Transgenic rabbits expressing human apolipoprotein A-I in the liver

Genetically modified rabbit models for cardiovascular medicine

Genetically modified (GM) rabbits are outstanding animal models for studying human genetic and acquired diseases. As such, GM rabbits that express human genes have been extensively used as models of cardiovascular disease. Rabbits are genetically modified via prokaryotic microinjection. Through this process, genes are randomly integrated into the rabbit genome. Moreover, gene targeting in embryonic stem (ES) cells is a powerful tool for understanding gene function. However, rabbits lack stable ES cell lines. Therefore, ES-dependent gene targeting is not possible in rabbits. Nevertheless, the RNA interference technique is rapidly becoming a useful experimental tool that enables researchers to knock down specific gene expression, which leads to the genetic modification of rabbits. Recently, with the emergence of new genetic technology, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR), and CRISPR-associated protein 9 (CRISPR/Cas9), major breakthroughs have been made in rabbit gene targeting. Using these novel genetic techniques, researchers have successfully modified knockout (KO) rabbit models. In this paper, we aimed to review the recent advances in GM technology in rabbits and highlight their application as models for cardiovascular medicine.

Genetically Modified Rabbits for Cardiovascular Research

Rabbits are one of the most used experimental animals for investigating the mechanisms of human cardiovascular disease and lipid metabolism because they are phylogenetically closer to human than rodents (mice and rats). Cholesterol-fed wild-type rabbits were first used to study human atherosclerosis more than 100 years ago and are still playing an important role in cardiovascular research. Furthermore, transgenic rabbits generated by pronuclear microinjection provided another means to investigate many gene functions associated with human disease. Because of the lack of both rabbit embryonic stem cells and the genome information, for a long time, it has been a dream for scientists to obtain knockout rabbits generated by homologous recombination-based genomic manipulation as in mice. This obstacle has greatly hampered using genetically modified rabbits to disclose the molecular mechanisms of many human diseases. The advent of genome editing technologies has dramatically extended the applications of experimental animals including rabbits. In this review, we will update genetically modified rabbits, including transgenic, knock-out, and knock-in rabbits during the past decades regarding their use in cardiovascular research and point out the perspectives in future.

Transgenic Rabbit Models: Now and the Future

Transgenic rabbits have contributed to the progress of biomedical science as human disease models because of their unique features, such as the lipid metabolism system similar to humans and medium body size that facilitates handling and experimental manipulation. In fact, many useful transgenic rabbits have been generated and used in research fields such as lipid metabolism and atherosclerosis, cardiac failure, immunology, and oncogenesis. However, there have been long-term problems, namely that the transgenic efficiency when using pronuclear microinjection is low compared with transgenic mice and production of knockout rabbits is impossible owing to the lack of embryonic stem cells for gene targeting in rabbits. Despite these limitations, the emergence of novel genome editing technology has changed the production of genetically modified animals including the rabbit. We are finally able to produce both transgenic and knockout rabbit models to analyze gain- and loss-of-functions of specific genes. It is expected that the use of genetically modified rabbits will extend to various research fields. In this review, we describe the unique features of rabbits as laboratory animals, the current status of their development and use, and future perspectives of transgenic rabbit models for human diseases.

Principles and Applications of Rabbit Models for Atherosclerosis Research

Rabbits are one of the most used experimental animals for biomedical research, particularly as a bioreactor for the production of antibodies. However, many unique features of the rabbit have also made it as an excellent species for examining a number of aspects of human diseases such as atherosclerosis. Rabbits are phylogenetically closer to humans than rodents, in addition to their relatively proper size, tame disposition, and ease of use and maintenance in the laboratory facility. Due to their short life spans, short gestation periods, high numbers of progeny, low cost (compared with other large animals) and availability of genomics and proteomics, rabbits usually serve to bridge the gap between smaller rodents (mice and rats) and larger animals, such as dogs, pigs and monkeys, and play an important role in many translational research activities such as pre-clinical testing of drugs and diagnostic methods for patients. The principle of using rabbits rather than other animals as an experimental model is very simple: rabbits should be used for research, such as translational research, that is difficult to accomplish with other species. Recently, rabbit genome sequencing and transcriptomic profiling of atherosclerosis have been successfully completed, which has paved a new way for researchers to use this model in the future. In this review, we provide an overview of the recent progress using rabbits with specific reference to their usefulness for studying human atherosclerosis.

Levels of prebeta-1 high-density lipoprotein are elevated in 3 phenotypes of dyslipidemia

BACKGROUND

Prebeta-1 high-density lipoprotein (HDL) is a small subspecies of HDL that functions as the HDL quantum particle and is the principal acceptor of cholesterol effluxed from macrophages through the ATP-binding cassette transporter, ABCA1. High levels of prebeta-1 HDL are associated with increased risk of structural coronary artery disease and myocardial infarction.

OBJECTIVE

We aimed to compare prebeta-1 HDL levels in normal subjects and in 3 phenotypes of dyslipidemia.

METHODS

We studied 2435 individuals (1388 women; 1047 men). Of these, 2018 were not taking lipid-lowering medication when enrolled: 392 were normolipidemic controls; 713 had elevated levels of low-density lipoprotein cholesterol; 623 had combined hyperlipidemia; and 290 had hypertriglyceridemia.

