Sunny H. Zhang

Atherosclerosis in mice lacking apo E. Evaluation of lesional development and progression.

Apolipoprotein E-deficient mice have spontaneous elevations of total plasma cholesterol and triglycerides and reduced high-density lipoprotein. The mice develop arterial lesions in a time-dependent manner. Lesional distribution was centered in the aortic sinus in young mice, and the lesions were widely distributed throughout the arterial tree in mice at 8 to 9 months of age. In young mice, subendothelial foam cell deposits were present in the aortic sinus adjacent to valve-attachment sites. By 5 months of age, foam cell deposits, free cholesterol, and admixed smooth muscle cells composed the developing atherosclerotic lesions. After 8 to 9 months of age, the arterial lesions showed increased complexity, and fibrous cap lesions were present. Transmission electron microscopy showed foam cells, smooth muscle cells (both contractile and synthetic varieties), cellular debris, and acicular cholesterol deposits within the plaques. By scanning electron microscopy, subendothelial collections of foam cells were present within the aortic sinus and ascending aorta. The results show that the complexity of the atherosclerotic lesions that develop in these apo E deficient-mice are similar to those described in other species and therefore represent an important model for studies of genetic and environmental influences on the atherosclerotic process.

Diet-induced atherosclerosis in mice heterozygous and homozygous for apolipoprotein E gene disruption.

With the aim of establishing whether a genetically reduced capability of producing apolipoprotein E (apo E) can affect atherogenesis, we have compared the consequences of dietary stress on normal mice and on mice heterozygous or homozygous for a disrupted apo E gene. A dramatically accelerated development of lesions occurred in the vasculature of the homozygous mutants as a result of feeding an atherogenic diet for 12 wk, and extensive deposition of lipid-filled macrophages was found outside the cardiovascular system. In nine heterozygotes fed the atherogenic diet for 12 wk, the amount of apo E in their total plasma lipoproteins increased to a level comparable to normal, but all nine developed much larger foam cell lesions in their proximal aorta than those found in 3 of 9 normal mice fed the same diet. The other six normals had no lesions. Our study demonstrates that heterozygous mice with only one functional apo E gene are more susceptible to diet-induced atherosclerosis than are normal, two-copy mice. Genetically determined quantitative limitations of apo E could, therefore, have similar effects in humans when they are stressed by an atherogenic diet.

Angiotensin AT1B receptor mediates calcium signaling in vascular smooth muscle cells of AT1A receptor-deficient mice.

Our studies on angiotensin II receptor subtype 1A (AT1A) knockout mice define how endogenous receptors other than AT1A receptors stimulate changes in cytosolic calcium concentration ([Ca2+]i) in cultured aortic vascular smooth muscle cells (VSMCs). Wild-type cells have a 1.7 ratio of AT1A/AT1B receptor mRNA as determined by semiquantitative reverse transcriptase-polymerase chain reaction. Mutant cells express AT1B receptor mRNA but not that for the AT1A receptor. In wild-type cells with AT1A present, Ang II (10(-7) mol/L) produces a characteristic rapid peak increase in [Ca2+]i of 150 to 180 nmol/L, followed by a plateau phase characterized by a sustained 70 to 80 nmol/L increase in [Ca2+]i. An unexpected finding was that the magnitude and time-dependent pattern of [Ca2+]i changes produced by Ang II were similar in cells that lacked AT1A receptors but possessed AT1B receptors. The response in mutant cells indicates effective coupling of an Ang II receptor to one or more second messenger systems. The similarity of response patterns between cells with and without AT1A receptors suggests that non-AT1A receptors are functionally linked to similar signal transduction pathways in mutant cells. The fact that mutant and wild-type cells exhibit similar patterns of calcium mobilization and entry supports the notion that AT1A and non-AT1A receptors share common signal transduction pathways. The AT2 receptor ligands PD-123319 and CGP-42112 do not alter Ang II effects in either VSMC type, suggesting a paucity of AT2 receptors and/or an absence of their linkage to [Ca2+]i pathways. The nonpeptide AT1 receptor blocker losartan antagonizes Ang II-induced [Ca2+]i increases in both cell groups, supporting mediation by native AT1B receptors and effective coupling of this subtype to second messenger systems leading to calcium entry and mobilization. Our results demonstrate that Ang II causes calcium signaling in AT1A-deficient VSMCs that is mediated by an endogenous losartan-sensitive AT1B receptor.

