Influences of aortic motion and curvature on vessel expansion in murine experimental aneurysms

Terazosin attenuates abdominal aortic aneurysm formation by downregulating Peg3 expression to inhibit vascular smooth muscle cell apoptosis and senescence

Abdominal aortic aneurysm (AAA), a vascular degenerative disease, is a potentially life-threatening condition characterised by the loss of vascular smooth muscle cells (VSMCs), degradation of extracellular matrix (ECM), inflammation, and oxidative stress. Despite the severity of AAA, effective drugs for treatment are scarce. At low doses, terazosin (TZ) exerts antiapoptotic and anti-inflammatory effects in several diseases, but its potential to protect against AAA remains unexplored. Herein, we investigated the effects of TZ in two AAA animal models: Angiotensin II (Ang II) infusion in Apoe-/- mice and calcium chloride application in C57BL/6J mice. Mice were orally administered with TZ (100 or 1000 μg/kg/day). The in vivo results indicated that low-dose TZ alleviated AAA formation in both models. Low-dose TZ significantly reduced aortic pulse wave velocity without exerting an apparent antihypertensive effect in the Ang II-induced AAA model. Paternally expressed gene 3 (Peg3) was identified via RNA sequencing as a novel TZ target. PEG3 expression was significantly elevated in both mouse and human AAA tissues. TZ suppressed PEG3 expression and reduced the abundance of matrix metalloproteinases (MMP2/MMP9) in the tunica media. Functional experiments and molecular analyses revealed that TZ (10 nM) treatment and Peg3 knockdown effectively prevented Ang II-induced VSMC senescence and apoptosis in vitro. Thus, Peg3, a novel target of TZ, mediates inflammation-induced VSMC apoptosis and senescence. Low-dose TZ downregulates Peg3 expression to attenuate AAA formation and ECM degradation, suggesting a promising therapeutic strategy for AAA.

Longitudinal investigation of aortic dissection in mice with computational fluid dynamics

Predicting late adverse events in aortic dissections is challenging. One commonly observed risk factor is partial thrombosis of the false lumen. In this study we investigated false lumen thrombus progression over 7 days in four mice with angiotensin II-induced aortic dissection. We performed computational fluid dynamic simulations with subject-specific boundary conditions from velocity and pressure measurements. We investigated endothelial cell activation potential, mean velocity, thrombus formation potential, and other hemodynamic factors. Our findings support the hypothesis that flow stagnation is the predominant hemodynamic factor driving a large thrombus ratio in false lumina, particularly those with a single fenestration.

Statistical shape representation of the thoracic aorta: accounting for major branches of the aortic arch

Statistical shape modeling (SSM) is an emerging tool for risk assessment of thoracic aortic aneurysm. However, the head branches of the aortic arch are often excluded in SSM. We introduced an SSM strategy based on principal component analysis that accounts for aortic branches and applied it to a set of patient scans. Computational fluid dynamics were performed on the reconstructed geometries to identify the extent to which branch model accuracy affects the calculated wall shear stress (WSS) and pressure. Surface-averaged and location-specific values of pressure did not change significantly, but local WSS error was high near branches when inaccurately modeled.

Biomechanical characterization of tissue types in murine dissecting aneurysms based on histology and 4D ultrasound-derived strain

Abdominal aortic aneurysm disease is the local enlargement of the aorta, typically in the infrarenal section, causing up to 200,000 deaths/year. In vivo information to characterize the individual elastic properties of the aneurysm wall in terms of rupture risk is lacking. We used a method that combines 4D ultrasound and direct deformation estimation to compute in vivo 3D Green-Lagrange strain in murine angiotensin II-induced dissecting aortic aneurysms, a commonly used mouse model. After euthanasia, histological staining of cross-sectional sections along the aorta was performed in areas where in vivo strains had previously been measured. The histological sections were segmented into intact and fragmented elastin, thrombus with and without red blood cells, and outer vessel wall including the adventitia. Meshes were then created from the individual contours based on the histological segmentations. The isolated contours of the outer wall and lumen from both imaging modalities were registered individually using a coherent point drift algorithm. 2D finite element models were generated from the meshes, and the displacements from the registration were used as displacement boundaries of the lumen and wall contours. Based on the resulting deformed contours, the strains recorded were grouped according to segmented tissue regions. Strains were highest in areas containing intact elastin without thrombus attachment. Strains in areas with intact elastin and thrombus attachment, as well as areas with disrupted elastin, were significantly lower. Strains in thrombus regions with red blood cells were significantly higher compared to thrombus regions without. We then compared this analysis to statistical distribution indices and found that the results of each aligned, elucidating the relationship between vessel strain and structural changes. This work demonstrates the possibility of advancing in vivo assessments to a microstructural level ultimately improving patient outcomes.

