Hypercholesterolemia in low density lipoprotein receptor knockout mice and its reversal by adenovirus-mediated gene delivery

Activin and TR3 orphan receptor: Two 'atheroprotective' genes as evidenced in dedicated mouse models

1. Atherosclerosis is a multifactorial, inflammatory disease of the arterial vessel wall that is promoted by various well-defined risk factors. Although numerous genes, expressed in different vascular and inflammatory cells, have been implicated in this disease, it is widely appreciated that most of the genes and gene products vital for initiation and progression of atherosclerosis are unknown. 2. We follow two strategies in an attempt to make up for the void of essential knowledge. First, we study candidate genes that have not been implied in human atherosclerosis before, notably the differentiation factor activin A. 3. Second, we performed a genome-wide search by differential display reverse transcription-polymerase chain reaction. This study indicated potential involvement of the TR3 orphan receptor transcription factor in smooth muscle cell (SMC) (patho)physiology. 4. To reveal functional involvement of these proteins in SMC during atherosclerosis, we performed experiments with mouse models, adjusted either to the characteristics of a secreted protein or to that of an intracellular transcription factor. 5. The secreted protein activin A was studied in mice infected systemically with recombinant adenoviral vehicles, resulting in predominant hepatic expression and subsequent high protein levels in the circulation. 6. To study the role of TR3 in atherosclerosis, we generated transgenic mice in which promoter sequences were applied that direct expression of the transgenes to SMC of the arterial tree. 7. Two approaches were taken to induce the formation of SMC-rich lesions: (i) activation of femoral artery SMC by placement of a loosely fitting cuff; and (ii) ligation of the carotid artery. 8. The aim of the present review is to illustrate the different approaches that can be taken to assess the potential relevance of genes in atherosclerosis in carefully selected mouse models. 9. Based on the results described, we propose that both activin A and TR3 prevent excessive SMC proliferation.

Early embryonic lethality caused by targeted disruption of the 3-hydroxy-3-methylglutaryl-CoA reductase gene

The endoplasmic reticulum (ER) enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, which converts HMG-CoA to mevalonate, catalyzes the ratelimiting step in cholesterol biosynthesis. Because this mevalonate pathway also produces several non-sterol isoprenoid compounds, the level of HMG-CoA reductase activity may coordinate many cellular processes and functions. We used gene targeting to knock out the mouse HMG-CoA reductase gene. The heterozygous mutant mice (Hmgcr+/-) appeared normal in their development and gross anatomy and were fertile. Although HMG-CoA reductase activities were reduced in Hmgcr+/- embryonic fibroblasts, the enzyme activities and cholesterol biosynthesis remained unaffected in the liver from Hmgcr+/- mice, suggesting that the haploid amount of Hmgcr gene is not rate-limiting in the hepatic cholesterol homeostasis. Consistently, plasma lipoprotein profiles were similar between Hmgcr+/- and Hmgcr+/+ mice. In contrast, the embryos homozygous for the Hmgcr mutant allele were recovered at the blastocyst stage, but not at E8.5, indicating that HMG-CoA reductase is crucial for early development of the mouse embryos. The lethal phenotype was not completely rescued by supplementing the dams with mevalonate. Although it has been postulated that a second, peroxisome-specific HMG-CoA reductase could substitute for the ER reductase in vitro, we speculate that the putative peroxisomal reductase gene, if existed, does not fully compensate for the lack of the ER enzyme at least in embryogenesis.

The low density lipoprotein receptor prevents secretion of dense apoB100-containing lipoproteins from the liver

The assembly and secretion of very low density lipoproteins (VLDL) require microsomal triglyceride transfer protein (MTP). Recent evidence also suggests a role for the low density lipoprotein (LDL) receptor in this process. However, the relative importance of MTP in the two steps of VLDL assembly and the specific role of the LDL receptor still remain unclear. To further investigate the role of MTP and the LDL receptor in VLDL assembly, we bred mice harboring "floxed" Mttp alleles (Mttpflox/flox) and a Cre transgene on a low-density lipoprotein receptor-deficient background to generate mice with double deficiency in the liver (Ldlr-/- MttpDelta/Delta). In contrast to the plasma of Ldlr+/+ MttpDelta/Delta mice, the plasma of Ldlr-/- MttpDelta/Delta mice contained apoB100. Accordingly, Ldlr-/- MttpDelta/Delta but not Ldlr+/+ MttpDelta/Delta hepatocytes secreted apoB100-containing lipoprotein particles. The secreted lipoproteins were of LDL and HDL sizes but no VLDL-sized lipoproteins could be detected. These findings indicate that hepatic LDL receptors function as "gatekeepers" targeting dense apoB100-containing lipoproteins for degradation. In addition, these results suggest that very low levels of MTP are insufficient to mediate the second step but sufficient for the first step of VLDL assembly.

Induction of atherosclerosis by low-fat, semisynthetic diets in LDL receptor-deficient C57BL/6J and FVB/NJ mice: comparison of lesions of the aortic root, brachiocephalic artery, and whole aorta (en face measurement)

OBJECTIVE

A semisynthetic diet with varying amounts of cholesterol was used to achieve hypercholesterolemia and atherosclerosis in LDL receptor-deficient (LDLR-/-) mice. Atherosclerotic lesions were measured as cross-sectional area at the aortic root and brachiocephalic artery and by en face analysis of aortic lesion area in 209 male and female animals on the C57BL/6J (B6.LDLR-/-) and FVB/NJ (FVB.LDLR-/-) backgrounds.

METHODS AND RESULTS

The semisynthetic diet containing 4.3% fat and 0.00% or 0.02% cholesterol was sufficient to induce hypercholesterolemia (12.6+/-2.4 mmol/L) and atherosclerosis in B6.LDLR-/- mice at the aortic root (98,980+/-37 727 microm2) and brachiocephalic artery (12,039+/-12,750 microm2) but did not produce significant lesions in the aorta measurable by the en face method. Raising dietary cholesterol to 0.15%, 0.30%, or 0.50% more than doubled plasma cholesterol levels (35.9+/-8.5 mmol/L) and resulted in significant en face lesions. It also led to a significant increase in atherosclerotic lesion area at the aortic root (547,753+/-182,151 microm2) and brachiocephalic arteries (125,666+/-59,339 microm2). Although FVB.LDLR-/- mice developed comparable cholesterol levels, they were relatively atherosclerosis resistant and had many-fold smaller lesions.

CONCLUSIONS

These results should aid investigations of atherosclerosis in LDLR-/- mice by informing the selection of diet to be used and the location of lesions to be scored.

Alpha2beta1 integrin and development of atherosclerosis in a mouse model: assessment of risk

OBJECTIVE

The alpha2beta1 integrin serves as a collagen or collagen/laminin receptor on many cell types, including endothelial cells and platelets. Many studies indicate that the alpha2beta1 integrin is a critical mediator of platelet adhesion to collagen. Epidemiologic studies suggest a direct correlation between the genetically determined platelet surface density of the alpha2beta1 integrin and the risk of thrombotic diseases, such as myocardial infarction and stroke, in the young, which are well-established complications of atherosclerosis. We have now used the alpha2beta1 integrin-deficient mouse to evaluate the contributions of the alpha2beta1 integrin to the development of atherosclerosis.

