Low Density Lipoprotein Aged in Plasma Forms Clusters Resembling Subendothelial Droplets: Aggregation via Surface Sites

Abstract

In early phases of atherogenesis, droplets and vesicles accumulate in the subendothelial extracellular space of arterial intima. There is much evidence to suggest that these droplets, ranging between 100 and 400 nm, derive from modified low-density lipoprotein (LDL). In investigations of the formation mechanism of these droplets, LDL fusion was previously induced in vitro by proteolysis, lipolysis, oxidation, and vigorous shaking, but all treatments failed to reproduce the size distribution range of in vivo droplets, mostly resulting, instead, in particles with a diameter intermediate between that of one and two LDL. Our approach was meant to mimic LDL aging in plasma. LDL isolated from plasma that was incubated overnight at 37 degrees C is slightly modified in the secondary structure of its protein component and is primed to form very large aggregates according to a reaction-limited mechanism. This mechanism requires interactions between selected surface sites, whereas massive fusion is ruled out. In the frame of the general theory for colloids, the aggregation of LDL aged in plasma fulfills all the requirements of the reaction-limited mechanism, encompassing 1), exponential growth; 2), fractal structure, with the dimension of elementary constituent still consistent with a single LDL; and 3), extreme polydispersity of aggregates, with shape and dimension very close to that of droplets observed in vivo.

2D-NMR reveals different populations of exposed lysine residues in the apoB-100 protein of electronegative and electropositive fractions of LDL particles

“…It has been reported that active Lys are clustered in basic microenvironments, some of them being involved in LDLr recognition (11)(12)(13). These conformational differences between LDL(+) and LDL(-) agree with the observation of misfolded apoB-100 in LDL(-) ( 9 ), a property that increases its overall amyloidogenic propensity ( 10 ) and could also be involved in its higher susceptibility to particle aggregation ( 7,8 ).…”

Kinetic analysis of thermal stability of human low density lipoproteins: a model for LDL fusion in atherogenesis

“…The latter may involve apoB residues 405-539 (MB24 binding site) or 1022-1031 (MB11 binding site) ( Fig. 5 ), which are located in relative proximity on LDL surface ( 31 ). We speculate that targeting conformational changes in apoB that prime LDL for fusion may provide a strategy for blocking this pathogenic reaction before it occurs.…”

“…Although the detailed structural basis for LDL fusion is unclear, it is thought to result from accumulation of packing defects on LDL surface that form upon mechanical, thermal, chemical or enzymatic perturbations (9)(10)(11)(12)(13)(14)(15)(29)(30)(31)(32). The latter include lipolysis of polar lipids, such as phosphatidylcholine or sphingomyelin, and proteolysis of apoB followed by dissociation of its proteolytic fragments (7)(8)(9)(10)(11)(12)(13)(14)(15)32 ).…”

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

“…For example, minimal lipolytic activity of secretary PLA 2 was observed when freshly isolated plasma LDLs were used as substrates; however, lipolytic activity increased up to 25-fold upon LDL storage at 6°C for 8 weeks or at 37°C for 15 h, which was probably due to PC oxidation (81). In another study, plasma incubation at 37°C produced a subpopulation of LDLs that were prone to aggregation, fusion, and lipid droplet formation as detected by dynamic light scattering and atomic force microscopy (82). Kinetics analysis suggested that particle aggregation in these experiments was driven by interactions between a limited number of specific surface sites on LDLs (82).…”