Biospectroscopy

Research Group at Laboratory of Computational Engineering

 

Molecular Structure of Lipoprotein Particles

Researchers: Peter Engelhardt, Jukka Heikkonen, Kimmo Kaski, Vibhor Kumar, Jussi Kumpula, Linda Kumpula, Niko Lankinen, Tomi Peltola, Pasi Soininen (a), Matti Jauhiainen (b), Petri Ingmanc, Katariina Lähdesmäki (d), Katariina Öörni (d), Petri T. Kovanen (d), Sanna Mäkelä (e), Minna Hannuksela (e), Markku Savolainen (e), Sarah Butcher (f), Mika Ala-Korpela*
(a) Department of Chemistry, University of Kuopio
(b) Department of Molecular Medicine, National Public Health Institute, Helsinki
(c) Department of Chemistry, Instrument Centre, University of Turku;
(d) Wihuri Research Institute, Helsinki;
(e) Department of Internal Medicine, University of Oulu;
(f) Institute of Biotechnology, Electron Microscopy Unit, University of Helsinki.
– *Correspondence to mika.ala-korpelaathut.fi

Lipids are carried in the circulation in water(blood)-soluble lipoprotein particles that consist of a hydrophobic core consisting mainly of esterified cholesterol and triglycerides, and a hydrophilic surface of mainly unesterified cholesterol, phospholipids and apolipoproteins (see Figure 1). Apolipoproteins (i.e., the protein molecules in various lipoprotein particles) maintain the structural integrity of lipoprotein particles and direct their metabolic interactions with cell-surface receptors, hydrolytic enzymes, and lipid transport proteins. The low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are still not well understood. The HDL particles are the key cholesterol carriers in the reverse cholesterol transport, i.e., transfer of accumulated cholesterol molecules from the arterial intima to liver for excretion. HDL particles have several documented functions, although the precise mechanism by which they prevent atherosclerosis still remains uncertain.

We have earlier brought together existing pieces of structural information on LDL particles and also combined computer models of the individual molecular components to give a detailed structural model and visualisation of the particles. We have presented strong evidence in favour of such molecular interactions between LDL lipid constituents that result in specific domain formation in the particles. We termed these local environments as nanodomains. It is becoming evident that the molecular structures of individual lipid molecules initiate interaction phenomena that intrinsically control the complex lipoprotein cascades in our bloodstream as well as in the intimal areas, the site of atherosclerotic LDL cholesterol and lipid accumulation. The very same lipid molecules also form HDL particles making the nanodomain approach also relevant to molecular studies of reverse cholesterol transport.

Recent findings suggest that small alterations in the chemical structure of lipoprotein lipids may also relate to the effects of alcohol and alcoholism on reverse cholesterol transport; it is known that alcohol does have beneficial effects on lipoprotein metabolism in general and that small amounts of “abnormal” lipids, e.g., phosphatidylethanol, are formed in the presence of ethanol and are associated with lipoproteins in plasma. Ethanol and ethanol-induced modifications of lipids are likely to modulate the effects of lipoproteins on the cells in the arterial wall. The molecular mechanisms involved in these processes are complex, requiring further study to better understand the specific effects of ethanol in the pathogenesis of atherosclerosis.

In collaboration with the Department of Internal Medicine, University of Oulu, we have detected, for the first time, that high density lipoprotein (HDL) particles of heavy alcohol drinkers have an enhanced capacity to promote cholesterol efflux. The enhanced efflux capacity was due to the HDL2 fraction. Specifically, the increase in phospholipids of HDL2, due to 2-fold increase in the large HDL2b, was associated with enhanced cholesterol efflux. The increase in efflux potent HDL2b in heavy alcohol drinkers could be one of the anti-atherogenic effects of alcohol and prevent the development of atherosclerosis in alcoholics.

Using proton NMR we have recently been able to identify and quantify lysophosphatidylcholine (lysoPC) (in addition to PC and sphingomyelin) in LDL particles. This finding is particularly important concerning studies of LDL particle modifications in various pH conditions. Recent evidence suggest that atherosclerotic plaques and plaque vulnerability are related to acidic pH and recent results have also pointed out remarkable differences in the LDL particle modifications at different pH after enzymatic modifications. LysoPC may also induce various cell related phenomena in the intima since it is known to have some functions in cell signalling.

Recently we have also been developing a computationally optimised, general structural model for spherical lipoprotein particles. Our preliminary modelling, based on extensive biochemical data on the molecular compositions of different lipoprotein particles, indicates new aspects in relation to the distribution of hydrophobic lipid molecules, such as triglycerides and cholesterol esters, in the particles. The obtained molecular distributions seem to be a characteristic of each metabolic lipoprotein category revealing a molecular rationale for the lipoprotein metabolism.