METHODS AND RESULTS

We generated wild-type (alpha2+/+) or alpha2beta1 integrin-deficient (alpha2-/-) mice that were also deficient in the apolipoprotein E (ApoE) gene (ApoE-/-) and compared atherosclerotic lesion development in alpha2+/+ ApoE-/- and alpha2-/- ApoE-/- mice that were fed a high-fat, cholesterol-containing diet for 6 or 15 weeks. Total lesional area did not differ significantly between the alpha2-null animals and the wild-type animals at either 6 or 15 weeks.

CONCLUSIONS

Our results suggest that risk for arterial thrombotic disease associated with high-level alpha2beta1 integrin expression is not attributable to enhanced development of atherosclerosis per se but may rather be a consequence of thrombotic complications at the plaques.

Hematopoietic NF-kappaB1 deficiency results in small atherosclerotic lesions with an inflammatory phenotype

Atherosclerosis is a chronic inflammatory disease characterized by the accumulation of lipid-laden macrophages in the vessel wall. One of the major transcription factors in inflammation is nuclear factor kappaB (NF-kappaB), and we have studied its role in the development of atherosclerosis. Bone marrow from mice targeted in the NF-kappaB1 gene encoding for the p50 subunit was used to reconstitute irradiated LDLR(-/-) mice as a model for atherosclerosis. After feeding the mice a high-fat diet, those deficient in NF-kappaB1 had a 41% lower rate of atherosclerosis than control mice, as judged by the sizes of the lesions. Furthermore, in the absence of NF-kappaB1, the lesions were characterized by an inflammatory phenotype, contained increased numbers of small cells, and were almost devoid of normal foam cells. In vitro studies using bone marrow (BM)-derived macrophages showed that macrophages lacking p50 had a prolonged production of tumor necrosis factor (TNF) in response to lipopolysaccharide (LPS), and other cytokines were also affected. Interestingly, the uptake of oxidized low-density lipoprotein (LDL) was greatly reduced in activated p50-deficient macrophages, probably because of a reduction in the expression of scavenger receptor class A. The effects on atherosclerosis might have resulted from the changes in cytokine production and the uptake of modified lipoproteins, making p50 a pivotal regulator of atherogenesis.

Overexpression of sterol regulatory element-binding protein-1a in mouse adipose tissue produces adipocyte hypertrophy, increased fatty acid secretion, and fatty liver

Sterol regulatory element-binding proteins (SREBPs) are a family of membrane-bound transcription factors that regulate cholesterol and fatty acid homeostasis. In mammals, three SREBP isoforms designated SREBP-1a, SREBP-1c, and SREBP-2 have been identified. SREBP-1a and SREBP-1c are derived from the same gene by virtue of alternatively spliced first exons. SREBP-1a has a longer transcriptional activation domain and is a more potent transcriptional activator than SREBP-1c in cultured cells and liver. Here, we describe the physiologic consequences of overexpressing the nuclear form of SREBP-1a (nSREBP-1a) in adipocytes of mice using the adipocyte-specific aP2 promoter (aP2-nSREBP-1a). The transgenic aP2-nSREBP-1a mice developed markedly enlarged white and brown adipocytes that were fully differentiated. Adipocytes isolated from aP2-nSREBP-1a mice had significantly increased rates of fatty acid synthesis and enhanced fatty acid secretion. The increased production and release of fatty acids from adipocytes led, in turn, to a fatty liver. Overexpression of the alternative SREBP-1 isoform, nSREBP-1c, in adipose tissue inhibits adipocyte differentiation; as a result, the transgenic nSREBP-1c mice develop a syndrome resembling human lipodystrophy, which includes a loss of peripheral white adipose tissue, diabetes, and fatty livers (Shimomura, I., Hammer, R. E., Richardson, J. A., Ikemoto, S., Bashmakov, Y., Goldstein, J. L., and Brown, M. S. (1998) Genes Dev. 12, 3182-3194). In striking contrast, nSREBP-1a overexpression in fat resulted in the hypertrophy of fully differentiated adipocytes, no diabetes, and mild hepatic steatosis. These results suggest that nSREBP-1a and nSREBP-1c have distinct roles in adipocyte fat metabolism in vivo.

Induction of fatal inflammation in LDL receptor and ApoA-I double-knockout mice fed dietary fat and cholesterol

Atherogenic response to dietary fat and cholesterol challenge was evaluated in mice lacking both the LDL receptor (LDLr(-/-)) and apoA-I (apoA-I(-/-)) gene, LDLr(-/-)/apoA-I(-/-) or double-knockout mice. Gender- and age-matched LDLr(-/-)/apoA-I(-/-) mice were fed a diet consisting of 0.1% cholesterol and 10% palm oil for 16 weeks and compared to LDLr(-/-) mice or single-knockout mice. The LDLr(-/-) mice showed a 6- to 7-fold increase in total plasma cholesterol (TPC) compared to their chow-fed mice counterparts, while LDLr(-/-)/apoA-I(-/-) mice showed only a 2- to 3-fold increase in TPC compared to their chow-fed controls. This differential response to the atherogenic diet was unanticipated, since chow-fed LDLr(-/-) and LDLr(-/-)/apoA-I(-/-) mice began the study with similar LDL levels and differed primarily in their HDL concentration. The 6-fold diet-induced increase in TPC observed in the LDLr(-/-) mice occurred mainly in VLDL/LDL and not in HDL. Mid-study plasma samples taken after 8 weeks of diet feeding showed that LDLr(-/-) mice had TPC concentrations approximately 60% of their 16-week level, while the LDLr(-/-)/apoA-I(-/-) mice had reached 100% of their 16-week TPC concentration after only 8 weeks of diet. Male LDLr(-/-) mice showed similar aortic cholesterol levels to male LDLr(-/-)/apoA-I(-/-) mice despite a 4-fold higher VLDL/LDL concentration in the LDLr(-/-) mice. A direct comparison of the severity of aortic atherosclerosis between female LDLr(-/-) and LDLr(-/-)/apoA-I(-/-) mice was compromised due to the loss of female LDLr(-/-)/apoA-I(-/-) mice between 10 and 14 weeks into the study. Diet-fed female and, with time, male LDLr(-/-)/apoA-I(-/-) mice suffered from severe ulcerated cutaneous xanthomatosis. This condition, combined with a complete depletion of adrenal cholesterol, manifested in fatal wasting of the affected mice. In conclusion, LDLr(-/-) and LDLr(-/-)/apoA-I(-/-) mice showed dramatic TPC differences in response to dietary fat and cholesterol challenge, while despite these differences both genotypes accumulated similar levels of aortic cholesterol.

Dyslipidaemia

The lowering of serum cholesterol is increasingly recognised as essential in the prevention of coronary heart disease and other atherosclerotic disease. The success of statin trials and the need to deploy these drugs effectively in the population has led increasingly to the identification of many people whose serum cholesterol, triglycerides, and HDL-cholesterol require clinical assessment, and frequently treatment. Lipid disorders are mainly straightforward, but some are complex or resistant to simple treatment strategies. I have reviewed the clinical manifestations of disordered lipid metabolism (dyslipidaemia) and its management.

