Contribution of Macromolecular Structure to the Retention of Low-Density Lipoprotein at Arterial Branch Points

Abstract

Extracellular deposition of low-density lipoprotein (LDL) in the arterial wall is an essential early step in atherosclerosis. This process preferentially occurs at arterial branch points, reflecting a regional variation in lipoprotein-arterial wall interactions. In this study, we characterized the submicron microstructure of arterial wall collagen and elastin to evaluate its potential role in regional LDL deposition. With 2-photon microscopy, we used the intrinsic optical properties of collagen and elastin to determine the arterial wall macromolecular microstructure in fresh porcine and murine arteries. This optical approach generated unique nondestructive en face 3-dimensional views of the wall. The collagen/elastin microstructure was found to vary with the topology of the arterial bed. A nearly confluent elastin surface layer was present throughout but was missing at atherosclerosis-susceptible branch points, exposing dense collagen-proteoglycan complexes. In LDL binding studies, this luminal elastin layer limited LDL penetration, whereas its absence at the branches resulted in extensive LDL binding. Furthermore, LDL colocalized with proteoglycans with a sigmoidal dose dependence (inflection point, approximately 130 mg LDL/dL). Ionic strength and competing anions studies were consistent with the initial interaction of LDL with proteoglycans to be electrostatic in nature. This optical sectioning approach provided a robust 3-dimensional collagen/elastin microstructure of the arterial wall in fresh samples. At atherosclerosis-susceptible vascular branch points, the absence of a luminal elastin barrier and the presence of a dense collagen/proteoglycan matrix contribute to increased retention of LDL.