Non-invasive MRI of mouse models of atherosclerosis

β-Aminopropionitrile-induced aortic aneurysm and dissection in mice

The mechanistic basis for the formation of aortic aneurysms and dissection needs to be elucidated to facilitate the development of effective medications. β-Aminopropionitrile administration in mice has been used frequently to study the pathologic features and mechanisms of aortic aneurysm and dissection. This mouse model mimics several facets of the pathology of human aortic aneurysms and dissection, although many variables exist in the experimental design and protocols that must be resolved to determine its application to the human disease. In the present brief review, we have introduced the development of this mouse model and provided insights into understanding its pathologic features.

Carotid arterial wall MRI of apolipoprotein e-deficient mouse at 7 T using DANTE-prepared variable-flip-angle rapid acquisition with relaxation enhancement

PURPOSE

To optimize a sequence combining the delay alternating with nutation for tailored excitation (DANTE) preparative module with the variable-flip-angle rapid acquisition with relaxation enhancement (VF-RARE) sequence (DANTE-VF-RARE) and to investigate its feasibility for vessel wall imaging in Apolipoprotein E-Deficient (ApoE^-/-) mouse at 7 Tesla (T).

MATERIALS AND METHODS

Specific T1/T2 values were used for producing a sharper vessel wall in the variable-flip-angle optimization scheme. The DANTE RF pulse flip angle and pulse train length were optimized for maximizing the wall-lumen contrast. ApoE^-/- (fed high fat diet for 20/40/ 60 weeks, n = 9/4/4) and wild-type mice (controls, n = 3) were imaged at 7 T using VF-RARE, DANTE-VF-RARE, time-of-flight (TOF) angiography, and multi-slice T1-weighted 2D RARE coupled with inflow outflow saturation bands (IOSB-RARE). Wall-lumen contrast-to-noise-ratio efficiency (CNR_eff), lumen area (LA), and wall area (WA) were compared between DANTE-VF-RARE and 2D IOSB-RARE sequences. Additionally, linear regression analysis was conducted between MR measurements and histomorphometric planimetry results.

RESULTS

Residual blood signal was observed in the four out of eighteen carotids on VF-RARE images, whereas it was significantly suppressed on DANTE-VF-RARE images. Compared with IOSB-RARE, DANTE-VF-RARE offered significantly improved CNR_eff (P < 0.001). The LA and WA were both comparable (P = 0.085 and 0.112, respectively) and showed excellent agreement between DANTE-VF-RARE and IOSB-RARE (ICC = 0.96 and 0.95, respectively). The luminal stenosis identified by DANTE-VF-RARE was in consistent with the results of TOF. Strong correlations were found between MR measurements and histopathological analysis for both WA (DANTE-VF-RARE: r = 0.92, slope = 0.94, P < 0.001; IOSB-RARE: r = 0.93, slope = 0.94, P < 0.001) and LA (DANTE-VF-RARE: r = 0.82, slope = 0.54, P < 0.001; IOSB-RARE: r = 0.78, slope = 0.50, P < 0.001).

CONCLUSION

DANTE-VF-RARE achieves effective blood signal suppression and is a feasible approach for the 3D carotid arterial wall imaging of ApoE^-/- mouse at 7 T.

[Carotid artery wall imaging using 7.0T magnetic resonance imaging with threedimensional DANTE-prepared FLASH in ApoE-/- mice]

OBJECTIVE

To explore the feasibility of three-dimensional (3D) vessel wall imaging of carotid atherosclerotic plaques in ApoE-/- mice using 7.0T magnetic resonance imaging (MRI) with delays alternating with nutations for tailored excitation (DANTE)-prepared fast low-angle shot (DANTE-FLASH) technique.

OBJECTIVE

Numerical simulations were performed for optimizing imaging parameters to maximize the wall-lumen contrast. Six ApoE-/- and three wild-type mice were scanned using a 7.0T MRI scanner with DANTE-FLASH and multi-slice 2D RARE coupled with inflow outflow saturation bands (2D-IOSBRARE). The wall signal-to-noise ratio (SNRwall), lumen SNR (SNRlumen), wall-lumen contrast-to-noise ratio (CNR), lumen area (LA), and wall area (WA) were compared between DANTE- FLASH and 2D-IOSB-RARE sequences. Linear regression analysis was performed to assess the correlation between the MRI measurements and histopathological measurements of LA and WA.

OBJECTIVE

Based on the simulation results, a flip angle of 15° and a train length of 150 were implemented in the live imaging study. Compared with 2D-IOSB-RARE, DANTE-FLASH provided a slightly reduced CNR (P < 0.001) but much improved slice resolution. The LA and WA measurements from the DANTE-FLASH and 2D-IOSB- RARE showed excellent agreement based on ICC analysis (LA: ICC=0.94, P < 0.001; WA: ICC=0.93, P < 0.001) and Bland-Altman plots. Strong correlations were observed between the MRI and histopathological measurements for both LA (P < 0.0001) and WA (P < 0.0001).

