Ultrastructural Evidence for Plaque Erosions in Hypercholesterolemic Apoe -/- Mice

Introduction: Up to 1/3 of acute myocardial infarctions are due to thrombosis initiated by plaque erosions. In the fat-fed ApoE-/- mouse, dramatic increases in plaque permeability and excessive procoagulant risk are observed after ~4 months on a high fat diet, which resolves within weeks of restoring a regular diet. However, whether endothelial barrier disruption is due to dissociation of lateral cell junctions or to frank plaque erosions with endothelial death and sloughing is not known. Methods and Results: 80 male ApoE-/- mice were divided into 8 groups, including high-fat diet for 4, 5, and 6 months, age-matched controls fed normal chow, and mice on a high-fat diet for 4 months followed by normal chow for 1 or 2 months. Aortas were collected for transmission (TEM) and scanning electron microscopy (SEM). Unlike the smooth and intact endothelial layer in younger ApoE-/- mice on normal chow (Fig A), high fat fed mice exhibited cholesterol crystals (Fig C) densely deposited on the surface of aortic plaques (Fig B), with no evidence of normal endothelium, consistent with extensive denudation. However after switching to normal chow for 2 months, endothelial recovery was notable (Fig D), in contrast to mice remaining on high-fat diet (Fig E) with persistent denudation. Denser intimal collagen fibers with fewer lipid droplets were observed in the healing plaques of the diet adjusted groups (Fig F) but not the high fat groups (Fig G). Conclusions: Evidence of plaque erosions and endothelial denudation is revealed as a cause of excessive permeability and thrombotic risk in ApoE-/- mice after prolonged fat feeding. These data argue for a more ubiquitous presence of hypercoagulable erosions than has previously been assumed in individuals at risk for acute vascular events. Accordingly, we hypothesize the plaque erosions may be present in all hypercholesterolemic individuals and some go on to rupture, but that the risk is reversible by dietary manipulation following endothelial recovery.