E-cigarette exposure augments murine abdominal aortic aneurysm development: role of Chil1

AIMS

Abdominal aortic aneurysm (AAA) is a common cardiovascular disease with a strong correlation to smoking, although underlying mechanisms have been minimally explored. Electronic cigarettes (e-cigs) have gained recent broad popularity and can deliver nicotine at comparable levels to tobacco cigarettes, but effects on AAA development are unknown.

METHODS AND RESULTS

We evaluated the impact of daily e-cig vaping with nicotine on AAA using two complementary murine models and found that exposure enhanced aneurysm development in both models and genders. E-cigs induced changes in key mediators of AAA development including cytokine chitinase-3-like protein 1 (CHI3L1/Chil1) and its targeting microRNA-24 (miR-24). We show that nicotine triggers inflammatory signalling and reactive oxygen species while modulating miR-24 and CHI3L1/Chil1 in vitro and that Chil1 is crucial to e-cig-augmented aneurysm formation using a knockout model.

CONCLUSIONS

In conclusion our work shows increased aneurysm formation along with augmented vascular inflammation in response to e-cig exposure with nicotine. Further, we identify Chil1 as a key mediator in this context. Our data raise concerns regarding the potentially harmful long-term effects of e-cig nicotine vaping.

Taxifolin ameliorates abdominal aortic aneurysm by preventing inflammation and apoptosis and extracellular matrix degradation via inactivating TLR4/NF-κB axis

BACKGROUND

Abdominal aortic aneurysm (AAA) is a serious aortic disease with high mortality. Vascular smooth muscle cells (VSMCs) loss is a prominent feature of AAA. Taxifolin (TXL) is a natural antioxidant polyphenol and possesses therapeutic functions in numerous human diseases. This study aimed to investigate TXL's impact on VSMC phenotype in AAA.

METHODS

In vitro and in vivo of VSMC injury model was induced by angiotensin II (Ang II). The potential function of TXL on AAA was determined using Cell Counting Kit-8, flow cytometry, Western blot, quantitative reverse transcription-PCR, and enzyme-linked immunosorbent assay. Meanwhile, TXL mechanism on AAA was checked by a series of molecular experiments. Also, TXL function on AAA in vivo was further evaluated using hematoxylin-eosin staining, TUNEL assay, Picric acid-Sirius red staining and immunofluorescence assay in C57BL/6 mice.

RESULTS

TXL alleviated Ang II-induced VSMC injury mainly by enhancing VSMC proliferation and weakening cell apoptosis, alleviating VSMC inflammation, and reducing extracellular matrix (ECM) degradation of VSMCs. Furthermore, mechanistic studies corroborated that TXL reversed the high levels of Toll-like receptor 4 (TLR4) and p-p65/p65 induced by Ang II. Also, TXL facilitated VSMC proliferation and reduced cell apoptosis, repressed inflammation, and ECM degradation of VSMCs, while these effects were reversed by TLR4 overexpression. In vivo studies further confirmed that TXL owned the function of alleviating AAA, such as alleviating collagen fiber hyperplasia and inflammatory cell infiltration in AAA mice, and repressing inflammation and ECM degradation.

CONCLUSION

TXL protected VSMCs against Ang II-induced injury through activating TLR4/noncanonical nuclear factor-kappaB(NF-κB).