RESULTS

Relative to controls, prebeta-1 HDL levels were increased in all 3 dyslipidemic phenotypes, particularly the combined and hypertriglyceridemia groups. This increase possibly reflects increased acceptor capacity of apolipoprotein B-100 containing lipoproteins for entropically driven transfer of cholesteryl esters from HDL via cholesteryl ester transfer protein. Multiple regression analysis revealed that the main predictor variables significantly associated with prebeta-1 HDL levels were apolipoprotein A-I (apoA-1) (β = 0.500), triglyceride (β = 0.285), HDL-C (β = -0.237), and age (β = -0.169). There was an interaction between apoA-1 and sex (female vs male; β = -0.110). Among postmenopausal women, estrogenized subjects had a similar level of prebeta-1 HDL compared to those not receiving estrogens.

CONCLUSIONS

Prebeta-1 HDL levels are elevated in the 3 most common types of hyperlipidemia and are most strongly influenced by the levels of apoA-1, triglyceride, and HDL-C.

Detection of recombinant human lactoferrin and lysozyme produced in a bitransgenic cow

Lactoferrin and lysozyme are 2 glycoproteins with great antimicrobial activity, being part of the nonspecific defensive system of human milk, though their use in commercial products is difficult because human milk is a limited source. Therefore, many investigations have been carried out to produce those proteins in biological systems, such as bacteria, yeasts, or plants. Mammals seem to be more suitable as expression systems for human proteins, however, especially for those that are glycosylated. In the present study, we developed a bicistronic commercial vector containing a goat β-casein promoter and an internal ribosome entry site fragment between the human lactoferrin and human lysozyme genes to allow the introduction of both genes into bovine adult fibroblasts in a single transfection. Embryos were obtained by somatic cell nuclear transfer, and, after 6 transferences to recipients, 3 pregnancies and 1 viable bitransgenic calf were obtained. The presence of the vector was confirmed by fluorescent in situ hybridization of skin cells. At 13 mo of life and after artificial induction of lactation, both recombinant proteins were found in the colostrum and milk of the bitransgenic calf. Human lactoferrin concentration in the colostrum was 0.0098 mg/mL and that in milk was 0.011 mg/mL; human lysozyme concentration in the colostrum was 0.0022 mg/mL and that in milk was 0.0024 mg/mL. The molar concentration of both human proteins revealed no differences in protein production of the internal ribosome entry site upstream and downstream protein. The enzymatic activity of lysozyme in the transgenic milk was comparable to that of human milk, being 6 and 10 times higher than that of bovine lysozyme present in milk. This work represents an important step to obtain multiple proteins or enhance single protein production by using animal pharming and fewer regulatory and antibiotic-resistant foreign sequences, allowing the design of humanized milk with added biological value for newborn nutrition and development. Transgenic animals can offer a unique opportunity to the dairy industry, providing starting materials suitable to develop specific products with high added value.

Experimental Biology for the Identification of Causal Pathways in Atherosclerosis

More than 60 genomic loci have been implicated by genome-wide association studies (GWAS) and exome-wide association studies as conferring an increased risk of myocardial infarction and coronary artery disease (CAD). However, the causal gene and variant is often unclear. Using the functional analysis of genetic variants in experimental animal models, we anticipate understanding which candidate gene at a specific locus is associated with atherosclerosis and revealing the underlying molecular and cellular mechanisms, ultimately leading to the identification of causal pathways in atherosclerosis and may provide novel therapeutic targets for the treatment of atherosclerotic cardiovascular disease.

Rabbit models for the study of human atherosclerosis: from pathophysiological mechanisms to translational medicine

Laboratory animal models play an important role in the study of human diseases. Using appropriate animals is critical not only for basic research but also for the development of therapeutics and diagnostic tools. Rabbits are widely used for the study of human atherosclerosis. Because rabbits have a unique feature of lipoprotein metabolism (like humans but unlike rodents) and are sensitive to a cholesterol diet, rabbit models have not only provided many insights into the pathogenesis and development of human atherosclerosis but also made a great contribution to translational research. In fact, rabbit was the first animal model used for studying human atherosclerosis, more than a century ago. Currently, three types of rabbit model are commonly used for the study of human atherosclerosis and lipid metabolism: (1) cholesterol-fed rabbits, (2) Watanabe heritable hyperlipidemic rabbits, analogous to human familial hypercholesterolemia due to genetic deficiency of LDL receptors, and (3) genetically modified (transgenic and knock-out) rabbits. Despite their importance, compared with the mouse, the most widely used laboratory animal model nowadays, the use of rabbit models is still limited. In this review, we focus on the features of rabbit lipoprotein metabolism and pathology of atherosclerotic lesions that make it the optimal model for human atherosclerotic disease, especially for the translational medicine. For the sake of clarity, the review is not an attempt to be completely inclusive, but instead attempts to summarize substantial information concisely and provide a guideline for experiments using rabbits.

