Murine ultrasound imaging for circumferential strain analyses in the angiotensin II abdominal aortic aneurysm model

Evolving Mural Defects, Dilatation, and Biomechanical Dysfunction in Angiotensin II-Induced Thoracic Aortopathies

BACKGROUND

Thoracic aortopathy associates with extracellular matrix remodeling and altered biomechanical properties. We sought to quantify the natural history of thoracic aortopathy in a common mouse model and to correlate measures of wall remodeling such as aortic dilatation or localized mural defects with evolving microstructural composition and biomechanical properties of the wall.

METHODS

We combined a high-resolution multimodality imaging approach (panoramic digital image correlation and optical coherence tomography) with histopathologic examinations and biaxial mechanical testing to correlate spatially, for the first time, macroscopic mural defects and medial degeneration within the ascending aorta with local changes in aortic wall composition and mechanical properties.

RESULTS

Findings revealed strong correlations between local decreases in elastic energy storage and increases in circumferential material stiffness with increasing proximal aortic diameter and especially mural defect size. Mural defects tended to exhibit a pronounced biomechanical dysfunction that is driven by an altered organization of collagen and elastic fibers.

CONCLUSIONS

While aneurysmal dilatation is often observed within particular segments of the aorta, dissection and rupture initiate as highly localized mechanical failures. We show that wall composition and material properties are compromised in regions of local mural defects, which further increases the dilatation and overall structural vulnerability of the wall. Identification of therapies focused on promoting robust collagen accumulation may protect the wall from these vulnerabilities and limit the incidence of dissection and rupture.

Molecular Imaging of Aortic Aneurysm and Its Translational Power for Clinical Risk Assessment

Aortic aneurysms (AAs) are dilations of the aorta, that are often fatal upon rupture. Diagnostic radiological techniques such as ultrasound (US), magnetic resonance imaging (MRI), and computed tomography (CT) are currently used in clinical practice for early diagnosis as well as clinical follow-up for preemptive surgery of AA and prevention of rupture. However, the contemporary imaging-based risk prediction of aneurysm enlargement or life-threatening aneurysm-rupture remains limited as these are restricted to visual parameters which fail to provide a personalized risk assessment. Therefore, new insights into early diagnostic approaches to detect AA and therefore to prevent aneurysm-rupture are crucial. Multiple new techniques are developed to obtain a more accurate understanding of the biological processes and pathological alterations at a (micro)structural and molecular level of aortic degeneration. Advanced anatomical imaging combined with molecular imaging, such as molecular MRI, or positron emission tomography (PET)/CT provides novel diagnostic approaches for in vivo visualization of targeted biomarkers. This will aid in the understanding of aortic aneurysm disease pathogenesis and insight into the pathways involved, and will thus facilitate early diagnostic analysis of aneurysmal disease. In this study, we reviewed these molecular imaging modalities and their association with aneurysm growth and/or rupture risk and their limitations. Furthermore, we outline recent pre-clinical and clinical developments in molecular imaging of AA and provide future perspectives based on the advancements made within the field. Within the vastness of pre-clinical markers that have been studied in mice, molecular imaging targets such as elastin/collagen, albumin, matrix metalloproteinases and immune cells demonstrate promising results regarding rupture risk assessment within the pre-clinical setting. Subsequently, these markers hold potential as a future diagnosticum of clinical AA assessment. However currently, clinical translation of molecular imaging is still at the onset. Future human trials are required to assess the effectivity of potentially viable molecular markers with various imaging modalities for clinical rupture risk assessment.

Salt-Sensitive Hypertension in GR+/- Rats Is Accompanied with Dysregulation in Adrenal Soluble Epoxide Hydrolase and Polyunsaturated Fatty Acid Pathways

Mutations within the glucocorticoid receptor (GR) gene locus lead to glucocorticoid resistance which is characterized by several clinical symptoms such as adrenal gland hyperplasia and salt-sensitive hypertension, although the underlying mechanisms are still unknown. We studied GR haploinsufficient (GR^+/−) Sprague Dawley rats which, on a standard diet, showed significantly increased plasma aldosterone and corticosterone levels and an adrenocortex hyperplasia accompanied by a normal systolic blood pressure. Following a high salt diet, these rats developed salt-sensitive hypertension and maintained elevated enzyme-soluble epoxide hydrolase (sEH) in adrenal glands, while sEH was significantly decreased in wild-type rats. Furthermore, GR^+/− rats showed dysregulation of the equilibrated linoleic and arachidonic acid pathways, with a significant increase of less active metabolites such as 8,9-DiHETrE. In Sprague Dawley rats, GR haploinsufficiency induced steroid disturbances, which provoked hypertension only in combination with high salt intake, which was accompanied by disturbances in sEH and fatty acid metabolism. Our results suggest that sEH inhibition could be a potential target to treat hypertension in patients with GR haploinsufficiency.

