Membrane Curvature Catalyzes Lipid Droplet Assembly

Area of Science:

• Cell Biology
• Membrane Biology
• Lipid Metabolism

Background:

• Lipid droplet (LD) biogenesis initiates at the endoplasmic reticulum (ER), but the role of ER membrane topology is not well understood.
• LD formation involves the nucleation of neutral lipids like triacylglycerols (TGs) and sterol esters (SEs).

Purpose of the Study:

• To investigate how the endoplasmic reticulum (ER) membrane topology influences lipid droplet (LD) nucleation.
• To determine the impact of membrane curvature and lipid composition on LD assembly.

Main Methods:

• Utilized an in vitro approach to study LD nucleation.
• Analyzed the distribution of TGs and SEs in different ER membrane regions (tubules vs. sheets).
• Investigated the role of the protein seipin in LD assembly.

Main Results:

• Lipid droplets (LDs) preferentially assemble at highly curved ER tubules rather than flat ER sheets.
• Lower triacylglycerol (TG) concentrations are required for LD nucleation at curved membranes.
• Increased membrane curvature alone can induce LD nucleation in vitro.
• Sterol esters (SEs) are favored in tubules and inhibit TG LD nucleation.
• Seipin localizes to ER tubules and regulates TG condensation, preventing aberrant nucleation.

Conclusions:

• Endoplasmic reticulum (ER) membrane curvature acts as a key catalyst for lipid droplet (LD) assembly.
• Seipin plays a crucial role in modulating curvature-driven LD nucleation, ensuring controlled lipid droplet formation.