Normal sorting but defective endocytosis of the low density lipoprotein receptor in mice with autosomal recessive hypercholesterolemia

Autosomal recessive hypercholesterolemia (ARH) is a genetic form of hypercholesterolemia that clinically resembles familial hypercholesterolemia (FH). As in FH, the rate of clearance of circulating low density lipoprotein (LDL) by the LDL receptor (LDLR) in the liver is markedly reduced in ARH. Unlike FH, LDL uptake in cultured fibroblasts from ARH patients is normal or only slightly impaired. The gene defective in ARH encodes a putative adaptor protein that has been implicated in linking the LDLR to the endocytic machinery. To determine the role of ARH in the liver, ARH-deficient mice were developed. Plasma levels of LDL-cholesterol were elevated in the chow-fed Arh-/- mice (83 +/- 8 mg/dl versus 68 +/- 8 mg/dl) but were lower than those of mice expressing no LDLR (Ldlr-/-) (197 +/- 8 mg/dl). Cholesterol feeding elevated plasma cholesterol levels in both strains. The fractional clearance rate of radiolabeled LDL was reduced to similar levels in the Arh-/- and Ldlr-/- mice, whereas the rate of removal of alpha2-macroglobulin by the LDLR-related protein, which also interacts with ARH, was unchanged. Immunolocalization studies revealed that a much greater proportion of immunodetectable LDLR, but not LDLR-related protein, was present on the sinusoidal surface of hepatocytes in the Arh-/- mice. Taken together, these results are consistent with ARH playing a critical and specific role in LDLR endocytosis in the liver.

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.

Lipoprotein subclass profiles of hyperlipidemic diabetic mice measured by nuclear magnetic resonance spectroscopy

Dyslipidemia accelerates vascular complications of diabetes. Nuclear magnetic resonance (NMR) analysis of lipoprotein subclasses is used to evaluate a mouse model of human familial hypercholesterolemia +/- streptozotocin (STZ)-induced diabetes. A double knockout (DKO) mouse (low-density lipoprotein receptor [LDLr] -/-; apolipoprotein B [apoB] mRNA editing catalytic polypeptide-1 [Apobec1] -/-) was studied. Wild-type (WT) and DKO mice received sham or STZ injections at age 7 weeks, yielding control (WT-C, DKO-C) and diabetic (WT-D, DKO-D) groups. Fasting serum was collected when the mice were killed (age 40 weeks) for Cholestech analysis (Cholestech Corp, Hayward, CA) and NMR lipoprotein subclass profile. By Cholestech, fasting triglyceride and total cholesterol increased in DKO-C versus WT-C. Diabetes further increased total cholesterol in DKO. High-density lipoprotein cholesterol (HDL-C) was similar among all groups. NMR revealed that LDL in all groups was present in a subclass the size of large human LDL and was increased 48-fold in DKO-C versus WT-C animals, but was unaffected by diabetes. HDL was found in a subclass equivalent to large human HDL, and was similar among groups. In conclusion, NMR analysis reveals lipoprotein subclass distributions and the effects of genetic modification and diabetes in mice, but lack of particles the size of human small LDL and small HDL may limit the relevance of the present animal model to human disease.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

The low density lipoprotein receptor-related protein complexes with cell surface heparan sulfate proteoglycans to regulate proteoglycan-mediated lipoprotein catabolism

It has been proposed that clearance of cholesterol-enriched very low density lipoprotein (VLDL) particles occurs through a multistep process beginning with their initial binding to cell-surface heparan sulfate proteoglycans (HSPG), followed by their uptake into cells by a receptor-mediated process that utilizes members of the low density lipoprotein receptor (LDLR) family, including the low density lipoprotein receptor-related protein (LRP). We have further explored the relationship between HSPG binding of VLDL and its subsequent internalization by focusing on the LRP pathway using a cell line deficient in LDLR. In this study, we show that LRP and HSPG are part of a co-immunoprecipitable complex at the cell surface demonstrating a novel association for these two cell surface receptors. Cell surface binding assays show that this complex can be disrupted by an LRP-specific ligand binding antagonist, which in turn leads to increased VLDL binding and degradation. The increase in VLDL binding results from an increase in the availability of HSPG sites as treatment with heparinase or competitors of glycosaminoglycan chain addition eliminated the augmented binding. From these results we propose a model whereby LRP regulates the availability of VLDL binding sites at the cell surface by complexing with HSPG. Once HSPG dissociates from LRP, it is then able to bind and internalize VLDL independent of LRP endocytic activity. We conclude that HSPG and LRP together participate in VLDL clearance by means of a synergistic relationship.

Infectious delivery of a 135-kb LDLR genomic locus leads to regulated complementation of low-density lipoprotein receptor deficiency in human cells

The ability to deliver efficiently a complete genomic DNA locus to human and rodent cells will likely find widespread application in functional genomic studies and novel gene therapy protocols. In contrast to a cDNA expression cassette, the use of a complete genomic DNA locus delivers a transgene intact with its native promoter, the exons, all the intervening introns, and the regulatory regions. The presence of flanking, noncoding genomic DNA sequences could prove critical for prolonged and appropriate gene expression. We have recently developed a technology for the rapid conversion of bacterial artificial chromosome (BAC) clones into high-capacity herpes simplex virus-based amplicon vectors. Here, we express the human low-density lipoprotein receptor (LDLR), mutated in familial hypercholesterolemia (FH), from a 135-kb BAC insert. The infectious LDLR genomic locus vectors were shown to express at physiologically appropriate levels in three contexts. First, the LDLR locus was expressed appropriately in the ldl(-/-)a7 Chinese hamster ovary (CHO) cell line immediately following infectious delivery; second, the locus was maintained within a replicating episomal vector and expressed at broadly physiological levels in CHO cells for 3 months following infectious delivery; and third, the locus was efficiently expressed in human fibroblasts derived from FH patients. Finally, we show that the infectious LDLR locus retains classical expression regulation by sterol levels in human cells. This long-term expression and physiological regulation of LDLR prepares the way for in vivo functional studies of infectious delivery of BAC inserts.

Human rhinovirus type 2 is internalized by clathrin-mediated endocytosis

Using several approaches, we investigated the importance of clathrin-mediated endocytosis in the uptake of human rhinovirus serotype 2 (HRV2). By means of confocal immunofluorescence microscopy, we show that K(+) depletion strongly reduces HRV2 internalization. Viral uptake was also substantially reduced by extraction of cholesterol from the plasma membrane with methyl-beta-cyclodextrin, which can inhibit clathrin-mediated endocytosis. In accordance with these data, overexpression of dynamin K44A in HeLa cells prevented HRV2 internalization, as judged by confocal immunofluorescence microscopy, and strongly reduced infection. We also demonstrate that HRV2 bound to the surface of HeLa cells is localized in coated pits but not in caveolae. Finally, transient overexpression of the specific dominant-negative inhibitors of clathrin-mediated endocytosis, the SH3 domain of amphiphysin and the C-terminal domain of AP180, potently inhibited internalization of HRV2. Taken together, these results indicate that HRV2 uses clathrin-mediated endocytosis to infect cells.

Intravenous gene therapy for familial hypercholesterolemia using ligand-facilitated transfer of a liposome:LDL receptor gene complex

Familial hypercholesterolemia (FH) is an autosomal dominant disorder because of a mutation in the low-density lipoprotein receptor (LDLR) gene. Although lowering plasma cholesterol decreases the risk of coronary artery disease, FH patients respond poorly to pharmacologic treatment. Transferrin-facilitated intravenous transfer of a cationic liposome rabbit LDLR cDNA complex alleviated hypercholesterolemia in Watanabe Heritable Hyperlipidemic Rabbits (WHHL), an animal model of FH. Intravenous treatment dose dependently decreased plasma total and LDL cholesterol levels, correlating with an increased level of LDLR mRNA transcripts in leukocytes. Transferrin-facilitated intravenous delivery of cationic liposome LDLR gene complexes could serve as an important adjunct therapy for the treatment of FH.

