Modeling of transport of triacylglycerols during lipid droplet formation and growth

Lipid droplets are cellular membrane-less organelles, consisting of a hydrophobic lipid core enclosed by a monolayer of phospholipids, which act as surfactants with their hydrophobic tails oriented toward the core and their hydrophilic head groups toward the aqueous cytoplasm. Formation and growth of lipid droplets allows cells to store excess fatty acids, thus providing a buffer for fluctuations in demands for energy or membrane lipid synthesis. Despite tremendous progress in understanding the molecular pathways of lipid droplet formation, the mechanisms of transport of triacylglycerols, the main component of the hydrophobic core, into a growing lipid droplet are not fully understood. We developed a model that accounts for the effects of surface tension, viscous flow, and diffusion of triacylglycerols in the endoplasmic reticulum, where their synthesis is taking place. The model predicts the droplet radius as a function of time and shows two phases of growth: a relatively rapid initial increase of the radius followed by slower evolution, in qualitative agreement with experimental observations. The effects of lipid degradation on the droplet surface are discussed.