Analysis of gender-specific atherosclerosis susceptibility in transgenic[hCETP]25DS rat model

Translating atherosclerosis research from bench to bedside: navigating the barriers for effective preclinical drug discovery

Cardiovascular disease (CVD) remains the leading cause of death worldwide. An ongoing challenge remains the development of novel pharmacotherapies to treat CVD, particularly atherosclerosis. Effective mechanism-informed development and translation of new drugs requires a deep understanding of the known and currently unknown biological mechanisms underpinning atherosclerosis, accompanied by optimization of traditional drug discovery approaches. Current animal models do not precisely recapitulate the pathobiology underpinning human CVD. Accordingly, a fundamental limitation in early-stage drug discovery has been the lack of consensus regarding an appropriate experimental in vivo model that can mimic human atherosclerosis. However, when coupled with a clear understanding of the specific advantages and limitations of the model employed, preclinical animal models remain a crucial component for evaluating pharmacological interventions. Within this perspective, we will provide an overview of the mechanisms and modalities of atherosclerotic drugs, including those in the preclinical and early clinical development stage. Additionally, we highlight recent preclinical models that have improved our understanding of atherosclerosis and associated clinical consequences and propose model adaptations to facilitate the development of new and effective treatments.

A new clinically relevant model for intracranial atherosclerosis in rats

INTRODUCTION

Intracranial atherosclerotic stenosis (ICAS) is one of the most common causes of stroke worldwide and, in particular, has been implicated as a leading cause of recurrent ischemic stroke. We developed a new rat model to study intracranial atherosclerosis.

METHODS

Twelve-week-old male Sprague-Dawley rats were divided into a control (on a maintain diet) and a high-cholesterol group (on a daily 1% cholesterol diet) for up to 6 weeks. During the first two weeks, NG-nitro-L-arginine methylester (L-NAME, 3 mg/mL) was added to the drinking water in the high-cholesterol group to induce intimal changes making the rats susceptible to atherosclerosis. Blood lipids, including low-density lipoprotein (LDL), cholesterol (CHO), triglycerides (TG), and high-density lipoprotein (HDL), were measured after 3 and 6 weeks. Histological sections of the brains, including internal carotid artery (ICA), middle cerebral artery (MCA), and basilar artery (BA), were prepared to study intracranial artery morphometry and intimal thickening. The levels of CD68, an inflammatory marker, within the vessel walls as determined by immunohistochemistry were also measured.

RESULTS

The high-cholesterol diet increased the levels of classic blood markers of atherosclerosis, LDL, CHO, and TG as well as decreased HDL, which became progressively more intensive with time. Rats showed increased intimal thickening in the ICA, MCA, and BA. This protocol also increased the levels of CD68 immunoreactivity within the vessel walls.

CONCLUSIONS

A rat model of intracranial atherosclerosis was effectively developed by high-cholesterol diet and L-NAME administration. This clinically relevant model would be beneficial for studying ICAS.

Molecular imaging of vasa vasorum neovascularization via DEspR-targeted contrast-enhanced ultrasound micro-imaging in transgenic atherosclerosis rat model

PURPOSE

Given that carotid vasa vasorum neovascularization is associated with increased risk for stroke and cardiac events, the present in vivo study was designed to investigate molecular imaging of carotid artery vasa vasorum neovascularization via target-specific contrast-enhanced ultrasound (CEU) micro-imaging.

PROCEDURES

Molecular imaging was performed in male transgenic rats with carotid artery disease and non-transgenic controls using dual endothelin1/VEGFsp receptor (DEspR)-targeted microbubbles (MB(D)) and the Vevo770 micro-imaging system and CEU imaging software.

RESULTS

DEspR-targeted CEU-positive imaging exhibited significantly higher contrast intensity signal (CIS)-levels and pre-/post-destruction CIS-differences in seven of 13 transgenic rats, in contrast to significantly lower CIS-levels and differences in control isotype-targeted microbubble (MB(C))-CEU imaging (n = 8) and in MB(D) CEU-imaging of five non-transgenic control rats (P < 0.0001). Ex vivo immunofluorescence analysis demonstrated binding of MB(D) to DEspR-positive endothelial cells; and association of DEspR-targeted increased contrast intensity signals with DEspR expression in vasa vasorum neovessel and intimal lesions. In vitro analysis demonstrated dose-dependent binding of MB(D) to DEspR-positive human endothelial cells with increasing %cells bound and number of MB(D) per cell, in contrast to MB(C) or non-labeled microbubbles (P < 0.0001).