Animal models of atherosclerosis

Cardiovascular disease is currently the predominant cause of mortality worldwide and its incidence is expected to increase significantly during the next decades owing to the unhealthy effects of modern lifestyle habits (e.g., obesity and lack of physical exercise). Cardiovascular death is frequently associated with acute myocardial infarction or stroke, which are generally the ultimate consequence of an underlying atherosclerotic process. Small and big animal models are valuable tools to understand the molecular mechanisms underlying atherosclerotic plaque formation and progression, as well as the occurrence of associated ischemic events. Moreover, animal models of atherosclerosis are pivotal for testing mechanistic hypothesis and for translational research, including the assessment of dietary and/or pharmacological interventions and the development of imaging technologies and interventional devices. In this chapter, we will describe the most widely used animal models that have permitted major advances in atherosclerosis research and significant improvements in the treatment and diagnosis of atherosclerotic disease.

Site-specific oxidation of apolipoprotein A-I impairs cholesterol export by ABCA1, a key cardioprotective function of HDL

The mechanisms that deprive HDL of its cardioprotective properties are poorly understood. One potential pathway involves oxidative damage of HDL proteins by myeloperoxidase (MPO) a heme enzyme secreted by human artery wall macrophages. Mass spectrometric analysis demonstrated that levels of 3-chlorotyrosine and 3-nitrotyrosine - two characteristic products of MPO - are elevated in HDL isolated from patients with established cardiovascular disease. When apolipoprotein A-I (apoA-I), the major HDL protein, is oxidized by MPO, its ability to promote cellular cholesterol efflux by the membrane-associated ATP-binding cassette transporter A1 (ABCA1) pathway is diminished. Biochemical studies revealed that oxidation of specific tyrosine and methionine residues in apoA-I contributes to this loss of ABCA1 activity. Another potential mechanism for generating dysfunctional HDL involves covalent modification of apoA-I by reactive carbonyls, which have been implicated in atherogenesis and diabetic vascular disease. Indeed, modification of apoA-I by malondialdehyde (MDA) or acrolein also markedly impaired the lipoprotein's ability to promote cellular cholesterol efflux by the ABCA1 pathway. Tandem mass spectrometric analyses revealed that these reactive carbonyls target specific Lys residues in the C-terminus of apoA-I. Importantly, immunochemical analyses showed that levels of MDA-protein adducts are elevated in HDL isolated from human atherosclerotic lesions. Also, apoA-I co-localized with acrolein adducts in such lesions. Thus, lipid peroxidation products might specifically modify HDL in vivo. Our observations support the hypotheses that MPO and reactive carbonyls might generate dysfunctional HDL in humans. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Transgenic rabbits expressing human lipoprotein lipase

To study the functions of lipoprotein lipase (LPL) in lipid and lipoprotein metabolism and the relationship between LPL and atherosclerosis, we generated transgenic rabbits expressing the human LPL gene. A total of 4045 Japanese whiterabbit embryos were microinjected with a 3.8-kb SalI/HindIII fragment containing the chicken beta-actin promoter, human LPL cDNA and rabbit beta-globin with poly (A) signals, and then transplanted into 116 recipient rabbits. Of the 166 pups born, six pups were transgenic as confirmed by Southern blot analysis. ANorthern blot analysis revealed that human LPL was expressed by a number of tissues including the heart, kidney, adrenal gland and intestine. One transgenic rabbit showed up to 3-foldincreased LPL activity in post-heparin plasma compared to thatin nontransgenic rabbits. Human LPL expression in various tissues of transgenic rabbits was further elucidated by in situ hybridization and immunostaining. Since rabbits are superior to mice as a model of atherosclerosis, this transgenicrabbit model should provide a valuable tool for the study of LPL in lipid metabolism and atherosclerosis.

Effect of leucine to phenylalanine substitution on the nonpolar face of a class A amphipathic helical peptide on its interaction with lipid: high resolution solution NMR studies of 4F-dimyristoylphosphatidylcholine discoidal complex

Model class A amphipathic helical peptides mimic several properties of apolipoprotein A-I (apoA-I), the major protein component of high density lipoproteins. Previously, we reported the NMR structures of Ac-18A-NH(2) (renamed as 2F because of two phenylalanines), the base-line model class A amphipathic helical peptide in the presence of lipid ( Mishra, V. K., Anantharamaiah, G. M., Segrest, J. P., Palgunachari, M. N., Chaddha, M., Simon Sham, S. W., and Krishna, N. R. (2006) J Biol. Chem. 281, 6511-6519 ). Substitution of two Leu residues on the nonpolar face (Leu(3) and Leu(14)) with Phe residues produced the peptide 4F (so named because of four phenylalanines), which has been extensively studied for its anti-inflammatory and antiatherogenic properties. Like 2F, 4F also forms discoidal nascent high density lipoprotein-like particles with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Since subtle structural changes in the peptide-lipid complexes have been shown to be responsible for their antiatherogenic properties, we undertook high resolution NMR studies to deduce detailed structure of 4F in 4F.DMPC discs. Like 2F, 4F adopts a well defined amphipathic alpha-helical structure in association with the lipid at a 1:1 peptide/lipid weight ratio. Nuclear Overhauser effect (NOE) spectroscopy revealed a number of intermolecular close contacts between the aromatic residues in the hydrophobic face of the helix and the lipid acyl chain protons. Similar to 2F, the pattern of observed peptide-lipid NOEs is consistent with a parallel orientation of the amphipathic alpha helix, with respect to the plane of the lipid bilayer, on the edge of the disc (the belt model). However, in contrast to 2F in 2F.DMPC, 4F in the 4F.DMPC complex is located closer to the lipid headgroup as evidenced by a number of NOEs between 4F and DMPC headgroup protons. These NOEs are absent in the 2F.DMPC complex. In addition, the conformation of the DMPC sn-3 chain in 4F.DMPC complex is different than in the 2F.DMPC complex as evidenced by the NOE between lipid 2.CH and betaCH(2) protons in 4F.DMPC, but not in 2F.DMPC, complex. Based on the results of this study, we infer that the antiatherogenic properties of 4F may result from its preferential interaction with lipid headgroups.