Coumarin-based turn-on fluorescence probe with a large Stokes shift for detection of endogenous neutrophil elastase in live cells and zebrafish

Neutrophil elastase (NE), a serine proteinase, is a significant biomarker which is closely related to the progress of diseases. However, only few probes have been reported for detection of NE activity and cell imaging. And these probes have exhibited small Stokes shift, which leads to high fluorescence interferences. Furthermore, only one probe among them is able to image NE in vivo successfully. To overcome the above problems, we designed a novel coumarin-based fluorescent probe HNCOU-NE with large Stokes shift to visualize NE activity in living cells and zebrafish. The new probe HNCOU-NE for NE contains fluorophore HNCOU as the reporter and pentafluoroethyl as the enzyme-active trigger moiety. As expected, HNCOU-NE displays perfect detecting performance for sensing of NE, including good water solubility, large Stokes shift, high affinity and wide linear response concentration. In addition, HNCOU-NE has been successfully utilized for NE real-time detection and imaging in different living cells, exhibiting low cytotoxicity and excellent biocompatibility. Most importantly, endogenous NE fluorescence imaging experiments reveals that HNCOU-NE can distinguish liver cancer cells (HepG2) and other cells (293T, HeLa and SKOV3), illustrating its specific ability to diagnose liver cancer cells. Besides, probe HNCOU-NE also has the ability to specifically detect endogenous NE activity in living zebrafish. All the results indicate that HNCOU-NE is a valuable probe for qualitative and quantitative sensing of NE activity in vitro and in vivo.

Twenty Years of Studying AngII (Angiotensin II)-Induced Abdominal Aortic Pathologies in Mice: Continuing Questions and Challenges to Provide Insight Into the Human Disease

AngII (angiotensin II) infusion in mice has been used to provide mechanistic insight into human abdominal aortic aneurysms for over 2 decades. This is a technically facile animal model that recapitulates multiple facets of the human disease. Although numerous publications have reported abdominal aortic aneurysms with AngII infusion in mice, there remain many fundamental unanswered questions such as uniformity of describing the pathological characteristics and which cell type is stimulated by AngII to promote abdominal aortic aneurysms. Extrapolation of the findings to provide insight into the human disease has been hindered by the preponderance of studies designed to determine the effects on initiation of abdominal aortic aneurysms, rather than a more clinically relevant scenario of determining efficacy on the established disease. The purpose of this review is to enhance understanding of AngII-induced abdominal aortic pathologies in mice, thereby providing greater insight into the human disease.

Experimental aortic aneurysm severity and growth depend on topical elastase concentration and lysyl oxidase inhibition

Abdominal aortic aneurysm (AAA) formation and expansion is highly complex and multifactorial, and the improvement of animal models is an important step to enhance our understanding of AAA pathophysiology. In this study, we explore our ability to influence aneurysm growth in a topical elastase plus β-Aminopropionitrile (BAPN) mouse model by varying elastase concentration and by altering the cross-linking capability of the tissue. To do so, we assess both chronic and acute effects of elastase concentration using volumetric ultrasound. Our results suggest that the applied elastase concentration affects initial elastin degradation, as well as long-term vessel expansion. Additionally, we assessed the effects of BAPN by (1) removing it to restore the cross-linking capability of tissue after aneurysm formation and (2) adding it to animals with stable aneurysms to interrupt cross-linking. These results demonstrate that, even after aneurysm formation, lysyl oxidase inhibition remains necessary for continued expansion. Removing BAPN reduces the aneurysm growth rate to near zero, resulting in a stable aneurysm. In contrast, adding BAPN causes a stable aneurysm to expand. Altogether, these results demonstrate the ability of elastase concentration and BAPN to modulate aneurysm growth rate and severity. The findings open several new areas of investigation in a murine model that mimics many aspects of human AAA.

Post-traumatic osteoarthritis progression is diminished by early mechanical unloading and anti-inflammatory treatment in mice

OBJECTIVE

Post-traumatic osteoarthritis (PTOA) is a degenerative joint disease initiated by injury. Early phase (0-7 days) treatments often include rest (unloading) and anti-inflammatory medications, but how those early interventions impact PTOA progression is unknown. We hypothesized that early unloading and anti-inflammatory treatment would diminish joint inflammation and slow PTOA progression.

DESIGN

Mice were injured with non-invasive ACL rupture followed by hindlimb unloading (HLU) or normal cage activity (ground control: GC) for 7 days, after which all mice were allowed normal cage activity. HLU and GC mice were treated with daily celecoxib (CXB; 10 mg/kg IP) or vehicle. Protease activity was evaluated using in vivo fluorescence imaging, osteophyte formation and epiphyseal trabecular bone were quantified using micro-computed tomography, and synovitis and articular cartilage were evaluated using whole-joint histology at 7, 14, 21, and 28 days post-injury.

