Capacity for LDL (Low-Density Lipoprotein) Retention Predicts the Course of Atherogenesis in the Murine Aortic Arch

Abstract

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.

Arterial Zones That Take a Pause in Early Plaque Development

“…Arrows are color coded to indicate crucial mechanisms in the retention and early modification of cholesterol-rich apoB lipoproteins within the arterial wall (yellow) and then strikingly maladaptive local responses to the retained and modified material that lead to plaque growth and progression (red). Lewis et al 8 focused on early LDL entry and retention, as well as early recruitment of macrophages. B , Summary of the findings by Lewis et al in the central zone of the inner curvature region of the proximal murine aorta after 3.5 weeks of hypercholesterolemia, with speculations about the physical state of the LDL retained there.…”

“…The initial goal of Lewis et al was to resolve a conflict in the literature between LDL entry 9 versus LDL retention 1,10–12 as the rate-limiting step in early atherogenesis. While pursuing these studies, Lewis et al 8 made an unexpected finding: a zone within the inner (lesser) curvature region of the proximal murine aorta with unusual properties during early atherogenesis. The authors found that 3.5 weeks of hypercholesterolemia decreased 18-hour retention of labeled LDL in the inner curvature region of the proximal aortic arch.…”

“…[1][2][3][4][5][6][7] See accompanying article on page 637 But we can still learn from early plaque development. In this issue of ATVB, Lewis et al 8 present the latest in a series of studies from the Bentzon laboratory using elegant techniques in animal models to investigate key questions concerning atherosclerosis initiation and its initial progression. In their current work, these authors assessed entry of exogenous fluorescently labeled LDL (low-density lipoprotein) into the murine aortic wall during the first hour after intravenous injection, as well as a snapshot of the most recent retention by examining accumulation of fluorescently labeled exogenous LDL within the aortic wall 18 hours after intravenous injection.…”

“…Remarkably, the answer appears to be that the aortic wall in the central zone had simply become clogged up with endogenous LDL that occupied binding sites on the extracellular matrix, thereby leaving 1-hour entry of labeled LDL intact but impairing 18-hour retention of recently injected labeled LDL (Figure [B]). Lewis et al 8 point out that, after 3.5 weeks of hypercholesterolemia, the total pool of arterial wall LDL, assessed by fluorescent staining for total apoB protein, was more pronounced in the central zone than in the border zones.…”

“…16 Does the content of normal resident intimal myeloid cells, expression of eNOS (endothelial nitric oxide synthase), or other features considered distinctive of the atherosclerosis-prone inner curvature of the proximal murine aorta 17 differ between the central zone and the border zones? Based on prior work from their laboratory and others, Lewis et al 8 hypothesized that short-range variations in the velocities and directions of blood flow in the proximal murine aortic arch could be an underlying cause. If so, the shapes of the endothelial cells might differ.…”