Ultrastructure of Early Lipid Accumulation in ApoE-Deficient Mice

Abstract

Abstract-Apolipoprotein (apo) E-deficient mice develop severe hypercholesterolemia and have lesions that progress from fatty streaks to fibrous plaques distributed in lesion-prone areas throughout the aorta. Lesions develop in apoE-deficient mice on a regular chow diet and will occur faster on a diet higher in cholesterol. Examination of the aortas from these mice on a chow diet by high-resolution, freeze-etch electron microscopy demonstrated lipid retention in the intima by 3 weeks of age. Lipid was retained in the matrix as individual particles between 33 and 48 nm in diameter, aligned along the collagen fibrils and in aggregates consisting of lipid particles with average diameters of 33 and 68 nm. Larger particles seemed to have formed from fusion of smaller particles. Lipid retention was more widespread in 5-and 9-week-old mice. Monocyte attachment to endothelial cells was observed by electron microscopy at 5 weeks of age. The appearance of the intimal lipid was similar to that previously described in rabbit models and suggests that lipid interaction with matrix filaments and subsequent aggregation of lipid particles are critical first steps in the process of foam cell formation. T he creation of apoE-deficient mice by homologous recombination of embryonic stem cells has provided a murine model of atherosclerosis. 1,2 The mice have severe hypercholesterolemia and have lesions that progress with age from fatty streaks to fibrous plaques distributed in lesionprone areas. Lesions develop even on a low-cholesterol, low-fat chow diet but occur sooner and become larger on a high-cholesterol, high-fat, Western-type diet.Two studies have provided detailed histological observations on the morphology of the development of atherosclerotic lesions in apoE-deficient mice. Nakashima et al 3 studied animals between 6 and 40 weeks of age fed Western-and chow-type diets. These authors showed that these animals developed the full range of lesions from fatty streaks to fibrous plaques and, most important, that the lesions were similar in their inflammatory-fibroproliferative features to those seen in rabbit models, nonhuman primates, and humans. Reddick et al 4 studied mice from 11 to 64 weeks of age fed solely a chow diet. Their morphological studies, at the light-microscope level of resolution, also showed that the progression of atherosclerotic lesions in mice lacking apoE paralleled that described in Watanabe heritable hyperlipidemic rabbits. These histological studies established that the light-microscopic appearance, cellular composition, and distribution of the lesions are typical of lesion development in animal models and similar in some respects to lesions in humans. However, these studies were at the light-microscope level of resolution, and the observations occurred after fatty streaks had begun to form. Given the importance of the apoE-knockout mouse model as an excellent system to study the pathogenesis and progression of atherosclerosis, the objective of the present study was to extend the structural observations...

Prominent Role of P-Selectin in the Development of Advanced Atherosclerosis in ApoE-Deficient Mice

“…Monocyte attachment to endothelial cells of the aortic sinus in apoE-deficient mice was observed before obvious fatty streak formation. 36 Macrophages loaded with fluorescent microspheres injected intravenously into 40-week-old apoE-deficient mice adhered to all stages of atherosclerotic plaques, including mature calcified fibrous lesions. 37 Macrophages in the subendothelium may at first protect against atherosclerosis but later also contribute to lesion progression.…”

Lipolysis of LDL by Human Secretory Phospholipase A 2 Induces Particle Fusion and Enhances the Retention of LDL to Human Aortic Proteoglycans

“…I nitiation of atherosclerosis in both humans 1 and experimental animals [2][3][4] is characterized by the appearance of extracellular lipid droplets in the proteoglycan (PG)-rich subendothelial layer. Experimental models have shown that similar droplets can be formed directly from LDL particles both in situ 2,5 and in vivo.…”

Inhibition of atherosclerosis in apolipoprotein-E-deficient mice following muscle transduction with adeno-associated virus vectors encoding human apolipoprotein-E

“…39 A defining stage in the development of atherosclerotic lesions that precedes the attachment of monocytes to the vessel wall, is the accumulation of remnant lipoproteins on the intimal extracellular matrix which occurs as early as 3 weeks of age in apoE -/-mice. 46 The presence of low levels of apoE in the circulation may prevent the recruitment of the remnant lipoproteins to the vessel wall and therefore protect against the development of atherosclerosis. 45 This may explain how low levels of human apoE in the circulation of apoE -/-mice can protect against early lesion development.…”