Assessment of the Global and Regional Circumferential Strain of Abdominal Aortic Aneurysm with Different Size by Speckle-Tracking Echocardiography

OBJECTIVES

We aimed to use speckle-tracking echocardiography (STE) to quantify circumferential aortic strain of abdominal aortic aneurysms (AAA) with different size.

METHODS

A total of 87 AAA patients were included. The morphological variables, including aortic maximum diameter (MD), end systolic area (ESA), end diastolic area (EDA), and thickness and area of intraluminal thrombus (ILT), were measured by ultrasound. STE was applied to calculate circumferential strain (CS) at 6 equally divided segments of the aorta at MD. We evaluated the mean value of peak strain along the 6 segments as global circumferential strain (GCS).

RESULTS

Large AAA (≥5.5 cm) patients had higher MD, ESA, EDA, AAA length, ILT thickness, and area, but lower fractional area change, GCS, and segmental CSs than small AAA (<5.5 cm) subjects (all P < .05). Compared with AAA <4.5 cm group, AAA patients ≥4.5 cm possessed increased MD, ESA, EDA, AAA length, ILT thickness, and area, which results were also reflected in the comparison between AAA <6.5 and ≥6.5 cm group. In small AAA patients, GCS and regional strains in CS1, CS3, and CS5 segments were lower in AAA subjects ≥4.5 cm than those <4.5 cm (all P<.05). However, no significant differences in the GCS and regional CS between ≥6.5 and <6.5 cm group were found. Correlation analysis revealed a significant negative association of GCS with MD, ESA, and EDA, even after adjusting the potential confounding factors (all P < .05).

CONCLUSIONS

Our findings may yield insight into the structural strain characteristics of AAA wall with different size, which adds the benefit of using simple echocardiography-derived biomechanics to stratify AAA patients.

Systemic delivery of targeted nanotherapeutic reverses angiotensin II-induced abdominal aortic aneurysms in mice

Abdominal aortic aneurysm (AAA) disease causes dilation of the aorta, leading to aortic rupture and death if not treated early. It is the 14th leading cause of death in the U.S. and 10th leading cause of death in men over age 55, affecting thousands of patients. Despite the prevalence of AAA, no safe and efficient pharmacotherapies exist for patients. The deterioration of the elastic lamina in the aneurysmal wall is a consistent feature of AAAs, making it an ideal target for delivering drugs to the AAA site. In this research, we conjugated nanoparticles with an elastin antibody that only targets degraded elastin while sparing healthy elastin. After induction of aneurysm by 4-week infusion of angiotensin II (Ang II), two biweekly intravenous injections of pentagalloyl glucose (PGG)-loaded nanoparticles conjugated with elastin antibody delivered the drug to the aneurysm site. We show that targeted delivery of PGG could reverse the aortic dilation, ameliorate the inflammation, restore the elastic lamina, and improve the mechanical properties of the aorta at the AAA site. Therefore, simple iv therapy of PGG loaded nanoparticles can be an effective treatment option for early to middle stage aneurysms to reverse disease progression and return the aorta to normal homeostasis.

Early Morphofunctional Changes in AngII-Infused Mice Contribute to Regional Onset of Aortic Aneurysm and Dissection

Aortic aneurysms and dissections are silent and lethal conditions, whose pathogenesis remains incompletely understood. Although angiotensin II (AngII)-infused ApoE-/- mice have been widely used to study aortic aneurysm and dissection, early morphofunctional alterations preceding the onset of these conditions remain unknown. The goal of this study was to unveil early morphofunctional changes underlying the onset of aneurysm and dissection. At 3 days post-AngII infusion, suprarenal abdominal aorta presented significant volumetric dilatation and microstructural damage. Ex vivo assessment of vascular reactivity of the suprarenal dissection-prone aorta and its side branches, showed an endothelial and contractile dysfunctions that were severe in the suprarenal aorta, moderate distally, and absent in the side branches, mirroring the susceptibility to dissection of these different vascular segments. Early and specific morphofunctional changes of the suprarenal aorta may contribute to the regional onset of aortic aneurysm and dissection by exacerbating the biomechanical burden arising from its side branches.

