A wash-free ER marker to monitor micropolarity during ER stress and visualize ER- Golgi transport

Cellular stress plays a key role to regulate and maintain organismal as well as microenvironmental homeostasis. The stress-induced response is also reflected in the micropolarity of any specific cellular compartment, which is essential to be quantified for early disease diagnosis. Alongside, coming up with a biocompatible small-molecule fluorophore NBD-Oct for exclusive endoplasmic reticulum (ER) localization simply driven by its hydrophobicity, in this contribution, we present a quantitative study of micropolarity alteration inside the ER during G1/S and G2/M phases. The cell cycle arrests caused by the induced ER stress led to the enhancement of the ER micropolarity in cells. NBD-Oct is selected among a series of analogous probes based on its fastest diffusion properties demonstrated by fluorescence recovery after the photobleaching experiment. The probe is a versatile staining agent as it could efficiently stain the ER in live/fixed mammalian cells, isolated ER, Caenorhabditis elegans, and mice tissues. Finally, a well-known biological event, ER to Golgi transport is also visualized by live-cell fluorescence microscopy using this probe. We believe this exhaustive investigation of micropolarity using NBD-based dye provides a new avenue to study ER stress that may unravel a deeper understanding of proteostasis in model systems and potentially even fixed patient samples.