CONCLUSION

In vivo DEspR-targeted molecular imaging detected increased DEspR-expression in carotid artery lesions and in expanded vasa vasorum neovessels in transgenic rats with carotid artery disease. Future studies are needed to determine predictive value for stroke or heart disease in this transgenic atherosclerosis rat model and translational applications.

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.

Prestroke proteomic changes in cerebral microvessels in stroke-prone, transgenic[hCETP]-Hyperlipidemic, Dahl salt-sensitive hypertensive rats

Stroke is the third leading cause of death in the United States with high rates of morbidity among survivors. The search to fill the unequivocal need for new therapeutic approaches would benefit from unbiased proteomic analyses of animal models of spontaneous stroke in the prestroke stage. Since brain microvessels play key roles in neurovascular coupling, we investigated prestroke microvascular proteome changes. Proteomic analysis of cerebral cortical microvessels (cMVs) was done by tandem mass spectrometry comparing two prestroke time points. Metaprotein-pathway analyses of proteomic spectral count data were done to identify risk factor-induced changes, followed by QSPEC-analyses of individual protein changes associated with increased stroke susceptibility. We report 26 cMV proteome profiles from male and female stroke-prone and non-stroke-prone rats at 2 months and 4.5 months of age prior to overt stroke events. We identified 1,934 proteins by two or more peptides. Metaprotein pathway analysis detected age-associated changes in energy metabolism and cell-to-microenvironment interactions, as well as sex-specific changes in energy metabolism and endothelial leukocyte transmigration pathways. Stroke susceptibility was associated independently with multiple protein changes associated with ischemia, angiogenesis or involved in blood brain barrier (BBB) integrity. Immunohistochemical analysis confirmed aquaporin-4 and laminin-α1 induction in cMVs, representative of proteomic changes with >65 Bayes factor (BF), associated with stroke susceptibility. Altogether, proteomic analysis demonstrates significant molecular changes in ischemic cerebral microvasculature in the prestroke stage, which could contribute to the observed model phenotype of microhemorrhages and postischemic hemorrhagic transformation. These pathways comprise putative targets for translational research of much needed novel diagnostic and therapeutic approaches for stroke.

Early-life sodium exposure unmasks susceptibility to stroke in hyperlipidemic, hypertensive heterozygous Tg25 rats transgenic for human cholesteryl ester transfer protein

BACKGROUND

Early-life risk factor exposure increases aortic atherosclerosis and blood pressure in humans and animal models; however, limited insight has been gained as to end-organ complications.

METHODS AND RESULTS

We investigated the effects of early-life Na exposure (0.23% versus 0.4% NaCl regular rat chow) on vascular disease outcomes using the inbred, transgenic [hCETP](25) Dahl salt-sensitive hypertensive rat model of male-predominant coronary atherosclerosis, Tg25. Rather than the expected increase in coronary heart disease, fetal 0.4% Na exposure (< or =2 g of Na per 2-kcal/d diet) induced adult-onset stroke in both sexes (ANOVA P<0.0001), with earlier stroke onset in Tg25 females. Analysis of later onset of 0.4% Na exposure resulted in decreased stroke risk and later stroke onset despite longer 0.4% Na exposure durations, which indicates increasing risk with earlier onset of 0.4% Na exposure. Histological analysis of stroke-positive rat brains revealed cerebral cortical hemorrhagic infarctions, microhemorrhages, neuronal ischemia, and microvascular injury. Ex vivo MRI of stroke-positive rat brains detected cerebral hemorrhages, microhemorrhages, and ischemia with middle cerebral artery distribution and cerebellar noninvolvement. Ultrasound microimaging detected carotid artery disease. Prestroke analysis detected neuronal ischemia and decreased mass of isolated cerebral but not cerebellar microvessels.