In the current multidisciplinary collaboration we will study the molecular structures of lipoprotein particles, focusing on HDL particles in relation to reverse cholesterol transport as well as on native and modified LDL particles in relation to early atherosclerotic lesion formation. On the biophysical side the role of optimised structural modelling in the studies of lipoprotein structure will be evaluated further. The applications of SOM analysis and the Bayesian methodology for NMR data as well as for clinical biochemistry data are to be extended. To reach the general goal – detailed molecular understanding of lipoprotein structure and dynamics – we will be applying and combining various experimental and computational approaches.

Figure 1
Figure 1: A schematic molecular model of an LDL particle. The colour coding for the molecules is: dark blue – phosphatidylcholine, light blue – sphingomyelin, dark yellow – cholesterol ester, red – cholesterol, green – triglyceride, and grey – apolipoprotein B-100. The molecular shapes and scales are derived from molecular dynamics simulations. For more details, see BBA 1488, 189, 2000.

Recent articles:

K. Öörni, P. Posio, M. Ala-Korpela, M. Jauhiainen, P. T. Kovanen: Sphingomyelinase induces aggregation and fusion of small VLDL and IDL particles and increases their retention to human arterial proteoglycans. Arteriosclerosis, Thrombosis, and Vascular Biology 25, 1678-1683, 2005.

Selected background articles:

M. T. Hyvönen, Y. Hiltunen, W. El-Deredy, T. Ojala, J. Vaara, P. T. Kovanen, M. Ala-Korpela: Application of self-organizing maps in conformational analysis of lipids. Journal of the American Chemical Society 123, 810-816, 2001.

T. Hevonoja, M. O. Pentikäinen, M. T. Hyvönen, P. T. Kovanen, M. Ala-Korpela: Structure of low density lipoprotein (LDL) particles. Basis for understanding molecular changes in modified LDL. Biochimica Biophysica Acta - Molecular and Cell Biology of Lipids 1488, 189-210, 2000.

K. Öörni, M. O. Pentikäinen, M. Ala-Korpela, P. T. Kovanen: Aggregation, fusion, and vesicle formation of modified LDL particles: molecular mechanisms and effects on matrix interactions. Journal of Lipid Research 41, 1703-1714, 2000.

J. K. Hakala, K. Öörni, M. Ala-Korpela, P. T. Kovanen: Lipolytic modification of LDL by phospholipase A2 induces particle aggregation in the absence and fusion in the presence of heparin. Arteriosclerosis, Thrombosis, and Vascular Biology 19, 1276-1283, 1999.

K. Öörni, J. K. Hakala, A. Annila, M. Ala-Korpela, P. T. Kovanen: Sphingomyelinase induces aggregation and fusion, but phospholipase A2 only aggregation, of low density lipoprotein particles: two distinct mechanisms leading to increased binding strength of LDL to human aortic proteoglycans. Journal of Biological Chemistry 273, 29127-29134, 1998.

A. Korhonen, M. Jauhiainen, C. Ehnholm, P. T. Kovanen, M. Ala-Korpela: Remodeling of HDL by phospholipid transfer protein: demonstration of particle fusion by 1H NMR spectroscopy. Biochemical and Biophysical Research Communications 249, 910-916, 1998.

M. Ala-Korpela, M. O. Pentikäinen, A. Korhonen, T. Hevonoja, J. Lounila, P. T. Kovanen: Detection of low density lipoprotein particle fusion by proton nuclear magnetic resonance spectroscopy. Journal of Lipid Research 39, 1705-1712, 1998.

M. T. Hyvönen, T. T. Rantala, M. Ala-Korpela: Structure and dynamic properties of diunsaturated PLPC lipid bilayer from molecular dynamics simulation. Biophysical Journal 73, 2907-2923, 1997.

H. C. Murphy, M. Ala-Korpela, J. J. White, A. Raoof, J. D. Bell, M. L. Barnard, S. P. Burns, R. A. Iles: Evidence for distinct behaviour of phosphatidylcholine and sphingomyelin at the low density lipoprotein surface. Biochemical and Biophysical Research Communications 234, 733-737, 1997.

J. Lounila, M. Ala-Korpela, J. Jokisaari, M. J. Savolainen, Y. A. Kesäniemi: Effects of orientational order and particle size on the NMR line positions of lipoproteins. Physical Review Letters 72, 4049-4052, 1994.