Quantitative trait loci that determine lipoprotein cholesterol levels in DBA/2J and CAST/Ei inbred mice

To investigate genetic contributions to individual variations of lipoprotein cholesterol concentrations, we performed quantitative trait locus/loci (QTL) analyses of an intercross of CAST/Ei and DBA/2J inbred mouse strains after feeding a high-cholesterol cholic acid diet for 10 weeks. In total, we identified four QTL for HDL cholesterol. Three of these were novel and were named Hdlq10 [20 centimorgans (cM), chromosome 4], Hdlq11 (48 cM, chromosome 6), and Hdlq12 (68 cM, chromosome 6). The fourth QTL, Hdl1 (48 cM, chromosome 2), confirmed a locus discovered previously using a breeding cross that employed different inbred mouse strains. In addition, we identified one novel QTL for total and non-HDL cholesterol (8 cM, chromosome 9) that we named Chol6. Hdlq10, colocalized with a mutagenesis-induced point mutation (Lch), also affecting HDL. We provide molecular evidence for Abca1 as the gene underlying Hdlq10 and Ldlr as the gene underlying Chol6 that, coupled with evidence generated by other researchers using knockout and transgenic models, causes us to postulate that polymorphisms of these genes, different from the mutations leading to Tangier's disease and familial hypercholesterolemia, respectively, are likely primary genetic determinants of quantitative variation of lipoprotein levels in mice and, by orthology, in the human population.

Activating signal cointegrator 2 required for liver lipid metabolism mediated by liver X receptors in mice

Activating signal cointegrator 2 (ASC-2), a cancer-amplified transcriptional coactivator of nuclear receptors and many other transcription factors, contains two LXXLL-type nuclear receptor interaction domains. Interestingly, the second LXXLL motif is highly specific to the liver X receptors (LXRs). In cotransfection, DN2, an ASC-2 fragment encompassing this motif, exerts a potent dominant-negative effect on transactivation by LXRs, which is rescued by ectopic coexpression of the full-length ASC-2 but not by other LXXLL-type coactivators, such as SRC-1 and TRAP220. In contrast, DN2/m, in which the LXXLL motif is mutated to LXXAA to abolish the interactions with LXRs, is without any effect. Accordingly, expression of DN2, but not DN2/m, in transgenic mice results in phenotypes that are highly homologous to those previously observed with LXRalpha(-/-) mice, including a rapid accumulation of large amounts of cholesterol and down-regulation of the known lipid-metabolizing target genes of LXRalpha in the liver upon being fed a high-cholesterol diet. These results identify ASC-2 as a physiologically important transcriptional coactivator of LXRs and demonstrate its pivotal role in the liver lipid metabolism.

Familial hypercholesterolemia--improving treatment and meeting guidelines

Familial hypercholesterolemia (FH) is a common, inherited disorder that affects around one in 500 individuals in the heterozygous form. By the year 2001, more people in the US had FH than were infected by the human immunodeficiency virus. The disease is caused by mutations within the low-density lipoprotein (LDL) receptor gene. FH is associated with elevated plasma LDL-cholesterol (LDL-C) levels, xanthomatosis, early onset of atherosclerosis and premature cardiac death. Patients with heterozygous FH commonly have plasma LDL-C levels that are two-fold higher than normal, while homozygotes have four- to five-fold elevations in plasma LDL-C. Although FH patients have a high risk of developing premature coronary heart disease (CHD), they remain underdiagnosed and undertreated. Early detection of FH is critical to prolonging the life of these patients. Once identified, patients with heterozygous FH can be placed on a diet and drug management program. As the most efficacious and well-tolerated agents, hydroxy methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are usually the drugs of first choice; bile acid sequestrants, niacin, and occasionally fibrates may be used as supplemental agents. Statins may also provide a realistic option for the treatment of some FH homozygotes with genes that produce partially functional LDL receptors. However, a number of patients are still failing to reach treatment guidelines even with the most effective of the currently available statins. The development of new more efficacious statins or the use of new combination therapies such as statins with the cholesterol absorption inhibitor, ezetimibe may help to reduce the current problem of undertreatment in FH patients.

LRP: role in vascular wall integrity and protection from atherosclerosis

Vascular smooth muscle cell (SMC) proliferation and migration are important events in the development of atherosclerosis. The low-density lipoprotein receptor-related protein (LRP1) mediates suppression of SMC migration induced by platelet-derived growth factor (PDGF). Here we show that LRP1 forms a complex with the PDGF receptor (PDGFR). Inactivation of LRP1 in vascular SMCs of mice causes PDGFR overexpression and abnormal activation of PDGFR signaling, resulting in disruption of the elastic layer, SMC proliferation, aneurysm formation, and marked susceptibility to cholesterol-induced atherosclerosis. The development of these abnormalities was reduced by treatment with Gleevec, an inhibitor of PDGF signaling. Thus, LRP1 has a pivotal role in protecting vascular wall integrity and preventing atherosclerosis by controlling PDGFR activation.

Lipid-lowering effects of TAK-475, a squalene synthase inhibitor, in animal models of familial hypercholesterolemia

The lipid-lowering effects of 1-[2-[(3R,5S)-1-(3-acetoxy-2,2-dimethylpropyl)-7-chloro-1,2,3,5-tetrahydro-2-oxo-5-(2,3-dimethoxyphenyl)-4,1-benzoxazepine-3-yl] acetyl] piperidin-4-acetic acid (TAK-475), a novel squalene synthase inhibitor, were examined in two models of familial hypercholesterolemia, low-density lipoprotein (LDL) receptor knockout mice and Watanabe heritable hyperlipidemic (WHHL) rabbits. Two weeks of treatment with TAK-475 in a diet admixture (0.02% and 0.07%; approximately 30 and 110 mg/kg/day, respectively) significantly lowered plasma non-high-density lipoprotein (HDL) cholesterol levels by 19% and 41%, respectively, in homozygous LDL receptor knockout mice. The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, simvastatin and atorvastatin (in 0.02% and 0.07% admixtures), also reduced plasma levels of non-HDL cholesterol. In homozygous WHHL rabbits, 4 weeks of treatment with TAK-475 (0.27%; approximately 100 mg/kg/day) lowered plasma total cholesterol, triglyceride and phospholipid levels by 17%, 52% and 26%, respectively. In Triton WR-1339-treated rabbits, TAK-475 inhibited to the same extent the rate of secretion from the liver of the cholesterol, triglyceride and phospholipid components of very-low-density lipoprotein (VLDL). These results suggest that the lipid-lowering effects of TAK-475 in WHHL rabbits are based partially on the inhibition of secretion of VLDL from the liver. TAK-475 had no effect on plasma aspartate aminotransferase and alanine aminotransferase activities. Thus, the squalene synthase inhibitor TAK-475 revealed lipid-lowering effects in both LDL receptor knockout mice and WHHL rabbits.

