has the fastest diffusion properties among a series of analogous probes. We found that induced ER stress caused cell cycle arrests leading to an increase in ER micropolarity which is well supported by lambda scanning experiments and fluorescence lifetime imaging microscopy (FLIM) as well. ER-Oct is a versatile staining agent that could effectively stain the ER in various living/fixed mammalian cells, isolated ER, Caenorhabditis elegans , and mice tissues. Furthermore, we used this probe to visualize a well-known biological event, ER to Golgi transport, by live-cell fluorescence microscopy. Our exhaustive investigation of micropolarity using ER-staining dye provides a new way to study ER stress, which could provide a deeper understanding of proteostasis in model systems and even in fixed patient samples.