Ostwald Ripening of Triacylglycerol Droplets Embedded in Glass-Supported Phospholipid Bilayers

Lipid droplets are fat storage organelles that consistof a neutrallipid core surrounded by a phospholipid monolayer. Because of theirimportant biological functions, reconstituting model lipid dropletsin synthetic phospholipid membranes is of great interest. In the presentstudy, we investigated the incorporation of triacylglycerol dropletsinto glass-supported phospholipid bilayers by using fluorescence microscopy.We adsorbed triolein emulsions onto a glass surface that was partiallycovered with planar bilayers. After adsorption, triolein dropletswere found to be immobilized in the bilayer membrane. The volume ofeach bound droplet varied over time. Large droplets grew, whereassmall droplets shrank. Additionally, data on fluorescence recoveryafter photobleaching obtained for a phospholipid probe indicate thatphospholipids on and near triolein droplets were fully mobile. Furthermore,photobleaching data obtained for a triacylglycerol probe indicatethat triolein molecules diffused between different droplets alongthe planar bilayer. These results demonstrate Ostwald ripening, wheretriolein molecules in a small droplet dissolved in the bilayer, diffusedlaterally, and eventually bound to the interfaces of larger droplets.We investigated the ripening rate by using the average of the cuberoot of the fluorescence emission obtained for individual droplets.The ripening slowed after the addition of trilinolein to the trioleinphase. Finally, we investigated the time dependence of the size distributionsof the triolein droplets. The distribution was initially nearly unimodaland subsequently became bimodal.

Automated summary

In this study, the adsorption of triacylglycerol droplets onto glass-supported phospholipid bilayers was investigated using fluorescence microscopy. The results showed that triacylglycerol droplets were immobilized in the bilayer membrane and their volume varied over time. Phospholipids on and near the droplets were fully mobile, and triacylglycerol molecules diffused between different droplets along the bilayer. The size distribution of the droplets also changed over time.