Deficiency of cathepsin S reduces atherosclerosis in LDL receptor-deficient mice

Human atherosclerotic lesions overexpress the lysosomal cysteine protease cathepsin S (Cat S), one of the most potent mammalian elastases known. In contrast, atheromata have low levels of the endogenous Cat S inhibitor cystatin C compared with normal arteries, suggesting involvement of this protease in atherogenesis. The present study tested this hypothesis directly by crossing Cat S-deficient (CatS(-/-)) mice with LDL receptor-deficient (LDLR(-/-)) mice that develop atherosclerosis on a high-cholesterol diet. Compared with LDLR(-/-) mice, double-knockout mice (CatS(-/-)LDLR(-/-)) developed significantly less atherosclerosis, as indicated by plaque size (plaque area and intimal thickening) and stage of development. These mice also had markedly reduced content of intimal macrophages, lipids, smooth muscle cells, collagen, CD4(+) T lymphocytes, and levels of IFN-gamma. CatS(-/-)LDLR(-/-) monocytes showed impaired subendothelial basement membrane transmigration, and aortas from CatS(-/-)LDLR(-/-) mice had preserved elastic laminae. These findings establish a pivotal role for Cat S in atherogenesis.

Dietary sodium chloride restriction enhances aortic wall lipid storage and raises plasma lipid concentration in LDL receptor knockout mice

This study aimed at measuring the influence of a low salt diet on the development of experimental atherosclerosis in moderately hyperlipidemic mice. Experiments were carried out on LDL receptor (LDLR) knockout (KO) mice, or apolipoprotein E (apoE) KO mice on a low sodium chloride diet (LSD) as compared with a normal salt diet (NSD). On LSD, the rise of the plasma concentrations of TG and nonesterified fatty acid (NEFA) was, respectively, 19% and 34% in LDLR KO mice, and 21% and 35% in apoE KO mice, and that of plasma cholesterol was limited to the LDLR KO group alone (15%). Probably due to the apoE KO severe hypercholesterolemia, the arterial inner-wall fat storage was not influenced by the diet salt content and was far more abundant in the apoE KO than in the LDLR KO mice. However, in the less severe hypercholesterolemia of the LDLR KO mice, lipid deposits on the LSD were greater than on the NSD. Arterial fat storage correlated with NEFA concentrations in the LDLR KO mice alone (n = 14, P = 0.0065). Thus, dietary sodium chloride restriction enhances aortic wall lipid storage in moderately hyperlipidemic mice.

Invited review: Identifying new mouse models of cardiovascular disease: a review of high-throughput screens of mutagenized and inbred strains

The mouse is a proven model for studying human disease. Many strains exist that exhibit either natural or engineered genetic variation and thereby enable the elucidation of pathways involved in the development of cardiovascular disease. Although those mouse models have been fundamental to advancing our knowledge base, we are still at an early stage in understanding how genes contribute to complex disorders. There remains a need for new animal models that closely represent human disease. To expedite their development, we have established the Center for New Mouse Models of Heart, Lung, Blood, and Sleep Disorders at The Jackson Laboratory. We are using a phenotype-driven approach to identify mutations leading to atherosclerosis, hypertension, obesity, blood disorders, lung dysfunction, thrombosis, and disordered sleep. Our high-throughput, comprehensive phenotyping draws from two sources for new models: 1) the natural variation among over 40 inbred mouse strains and 2) chemically induced, whole-genome mutagenized mice. Here, we review our cardiovascular screens and present some hypertensive, obese, and cardiovascular models identified with this approach.

Severe hypercholesterolemia, impaired fat tolerance, and advanced atherosclerosis in mice lacking both low density lipoprotein receptor-related protein 5 and apolipoprotein E

LDL receptor-related protein 5 (LRP5) plays multiple roles, including embryonic development and bone accrual development. Recently, we demonstrated that LRP5 is also required for normal cholesterol metabolism and glucose-induced insulin secretion. To further define the role of LRP5 in the lipoprotein metabolism, we compared plasma lipoproteins in mice lacking LRP5, apolipoprotein E (apoE), or both (apoE;LRP5 double knockout). On a normal chow diet, the apoE;LRP5 double knockout mice (older than 4 months of age) had approximately 60% higher plasma cholesterol levels compared with the age-matched apoE knockout mice. In contrast, LRP5 deficiency alone had no significant effects on the plasma cholesterol levels. High performance liquid chromatography analysis of plasma lipoproteins revealed that cholesterol levels in the very low density lipoprotein and low density lipoprotein fractions were markedly increased in the apoE;LRP5 double knockout mice. There were no apparent differences in the pattern of apoproteins between the apoE knockout mice and the apoE;LRP5 double knockout mice. The plasma clearance of intragastrically loaded triglyceride was markedly impaired by LRP5 deficiency. The atherosclerotic lesions of the apoE;LRP5 double knockout mice aged 6 months were approximately 3-fold greater than those in the age-matched apoE-knockout mice. Furthermore, histological examination revealed highly advanced atherosclerosis, with remarkable accumulation of foam cells and destruction of the internal elastic lamina in the apoE;LRP5 double knockout mice. These data suggest that LRP5 mediates both apoE-dependent and apoE-independent catabolism of plasma lipoproteins.

Deficiency of cathepsin S reduces atherosclerosis in LDL receptor-deficient mice

Human atherosclerotic lesions overexpress the lysosomal cysteine protease cathepsin S (Cat S), one of the most potent mammalian elastases known. In contrast, atheromata have low levels of the endogenous Cat S inhibitor cystatin C compared with normal arteries, suggesting involvement of this protease in atherogenesis. The present study tested this hypothesis directly by crossing Cat S-deficient (CatS(-/-)) mice with LDL receptor-deficient (LDLR(-/-)) mice that develop atherosclerosis on a high-cholesterol diet. Compared with LDLR(-/-) mice, double-knockout mice (CatS(-/-)LDLR(-/-)) developed significantly less atherosclerosis, as indicated by plaque size (plaque area and intimal thickening) and stage of development. These mice also had markedly reduced content of intimal macrophages, lipids, smooth muscle cells, collagen, CD4(+) T lymphocytes, and levels of IFN-gamma. CatS(-/-)LDLR(-/-) monocytes showed impaired subendothelial basement membrane transmigration, and aortas from CatS(-/-)LDLR(-/-) mice had preserved elastic laminae. These findings establish a pivotal role for Cat S in atherogenesis.

Eliminating atherogenesis in mice by switching off hepatic lipoprotein secretion

BACKGROUND

LDL receptor-deficient "apolipoprotein (apo)-B100-only" mice (Ldlr-/-Apob100/100 have elevated LDL cholesterol levels on a chow diet and develop severe aortic atherosclerosis. We hypothesized that both the hypercholesterolemia and the susceptibility to atherosclerosis could be eliminated by switching off hepatic lipoprotein production.

METHODS AND RESULTS

We bred Ldlr-/-Apob100/100 mice that were homozygous for a conditional allele for Mttp (the gene for microsomal triglyceride transfer protein) and the inducible Mx1-Cre transgene. In these animals, which we called "Reversa mice," the hypercholesterolemia could be reversed, without modifying the diet or initiating a hypolipidemic drug, by the transient induction of Cre expression in the liver. After Cre induction, hepatic Mttp expression was virtually eliminated (as judged by quantitative real-time PCR), hepatic lipoprotein secretion was abolished (as judged by electron microscopy), and LDLs were virtually eliminated from the plasma. Intestinal lipoprotein production was unaffected. In mice fed a chow diet, Cre induction reduced plasma cholesterol levels from 233.9+/-46.0 to 37.2+/-6.5 mg/dL. In mice fed a high-fat diet, cholesterol levels fell from 525.7+/-32.2 to 100.6+/-14.3 mg/dL. The elimination of hepatic lipoprotein production completely prevented both the development of atherosclerosis and the changes in gene expression that accompany atherogenesis.