OBJECTIVE

As a 3D black-blood MR sequence, DANTE-FLASH provides isotropic high spatial resolution to allow reliable visualization and quantitative evaluation of the arteriosclerotic lesions within the carotid artery of ApoE-/- mice using a 7.0T MRI scanner.

Imaging and nanomedicine in inflammatory atherosclerosis

Bioengineering provides unique opportunities to better understand and manage atherosclerotic disease. The field is entering a new era that merges the latest biological insights into inflammatory disease processes with targeted imaging and nanomedicine. Preclinical cardiovascular molecular imaging allows the in vivo study of targeted nanotherapeutics specifically directed toward immune system components that drive atherosclerotic plaque development and complication. The first multicenter trials highlight the potential contribution of multimodality imaging to more efficient drug development. This review describes how the integration of engineering, nanotechnology, and cardiovascular immunology may yield precision diagnostics and efficient therapeutics for atherosclerosis and its ischemic complications.

A mouse model for pathogen-induced chronic inflammation at local and systemic sites

Chronic inflammation is a major driver of pathological tissue damage and a unifying characteristic of many chronic diseases in humans including neoplastic, autoimmune, and chronic inflammatory diseases. Emerging evidence implicates pathogen-induced chronic inflammation in the development and progression of chronic diseases with a wide variety of clinical manifestations. Due to the complex and multifactorial etiology of chronic disease, designing experiments for proof of causality and the establishment of mechanistic links is nearly impossible in humans. An advantage of using animal models is that both genetic and environmental factors that may influence the course of a particular disease can be controlled. Thus, designing relevant animal models of infection represents a key step in identifying host and pathogen specific mechanisms that contribute to chronic inflammation. Here we describe a mouse model of pathogen-induced chronic inflammation at local and systemic sites following infection with the oral pathogen Porphyromonas gingivalis, a bacterium closely associated with human periodontal disease. Oral infection of specific-pathogen free mice induces a local inflammatory response resulting in destruction of tooth supporting alveolar bone, a hallmark of periodontal disease. In an established mouse model of atherosclerosis, infection with P. gingivalis accelerates inflammatory plaque deposition within the aortic sinus and innominate artery, accompanied by activation of the vascular endothelium, an increased immune cell infiltrate, and elevated expression of inflammatory mediators within lesions. We detail methodologies for the assessment of inflammation at local and systemic sites. The use of transgenic mice and defined bacterial mutants makes this model particularly suitable for identifying both host and microbial factors involved in the initiation, progression, and outcome of disease. Additionally, the model can be used to screen for novel therapeutic strategies, including vaccination and pharmacological intervention.

Regression of atherosclerosis: insights from animal and clinical studies

BACKGROUND

Based on studies that date back to the 1920s, regression and stabilization of atherosclerosis in humans has gone from just a dream to one that is achievable. Review of the literature indicates that the successful attempts at regression generally applied robust measures to improve plasma lipoprotein profiles. Examples include extensive lowering of plasma concentrations of atherogenic apolipoprotein B and enhancement of reverse cholesterol transport from atheromata to the liver.

FINDINGS

Possible mechanisms responsible for lesion shrinkage include decreased retention of atherogenic apolipoprotein B within the arterial wall, efflux of cholesterol and other toxic lipids from plaques, emigration of lesional foam cells out of the arterial wall, and influx of healthy phagocytes that remove necrotic debris as well as other components of the plaque. This review will highlight the role key players such as LXR, HDL and CCR7 have in mediating regression.

CONCLUSION

Although much progress has been made, there are many unanswered questions. There is, therefore, a clear need for preclinical and clinical testing of new agents expected to facilitate atherosclerosis regression with the hope that additional mechanistic insights will allow further progress.

Local arterial stiffening assessed by MRI precedes atherosclerotic plaque formation

BACKGROUND

Atherosclerosis is known to impair vascular function and cause vascular stiffening. The aim of this study was to evaluate the potential predictive role of vascular stiffening in the early detection of atherosclerosis. Therefore, we investigated the time course of early functional and morphological alterations of the vessel wall in a murine atherosclerosis model. Because initial lesions are distributed inhomogeneously in early-stage atherosclerosis, MR microscopy was performed to measure vascular elasticity locally, specifically the local pulse wave velocity and the arterial wall thickness.