Co-localization of microstructural damage and excessive mechanical strain at aortic branches in angiotensin-II-infused mice

Animal models of aortic aneurysm and dissection can enhance our limited understanding of the etiology of these lethal conditions particularly because early-stage longitudinal data are scant in humans. Yet, the pathogenesis of often-studied mouse models and the potential contribution of aortic biomechanics therein remain elusive. In this work, we combined micro-CT and synchrotron-based imaging with computational biomechanics to estimate in vivo aortic strains in the abdominal aorta of angiotensin-II-infused ApoE-deficient mice, which were compared with mouse-specific aortic microstructural damage inferred from histopathology. Targeted histology showed that the 3D distribution of micro-CT contrast agent that had been injected in vivo co-localized with precursor vascular damage in the aortic wall at 3 days of hypertension, with damage predominantly near the ostia of the celiac and superior mesenteric arteries. Computations similarly revealed higher mechanical strain in branching relative to non-branching regions, thus resulting in a positive correlation between high strain and vascular damage in branching segments that included the celiac, superior mesenteric, and right renal arteries. These results suggest a mechanically driven initiation of damage at these locations, which was supported by 3D synchrotron imaging of load-induced ex vivo delaminations of angiotensin-II-infused suprarenal abdominal aortas. That is, the major intramural delamination plane in the ex vivo tested aortas was also near side branches and specifically around the celiac artery. Our findings thus support the hypothesis of an early mechanically mediated formation of microstructural defects at aortic branching sites that subsequently propagate into a macroscopic medial tear, giving rise to aortic dissection in angiotensin-II-infused mice.

Gold nanoparticles that target degraded elastin improve imaging and rupture prediction in an AngII mediated mouse model of abdominal aortic aneurysm

Background: Abdominal aortic aneurysms (AAA) are characterized by a progressive disruption and weakening of the extracellular matrix (ECM) leading to dilation of the aorta which can be fatal if not treated. Current diagnostic imaging modalities provides little insight on the varying degree of ECM degeneration that precedes rupture in AAAs. Targeted delivery of contrast agents such as gold nanoparticles (GNPs) that bind to degraded matrix could prove useful when combined with computed tomography (CT) to provide a non-invasive surrogate marker of AAA rupture potential.

Methods: AAAs were induced by chronic infusion of angiotensin II (AngII) into low density-lipoprotein receptor-deficient (LDLr -/-) mice in combination with a high-fat diet. Abdominal ultrasound was used to monitor disease progression and to assess the circumferential strain throughout the cardiac cycle. At six weeks, GNPs conjugated with an elastin antibody (EL-GNP) were injected retro-orbitally. Mice were euthanized 24 hours after EL-GNP injection, and aortas were explanted and scanned ex-vivo with a micro-CT system. Histological assessment and 3D models of the aneurysms with micro-CT were used to determine the EL-GNPs distribution. Isolated vessel burst pressure testing was performed on each aneurysmal aorta to quantify rupture strength and to assess rupture location.

Results: Aneurysms were found along the suprarenal aorta in AngII infused mice. Darkfield microscopy indicated EL-GNPs accumulation around the site of degraded elastin while avoiding the healthy and intact elastin fibers. Using nonlinear regression, the micro-CT signal intensity of EL-GNPs along the suprarenal aortas correlated strongly with burst pressures (R²=0.9415) but not the dilation as assessed by ultrasound measurements.

Conclusions: Using an established mouse model of AAA, we successfully demonstrated in vivo targeting of EL-GNPs to damaged aortic elastin and correlated micro-CT-based signal intensities with burst pressures. Thus, we show that this novel targeting technique can be used as a diagnostic tool to predict the degree of elastin damage and therefore rupture potential in AAAs better than the extent of dilation.

A preclinical ultrasound method for the assessment of vascular disease progression in murine models

Introduction

The efficacy of preclinical ultrasound at providing a quantitative assessment of mouse models of vascular disease is relatively unknown. In this study, preclinical ultrasound was used in combination with a semi-automatic image processing method to track arterial distension alterations in mouse models of abdominal aortic aneurysm and atherosclerosis.

Methods

Longitudinal B-mode ultrasound images of the abdominal aorta were acquired using a preclinical ultrasound scanner. Arterial distension was assessed using a semi-automatic image processing algorithm to track vessel wall motion over the cardiac cycle. A standard, manual analysis method was applied for comparison.

Results

Mean arterial distension was significantly lower in abdominal aortic aneurysm mice between day 0 and day 7 post-onset of disease (p < 0.01) and between day 0 and day 14 (p < 0.001), while no difference was observed in sham control mice. Manual analysis detected a significant decrease (p < 0.05) between day 0 and day 14 only. Atherosclerotic mice showed alterations in arterial distension relating to genetic modification and diet. Arterial distension was significantly lower (p < 0.05) in Ldlr^-/- (++/--) mice fed high-fat western diet when compared with both wild type (++/++) mice and Ldlr^-/- (++/--) mice fed chow diet. The manual method did not detect a significant difference between these groups.

Conclusions

Arterial distension can be used as an early marker for the detection of arterial disease in murine models. The semi-automatic analysis method provided increased sensitivity to differences between experimental groups when compared to the manual analysis method.

