Human LDL core cholesterol ester packing: three-dimensional image reconstruction and SAXS simulation studies

Abstract

This article is available online at http://www.jlr.org accumulate in the circulation as a result of a defi ciency in the LDL receptor, for example, the LDL can be oxidized and subsequentially interacts with macrophages. The accumulation of cholesterol ester transforms the macrophage into an atherogenic foam cell and contributes to the pathophysiology of atherosclerosis ( 6 ).Previous studies have shown that the cholesterol ester in the core of the LDL particle can exist in two phases accompanying change in temperature ( 7-9 ). It also has been shown that the phase transition and lipid packing may affect the rate of LDL oxidation ( 10 ). Thus, the structure of the LDL lipid core has been intensively studied to shed light on its involvement in the processes of the early atherogenic events.In the 1970s, a model for the cholesterol ester packing was proposed primarily based on X-ray and neutron scattering studies. These studies suggested that the cholesterol ester packed in concentric shells in the core at a temperature below the phase transition, and in an isotropic state at a temperature above the phase transition with the underlying assumption that the LDL is a spherical particle for both core phases ( 7-9, 11 ). More recent work from electron cryo-microscopy (cryo-EM) studies have shown in threedimensional (3D) images that the LDL particle exhibits a disk shape instead of a sphere at temperatures below the phase transition, and the cholesterol ester forms a fl at layered density ( 12-14 ) as opposed to concentric shells. However, the LDL lipid core structure above the phase transition, a more physiologically relevant state, remains controversial from image evidence. Two cryo-EM studies that attempted to freeze the LDL sample from a temperature above the phase transition have reached different conclusions about the changes of the cholesterol ester packing upon the phase transition ( 15,16 ). This discrepAbstract Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Structural changes associated with this phase transition have been shown to affect the resistance of LDL to oxidation in vitro studies. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three fl at layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. The LDL frozen from 53°C adopted a different morphology. The central density layer was disrupted and the outer two layers formed a "disrupted shell"-shaped density, located concentrically underneath the surface density of the LDL particle. LDL is a micro emulsion that transports lipid, mainly cholesterol ester, from liver to the peripheral tissues ( 1, 2 ). LDL particles ar...

Lipoprotein Structure and Dynamics: Low Density Lipoprotein Viewed as a Highly Dynamic and Flexible Nanoparticle

“…It has been shown that the physical state of core lipids changes within milliseconds [44]. This fast kinetics has caused experimental difficulties for long time, however, a recent experimental approach by speeding up freezing allowed to trap the lipids in the molten state [45]. The authors report on a co-existing phase of layered and broken shells for LDL particles, which are shock-frozen in a state above the phase transition.…”

Immuno-electron cryo-microscopy imaging reveals a looped topology of apoB at the surface of human LDL

“…In comparison to studies based on other methodologies ( 8,9,14,15 ), cryo-EM and image reconstruction have more advantages in preserving the LDL particle in the native state, and intuitively reveal the density distribution of the LDL structure in three-dimensional (3D) images (10)(11)(12)16 ). Previously, we have shown, in the 3D density map, that the apoB protein component corresponds to the high-density regions and distributes on the edge of the particle ( 16 ).…”

“…Chatterton et al ( 8,9 ) pioneered studies to determine the distribution of apoB on the surface of LDL by employing the strategy of using paired monoclonal antibody (MAb) labeling to obtain projection images with negative stain electron microscopy followed by triangulation to determine the corresponding MAb locations. However, later work from several cryo-electron microscopy (cryo-EM) groups has shown that LDL is a discoidal-shaped particle at a temperature below the phase transition associated with the core-located cholesteryl esters (10)(11)(12)(13). Thus, the underlying assumption that LDL is a spherical particle limits the accuracy of the triangulation measurements.…”

Human low density lipoprotein: the mystery of core lipid packing

“…However, as reported in this issue of the Journal of Lipid Research , Liu et al ( 16 ) were successful in establishing an advanced fast freezing procedure to freeze LDL in a state above the phase transition. By doing so, they were able to capture an intermediate state in the transition from isotroHuman low density lipoprotein: the mystery of core lipid packing 1 pic to liquid-crystalline.…”

“…Indeed, this patch nucleation behavior permits the temporary formation of local molecular microenvironments as suggested previously in terms of trigylceride segregation ( 17 ). Beyond detailed information on the internal core lipid organization, the paper by Liu et al ( 16 ) describes the direct impact of the core lipid phase transition on particle morphology and surface components. The fi ndings are consistent with earlier studies, which provided evidence that the physical state of the core lipids induces changes in the local molecular dynamics of the surface monolayer ( 18 , 19 ).…”