CONCLUSIONS

We developed mice in which hypercholesterolemia can be reversed with a genetic switch. These mice will be useful for understanding gene-expression changes that accompany the reversal of hypercholesterolemia and atherosclerosis.

Molecular biology and gene transfer in atherosclerosis in the stenting era

Atherosclerosis is the major cause of death in the developed world. Understanding the pathogenesis of atherosclerosis has been a major challenge to cardiovascular research over the past several decades. During this period a number of advances in various scientific disciplines has increased our understanding of this disease. These include improved understanding of the structural and functional components of normal vessel wall and more recently the use of cell biology and molecular biology techniques to elucidate the pathogenesis of atherosclerosis. None of these advances has been more dramatic nor has potentially more far reaching consequences as the application of molecular biology and gene technology to the practice of cardiovascular medicine. These developments have already opened new and exciting areas of vascular research and may in the future provide for earlier identification of genetic predisposition to atherosclerosis, strategic planning of preventive therapy and more tailored pharmacologic approaches for established disease.

A rapid PCR method of genotyping LDL receptor mutations in WHHL rabbits

WHHL rabbits are a valuable model for the study of human familial hypercholesterolemia and atherosclerosis. To use this animal model, it is often necessary to confirm LDL receptor status in WHHL rabbits. Here, we described a simple and rapid PCR method to detect LDL mutations in WHHL rabbits.

Influence of interferon-gamma on the extent and phenotype of diet-induced atherosclerosis in the LDLR-deficient mouse

OBJECTIVE

The aim of this study was to investigate the influence of interferon-gamma (IFN-gamma) on atherosclerosis in low density lipoprotein receptor (LDLR)-null mice.

METHODS AND RESULTS

We cross-bred IFN-gamma-deficient mice with LDLR-null mice and analyzed lipoprotein profiles and atherosclerosis in the compound mutant progeny after 8 and 20 weeks on a cholesterol-enriched diet. IFN-gamma deficiency did not affect serum cholesterol levels or lipoprotein profiles, but it did affect the extent and phenotype of atherosclerosis. Atherosclerotic lesions in IFN-gamma-deficient mice were reduced by 75% in the aortic arch and by 46% in the descending aorta compared with control mice after 8 weeks on the diet. After 20 weeks, arch lesions were reduced by 43%, and descending aorta lesions were reduced by 65% in IFN-gamma-deficient mice compared with controls. At 8 weeks, percent lesional macrophage and smooth muscle content was significantly less in the IFN-gamma-deficient mice, but not at 20 weeks. Although there were fewer class II major histocompatibility complex-positive cells in the lesions of IFN-gamma-deficient animals compared with controls, class II major histocompatibility complex expression on endothelial cells overlying lesions persisted in the absence of IFN-gamma.

CONCLUSIONS

These data provide direct evidence that IFN-gamma influences atherosclerosis development and phenotype in the LDLR-deficient mouse, independent of changes in blood lipoprotein profiles.

Expression cloning and characterization of a novel glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein, GPI-HBP1

By expression cloning using fluorescent-labeled high density lipoprotein (HDL), we isolated two clones that conferred the cell surface binding of HDL. Nucleotide sequence of the two clones revealed that one corresponds to scavenger receptor class B, type 1 (SRBI) and the other encoded a novel protein with 228 amino acids. The primary structure of the newly identified HDL-binding protein resembles GPI-anchored proteins consisting of an N-terminal signal sequence, an acidic region with a cluster of aspartate and glutamate residues, an Ly-6 motif highly conserved among the lymphocyte antigen family, and a C-terminal hydrophobic region. This newly identified HDL-binding protein designated GPI-anchored HDL-binding protein 1 (GPI-HBP1), was susceptible to phosphatidylinositol-specific phospholipase C treatment and binds HDL with high affinity (calculated K(d) = 2-3 microg/ml). Similar to SRBI, GPI-HBP1 mediates selective lipid uptake but not the protein component of HDL. Among various ligands for SRBI, HDL was most preferentially bound to GPI-HBP1. In contrast to SRBI, GPI-HBP1 lacked HDL-dependent cholesterol efflux. The GPI-HBP1 transcripts were detected with the highest levels in heart and, to a much lesser extent, in lung and liver. In situ hybridization revealed the accumulation of GPI-HBP1 transcripts in cardiac muscle cells, hepatic Kupffer cells and sinusoidal endothelium, and bronchial epithelium and alveolar macrophages in the lung.

Mouse models as tools for dissecting disorders of lipoprotein metabolism

The overexpression of proteins as transgenes or by adenovirus-mediated gene transfer as well as the disruption of genes by homologous DNA recombination in the mouse provide powerful tools to dissect the role of individual proteins in complex biological pathways. These and similar techniques have been widely used to characterize the function of most of the players involved in lipoprotein metabolism. These models are expected to greatly advance the finding of new therapeutic strategies for the treatment of disorders of lipoprotein metabolism.

Sepsis syndrome stimulates proximal tubule cholesterol synthesis and suppresses the SR-B1 cholesterol transporter

BACKGROUND

Previous studies demonstrate that renal cortical/proximal tubule cholesterol accumulation is part of the renal "stress response." The present study was performed to help define underlying mechanisms, using experimental sepsis as a test model.

METHODS

Male CD-1 mice and female low-density lipoprotein receptor (LDLR) knockout mice were injected with a heat-killed Escherichia coli suspension. Renal cortex and serum were obtained from these and control mice either 4, 6, or 18 hours later. Tissues samples were assayed for free cholesterol (FC), cholesteryl esters (CE), HMG CoA reductase (HMGCR) mRNA, and SR-B1 [the high-density lipoprotein (HDL) receptor/cholesterol transporter]. Statin effects on renal cortical HMGCR mRNA and FC/CE levels also were assessed. Finally, the impact of serum from septic versus normal mice on cultured proximal tubule (HK-2) cell cholesterol levels was assessed.

RESULTS

Sepsis induced approximately 30% and 300 to 500% increases in renal FC and CE content, respectively. Cholesterol accumulation was not blunted in LDLR-/- mice versus their controls. Statin therapy also did not alter sepsis-induced renal FC/CE accumulation. However, statin treatment exerted no discernible intra-renal activity (for example, no rise in renal HMGCR mRNA), despite significant extra-renal activity (25% reduction in serum cholesterol; 400% increase in hepatic HMGCR mRNA). HK-2 cells exposed to septic serum sustained a 40% cholesterol increase, compared to cells exposed to control serum. This response was completely statin inhibited, proving that de novo synthesis was involved. Sepsis markedly suppressed renal levels of SR-B1 (an FC efflux protein). Renal HMGCR mRNA did not fall despite sepsis triggered cholesterol loading, indicating a failure of negative feedback activity.