The strange case of Dr HDL and Mr HDL: Does a NO’s story illuminate the mystery of HDL’s dark side uncovered by Dr HDL’s drug targeting CETP?

Recently, the first large-scale morbidity and mortality trial (ILLUMINATE) to evaluate the cardiovascular end points of a CETP inhibitor (torcetrapib) has been prematurely stopped because the mortality was significantly increased in the treated group. Why torcetrapib caused excess death is not known. Based on the fact that HDL interacts with endothelial nitric oxyde synthase (eNOS) and nitric oxide (NO) secretion, which partly controlled blood pressure and than torcetrapib could increase blood pressure among some patients, we hypothesize that CETP inhibition could have significantly inhibit eNOS. CETP inhibition would have enlarged HDL size resulting in a deficit in the interaction between HDL and the Scavenger Receptor class B type I (SR-BI), which is an important link between HDL and eNOS activation. We suggest than the deficit in NO secretion would have been sufficient among all patients to induce a destabilization of the plaques of atheroma, but could have induced a pathogenic increase in blood pressure only in patients whose eNOS activity was naturally weak due to genetic polymorphisms of this enzyme. We also hypothesize that the increase in HDL levels, induced by CETP inhibition, coupled with the capacity of HDL to induce endothelin-1 secretion would have aggravated the cardiovascular risks under this CETP inhibitor treatment.

Functionally defective high-density lipoprotein: a new therapeutic target at the crossroads of dyslipidemia, inflammation, and atherosclerosis

High-density lipoproteins (HDL) possess key atheroprotective biological properties, including cellular cholesterol efflux capacity, and anti-oxidative and anti-inflammatory activities. Plasma HDL particles are highly heterogeneous in physicochemical properties, metabolism, and biological activity. Within the circulating HDL particle population, small, dense HDL particles display elevated cellular cholesterol efflux capacity, afford potent protection of atherogenic low-density lipoprotein against oxidative stress and attenuate inflammation. The antiatherogenic properties of HDL can, however be compromised in metabolic diseases associated with accelerated atherosclerosis. Indeed, metabolic syndrome and type 2 diabetes are characterized not only by elevated cardiovascular risk and by low HDL-cholesterol (HDL-C) levels but also by defective HDL function. Functional HDL deficiency is intimately associated with alterations in intravascular HDL metabolism and structure. Indeed, formation of HDL particles with attenuated antiatherogenic activity is mechanistically related to core lipid enrichment in triglycerides and cholesteryl ester depletion, altered apolipoprotein A-I (apoA-I) conformation, replacement of apoA-I by serum amyloid A, and covalent modification of HDL protein components by oxidation and glycation. Deficient HDL function and subnormal HDL-C levels may act synergistically to accelerate atherosclerosis in metabolic disease. Therapeutic normalization of attenuated antiatherogenic HDL function in terms of both particle number and quality of HDL particles is the target of innovative pharmacological approaches to HDL raising, including inhibition of cholesteryl ester transfer protein, enhanced lipidation of apoA-I with nicotinic acid and infusion of reconstituted HDL or apoA-I mimetics. A preferential increase in circulating concentrations of HDL particles possessing normalized antiatherogenic activity is therefore a promising therapeutic strategy for the treatment of common metabolic diseases featuring dyslipidemia, inflammation, and premature atherosclerosis.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Unusual metabolic characteristics in skeletal muscles of transgenic rabbits for human lipoprotein lipase

BACKGROUND

The lipoprotein lipase (LPL) hydrolyses circulating triacylglycerol-rich lipoproteins. Thereby, LPL acts as a metabolic gate-keeper for fatty acids partitioning between adipose tissue for storage and skeletal muscle primarily for energy use. Transgenic mice that markedly over-express LPL exclusively in muscle, show increases not only in LPL activity, but also in oxidative enzyme activities and in number of mitochondria, together with an impaired glucose tolerance. However, the role of LPL in intracellular nutrient pathways remains uncertain. To examine differences in muscle nutrient uptake and fatty acid oxidative pattern, transgenic rabbits harboring a DNA fragment of the human LPL gene (hLPL) and their wild-type littermates were compared for two muscles of different metabolic type, and for perirenal fat.

RESULTS

Analyses of skeletal muscles and adipose tissue showed the expression of the hLPL DNA fragment in tissues of the hLPL group only. Unexpectedly, the activity level of LPL in both tissues was similar in the two groups. Nevertheless, mitochondrial fatty acid oxidation rate, measured ex vivo using [1-(14C)]oleate as substrate, was lower in hLPL rabbits than in wild-type rabbits for the two muscles under study. Both insulin-sensitive glucose transporter GLUT4 and muscle fatty acid binding protein (H-FABP) contents were higher in hLPL rabbits than in wild-type littermates for the pure oxidative semimembranosus proprius muscle, but differences between groups did not reach significance when considering the fast-twitch glycolytic longissimus muscle. Variations in both glucose uptake potential, intra-cytoplasmic binding of fatty acids, and lipid oxidation rate observed in hLPL rabbits compared with their wild-type littermates, were not followed by any modifications in tissue lipid content, body fat, and plasma levels in energy-yielding metabolites.