METHODS AND RESULTS

Local pulse wave velocity and the mean arterial wall thickness were determined in the ascending and the abdominal aortae of ApoE(-/-) and wild-type mice. In vivo MRI revealed that baseline pulse wave velocity and morphology were similar in 6-week-old ApoE(-/-) and WT mice, whereas at the age of 18 weeks, local pulse wave velocity was significantly elevated in ApoE(-/-) mice. Significantly increased vessel wall thickness was not found in ApoE(-/-) mice until the age of 30 weeks. Histological analysis of the aortae of ApoE(-/-) and WT mice showed that increased pulse wave velocity coincided with the fragmentation of the elastic laminae in the arterial wall, which is hypothesized to induce early vascular stiffening and may be promoted by macrophage-mediated matrix degradation.

CONCLUSIONS

We newly report that the assessment of local pulse wave velocity via MRI provides early information about the local progression of atherosclerosis before macroscopic alterations of the vessel wall occur.

Strategies for the preparation of bifunctional gadolinium(III) chelators

The development of gadolinium chelators that can be easily and readily linked to various substrates is of primary importance for the development high relaxation efficiency and/or targeted magnetic resonance imaging (MRI) contrast agents. Over the last 25 years a large number of bifunctional chelators have been prepared. For the most part, these compounds are based on ligands that are already used in clinically approved contrast agents. More recently, new bifunctional chelators have been reported based on complexes that show a more potent relaxation effect, faster complexation kinetics and in some cases simpler synthetic procedures. This review provides an overview of the synthetic strategies used for the preparation of bifunctional chelators for MRI applications.

Animal models of cardiovascular diseases

Cardiovascular diseases are the first leading cause of death and morbidity in developed countries. The use of animal models have contributed to increase our knowledge, providing new approaches focused to improve the diagnostic and the treatment of these pathologies. Several models have been developed to address cardiovascular complications, including atherothrombotic and cardiac diseases, and the same pathology have been successfully recreated in different species, including small and big animal models of disease. However, genetic and environmental factors play a significant role in cardiovascular pathophysiology, making difficult to match a particular disease, with a single experimental model. Therefore, no exclusive method perfectly recreates the human complication, and depending on the model, additional considerations of cost, infrastructure, and the requirement for specialized personnel, should also have in mind. Considering all these facts, and depending on the budgets available, models should be selected that best reproduce the disease being investigated. Here we will describe models of atherothrombotic diseases, including expanding and occlusive animal models, as well as models of heart failure. Given the wide range of models available, today it is possible to devise the best strategy, which may help us to find more efficient and reliable solutions against human cardiovascular diseases.

Gadolinium Enhanced MR Coronary Vessel Wall Imaging at 3.0 Tesla

Purpose. We evaluated the influence of the time between low-dose gadolinium (Gd) contrast administration and coronary vessel wall enhancement (LGE) detected by 3T magnetic resonance imaging (MRI) in healthy subjects and patients with coronary artery disease (CAD). Materials and Methods. Four healthy subjects (4 men, mean age 29 ± 3 years and eleven CAD patients (6 women, mean age 61 ± 10 years) were studied on a commercial 3.0 Tesla (T) whole-body MR imaging system (Achieva 3.0 T; Philips, Best, The Netherlands). T1-weighted inversion-recovery coronary magnetic resonance imaging (MRI) was repeated up to 75 minutes after administration of low-dose Gadolinium (Gd) (0.1 mmol/kg Gd-DTPA). Results. LGE was seen in none of the healthy subjects, however in all of the CAD patients. In CAD patients, fifty-six of 62 (90.3%) segments showed LGE of the coronary artery vessel wall at time-interval 1 after contrast. At time-interval 2, 34 of 42 (81.0%) and at time-interval 3, 29 of 39 evaluable segments (74.4%) were enhanced. Conclusion. In this work, we demonstrate LGE of the coronary artery vessel wall using 3.0 T MRI after a single, low-dose Gd contrast injection in CAD patients but not in healthy subjects. In the majority of the evaluated coronary segments in CAD patients, LGE of the coronary vessel wall was already detectable 30–45 minutes after administration of the contrast agent.

Accurate assessment of carotid artery stenosis in atherosclerotic mice using accelerated high-resolution 3D magnetic resonance angiography

OBJECT

High-resolution magnetic resonance angiography (MRA) enables non-invasive detection and longitudinal monitoring of atherosclerosis in mouse models of human disease. However, MRA is hampered by long acquisition times putting high demands on the physiological stability of the animal. Therefore, we evaluated the feasibility of accelerated MRA using the parallel imaging technique SENSE with regard to both lesion detection and quantification.

MATERIALS AND METHODS

MRA acquisitions of supra-aortic vessels were performed in ApoE (-/-) mice that have been shown to develop atherosclerotic plaques. Findings obtained from accelerated data sets were compared to fully sampled reference data sets and histology.