CONCLUSIONS

Early-life Na exposure exacerbated hypertension and unmasked stroke susceptibility, with greater female vulnerability in hypertensive, hyperlipidemic Tg25 rats. The reproducible modeling in stroke-prone Tg25 rats of carotid artery disease, cerebral hemorrhagic infarctions, neuronal ischemia, microhemorrhages, and microvascular alterations suggests a pathogenic spectrum with causal interrelationships. This "mixed-stroke" spectrum could represent paradigms of ischemic-hemorrhagic transformation and/or a microangiopathic basis for the association of ischemic lesions, microhemorrhages, and strokes in humans. Together, the data reveal early-life Na exposure to be a significant modifier of hypertension and stroke disease course and hence a potentially modifiable prevention target that deserves systematic study.

Sex-specific hippocampus-dependent cognitive deficits and increased neuronal autophagy in DEspR haploinsufficiency in mice

Aside from abnormal angiogenesis, dual endothelin-1/VEGF signal peptide-activated receptor deficiency (DEspR(-/-)) results in aberrant neuroepithelium and neural tube differentiation, thus elucidating DEspR's role in neurogenesis. With the emerging importance of neurogenesis in adulthood, we tested the hypothesis that nonembryonic-lethal DEspR haploinsufficiency (DEspR(+/-)) perturbs neuronal homeostasis, thereby facilitating aging-associated neurodegeneration. Here we show that, in male mice only, DEspR-haploinsufficiency impaired hippocampus-dependent visuospatial and associative learning and induced noninflammatory spongiform changes, neuronal vacuolation, and loss in the hippocampus, cerebral cortex, and subcortical regions, consistent with autophagic cell death. In contrast, DEspR(+/-) females exhibited better cognitive performance than wild-type females and showed absence of neuropathological changes. Signaling pathway analysis revealed DEspR-mediated phosphorylation of activators of autophagy inhibitor mammalian target of rapamycin (mTOR) and dephosphorylation of known autophagy inducers. Altogether, the data demonstrate DEspR-mediated diametrical, sex-specific modulation of cognitive performance and autophagy, highlight cerebral neuronal vulnerability to autophagic dysregulation, and causally link DEspR haploinsufficiency with increased neuronal autophagy, spongiosis, and cognitive decline in mice.

Cholesteryl ester transfer protein (CETP) genetic variation and early onset of non-fatal myocardial infarction

Although Cholesteryl Ester Transfer Protein (CETP) mediates the transfer of cholesteryl esters and triglycerides between lipoprotein particles and thus plays a crucial role in reverse cholesterol transport, the association of variations in the CETP gene with acute myocardial infarction (MI) remains unclear. In this study we examined whether common genetic variation in the CETP gene is related to early-onset non-fatal MI risk in a population-based case-control study from western Washington State. Genotyping for the CETP -2708 G/A, -971 A/G, -629 A/C, Intron-I TaqI G/A and exon-14 A/G (I405V) SNPs was performed in 578 cases with first acute non-fatal MI and in 666 demographically similar controls, free of clinical cardiovascular disease, identified randomly from the community. In-person interviews and non-fasting blood specimens provided data on coronary heart disease risk factors. In men, there was little evidence for an association between single SNPs and MI risk, but in women the age- and race-adjusted OR was found to be significant in 4 out of the 5 CETP single variants. Haplotype analysis revealed two haplotypes associated with MI risk among men. As compared to men homozygous for the most common haplotype D (-2708 G, -971 G, -629 C, TaqI G and exon-14 A), the fully-adjusted multiplicative model identified haplotype G (-2708 G, -971 A, -629 A, TaqI G and exon-14 G) was associated with a 4.0-6.0-fold increased risk of MI for each additional copy; [95%CI 2.4-14.8] and haplotype B (-2708 G, -971 G, -629 A, TaqI A and exon-14 A) showed a significant decreased risk for early onset MI [OR = 0.18; 95%CI 0.04 - 0.75]. An evolutionary-based haplotype analysis indicated that the two haplotypes associated with the MI risk are most evolutionarily divergent from the other haplotypes. Variation at the CETP gene locus is associated with the risk of early-onset non-fatal MI. This association was found to be independent of HDL-C levels. These data and the sex-specific findings require confirmation in other populations.