Aortic Strain Correlates with Elastin Fragmentation in Fibrillin-1 Hypomorphic Mice

Background

High frequency ultrasound has facilitated in vivo measurements of murine ascending aortas, allowing aortic strains to be gleaned from two-dimensional images. Thoracic aortic aneurysms associated with mutations in fibrillin-1 (FBN1) display elastin fragmentation, which may impact aortic strain. In this study, we determined the relationship between elastin fragmentation and aortic circumferential strain in wild type and fibrillin-1 hypomorphic (FBN1 ^mgR/mgR) mice.

Methods and Results

Luminal diameters of the ascending aorta from wild type and FBN1 hypomorphic (FBN1 ^mgR/mgR) mice were measured in systole and diastole. Expansion of the ascending aorta during systole in male and female wild type mice was 0.21±0.02 mm (16.3%) and 0.21±0.01 mm (17.0%) respectively, while expansion in male and female FBN1 ^mgR/mgR mice was 0.11±0.04 mm (4.9%) and 0.07±0.02 mm (4.5%) respectively. Reduced circumferential strain was observed in FBN1 ^mgR/mgR mice compared to wild type littermates. Elastin fragmentation was inversely correlated to circumferential strain (R^2 = 0.628 p = 0.004) and significantly correlated with aortic diameter. (R^2 = 0.397, p = 0.038 in systole and R^2 = 0.515, p =0.013 in diastole).

Conclusions

FBN1 ^mgR/mgR mice had increased aortic diameters, reduced circumferential strain, and increased elastin fragmentation. Elastin fragmentation in FBN1 ^mgR/mgR and their wild type littermates was correlated with reduced circumferential strain.

Site-specific chelation therapy with EDTA-loaded albumin nanoparticles reverses arterial calcification in a rat model of chronic kidney disease

Medial arterial calcification (MAC) is a common outcome in diabetes and chronic kidney disease (CKD). It occurs as linear mineral deposits along the degraded elastin lamellae and is responsible for increased aortic stiffness and subsequent cardiovascular events. Current treatments for calcification, particularly in CKD, are predominantly focused on regulating the mineral disturbance and other risk factors. Ethylene diamine tetraacetic acid (EDTA), a chelating agent, can resorb mineral deposits, but the systemic delivery of EDTA may cause side effects such as hypocalcemia and bone resorption. We have developed elastin antibody conjugated albumin nanoparticles that target only degraded elastin in vasculature while sparing healthy tissues. In this study, we tested a targeted nanoparticle-based EDTA chelation therapy to reverse CKD-associated MAC. Renal failure was induced in Sprague-Dawley rats by a high adenine diet supplemented by high P and Ca for 28 days that led to MAC. Intravenous delivery of DiR dye-loaded nanoparticles confirmed targeting to vascular degraded elastin and calcification sites within 24 hours. Next, EDTA-loaded albumin nanoparticles conjugated with an anti-elastin antibody were intravenously injected twice a week for two weeks. The targeted nanoparticles delivered EDTA at the site of vascular calcification and reversed mineral deposits without any untoward effects. Systemic EDTA injections or blank nanoparticles were ineffective in reversing MAC. Reversal of calcification seems to be stable as it did not return after the treatment was stopped for an additional four weeks. Targeted EDTA chelation therapy successfully reversed calcification in this adenine rat model of CKD. We consider that targeted NP therapy will provide an attractive option to reverse calcification and has a high potential for clinical translation.

Early pathological characterization of murine dissecting abdominal aortic aneurysms

We report here on the early pathology of a well-established murine model of dissecting abdominal aortic aneurysms (AAAs). Continuous infusion of angiotensin II (AngII) into apolipoprotein E-deficient mice induces the formation of aortic dissection and expansion at some point after implantation of miniosmotic pumps containing AngII. While this model has been studied extensively at a chronic stage, we investigated the early pathology of dissecting AAA formation at multiple scales. Using high-frequency ultrasound, we screened 12-week-old male mice daily for initial formation of these aneurysmal lesions between days 3 and 10 post-implantation. We euthanized animals on the day of diagnosis of a dissecting AAA or at day 10 if no aneurysmal lesion developed. Aortic expansion and reduced vessel wall strain occurred in animals regardless of whether a dissecting AAA developed by day 10. The aortas of mice that did not develop dissecting AAAs showed intermediate changes in morphology and biomechanical properties. RNA sequencing and gene expression analysis revealed multiple proinflammatory and matrix remodeling genes to be upregulated in the suprarenal aorta of AngII-infused mice as compared to saline-infused controls. Histology and immunohistochemistry confirmed that extracellular matrix remodeling and inflammatory cell infiltration, notably neutrophils and macrophages, occurred in AngII-infused mice with and without dissecting AAAs but not saline-infused controls. Understanding early disease processes is a critical step forward in translating experimental results in cardiovascular disease research. This work advances our understanding of this well-established murine model with applications for improving early diagnosis and therapy of acute aortic syndrome in humans.

