A reporter for tracking the UPR in vivo reveals patterns of temporal and cellular stress during atherosclerotic progression

Progression of human arteriosclerosis is associated with and promoted by induction of the endoplasmic reticulum (ER) stress pathway known as the unfolded protein response (UPR). Most studies that assess UPR markers in atherosclerosis rely on methodologies that suffer from low signal sensitivity, nonspecific immunohistochemistry, or inability to resolve differences between cellular subsets. To accurately monitor the UPR independently of artifacts generated postmortem, we describe here the fi rst in vivo reporter for ER stress during atherosclerosis. Mice transgenic for the fl uorescent XBP-1 ER stress indicator Erai were bred onto the Ldlr ̃ / ̃ background and fed an atherogenic diet. Subsequently, ERAI fl uorescence at aortic roots was quantifi ed and colocalized with lesional cell type. We found that the ERAI fl uorescent signal increased as a function of time on the atherogenic diet and, in advanced lesions, was found close to necrotic cores. The majority of ERAI fl uorescence localized to macrophages, and to a lesser extent, to intimal smooth muscle cells and patches of endothelial cells. These mice provide a valuable tool to monitor activation of the UPR in atherosclerosis and will be useful for future studies investigating relationships between pharmacologic and genetic modulators of UPR and atherosclerosis.