Mouse models for atherosclerosis and pharmaceutical modifiers

Atherosclerosis is a multifactorial highly-complex disease with numerous etiologies that work synergistically to promote lesion development. The ability to develop preventive and ameliorative treatments will depend on animal models that mimic the human subject metabolically and pathophysiologically and will develop lesions comparable to those in humans. The mouse is the most useful, economic, and valid model for studying atherosclerosis and exploring effective therapeutic approaches. Among the most widely used mouse models for atherosclerosis are apolipoprotein E-deficient (ApoE-/-) and LDL receptor-deficient (LDLr-/-) mice. An up-and-coming model is the ApoE*3Leiden (E3L) transgenic mouse. Here, we review studies that have explored how and to what extent these mice respond to compounds directed at treatment of the risk factors hypercholesterolemia, hypertriglyceridemia, hypertension, and inflammation. An important outcome of this survey is that the different models used may differ markedly from one another in their response to a specific experimental manipulation. The choice of a model is therefore of critical importance and should take into account the risk factor to be studied and the working spectrum of the compounds tested.

CETP inhibition in cardiovascular risk management: a critical appraisal

In view of the cardioprotective effect of high-density lipoproteins (HDL) and the limited effects of statin and fibrate therapy on HDL cholesterol, it is clinically relevant to test whether pharmacological treatment aimed at raising HDL lowers cardiovascular risk. Cholesteryl ester transfer protein (CETP) is a new therapeutic target, because the cholesteryl ester transfer process lowers HDL cholesterol and contributes to an atherogenic lipoprotein profile, particularly when plasma triglycerides are high. Clinical evidence suggests that coronary artery calcification as well as intima media thickness is positively related to plasma cholesteryl ester transfer, and that high plasma CETP concentration is associated with increased cardiovascular risk in hypertriglyceridaemia. However, CETP could also have anti-atherogenic potential, since it provides a potentially beneficial route for delivery of HDL-derived cholesteryl esters to the liver. In addition, CETP could also favourably stimulate peripheral cell cholesterol removal and enhance hepatic cholesterol uptake. Recent evidence suggests that a high CETP level may confer lower cardiovascular risk in the context of low triglycerides. At maximal doses, the CETP inhibitors JTT-705 and torcetrapib elicit a marked rise in HDL cholesterol of up to 34% and 91-106%, respectively. The effectiveness of these drugs on (intermediate) clinical outcome measures is currently being tested in large-scale phase III clinical trials, with torcetrapib being only evaluated in combination therapy with atorvastatin. When and how to use CETP inhibitors, e.g. in combination with a statin or a fibrate, is a major challenge. We propose that low HDL cholesterol in the context of high triglycerides, such as found in type 2 diabetes mellitus, could become an important indication area for this new class of drugs.

Genetic studies in rat models: insights into cardiovascular disease

PURPOSE OF REVIEW

Limited to 2003-2004 publications, this review focuses on 'big picture' concepts learned from rat genetic studies of cardiovascular disease.

RECENT DEVELOPMENTS

Analysis reveals insights into pathogenic paradigms, as well as experimental perspectives into rat-based systems of analyses of complex cardiovascular disease. Key concepts are forwarded. Multiple susceptibility genes underlie several quantitative trait loci for blood pressure suggesting a 'quantitative trait loci cluster' concept; hypertension end-organ disease quantitative trait loci are distinct from blood pressure quantitative trait loci indicating differential susceptibility paradigms for hypertension and each complication (stroke, renal disease, cardiac hypertrophy); distinct blood pressure quantitative trait loci are found in males and females indicating gender-specific susceptibility; and genetic subtypes comprise polygenic hypertension in rat models suggesting a genetic basis for clinical heterogeneity of human essential hypertension. Gender specific genetic susceptibility plays a key role in coronary artery disease susceptibility; multiple distinct quantitative trait loci underlie hyperlipidemia and type-2 diabetes, indicating multiple susceptibilities in risk factors for cardiovascular disease. Studies in transgenic inbred rat-strain models demonstrate value for serial, complex, cardiovascular pathophysiological analyses within a genetic context.

SUMMARY

Cognizant of the limitations of animal model studies, observations from rat genetic studies provide insight into respective modeled human cardiovascular diseases and risk factor susceptibility, as well as systematically dissect the multifaceted complexities apparent in human complex cardiovascular disease. Given the recapitulation of many features of human cardiovascular disease, the value of rat model-based genetic studies for complex cardiovascular disease is unequivocal, thus mandating the expansion of resources for maximization of rat-based genetic studies.