Combining in vivo and in vitro biomechanical data reveals key roles of perivascular tethering in central artery function

Considerable insight into effectors of cardiovascular function can be gleaned from controlled studies on mice, especially given the diverse models that are available. Toward this end, however, there is a need for consistent and complementary methods of in vivo and in vitro data analysis, synthesis, and interpretation. The overall objective of this study is twofold. First, we present new semi-automated methods to quantify in vivo measurements of vascular function in anesthetized mice as well as new approaches to synthesize these data with those from in vitro biaxial mechanical characterizations. Second, we contrast regional differences in biomechanical behaviors along the central vasculature by combining biaxial strains measured in vivo with data on the unloaded geometry and biaxial material properties measured in vitro. Results support the hypothesis that the healthy ascending aorta stores significant elastic energy during systole, which is available to work on the heart and blood during diastole, particularly during periods of physical exertion, and further suggest that perivascular tethering allows arteries to work at lower values of wall stress and material stiffness than often assumed. The numerous measurements of vascular function and properties provided herein can also serve as reference values for normal wild-type male and female mice, to which values for myriad genetic, surgical, and pharmacological models can be compared in future studies.

Echocardiographic Strain Analysis for the Early Detection of Myocardial Structural Abnormality and Initiation of Drug Therapy in a Mouse Model of Dilated Cardiomyopathy

This study aimed to evaluate the role of echocardiography-based strain analysis in the early diagnosis and guidance for management of dilated cardiomyopathy (DCM). Muscular dystrophy mice (which spontaneously develop DCM) and control (C57 BL/6 J) mice were sequentially evaluated by ultrasound biomicroscopy, conventional left ventricle (LV) measurement, two-dimensional (2-D) strain analysis and myocardial histologic analysis for 12 consecutive months. Significant alternation of LV remodeling and dysfunction could be detected by conventional echocardiography after 9 mo, by strain analysis after 5 mo and by histologic analysis after 4 mo. The global longitudinal systolic peak strain (PK) was the most sensitive strain marker for early detection of myocardial structural abnormality in the subclinical stage. Moreover, losartan administration before the PK decrease was associated with significantly preserved LV function. These results suggest that myocardial strain analysis (particularly longitudinal PK) is sensitive for the early detection of LV dysfunction in mice with dilated cardiomyopathy.

Multi-Modality Imaging Enables Detailed Hemodynamic Simulations in Dissecting Aneurysms in Mice

A multi-modality imaging-based modeling approach was used to study complex unsteady hemodynamics and lesion growth in a dissecting abdominal aortic aneurysm model. We combined in vivo ultrasound (geometry and flow) and in vitro optical coherence tomography(OCT) (geometry) to obtain the high resolution needed to construct detailed hemodynamic simulations over large portions of the murine vasculature, which include fine geometric complexities. We illustrate this approach for a spectrum of dissecting abdominal aortic aneurysms induced in male apolipoprotein E-null mice by high-dose angiotensin II infusion. In vivo morphological and hemodynamic data provide information on volumetric lesion growth and changes in blood flow dynamics, respectively, occurring from the day of initial aortic expansion. We validated the associated computational models by comparing results on time-varying outlet flows and vortical structures within the lesions. Three out of four lesions exhibited abrupt formation of thrombus, though different in size. We determined that a lesion without thrombus formed with a thickened vessel wall, which was resolvable by OCT and histology. We attribute differences in final sizes and compositions of these lesions to the different computed flow and vortical structures we obtained in our mouse-specific fluid dynamic models. Differences in morphology and hemodynamics play crucial roles in determining the evolution of dissecting abdominal aortic aneurysms. Coupled high resolution in vivo and in vitro imaging approaches provide much-improved geometric models for hemodynamic simulations. Our imaging-based computational findings suggest a link between perturbations in hemodynamic metrics and aneurysmal disease heterogeneity.

Reversal of Vascular Calcification and Aneurysms in a Rat Model Using Dual Targeted Therapy with EDTA- and PGG-Loaded Nanoparticles

Degeneration of elastic lamina and vascular calcification are common features of vascular pathology such as aortic aneurysms. We tested whether dual therapy with targeted nanoparticles (NPs) can remove mineral deposits (by delivery of a chelating agent, ethylene diamine tetraacetic acid (EDTA)) and restore elastic lamina (by delivery of a polyphenol, pentagalloyl glucose (PGG)) to reverse moderate aneurysm development. EDTA followed by PGG NP delivery led to reduction in macrophage recruitment, matrix metalloproteinase (MMP) activity, elastin degradation and calcification in the aorta as compared to delivery of control blank NPs. Such dual therapy restored vascular elastic lamina and improved vascular function as observed by improvement in circumferential strain. Therefore, dual targeted therapy may be an attractive option to remove mineral deposits and restore healthy arterial structures in moderately developed aneurysms.