RESULTS

HLU significantly reduced protease activity (-22-30% compared to GC) and synovitis (-24-50% relative to GC) at day 7 post-injury (during unloading), but these differences were not maintained at later timepoints. Similarly, trabecular bone volume was partially preserved in HLU mice at during unloading (-14-15% BV/TV for HLU mice, -21-22% for GC mice relative to uninjured), but these differences were not maintained during reloading. Osteophyte volume was reduced by both HLU and CXB, but there was not an additive effect of these treatments (HLU: -46%, CXB: -30%, HLU + CXB: -35% relative to vehicle GC at day 28).

CONCLUSIONS

These data suggest that early unloading following joint injury can reduce inflammation and potentially slow PTOA progression.

Translating mouse models of abdominal aortic aneurysm to the translational needs of vascular surgery

Introduction

Abdominal aortic aneurysm (AAA) is a condition that has considerable socioeconomic impact and an eventual rupture is associated with high mortality and morbidity. Despite decades of research, surgical repair remains the treatment of choice and no medical therapy is currently available. Animal models and, in particular, murine models, of AAA are a vital tool for experimental in vivo research. However, each of the different models has individual limitations and provide only partial mimicry of human disease. This narrative review addresses the translational potential of the available mouse models, highlighting unanswered questions from a clinical perspective. It is based on a thorough presentation of the available literature and more than a decade of personal experience, with most of the available models in experimental and translational AAA research.

Results

From all the models published, only the four inducible models, namely the angiotensin II model (AngII), the porcine pancreatic elastase perfusion model (PPE), the external periadventitial elastase application (ePPE), and the CaCl₂ model have been widely used by different independent research groups. Although the angiotensin II model provides features of dissection and aneurysm formation, the PPE model shows reliable features of human AAA, especially beyond day 7 after induction, but remains technically challenging. The translational value of ePPE as a model and the combination with β-aminopropionitrile to induce rupture and intraluminal thrombus formation is promising, but warrants further mechanistic insights. Finally, the external CaCl₂ application is known to produce inflammatory vascular wall thickening. Unmet translational research questions include the origin of AAA development, monitoring aneurysm growth, gender issues, and novel surgical therapies as well as novel nonsurgical therapies.

Conclusion

New imaging techniques, experimental therapeutic alternatives, and endovascular treatment options provide a plethora of research topics to strengthen the individual features of currently available mouse models, creating the possibility of shedding new light on translational research questions.

The Association Between Curvature and Rupture in a Murine Model of Abdominal Aortic Aneurysm and Dissection

BACKGROUND

Mouse models of abdominal aortic aneurysm (AAA) and dissection have proven to be invaluable in the advancement of diagnostics and therapeutics by providing a platform to decipher response variables that are elusive in human populations. One such model involves systemic Angiotensin II (Ang-II) infusion into low density-lipoprotein receptor-deficient (LDLr-/-) mice leading to intramural thrombus formation, inflammation, matrix degradation, dilation, and dissection. Despite its effectiveness, considerable experimental variability has been observed in AAAs taken from our Ang-II infused LDLr-/- mice (n=12) with obvious dissection occurring in 3 samples, outer bulge radii ranging from 0.73 to 2.12 mm, burst pressures ranging from 155 to 540 mmHg, and rupture location occurring 0.05 to 2.53 mm from the peak bulge location.

OBJECTIVE

We hypothesized that surface curvature, a fundamental measure of shape, could serve as a useful predictor of AAA failure at supra-physiological inflation pressures.

METHODS

To test this hypothesis, we fit well-known biquadratic surface patches to 360° micro-mechanical test data and used Spearman's rank correlation (rho) to identify relationships between failure metrics and curvature indices.