CONCLUSIONS

Expression of intracellular binding proteins for both fatty acids and glucose, and their following oxidation rates in skeletal muscles of hLPL rabbits were not fully consistent with the physiology rules. The modifications observed in muscle metabolic properties might not be directly associated with any LPL-linked pathways, but resulted likely of transgene random insertion into rabbit organism close to any regulatory genes. Our findings enlighten the risks for undesirable phenotypic modifications in micro-injected animals and difficulties of biotechnology in mammals larger than mice.

Therapeutic approaches to raising plasma HDL-cholesterol levels

Epidemiologic data from the Framingham and Prospective Cardiovascular Munster studies, demonstrating an inverse correlation between the plasma concentration of HDLs and the incidence of cardiovascular disease, have driven research to explore precisely how HDLs confer this cardioprotective effect. HDLs are anti-inflammatory, antithrombogenic and have vasoactive effects, as well as being efficient cholesterol acceptors enabling the removal of cholesterol from peripheral tissues, all functions that are likely to protect the vasculature. The first part of this article will review the clinical evidence in support of the pleiotropic effects of HDLs, along with laboratory-based investigations of the molecular mechanisms of action. As the evidence of clinical benefits of raising plasma HDL concentration has increased, so has the number of strategies currently being considered to achieve this goal. The second part of this article will review three current strategies: infusion of HDL-like products, comparing physicopharmacologic characteristics of the two commercial products currently under trial; the use of fibrates to raise plasma HDLs (although fibrates primarily reduce triglyceride levels, certain derivatives are able to induce significant increases in plasma HDLs); and the use of drugs that inhibit cholesterol ester transfer protein (these drugs increase plasma HDL concentration either alone or as an adjunct therapy with statins). The clinical efficacy and mechanism of action of fibrates and inhibitors of cholesterol ester transfer protein will be reviewed.

Human free apolipoprotein A-I and artificial pre-beta-high-density lipoprotein inhibit eNOS activity and NO release

Little is known about the effects of human free apolipoprotein A-I (Free-Apo A-I) and pre-beta-high density lipoprotein (pre-beta-HDL) on the endothelium function. In this study, we have investigated the effects of Free-Apo A-I and artificial pre-beta-HDL on endothelial NO synthase (eNOS) activity and on NO production by endothelial cells. Free-Apo A-I drastically inhibited NO production in human umbilical cord vein endothelial cells (HUVECs) and eNOS activity in bovine aortic endothelial cells (BAECs). Pre-beta-HDL and serum from human apolipoprotein A-I transgenic rabbits inhibited eNOS activity in BAECs but HDL3 did not. Free-Apo A-I displaced eNOS from BAEC plasma membrane towards intracellular pools without affecting eNOS activity and eNOS mass in BAEC crude homogenates. Free-Apo A-I and HDL3 did not decrease either caveolin bound to BAEC plasma membrane or caveola cholesterol content. As previously described, we showed that HDL3 directly induced endothelium-dependent relaxation of rings from rat aorta. We observed that pre-beta-HDL significantly decreased endothelium-dependent relaxation of rat aortic rings ex vivo.

The transgenic rabbit as model for human diseases and as a source of biologically active recombinant proteins

Until recently, transgenic rabbits were produced exclusively by pronuclear microinjection which results in additive random insertional transgenesis; however, progress in somatic cell cloning based on nuclear transfer will soon make it possible to produce rabbits with modifications to specific genes by the combination of homologous recombination and subsequent prescreening of nuclear donor cells. Transgenic rabbits have been found to be excellent animal models for inherited and acquired human diseases including hypertrophic cardiomyopathy, perturbed lipoprotein metabolism and atherosclerosis. Transgenic rabbits have also proved to be suitable bioreactors for the production of recombinant protein both on an experimental and a commercial scale. This review summarizes recent research based on the transgenic rabbit model.

Transgenic rabbits as therapeutic protein bioreactors and human disease models

Genetically modified laboratory animals provide a powerful approach for studying gene expression and regulation and allow one to directly examine structure-function and cause-and-effect relationships in pathophysiological processes. Today, transgenic mice are available as a research tool in almost every research institution. On the other hand, the development of a relatively large mammalian transgenic model, transgenic rabbits, has provided unprecedented opportunities for investigators to study the mechanisms of human diseases and has also provided an alternative way to produce therapeutic proteins to treat human diseases. Transgenic rabbits expressing human genes have been used as a model for cardiovascular disease, AIDS, and cancer research. The recombinant proteins can be produced from the milk of transgenic rabbits not only at lower cost but also on a relatively large scale. One of the most promising and attractive recombinant proteins derived from transgenic rabbit milk, human alpha-glucosidase, has been successfully used to treat the patients who are genetically deficient in this enzyme. Although the pronuclear microinjection is still the major and most popular method for the creation of transgenic rabbits, recent progress in gene targeting and animal cloning has opened new avenues that should make it possible to produce transgenic rabbits by somatic cell nuclear transfer in the future. Based on a computer-assisted search of the studies of transgenic rabbits published in the English literature here, we introduce to the reader the achievements made thus far with transgenic rabbits, with emphasis on the application of these rabbits as human disease models and live bioreactors for producing human therapeutic proteins and on the recent progress in cloned rabbits.