Local Application of Leptin Antagonist Attenuates Angiotensin II-Induced Ascending Aortic Aneurysm and Cardiac Remodeling

Background

Ascending thoracic aortic aneurysm (ATAA) is driven by angiotensin II (AngII) and contributes to the development of left ventricular (LV) remodeling through aortoventricular coupling. We previously showed that locally available leptin augments AngII‐induced abdominal aortic aneurysms in apolipoprotein E–deficient mice. We hypothesized that locally synthesized leptin mediates AngII‐induced ATAA.

Methods and Results

Following demonstration of leptin synthesis in samples of human ATAA associated with different etiologies, we modeled in situ leptin expression in apolipoprotein E–deficient mice by applying exogenous leptin on the surface of the ascending aorta. This treatment resulted in local aortic stiffening and dilation, LV hypertrophy, and thickening of aortic/mitral valve leaflets. Similar results were obtained in an AngII‐infusion ATAA mouse model. To test the dependence of AngII‐induced aortic and LV remodeling on leptin activity, a leptin antagonist was applied to the ascending aorta in AngII‐infused mice. Locally applied single low‐dose leptin antagonist moderated AngII‐induced ascending aortic dilation and protected mice from ATAA rupture. Furthermore, LV hypertrophy was attenuated and thickening of aortic valve leaflets was moderated. Last, analysis of human aortic valve stenosis leaflets revealed de novo leptin synthesis, whereas exogenous leptin stimulated proliferation and promoted mineralization of human valve interstitial cells in culture.

Conclusions

AngII‐induced ATAA is mediated by locally synthesized leptin. Aortoventricular hemodynamic coupling drives LV hypertrophy and promotes early aortic valve lesions, possibly mediated by valvular in situ leptin synthesis. Clinical implementation of local leptin antagonist therapy may attenuate AngII‐induced ATAA and moderate related LV hypertrophy and pre–aortic valve stenosis lesions.

Clinical Trial Registration

URL: https://www.clinicaltrials.gov/. Unique identifier: NCT00449306.

Geroscience approaches to increase healthspan and slow aging

For decades, researchers in the biology of aging have focused on defining mechanisms that modulate aging by primarily studying a single metric, sometimes described as the "gold standard" lifespan. Increasingly, geroscience research is turning towards defining functional domains of aging such as the cardiovascular system, skeletal integrity, and metabolic health as being a more direct route to understand why tissues decline in function with age. Each model used in aging research has strengths and weaknesses, yet we know surprisingly little about how critical tissues decline in health with increasing age. Here I discuss popular model systems used in geroscience research and their utility as possible tools in preclinical studies in aging.

Ascending Aortic Aneurysm in Angiotensin II-Infused Mice: Formation, Progression, and the Role of Focal Dissections

OBJECTIVE

To understand the anatomy and physiology of ascending aortic aneurysms in angiotensin II-infused ApoE(-/-) mice.

APPROACH AND RESULTS

We combined an extensive in vivo imaging protocol (high-frequency ultrasound and contrast-enhanced microcomputed tomography at baseline and after 3, 10, 18, and 28 days of angiotensin II infusion) with synchrotron-based ultrahigh resolution ex vivo imaging (phase contrast X-ray tomographic microscopy) in n=47 angiotensin II-infused mice and 6 controls. Aortic regurgitation increased significantly over time, as did the luminal volume of the ascending aorta. In the samples that were scanned ex vivo, we observed one or several focal dissections, with the largest located in the outer convex aspect of the ascending aorta. The volume of the dissections moderately correlated to the volume of the aneurysm as measured in vivo (r(2)=0.46). After 3 days of angiotensin II infusion, we found an interlaminar hematoma in 7/12 animals, which could be linked to an intimal tear. There was also a significant increase in single laminar ruptures, which may have facilitated a progressive enlargement of the focal dissections over time. At later time points, the hematoma was resorbed and the medial and adventitial thickness increased. Fatal transmural dissection occurred in 8/47 mice at an early stage of the disease, before adventita remodeling.

CONCLUSIONS

We visualized and quantified the dissections that lead to ascending aortic aneurysms in angiotensin II-infused mice and provided unique insight into the temporal evolution of these lesions.

Performance comparison of ultrasound-based methods to assess aortic diameter and stiffness in normal and aneurysmal mice

OBJECTIVE

Several ultrasound-based methods are currently used to assess aortic diameter, circumferential strain and stiffness in mice, but none of them is flawless and a gold standard is lacking. We aimed to assess the validity and sensitivity of these methods in control animals and animals developing dissecting abdominal aortic aneurysm.