RESULTS

Our results revealed only minor differences in detecting vascular lesions for data collections accelerated by factors of up to 3.3 using a four-element coil array. For vessels with a mean lumen diameter of 500 μm, morphometry of stenotic lesions revealed no substantial deviations from reference (fully sampled) data for all investigated acceleration factors. For the highest acceleration factor of 3.3, an average deviation of the degree of stenosis of 4.9 ± 3.6% was found. Common carotid stenoses assessed by in vivo MRA displayed a good correlation with histological analyses (slope of linear regression = 0.97, R (2) = 0.98).

CONCLUSION

According to the results of this work, we have demonstrated the feasibility and accuracy of accelerated high-resolution 3D ToF MRA in mice suitable for detailed depiction of mouse supra-aortic vessels and amenable to non-invasive quantification of small atherosclerotic lesions.

Two-photon intravital imaging of thrombus development

Thrombus development in mouse mesenteric vessels following laser-induced injury was monitored by high-resolution, near-real-time, two-photon, intravital microscopy. In addition to the use of fluorescently tagged fibrin(ogen) and platelets, plasma was labeled with fluorescently tagged dextran. Because blood cells exclude the dextran in the single plane, blood cells appear as black silhouettes. Thus, in addition to monitoring the accumulation of platelets and fibrin in the thrombus, the protocol detects the movement and incorporation of unlabeled cells in and around it. The developing thrombus perturbs the blood flow near the thrombus surface, which affects the incorporation of platelets and blood cells into the structure. The hemodynamic effects and incorporation of blood cells lead to the development of thrombi with heterogeneous domain structures. Additionally, image processing algorithms and simulations were used to quantify structural features of developing thrombi. This analysis suggests a novel mechanism to stop the growth of developing thrombus.

Effects of deep sedation or general anesthesia on cardiac function in mice undergoing cardiovascular magnetic resonance

BACKGROUND

Genetically engineered mouse models of human cardiovascular disease provide an opportunity to understand critical pathophysiological mechanisms. Cardiovascular magnetic resonance (CMR) provides precise reproducible assessment of cardiac structure and function, but, in contrast to echocardiography, requires that the animal be immobilized during image acquisition. General anesthetic regimens yield satisfactory images, but have the potential to significantly perturb cardiac function. The purpose of this study was to assess the effects of general anesthesia and a new deep sedation regimen, respectively, on cardiac function in mice as determined by CMR, and to compare them to results obtained in mildly sedated conscious mice by echocardiography.

RESULTS

In 6 mildly sedated normal conscious mice assessed by echo, heart rate was 615 +/- 25 min-1 (mean +/- SE) and left ventricular ejection fraction (LVEF) was 0.94 +/- 0.01. In the CMR studies of normal mice, heart rate was slightly lower during deep sedation with morphine/midazolam (583 +/- 30 min-1), but the difference was not statistically significant. General anesthesia with 1% inhaled isoflurane significantly depressed heart rate (468 +/- 7 min-1, p < 0.05 vs. conscious sedation). In 6 additional mice with ischemic LV failure, trends in heart rate were similar, but not statistically significant. In normal mice, deep sedation depressed LVEF (0.79 +/- 0.04, p < 0.05 compared to light sedation), but to a significantly lesser extent than general anesthesia (0.60 +/- 0.04, p < 0.05 vs. deep sedation). In mice with ischemic LV failure, ejection fraction measurements were comparable when performed during light sedation, deep sedation, and general anesthesia, respectively. Contrast-to-noise ratios were similar during deep sedation and during general anesthesia, indicating comparable image quality. Left ventricular mass measurements made by CMR during deep sedation were nearly identical to those made during general anesthesia (r2 = 0.99, mean absolute difference < 4%), indicating equivalent quantitative accuracy obtained with the two methods. The imaging procedures were well-tolerated in all mice.

CONCLUSION

In mice with normal cardiac function, CMR during deep sedation causes significantly less depression of heart rate and ejection fraction than imaging during general anesthesia with isoflurane. In mice with heart failure, the sedation/anesthesia regimen had no clear impact on cardiac function. Deep sedation and general anesthesia produced CMR with comparable image quality and quantitative accuracy.

The application of discovery toxicology and pathology towards the design of safer pharmaceutical lead candidates

Toxicity is a leading cause of attrition at all stages of the drug development process. The majority of safety-related attrition occurs preclinically, suggesting that approaches to identify 'predictable' preclinical safety liabilities earlier in the drug development process could lead to the design and/or selection of better drug candidates that have increased probabilities of becoming marketed drugs. In this Review, we discuss how the early application of preclinical safety assessment--both new molecular technologies as well as more established approaches such as standard repeat-dose rodent toxicology studies--can identify predictable safety issues earlier in the testing paradigm. The earlier identification of dose-limiting toxicities will provide chemists and toxicologists the opportunity to characterize the dose-limiting toxicities, determine structure-toxicity relationships and minimize or circumvent adverse safety liabilities.