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.

HDL therapy for the acute treatment of atherosclerosis

Although pharmacologic intervention to treat atherosclerosis originally focused on lowering LDL-cholesterol levels as a therapeutic target, a number of intervention trials have also highlighted the powerful effect of elevating HDL-cholesterol levels to reduce cardiovascular morbidity and mortality. Although the mechanism(s) by which HDL beneficially alters the atherosclerotic disease process is (are) still unknown, it is presumed that high levels of HDL facilitate the efflux of cholesterol from the arterial wall, thereby enhancing the transport of cholesterol and other lipids from arteries back to the liver for biliary excretion as fecal sterols and bile acids. It has therefore been hypothesized that through a rapid facilitation of HDL mediated cholesterol efflux from arteries by infusion of synthetic apolipoprotein A-I (apoA-I)/phospholipid (A-I/PL) complexes, HDL therapy could have an acute therapeutic application to treat cardiovascular disease at the site of action, namely the vulnerable, unstable atherosclerotic plaque. Single high dose infusions and repeated injections of lower doses of apoA-I variants or mimetics complexed to phospholipids have produced remarkable effects on the progression and regression of atherosclerosis in animal models. The positive results of these preclinical experiments have compelled researchers to perform exploratory studies in human subjects in which reconstituted HDL and synthetic A-I/PL complexes are infused through a peripheral vein. These clinical studies are testing the hypothesis and the potential use of synthetic HDL as a new treatment modality for acute coronary syndromes. Given that there is an unmet medical need for new and more effective therapies to elevate HDL-cholesterol levels and improve HDL function, a historical review, update and discussion of the preclinical and clinical studies which support the use of HDL therapy for reducing cardiovascular morbidity and mortality is warranted.

Gene therapy in cardiovascular disease. Current status

Cardiovascular disease is the leading cause of mortality and morbidity in developed countries. Most conventional therapy is often inefficacious and tends to treat the symptoms rather than the underlying causes of the disorder. Gene therapy offers a novel approach for prevention and treatment of cardiovascular diseases. Technical advances in viral vector systems and the development of fusigenic liposome vectors have been crucial to the development of effective gene therapy strategies directed at the vasculature and myocardium in animal models. Gene transfer techniques are being evaluated as potential treatment alternatives for both genetic (familial hypercholesterolemia) and acquired occlusive vascular diseases (atherosclerosis, restenosis, arterial thrombosis) as well as for cardiac disorders including heart failure, myocardial ischemia, graft coronary arteriosclerosis and hypertension. Continued technologic advances in vector systems and promising results in human and animal gene transfer studies make the use of gene therapy a promising strategy for the treatment of cardiovascular disorders.

Factors influencing efficient production of transgenic rabbits

Factors that influence the efficient production of transgenic rabbits are described. The effects of the number of embryos transferred to the recipient, of recipient age, of a variety of gene constructs and of a dual use of donors as recipients (donor-recipient (DR) method) were statistically evaluated from the data collected in three experiments with three different genes. Higher survival rates of microinjected embryos were obtained in younger recipients (6-17 months), while the rates were-markedly decreased in recipients over 18 months old. Integration efficiencies (transgenic rabbits per newborn) were significantly different from the gene constructs used, but not related to either the number of embryos transferred or the number of newborns obtained. No significant differences in the survival rate of embryos of injected embryos and the integration efficiency were observed in both the DR embryo transfer method and the traditional method using pseudopregnant recipients (PR). Our results suggest that the gene construct and the survival rate of injected embryos were important factors affecting the efficiency of producing transgenic rabbits, and the age of recipients was one of the important factors affecting the survival rate of the injected embryos. The DR method was useful for reducing the number of animals required for production of transgenic rabbits.

Structural and functional properties of apolipoprotein A-I mutants

Apolipoprotein (apo) A-I is composed of 243 amino acid residues that fold into amphipathic helixes, and plays a central role in the high density lipoprotein (HDL) metabolism. Familial apoA-I deficiency is a rare metabolic disorder of which three cases have been characterized at a molecular level in western Japan. However, in subjects with apoA-I deficiency, coronary artery disease was not always present. One apo A-I deficiency was compound heterozygous apoA-I mutant for a TATA box mutation and a structural nonsense mutation. To date, screening analysis in our laboratory has identified nine genetically-determined structural mutations of apo A-I. We have also characterized these apo A-I mutations, including apoA-I (Glu235del) Nichinan. Few structural mutations were associated with altered HDL cholesterol levels.