METHODS AND RESULTS

We first compared systolic and diastolic diameters as well as local circumferential strains obtained in 47 Angiotensin II-infused ApoE(-/-) mice with three different techniques (BMode, short axis MMode, long axis MMode), at two different abdominal aortic locations (supraceliac and paravisceral), and at three different time points of abdominal aneurysm formation (baseline, 14 days and 28 days). We found that short axis BMode was preferred to assess diameters, but should be avoided for strains. Short axis MMode gave good results for diameters but high standard deviations for strains. Long axis MMode should be avoided for diameters, and was comparable to short axis MMode for strains. We then compared pulse wave velocity measurements using global, ultrasound-based transit time or regional, pressure-based transit time in 10 control and 20 angiotensin II-infused, anti-TGF-Beta injected C57BL/6 mice. Both transit-time methods poorly correlated and were not able to detect a significant difference in PWV between controls and aneurysms. However, a combination of invasive pressure and MMode diameter, based on radio-frequency data, detected a highly significant difference in local aortic stiffness between controls and aneurysms, with low standard deviation.

CONCLUSIONS

In small animal ultrasound the short axis view is preferred over the long axis view to measure aortic diameters, local methods are preferred over transit-time methods to measure aortic stiffness, invasive pressure-diameter data are preferred over non-invasive strains to measure local aortic stiffness, and the use of radiofrequency data improves the accuracy of diameter, strain as well as stiffness measurements.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

Morphological and Biomechanical Differences in the Elastase and AngII apoE(-/-) Rodent Models of Abdominal Aortic Aneurysms

An abdominal aortic aneurysm (AAA) is a potentially fatal cardiovascular disease with multifactorial development and progression. Two preclinical models of the disease (elastase perfusion and angiotensin II infusion in apolipoprotein-E-deficient animals) have been developed to study the disease during its initiation and progression. To date, most studies have used ex vivo methods to examine disease characteristics such as expanded aortic diameter or analytic methods to look at circulating biomarkers. Herein, we provide evidence from in vivo ultrasound studies of the temporal changes occurring in biomechanical parameters and macromolecules of the aortic wall in each model. We present findings from 28-day studies in elastase-perfused rats and AngII apoE(-/-) mice. While each model develops AAAs specific to their induction method, they both share characteristics with human aneurysms, such as marked changes in vessel strain and blood flow velocity. Histology and nonlinear microscopy confirmed that both elastin and collagen, both important extracellular matrix molecules, are similarly affected in their levels and spatial distribution. Future studies could make use of the differences between these models in order to investigate mechanisms of disease progression or evaluate potential AAA treatments.

Ultrasound speckle tracking for radial, longitudinal and circumferential strain estimation of the carotid artery--an in vitro validation via sonomicrometry using clinical and high-frequency ultrasound

Ultrasound speckle tracking for carotid strain assessment has in the past decade gained interest in studies of arterial stiffness and cardiovascular diseases. The aim of this study was to validate and directly contrast carotid strain assessment by speckle tracking applied on clinical and high-frequency ultrasound images in vitro. Four polyvinyl alcohol phantoms mimicking the carotid artery were constructed with different mechanical properties and connected to a pump generating carotid flow profiles. Gray-scale ultrasound long- and short-axis images of the phantoms were obtained using a standard clinical ultrasound system, Vivid 7 (GE Healthcare, Horten, Norway) and a high-frequency ultrasound system, Vevo 2100 (FUJIFILM, VisualSonics, Toronto, Canada) with linear-array transducers (12L/MS250). Radial, longitudinal and circumferential strains were estimated using an in-house speckle tracking algorithm and compared with reference strain acquired by sonomicrometry. Overall, the estimated strain corresponded well with the reference strain. The correlation between estimated peak strain in clinical ultrasound images and reference strain was 0.91 (p<0.001) for radial strain, 0.73 (p<0.001) for longitudinal strain and 0.90 (p<0.001) for circumferential strain and for high-frequency ultrasound images 0.95 (p<0.001) for radial strain, 0.93 (p<0.001) for longitudinal strain and 0.90 (p<0.001) for circumferential strain. A significant larger bias and root mean square error was found for circumferential strain estimation on clinical ultrasound images compared to high frequency ultrasound images, but no significant difference in bias and root mean square error was found for radial and longitudinal strain when comparing estimation on clinical and high-frequency ultrasound images. The agreement between sonomicrometry and speckle tracking demonstrates that carotid strain assessment by ultrasound speckle tracking is feasible.

Acute reductions in mechanical wall strain precede the formation of intimal hyperplasia in a murine model of arterial occlusive disease

OBJECTIVE

Intimal hyperplasia (IH) continues to plague the durability of vascular interventions. Employing a validated murine model, ultrasound biomicroscopy, and speckle-tracking algorithms, we tested the hypothesis that reduced cyclic arterial wall strain results in accentuated arterial wall IH.

