Membrane lipids and the endoplasmic reticulum unfolded protein response: An interesting relationship.

The unfolded protein response of the endoplasmic reticulum (UPR(ER)) is a conserved signaling circuit that ensures ER protein homeostasis (proteostasis). In the UPR(ER) of higher eukaryotes, multiple sensors cooperatively perceive proteostatic disturbances in the ER lumen and induce downstream adaptive changes. Besides direct proteotoxic insults, altered lipid profiles can also lead to UPR(ER) activation, evidently because abnormal lipid composition impairs protein folding. However, 2 recent studies propose an alternative mechanism of UPR(ER) sensor activation. In one report, UPR(ER) activation occurred in cells expressing UPR(ER) sensors lacking the very domains that sense unfolded proteins; the other study found that Caenorhabditis elegans worms displayed UPR(ER) activation without apparent proteostatic imbalance in the ER lumen. Collectively, these studies suggest that lipid disequilibrium-activated UPR(ER) is not strictly accompanied by compromised ER proteostasis and hint at a lipid membrane-monitoring role of the UPR(ER). These discoveries raise several important questions: does the UPR(ER) monitor and maintain homeostasis of the ER membrane and/or its lipids? In turn, does the UPR(ER) initiate downstream regulatory events that specifically alleviate lipid or proteostatic imbalance? And what is the physiological significance of proteostasis-independent UPR(ER) activation? In this commentary, we will discuss these issues and highlight the utility of C. elegans as an in vivo model to study lipid disequilibrium-induced UPR(ER) and related pathways.