Accumulation of cholestatic lipoproteins in ANIT-treated human apolipoprotein A-I transgenic rats is diminished through dose-dependent apolipoprotein A-I activation of LCAT

Administration of alpha-naphthylisothiocyanate (ANIT) to rats induces changes to plasma lipids consistent with cholestasis. We have previously shown (J. Lipid Res. 37 (1996) 1086) that animals treated with ANIT accumulate large amounts of free cholesterol (FC) and phospholipid (PL)-rich cholestatic lipoproteins in the LDL density range by 48 h. This lipid was cleared by 120 h through apparent movement into HDL with concomitant cholesteryl ester (CE) production. It was hypothesised that the clearance was mediated through the movement of the PL and FC into apolipoprotein A-I (apo A-I) containing lipoproteins followed by LCAT esterification to form CE. To test this hypothesis, rats overexpressing various amounts of human apo A-I (TgR[HuAI] rats) were treated with ANIT (100 mg/kg) and the effect of plasma apo A-I concentration on plasma lipids and lipoprotein distribution was examined. In untreated TgR[HuAI] rats, human apo A-I levels were strongly correlated to plasma PL (r(2)=0. 94), FC (r(2)=0.93) and CE (r(2)=0.90), whereas in ANIT-treated TgR[HuAI] rats, human apo A-I levels were most strongly correlated to CE levels (r(2)=0.80) and an increased CE/FC ratio (r(2)=0.62) and the movement of cholestatic lipid in the LDL to HDL. Since LCAT activity was not affected by ANIT treatment, these results demonstrate that the ability of LCAT to esterify the plasma FC present in cholestatic liver disease is limited by in vivo apo A-I activation of the cholestatic lipid and not by the catalytic capacity of LCAT.

The mechanisms of coronary restenosis: insights from experimental models

Since its introduction into clinical practice, more than 20 years ago, percutaneous transluminal coronary angioplasty (PTCA) has proven to be an effective, minimally invasive alternative to coronary artery bypass grafting (CABG). During this time there have been great improvements in the design of balloon catheters, operative procedures and adjuvant drug therapy, and this has resulted in low rates of primary failure and short-term complications. However, the potential benefits of angioplasty are diminished by the high rate of recurrent disease. Up to 40% of patients undergoing angioplasty develop clinically significant restenosis within a year of the procedure. Although the deployment of endovascular stents at the time of angioplasty improves the short-term outcome, 'in-stent' stenosis remains an enduring problem. In order to gain an insight into the mechanisms of restenosis, several experimental models of angioplasty have been developed. These have been used together with the tools provided by recent advances in molecular biology and catheter design to investigate restenosis in detail. It is now possible to deliver highly specific molecular antagonists, such as antisense gene sequences, to the site of injury. The knowledge provided by these studies may ultimately lead to novel forms of intervention. The present review is a synopsis of our current understanding of the pathological mechanisms of restenosis.

Paraoxonase activity is reduced by a pro-atherosclerotic diet in rabbits

Serum paraoxonase (PON1) is believed to protect against the development of atherosclerosis because of its ability to retard the oxidation of low-density lipoprotein (LDL) by hydrolysing LDL-associated phospholipid and cholesteryl-ester hydroperoxides. We have examined the relationship between PON1 and atherosclerosis development in transgenic rabbits overexpressing human apolipoprotein (apo) A-I and nontransgenic littermates fed a pro-atherogenic diet. PON1 activity was higher in transgenic (4006.1 +/- 716.7 nmol/min/ml) compared to control (3078.5 +/- 623.3 nmol/min/ml) rabbits (P < 0.01) while high-density lipoprotein (HDL) cholesterol was 1.84 +/- 0.54 mmol/L in transgenic rabbits and 0.57 +/- 0.21 mmol/L in control rabbits (P = 0.0001). After feeding rabbits a high-cholesterol diet for 14 weeks HDL-cholesterol fell by 70% in both transgenic and control rabbits (P < 0.001 compared to week 0) PON1 activity fell by 50% in both groups of rabbits (P < 0. 01 compared to week 0). The amount of thoracic aortic surface area covered by lesions was 29 +/- 16% in the control group and 26 +/- 15% in the transgenic group (P = NS). A pro-atherosclerotic diet reduces PON1 which may exaggerate the effects of the diet on the development of atherosclerosis.

High-density lipoprotein subclasses and apolipoprotein A-I

Epidemiological and clinical studies showing an association between decreased concentrations of high-density lipoprotein (HDL) cholesterol and increased risk of premature coronary artery disease have generated interest in the mechanism through which HDL prevents atherosclerosis. Recognition of the importance of apolipoproteins (apo(s)) has led to the separation of HDL into subpopulations according to their apolipoprotein composition. It is now recognised that HDL comprises at least two types of apo A-I-containing lipoproteins: LpA-I:A-II containing both apo A-I and apo A-II and LpA-I containing apo A-I but not apo A-II. A majority of studies support the fact that LpA-I is more effective than LpA-I:A-II in promoting cellular cholesterol efflux, the first step in reverse cholesterol transport. Studies in transgenic animals have revealed that the gene transfer of human apo A-I in mice and rabbits increases plasma apo A-I and HDL cholesterol levels and particularly apo A-I-rich HDL particle concentrations, leading to inhibition of the development of dietary or genetically induced atherosclerosis. On the other hand, gene transfer of apo A-II in mice gives conflicting results. The conclusions of some experiments indicate either an atherogenic, or a poorly anti-atherogenic, or even a strongly anti-atherogenic role for apo A-II and for apo A-II-rich HDL lipoproteins. Although these experimental results have been obtained in animals, they confirm previous studies obtained in human clinical studies, indicating that apo A-I-rich HDL (tested as LpA-I in clinical studies) are generally strong plasma markers of atherosclerosis protection while the clinical significance of apo A-I + apo A-II HDL (tested as LpA-I:A-II in clinical studies) is more controversial. The introduction of immunological methods to measure LpA-I and LpA-I:A-II levels in blood make large-scale studies feasible to confirm the clinical significance of these HDL particles.