METHODS

A 9-0 suture was tied around the left mouse (n = 10) common carotid artery and a 35-gauge (outer diameter = 0.14 mm) blunt mandrel. We previously showed that mandrel removal results in a ∼78% reduction in diameter and ∼85% reduction in flow, with subsequent delayed induction of IH by day 28. Preoperative, postoperative day-4 (before measurable IH), and postoperative day-27 circumferential wall strains were measured in locations 1, 2, and 3 mm proximal to the stenosis and in the same locations on the contralateral (nonstenosed) carotid. At postoperative day 28, arteries were perfusion fixed and arterial wall morphology was assessed microscopically in the same regions.

RESULTS

Strains were the same in all locations preoperatively. Wall strain was decreased in all regions proximal to the stenosis by day 4 (0.26 ± 0.01 to 0.11 ± 0.02; P < .001), while strains remained unchanged for the contralateral artery (P = .45). No statistical regional differences in mean strain or IH were noted at any time point for the experimental or contralateral artery. Based on the median, regions were divided into those with low strain (≤0.1) and high strain (>0.1). Average preoperative strains in both groups were the same (0.27 ± 0.09 and 0.27 ± 0.08). All segments in the low-strain group (n = 13) demonstrated significant IH formation by day 28, while only 31% of the high strain group demonstrated any detectable IH at day 28. (Mean low-strain intimal thickness = 32 ± 20 μm, high strain = 8.0 ± 16 μm; P < .01). Changes in cross-sectional area at diastole drove the reduction in strain in the low-strain group, increasing significantly from preoperatively to day 4 (P = .04), while lumen cross-section at systole remained unchanged (P = .46). Cross-sectional area at diastole and systole in the high-strain group remained unchanged from preoperatively to day 4 (P = .67).

CONCLUSIONS

Early reduction in arterial wall strain is associated with subsequent development of hemodynamically induced IH.

Long-term renin-angiotensin blocking therapy in hypertensive patients with normal aorta may attenuate the formation of abdominal aortic aneurysms

Renin-angiotensin system (RAS) has been implicated in the pathogenesis of abdominal aortic aneurysm (AAA). Angiotensin II type 1 receptor blocker (ARB), when given with angiotensin II prevents AAA formation in mice, but found ineffective in attenuating the progression of preexisting AAA. This study was designed to evaluate the effect of chronic RAS blockers on abdominal aortic diameter in hypertensive patients without known aortic aneurysm. Consecutive hypertensive outpatients (n = 122) were stratified according to antihypertensive therapy they received for 12 months or more, consisting of ARB (n = 45), angiotensin converting enzyme inhibitor (ACE-I; n = 45), or nonARB/nonACE-I (control therapy; n = 32). Abdominal ultrasonography was performed to measure maximal subrenal aortic diameter. Eighty-four patients were reexamined by ultrasonography 8 months later. The correlation between the different antihypertensive therapies and aortic diameter was examined. Aortic diameters were significantly smaller in ARB than in control patients in the baseline and follow-up measurements (P = .004; P = .0004, respectively). Risk factor adjusted covariance analysis showed significant differences between ARB or ACE-I treated groups and controls (P = .006 or P = .046, respectively). Ultrasound that was performed 8 months later showed smaller increases in mean aortic diameters of the ARB and ACE-I groups than in controls. Both ARB and ACE-I therapy attenuated expansion of nonaneurysmal abdominal aorta in humans. These results indicate that RAS blockade given before advancement of aortic medial remodeling may slow down the development of AAA.

Molecular imaging of experimental abdominal aortic aneurysms

Current laboratory research in the field of abdominal aortic aneurysm (AAA) disease often utilizes small animal experimental models induced by genetic manipulation or chemical application. This has led to the use and development of multiple high-resolution molecular imaging modalities capable of tracking disease progression, quantifying the role of inflammation, and evaluating the effects of potential therapeutics. In vivo imaging reduces the number of research animals used, provides molecular and cellular information, and allows for longitudinal studies, a necessity when tracking vessel expansion in a single animal. This review outlines developments of both established and emerging molecular imaging techniques used to study AAA disease. Beyond the typical modalities used for anatomical imaging, which include ultrasound (US) and computed tomography (CT), previous molecular imaging efforts have used magnetic resonance (MR), near-infrared fluorescence (NIRF), bioluminescence, single-photon emission computed tomography (SPECT), and positron emission tomography (PET). Mouse and rat AAA models will hopefully provide insight into potential disease mechanisms, and the development of advanced molecular imaging techniques, if clinically useful, may have translational potential. These efforts could help improve the management of aneurysms and better evaluate the therapeutic potential of new treatments for human AAA disease.