CONCLUSIONS

Sepsis-induced renal cholesterol accumulation is not simply an intrinsic renal response, since it can be enhanced by circulating "stress factors" that drive HMGCR activity. Sepsis also down-regulates SR-B1. Thus, decreased cell FC efflux, coupled with increased synthesis, may synergistically induce the post-sepsis cholesterol overload state.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Estradiol suppresses vascular monocyte chemotactic protein-1 expression during early atherogenesis

OBJECTIVE

This study was undertaken to determine whether estrogen down-regulates vascular monocyte chemotactic protein-1 expression during the development of atherosclerosis in vivo and to identify the cellular localization of monocyte chemotactic protein-1 expression under baseline conditions and in response to atherogenic stimuli.

STUDY DESIGN

Female, homozygous low-density lipoprotein-receptor-deficient mice (n = 68) in a C57BL/6 background underwent ovariectomy, were implanted subcutaneously with 17beta-estradiol or placebo pellets, and were changed to a high cholesterol (1.25%) diet. Thereafter, four mice from each group were killed weekly for 8 weeks, and their aortae were frozen for immunohistochemical analysis. The lipid deposition was identified by Sudan black B staining. Monocyte chemotactic protein-1 expression was detected with a rabbit anti-mice monocyte chemotactic protein-1 antibody, and semiquantitative analysis was performed.

RESULTS

Consistent with previous reports, estradiol resulted in diminished vascular lipid deposition (22% +/- 7% vs 15% +/- 6% at 8 weeks of gestation, P <.05). We found that the inhibition of lipid deposition in aortae of animals that were treated with estrogen is associated with a concomitant down-regulation of monocyte chemotactic protein-1 immunoreactivity in aortic endothelial and smooth muscle cells (P <.05). Serum total cholesterol concentrations did not differ between the two treatment groups, which suggests a direct effect of estradiol on the aorta.

CONCLUSION

Our findings suggest that one of the mechanisms by which estrogen down-regulates atherogenesis is by the suppression of vascular monocyte chemotactic protein-1 expression, which leads to decreased macrophage recruitment to the arterial wall early in the process.

Adenovirus and adeno-associated virus vectors

Recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) are among the most extensively used vectors in gene therapy studies to date. These two vectors share some similar features such as a broad host range and ability to infect both proliferating and quiescent cells. However, they also possess their own unique set of properties that render them particularly attractive for gene therapy applications. rAd vectors can accommodate larger inserts, mediate transient but high levels of protein expression, and can be easily produced at high titers. Development of gutted rAd vectors has further increased the cloning capacity of these vectors. The gaining popularity of rAAV use in gene therapy can be attributed to its lack of pathogenicity and added safety due to its replication defectiveness, and its ability to mediate long-term expression in a variety of tissues. Site-specific integration, as occurs with wild-type AAV, will be a unique and valuable feature if incorporated into rAAV vectors, further improving their safety. This paper describes these properties of rAd and rAAV vectors, and discusses further development and vector improvements that continue to extend the utility of these vectors, such as cell retargeting by capsid modification, differential transduction by use of serotypes, and extension of the cloning capacity of rAAV vectors by dual vector heterodimerization.

Lipoprotein receptors in the nervous system

The low-density-lipoprotein (LDL) receptor family is an evolutionarily ancient gene family of structurally closely related cell-surface receptors. Members of the family are involved in the cellular uptake of extracellular ligands and regulate diverse biological processes including lipid and vitamin metabolism and cell-surface protease activity. Some members of the family also participate in cellular signaling and regulate the development and functional maintenance of the nervous system. Here we review the roles of this family of multifunctional receptors in the nervous system and focus on recent advances toward the understanding of the mechanisms by which lipoprotein receptors and their ligands transmit and modulate signals in the brain.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Molecular approaches for the treatment of atherosclerosis

Advances in vascular biology and the study of molecular pathophysiology have enabled the design and initial testing of therapeutic principles for cardiovascular intervention at the level of gene expression. This approach can offer an avenue to greatly impact the onset and progression of vascular disease at its roots. Early translations of basic research into human clinical protocols might provide novel alternatives for patients without traditional therapeutic options and might provide means of improving and prolonging the success of standard therapies. As the understanding of the genetic basis of vascular disease continues to grow and the tools for in vivo genetic manipulation continue to improve, vascular gene therapies might someday become a part of routine patient care.

Combined effects of cholesterol reduction and apolipoprotein A-I expression on atherosclerosis in LDL receptor deficient mice

Reduction of total and LDL cholesterol reduces atherosclerosis and clinical cardiovascular events. High density lipoprotein (HDL) cholesterol levels have a strong inverse association with atherosclerosis, and overexpression of apolipoprotein A-I (apoA-I), the major protein component of HDL, reduces atherosclerosis in hypercholesterolemic animals. However, little is known about the potential for additive or synergistic effects between cholesterol reduction and apoA-I overexpression on atherosclerosis. In the current study, we tested the hypothesis that significant reduction of plasma cholesterol combined with overexpression of apoA-I would reduce atherosclerosis to a greater extent than either one alone. We used somatic gene transfer of the LDL receptor (to induce cholesterol reduction) and apoA-I in LDL receptor deficient mice fed a Western type diet and compared the combination to expression of each gene alone and to controls. Atherosclerosis was quantitated using two independent methods, by en face analysis of the entire aorta and by cross-sectional analysis of the aortic root. Although the reduction of cholesterol was transient, expression of the LDL receptor alone significantly reduced atherosclerosis by 45% in the aorta and 44% in the aortic root compared with controls. Overexpression of human apoA-I alone reduced atherosclerosis by 42% in the aorta and 44% in the aortic root compared with controls. Co-expression of the LDL receptor with apoA-I resulted in significantly higher levels of apoA-I than expression of apoA-I alone. Although co-expression of the LDL receptor and apoA-I reduced atherosclerosis by 37% in the aorta and 32% in the aortic root compared with controls, the reduction in atherosclerosis was no different than that seen with expression of the LDL receptor alone or apoA-I alone. In summary, in this relatively short-term murine model, simultaneous reduction of cholesterol and expression of apoA-I was associated with higher levels of apoA-I than expression of apoA-I alone but did not result in greater reduction in atherosclerosis compared with either one alone.

Oral tolerance with heat shock protein 65 attenuates Mycobacterium tuberculosis-induced and high-fat-diet-driven atherosclerotic lesions

OBJECTIVE

The goal of this study was to explore the efficacy of oral tolerance with heat shock protein (HSP) 65 in two apparently non-overlapping models of murine atherosclerosis.

BACKGROUND

Atherosclerosis is considered to be a chronic inflammatory process. Autoimmune mechanisms have been shown to influence atherogenesis in experimental animal models. Heat shock protein 65 is a candidate antigen thought to drive a proatherogenic immune-mediated response. Mucosal tolerance is a therapeutic means of accomplishing immune unresponsiveness toward a given antigen by feeding it before active induction of the disorder.

METHODS

Low-density lipoprotein receptor deficient mice were fed with different doses of HSP65 every other day for 10 days. Feeding with either bovine serum albumin (BSA) or phosphate buffered saline (PBS) served as control. One day after the last feeding, mice were challenged either by immunization with heat killed Mycobacterium tuberculosis or by a high fat diet.