Transgenic rabbit models for biomedical research: current status, basic methods and future perspectives

The creation of genetically modified laboratory and livestock animals is one of the most dramatic advances derived from recombinant DNA technology. Over the past decade, the development of a large mammal transgenic model, transgenic rabbits, has provided unprecedented opportunities for investigators to study the mechanisms of human diseases and has also provided a novel way to produce foreign proteins for both therapeutic and commercial purposes. Recent progress in gene targeting and animal cloning has opened new avenues for production of transgenic rabbits. In this review, we will introduce the reader to the progress that has been achieved in transgenic rabbits with emphasis on the application of these rabbits as human disease models and bioproducers of human therapeutic proteins.

High density lipoproteins and coronary heart disease. Future prospects in gene therapy

Atherosclerosis cardiovascular disease is the leading cause of death in industrial societies. For coronary heart disease, hypercholesterolemia and dyslipoproteinemia are the major risk factors. Low serum levels of cholesterol in the HDL fraction is the most common abnormality found in patients with confirmed coronary artery disease. A therapeutical strategy consists in increasing the serum HDL cholesterol concentration in order to improve the 'reverse cholesterol transport'. Studies in transgenic mice and rabbits for human apo A-I or human lecithin cholesterol acyl-transferase showed that overexpression of these proteins increases serum HDL cholesterol concentration and reduces diet induced atherogenesis. Furthermore, adenovirus-mediated transfer of human apo A-I and LCAT genes in mice also increases circulating apo A-I and LCAT. Apo A-I and LCAT are two potential targets for gene therapy of patients with atherosclerosis associated with a low HDL cholesterol level.

Apolipoprotein(a) yeast artificial chromosome transgenic rabbits. Lipoprotein(a) assembly with human and rabbit apolipoprotein B

The in vivo analysis of lipoprotein(a) (Lp(a)), an independent atherosclerosis risk factor in humans, has been limited in part by its restricted distribution among mammals. Although transgenic mice have been created containing Lp(a), the relatively small size of the mouse has precluded some studies. To examine the properties of this molecule in a significantly larger mammal, we have used a 270-kilobase yeast artificial chromosome clone containing the human apolipoprotein(a) (apo(a)) gene and a 90-kilobase P1 phagemid clone containing the human apolipoprotein B (apoB) gene to create transgenic rabbits that express either or both transgenes. Expression of both transgenes was tissue specific and localized predominantly to the liver. Average apolipoprotein plasma levels in the rabbits were 2.5 mg/dl for apo(a) and 17.6 mg/dl for human apoB. In contrast to observations in apo(a) transgenic mice, we found that apo(a) plasma levels in the rabbits were stable throughout sexual maturity. Also, apo(a) formed a covalent association with the endogenous rabbit apoB albeit with a lower efficiency than its association with human apoB. The analysis of Lp(a) transgenic rabbits has provided new insights into apo(a) expression and Lp(a) assembly. In addition, these transgenic rabbits potentially will provide an improved experimental model for the in vivo analysis of Lp(a) and its role in promoting atherosclerosis and restenosis.

Endothelial derived vasorelaxation is impaired in human APO A-I transgenic rabbits

Endothelium-derived relaxing factor (nitric oxide: NO) may provide an endogenous defence against atherosclerosis which impairs endothelium-dependent vascular relaxation. Atherosclerosis development is inhibited in cholesterol fed human apo A-I transgenic rabbits (Duverger, N., Circulation, 1996, 94, 713-717). We investigated if endothelium-dependent vascular relaxation is modified in human apo A-I transgenic rabbits by testing in vitro endothelium-dependent receptor-dependent vascular relaxation to acetylcholine and endothelium-dependent receptor-independent vascular relaxation to A23187 of abdominal aorta, precontracted with phenylephrine, in human apo A-I transgenic rabbits (n=4) versus non transgenic littermates (n=4). Endothelium-independent vascular relaxation was investigated with sodium nitroprusside. Vascular precontraction to phenylephrine was significantly increased in human apo A-I transgenic rabbits (p<0.05) while endothelium-independent vascular relaxation to nitroprusside was similar between human apo A-I transgenic rabbits and control rabbits. Endothelium-dependent receptor-dependent and receptor-independent vascular relaxations were reduced in human apo A-I transgenic rabbits (p<0.05). Maximum endothelium-dependent receptor-dependent vascular relaxation was negatively correlated with HDL-cholesterol and total apo A-I (rabbit+ human) plasma levels (r=0.87 and 0.86, p=0.01, respectively) but not with atherogenic plasma lipid (VLDL-cholesterol, LDL-cholesterol, VLDL+LDL cholesterol, triglycerides, apolipoprotein B) levels. These results suggest that the transgenesis of human apo A-I in rabbits impairs signal transduction of endothelial NO synthesis.