Background: To cause atherosclerosis, LDLs (low-density lipoproteins) must first pass through the endothelium and then become retained in the arterial matrix. Which of these two processes is rate-limiting and predicts the topography of plaque formation remains controversial. To investigate this issue, we performed high-resolution mapping of LDL entry and retention in murine aortic arches before and during atherosclerosis development.
Methods: Maps of LDL entry and retention were created by injecting fluorescently labeled LDL followed by near-infrared scanning and whole-mount confocal microscopy after 1 hour (entry) and 18 hours (retention). By comparing arches between normal mice and mice with short-term hypercholesterolemia, we analyzed changes in LDL entry and retention during the LDL accumulation phase that precedes plaque formation. Experiments were designed to secure equal plasma clearance of labeled LDL in both conditions.
Results: We found that LDL retention is the overall limiting factor for LDL accumulation but that the capacity for LDL retention varied substantially over surprisingly short distances. The inner curvature region, previously considered a homogenous atherosclerosis-prone region, consisted of dorsal and ventral zones with high capacity and a central zone with low capacity for continued LDL retention. These features predicted the temporal pattern of atherosclerosis, which first appeared in the border zones and later in the central zone. The limit to LDL retention in the central zone was intrinsic to the arterial wall, possibly caused by saturation of the binding mechanism, and was lost upon conversion to atherosclerotic lesions.
Conclusions: Capacity for continued LDL retention varies over short distances and predicts where and when atherosclerosis develops in the mouse aortic arch.
Keywords: animals; atherosclerosis; endothelium; hypercholesterolemia; mice.