In Vivo Dynamics and Phase State of Natural Lipid Droplets

Abstract

All organisms store lipids as energy resource for metabolism. Such lipids are accumulated in the form of intracellular droplets. The structure of the lipid droplet is complex. The hydrophobic core represents a mix of un- and saturated triglycerides, enclosed by an amphiphilic shell of phospholipids with embedded proteins. Uncovering intrinsic biophysical properties of lipid droplets in living cells of various organisms is crucial for metabolism manipulation or reducing lipid storage. The type and organization of lipids affect their phase state and microscopic dynamics in the droplet. We characterized lipid droplets in the cells of fresh human and porcine subcutaneous fat tissues, as well as yeast cells using quasi-elastic neutron scattering, probing the molecular motions in a time scale of 6-400 picoseconds and a length scale of 3-20 Angstroms. The detected two-component dynamics in the droplet is associated with lipid unrestricted diffusion (D ∼ 0.006 A2/ps) and motions of its hydrocarbon chains (D ∼ 0.2 A2/ps) in a restricted volume (5-12 A). The dynamics of lipids is significantly reduced below 305 K and 266 K in porcine and human tissues, respectively, due to a first-order phase transition of lipids from fluid to gel (similar to synthetic lipid bilayer). The lipid phase change in human tissue is confirmed and enhanced at applied pressure of 90 bars. However, lipid droplets in the yeast cells remain in a fluid-like state within range of 280-310 K. We believe that phase behavior of lipid droplets is different in the cells of tissues and microorganism due to lipid composition, which was investigated by mass spectrometry. Lipid packing in the droplet is tight in the tissues, having mostly saturated lipids, and loose in the yeast droplet, having equal proportions of un- and saturated lipids.