HDL-Lipid Uptake is Regulated by Elastic Properties of the Plasma Membrane

Abstract

Selective uptake of high density lipoprotein (HDL)-associated cholesteryl-esters (CE) by hepatocytes is a crucial process for the removal of cholesterol from the circulation. Together with triglycerides, CE is transported in the hydrophobic core of HDL particles; free cholesterol, phospholipids, and apolipoproteins build up the particles' amphiphilic surface monolayer. The lipid transfer process itself is highly speculative: it is still unclear how lipids are removed from the circulation and transferred from HDL - a main carrier of cholesterol in the blood stream - to cells.In this study we provide a mechanistic understanding of the cargo exchange process between HDL and biomembranes. The interaction between HDL and synthetic lipid membranes was investigated with force spectroscopy and high speed atomic force microscopy; the transfer of single cargo molecules was directly visualized using a combined and simultaneously operating fluorescence and force microscope. Experimental evidence points to the fact that i) cargo transfer requires contact; ii) only amphiphilic cargo is transferred; iii) upon contact the particle incorporates into the hydrophobic core of the bilayer where it can diffuse. In particular, we compared the transfer of the fluorescently labelled lipids DiI, and Bodipy-labelled cholesterol and cholesteryl-ester. Live cell experiments confirmed the data obtained on the synthetic systems and on GPMV's (giant plasma membrane vesicles). Particle incorporation and cargo transfer was abolished at increased membrane cholesterol levels, as a consequence of the reduced membrane elasticity. These observations reveal a mechanism for regulation of lipid uptake based on sensing plasma membrane cholesterol levels. The function of the corresponding receptor SR-B1 is primarily an anchor to hold the particle close to the plasma membrane; once in proximity, elastic properties of the membrane regulate the fusion of the particle.