Detecting and quantitating physiological endoplasmic reticulum stress.

Unfolded protein response (UPR) is a key cellular defense mechanism associated with many human "conformational" diseases, including heart diseases, neurodegeneration, and metabolic syndrome. One of the major obstacles that have hindered our further understanding of physiological UPR and its future therapeutic potential is our inability to detect and quantitate ER stress and UPR activation under physiological and pathological conditions, where ER stress is perceivably very mild. Here, we describe a Phos-tag-based Western blot approach that allows for direct visualization and quantitative assessment of mild ER stress and UPR signaling, directly at the levels of UPR sensors, in various in vivo conditions. This method will likely pave the foundation for future studies on physiological UPR, aid in the diagnosis of ER-associated diseases, and facilitate therapeutic strategies targeting UPR in vivo.