Artificial endoplasmic reticulum-lipid droplet tethers facilitate lipid incorporation into lipid droplets

Abstract

Lipid droplets (LDs) are ubiquitous organelles that store neutral lipids to meet cellular energetic and signaling needs. As a unique monolayer structure, LDs arise from the endoplasmic reticulum (ER) and acquire proteins and lipids through their membrane contact sites (MCSs) with the ER. In this study, we exogenously induce ER-LD MCSs using a dimerization-dependent fluorescent protein (ddFP) system. Strikingly, inducing these MCSs increases LD size without influencing LD total amount per cell, in a manner that is distinct from LD biogenesis induced by the dietary fatty acid oleic acid. By examining the trafficking of the triacylglycerol synthesis enzyme DGAT2 under ddFP induction, we found that artificial tethering recruits LD proteins to the ER-LD interface but not to the LD surface, unlike oleic acid supplementation. However, by supplementing ddFP-transfected cells with fluorescent fatty acids, we found that ddFP-positive LDs preferentially incorporate exogenous lipid, suggesting that inducing MCSs can facilitate ER-to-LD lipid transfer. These results demonstrate ddFPs as a tool for manipulating LD MCSs and elucidate the role of ER-LD MCSs following LD biogenesis to continue to promote LD growth.