Nerve Regeneration Occurs in the Absence of Apolipoprotein E in Mice

Abstract: The concentration of apolipoprotein E (apoE), a high‐affinity ligand for the low‐density lipoprotein receptor, increases dramatically in peripheral nerve following injury. This endoneurial apoE is thought to play an important role in the redistribution of lipids from the degenerating axonal and myelin membranes to the regenerating axons and myelin sheaths. The importance of apoE in nerve repair was examined using mutant mice that lack apoE. We show that at 2 and 4 weeks following sciatic nerve crush, regenerating nerves in apoE‐deficient mice were morphologically similar to regenerating nerves in control animals, indicating that apoE is not essential for peripheral nerve repair. Moreover, cholesterol synthesis was reduced in regenerating nerves of apoE‐deficient mice as much as in regenerating nerves of control animals. These results suggest that the intraneural conservation and reutilization of cholesterol following nerve injury do not require apoE.

Nerve Regeneration and Cholesterol Reutilization Occur in the Absence of Apolipoproteins E and A-I in Mice

Abstract: Apolipoproteins have been implicated in the salvage and reutilization of myelin cholesterol during Wallerian degeneration and the subsequent nerve regeneration. Current evidence suggests that myelin cholesterol complexes with apolipoproteins E and A‐I to form lipoproteins that are taken up via low‐density lipoprotein receptors on myelinating Schwann cells. We recently reported, however, that apolipoprotein E is not required for nerve regeneration or reutilization of myelin cholesterol. We have now investigated nerve regeneration and the reutilization of cholesterol in mutant mice deficient in both apolipoproteins E and A‐I. Morphologic examination of nerves 4 and 12 weeks after crush injury revealed that regeneration proceeded at a normal rate in the absence of these apolipoproteins. Autoradiography of regenerating nerves indicated that prelabeled myelin lipid was reutilized in the regenerating myelin. 3‐Hydroxy‐3‐methylglutaryl‐CoA reductase, the rate‐limiting enzyme in cholesterol synthesis, was down‐regulated in the regenerating nerves, indicative of cholesterol uptake via lipoproteins. Prelabeled myelin cholesterol was present in lipoprotein fractions isolated from crushed nerves of mutant mice. These data suggest that there is considerable redundancy in the process of cholesterol reutilization within nerve, and that apolipoproteins other than apolipoproteins E and A‐I may be involved in the recycling of myelin cholesterol.

Differential Expression of the α1 Type VIII Collagen Gene by Smooth Muscle Cells from Atherosclerotic Plaques of Apolipoprotein-E-Deficient Mice

We have investigated the morphology, growth and gene expression of smooth muscle cell (SMC) cultures derived from advanced atherosclerotic plaques and from non-plaque-containing aorta of individual apolipoprotein-E-deficient mice. The initial outgrowth of cells was faster from plaques (P) than from non-plaque segments (NP), but the cells in P cultures divided more slowly than NP cells in subcultures. By the 6th passage, the general growth pattern, morphology, ploidy and response to mitogenic factors of the cells were no longer consistently different in P and NP cultures. However, by the use of differential display, several transcripts were identified that were differentially expressed in three independent pairs of P and NP cultures. One of the transcripts, from a type VIII collagen gene, was elevated in all the P cultures compared to their NP counterparts even at the 40th passage. The α1 type VIII collagen transcripts were also readily detectable by RT-PCR in freshly dissected plaques, but not in the normal parts of aortas from the apolipoprotein-E-deficient mice. In situ hybridization showed that the transcripts were limited to the fibrous cap of plaques. Thus, SMCs from atherosclerotic plaques produce type VIII collagen and this differential expression continues when the cells are maintained in tissue culture.

Hepatic clearance of chylomicron remnants m mice lacking apoprotein E

Control and apoprotein E-deficient mice generated by gene targeting in embryonic stem cells were fed a fat load containing 3H-labeled retinol and the absorbed radioactivity present in the plasma, liver, and carcass measured 6 h later. The radioactivity in the plasma of the apoprotein E-deficient mice was several fold higher than in control animals, but it accounted for less than 1/5 of the absorbed radioactivity. In both groups of animals most of the absorbed radioactivity was recovered in the livers. These findings indicate that in apoprotein E-deficient mice chylomicron remnants can be taken up by the liver by a process that does not require the mediation of apoprotein E.