LIPOPROTEIN PARTICLE INTERACTION WITH BIOMEMBRANES

Cholesterol is an essential component of mammalian cells and its distribution in the organism is facilitated by so-called lipoprotein particles. Excess cholesterol from the cells is transferred to high- density lipoproteins (HDLs) and then disposed of by the liver with the bile. An imbalance in the cholesterol level caused by food intake or genetic factors leads to its accumulation on the arterial wall; this leads to cell membrane stiffening and influences vascular integrity, signal transduction and absorption and transport processes and finally to arteriosclerosis, one of the most common causes of death in industrialized countries. The challenge is still to understand the function of lipoproteins on a molecular level in order to describe physiological processes and thus be able to treat disorders such as hypercholesterolemia, heart disease, stroke and neurodegenerative diseases. In addition, the production and/or targeted manipulation of lipoproteins offers new possibilities for the transport of therapeutic agents, as lipoproteins can be remodeled. Our hypothesis is that, on the one hand, the physical properties of the cellular membrane and, on the other hand, the specific structure of lipoproteins can strongly influence the transfer of cholesterol and thus impair the removal of excess lipids from the bloodstream. New knowledge about the uptake mechanism will clarify how cargo molecules like cholesterol are transferred from the outer envelope of lipoproteins to cells. Therefore, in the present project the relationship between receptor-mediated and direct lipoprotein interaction and the subsequent transfer process with model membranes and finally living cells will be analyzed and quantified. Using a broad spectrum of complementary and sophisticated measurement techniques such as combined single-molecule fluorescence and atomic force microscopy, high-speed scanning force microscopy and fluorescence cross-correlation spectroscopy, it will be possible to gain a more detailed insight into the underlying biomolecular mechanisms.

Cholesterol is an essential biomolecule for cellular function. It facilitates the flexibility of cell membranes and serves as a precursor for a variety of other biomolecules, including hormones. It is transported to cells by special delivery vehicles, namely lipoprotein particles, which are capable of carrying varying quantities of fats and non-water-soluble molecules, including some vitamins. The manner in which these lipoprotein particles facilitate the exchange of cholesterol with cells is of paramount importance for maintaining equilibrium in cholesterol levels. An imbalance may result in significant health complications, including cardiovascular disease, diabetes, and cancer. The objective of our study was to gain insight into the mechanisms underlying this transfer, with a particular focus on two key factors: (1) specific proteins found on lipoprotein particles, and (2) the surrounding environment of the cell membrane, which affects its properties. To investigate this, we employed advanced techniques, including state-of-the-art microscopes and ultra-sensitive scales, to observe the transfer of cholesterol in real time. Our findings indicate that cholesterol can be transferred directly between lipoprotein particles and cell membranes, even in the absence of specific receptors, which are typically believed to be essential for this process. The transfer mechanism was verified with the assistance of fluorescent markers, which permitted the tracking of cholesterol movement from lipoprotein particles to cell membranes. The results of our study indicate that all lipoprotein particles are capable of integrating with lipid membranes, enabling the transfer of their lipid cargo through a rapid, receptor-independent fusion mechanism. Previously, it was assumed that receptors were the sole means by which lipoproteins could deliver cholesterol. However, the transfer capability is influenced by the lipid composition within the target membrane and also by the Lipoprotein variants. This finding may have implications for the development of novel therapies for dyslipidemia and conditions such as familial hypercholesterolemia. Furthermore, we demonstrated the successful loading of insulin onto lipoprotein particles, which enhances the targeted delivery of therapeutic agents while maintaining the particles' structural integrity and physiological function. This research was led by B. Plochberger in collaboration with experts H. Stangl and E. Sezgin.