RESULTS

Lymphocyte reactivity from mice fed with HSP65 and immunized either against HSP65 or M. tuberculosis was significantly reduced in comparison with BSA-fed mice. Moreover, co-incubation of splenocytes-from mice with tolerance induced with HSP65 but not BSA-with HSP65-reactive lymphocytes resulted in the suppression of HSP65 reactivity by the latter cells. Interleukin-4 production by HSP65-fed and immunized mice was increased upon priming with respective protein. Early atherosclerosis was attenuated in HSP65-fed mice, compared with either BSA- or PBS-fed mice, regardless of the method employed to induce fatty streaks (M. tuberculosis immunization or high-fat diet).

CONCLUSIONS

Oral tolerance induced with HSP65 could prove to be a novel means of suppressing atherogenesis.

Repeated Three-Dimensional Magnetic Resonance Imaging of Atherosclerosis Development in Innominate Arteries of Low-Density Lipoprotein Receptor-Knockout Mice

Background— In vivo methods to evaluate the size and composition of atherosclerotic lesions in animal models of atherosclerosis would assist in the testing of antiatherosclerotic drugs. We have developed an MRI method of detecting atherosclerotic plaque in the major vessels at the base of the heart in low-density lipoprotein (LDL) receptor-knockout (LDLR −/− ) mice on a high-fat diet.

Methods and Results— Three-dimensional fast spin-echo magnetic resonance images were acquired at 7 T by use of cardiac and respiratory triggering, with ≈140-μm isotropic resolution, over 30 minutes. Comparison of normal and fat-suppressed images from female LDLR −/− mice 1 week before and 8 and 12 weeks after the transfer to a high-fat diet allowed visualization and quantification of plaque development in the innominate artery in vivo. Plaque mean cross-sectional area was significantly greater at week 12 in the LDLR −/− mice (0.14±0.086 mm 2 [mean±SD]) than in wild-type control mice on a normal diet (0.017±0.031 mm 2 , P <0.01). In the LDLR −/− mice, but not control mice, increase in plaque burden at week 12 relative to week 1 was also highly significant ( P =0.001). Lumen cross section was not significantly different between time points or groups. MRI and histological assessments of plaque size were closely correlated ( R =0.8). The lumen of proximal coronary arteries could also be visualized.

Conclusions— This is the first report of in vivo detection of aortic arch atherosclerosis in any animal model. The method could significantly assist rapid evaluation of experimental antiatherosclerotic therapies.

Experimental control of pancreatic development and maintenance

To investigate the role of the HOX-like homeoprotein PDX1 in the formation and maintenance of the pancreas, we have genetically engineered mice so that the only source of PDX1 is a transgene that can be controlled by the application of tetracycline or its analogue doxycycline. In these mice the coding region for the tetracycline-regulated transactivator (tTA(off)) has replaced the coding region of the endogenous Pdx1 gene to ensure correct temporal and spatial expression of the regulatable transactivator. In the absence of doxycycline, tTA(off) activates the transcription of a bicistronic transgene encoding PDX1 and an enhanced green fluorescent protein reporter, which acts as a visual marker of transgene expression in living cells. Expression of the transgene-encoded PDX1 rescues the Pdx1-null phenotype; the pancreata of these mice develop and function normally. The rescue is conditional; doxycycline-mediated repression of the transgenic Pdx1 throughout gestation recapitulates the Pdx1 null phenotype. Moreover, application of doxycycline at mid-pancreogenesis blocks further development. Adult animals of the rescue genotype that were treated with doxycycline for 3 weeks shut off Pdx1 expression, decreased insulin production, and lost the ability to maintain glucose homeostasis. These results demonstrate the feasibility of controlling the formation of an organ during embryogenesis in utero and the maintenance of the mature organ through the experimental manipulation of a key developmental regulator.

Insulin suppression of VLDL apo B secretion is not mediated by the LDL receptor

Insulin inhibits hepatic very low density lipoprotein (VLDL) apo B secretion in rats. Current studies test whether the insulin effect is LDL receptor-mediated by examining the effect of insulin on VLDL apo B secretion in hepatocytes derived from Ldlr-/- and control mice. Primary hepatocytes were incubated overnight with media containing 14C-leucine and either 0.1nM (basal) or 200nM insulin. Afterwards, secreted VLDL B100 and B48 were quantitated. Insulin reduced 14C-labeled B100 and B48 comparably in control and Ldlr-/- hepatocytes with a 62+/-12% vs. 59+/-12% decrease in B100, and a 56+/-11% vs. 61+/-9% decrease in B48. Results indicate: (1) mouse hepatocytes respond to insulin by reducing VLDL apo B output; (2) both VLDL B100 and B48 secretion are suppressed; and (3) insulin inhibition of VLDL apo B secretion is retained in Ldlr-/- hepatocytes.

A new model to evaluate the diet-induced prothrombotic state, using He-Ne laser-induced thrombogenesis in the carotid artery of apolipoprotein E-deficient and low-density lipoprotein receptor-deficient mice

Epidemiological studies suggest the high fat content of the Western diet to be responsible for atherosclerosis and its thrombotic complications. Despite such a prevailing view, few animal experiments have so far succeeded in demonstrating enhanced thrombogenicity due to a high fat diet. Even a high fat and a very high cholesterol (1%) diet has failed to demonstrate an enhanced thrombotic reaction in rodents and rabbits. The aim of the present study was twofold. First, we wanted to establish a new, sensitive and specific thrombosis model in mice, which can then be used to study the effect of diets. Second, we intended to employ such a thrombosis model in investigations into the effect of high or low fat diets on thrombosis. The technique described uses a laser to induce thrombus formation in the exposed carotid artery of apolipoprotein E-deficient and low-density lipoprotein receptor-deficient mice. Thrombus formation was recorded on video, analysed by computer, and the size of thrombus was calculated by image analysis software. Thrombotic status was evaluated by analysing a total of 61 individual images of the thrombotic reaction, which were taken over 600 s. The severity of atherosclerosis was assessed by image analysis of the stained elastic fibers. Two kinds of diets were used: the Western type, high fat diet contained 20% fat (w/w) and 0.05% cholesterol (w/w); the low fat diet contained 7% fat, without cholesterol. These diets were on the basis of AIN93G and were given to mice for 4 or 8 weeks. The high fat diet significantly enhanced both the thrombotic reaction and the development of atherosclerosis as compared with the low fat diet.

Fatty acids differentially regulate hepatic cholesteryl ester formation and incorporation into lipoproteins in the liver of the mouse

These experiments tested the hypothesis that fatty acids (FAs) that drive cholesterol esterification also enhance sterol secretion and were undertaken using a mouse model where lipoprotein-cholesterol output by the liver could be assessed in vivo. The turnover of sterol in the animals was kept constant ( approximately 160 mg/d per kg) while the liver was enriched with the single FAs 8:0, 14:0, 18:1, or 18:2. Under these conditions, the steady-state concentration of cholesteryl ester in the liver varied 6-fold, from 1.2 to 7.9 mg/g, and the expansion of this pool was directly related to the specific FA enriching the liver (FA 18:1>18:2>8:0> 14:0). Secretion of lipoprotein-cholesterol varied 5-fold and was a linear function of the concentration of cholesteryl ester in the liver. These studies demonstrate that unsaturated FAs drive the esterification reaction and enhance lipoprotein cholesterol secretion by the liver under conditions where cholesterol balance across this organ is constant. Thus, individual FAs interact with cholesterol to profoundly regulate both the output and uptake of sterol by the liver, and these effects are articulated through the esterification reaction.