RESULTS

We found the strongest associations between burst pressure and the maximum value of the first principal curvature (rho=-0.591, p-val=0.061), the maximum value of Mean curvature (rho=-0.545, p-val=0.087), and local values of Mean curvature at the burst location (rho=-0.864, p-val=0.001) with only the latter significant after Bonferroni correction. Additionally, the surface profile at failure was predominantly convex and hyperbolic (saddle-shaped) as indicated by a negative sign in the Gaussian curvature. Findings reiterate the importance of shape in experimental models of AAA.

Effects of Braiding Parameters on Tissue Engineered Vascular Graft Development

Tissue engineered vascular grafts (TEVGs) using scaffolds fabricated from braided poly(glycolic acid) (PGA) fibers coated with poly(glycerol sebacate) (PGS) are developed. The approach relies on in vivo tissue engineering by which neotissue forms solely within the body after a scaffold has been implanted. Herein, the impact of altering scaffold braid design and scaffold coating on neotissue formation is investigated. Several combinations of braiding parameters are manufactured and evaluated in a Beige mouse model in the infrarenal abdominal aorta. Animals are followed with 4D ultrasound analysis, and 12 week explanted vessels are evaluated for biaxial mechanical properties as well as histological composition. Results show that scaffold parameters (i.e., braiding angle, braiding density, and presence of a PGS coating) have interdependent effects on the resulting graft performance, namely, alteration of these parameters influences levels of inflammation, extracellular matrix production, graft dilation, neovessel distensibility, and overall survival. Coupling carefully designed in vivo experimentation with regression analysis, critical relationships between the scaffold design and the resulting neotissue that enable induction of favorable cellular and extracellular composition in a controlled manner are uncovered. Such an approach provides a potential for fabricating scaffolds with a broad range of features and the potential to manufacture optimized TEVGs.

AT2R agonist NP-6A4 mitigates aortic stiffness and proteolytic activity in mouse model of aneurysm

Clinical and experimental studies show that angiotensin II (AngII) promotes vascular pathology via activation of AngII type 1 receptors (AT1Rs). We recently reported that NP-6A4, a selective peptide agonist for AngII type 2 receptor (AT2R), exerts protective effects on human vascular cells subjected to serum starvation or doxorubicin exposure. In this study, we investigated whether NP-6A4-induced AT2R activation could mitigate AngII-induced abdominal aortic aneurism (AAA) using AngII-treated Apoe^-/- mice. Male Apoe^-/- mice were infused with AngII (1 µg/kg/min) by implanting osmotic pumps subcutaneously for 28 days. A subset of mice was pre-treated subcutaneously with NP-6A4 (2.5 mg/kg/day) or vehicle for 14 days prior to AngII, and treatments were continued for 28 days. NP-6A4 significantly reduced aortic stiffness of the abdominal aorta induced by AngII as determined by ultrasound functional analyses and histochemical analyses. NP-6A4 also increased nitric oxide bioavailability in aortic tissues and suppressed AngII-induced increases in monocyte chemotactic protein-1, osteopontin and proteolytic activity of the aorta. However, NP-6A4 did not affect maximal intraluminal aortic diameter or AAA incidences significantly. These data suggest that the effects of AT2R agonist on vascular pathologies are selective, affecting the aortic stiffness and proteolytic activity without affecting the size of AAA.

Assessing structural and functional response of murine vasculature to acute β-adrenergic stimulation in vivo during hypothermic and hyperthermic conditions

Background: Because of the importance of adrenoreceptors in regulating the cardiovascular (CV) system and the role of the CV system in thermoregulation, understanding the response to these two stressors is of interest. The purpose of this study was to assess changes of arterial geometry and function in vivo during thermal and β-adrenergic stress induced in mice and quantified by MRI.Methods: Male mice were anesthetized and imaged at 7 T. Anatomical and functional data were acquired from the neck (carotid artery), torso (suprarenal and infrarenal aorta and iliac artery) and periphery (femoral artery). Intravenous dobutamine (tail vein catheter, 40 µg/kg/min, 0.12 mL/h) was used as β-adrenergic stressor. Baseline and dobutamine data were acquired at minimally hypothermic (35 °C) and minimally hyperthermic (38 °C) core temperatures. Cross-sectional vessel area and maximum cyclic strain were measured across the cardiac cycle.Results: Vascular response varied by location and by core temperature. For minimally hypothermic conditions (35 °C), average, maximum and minimum areas decreased with dobutamine only at the suprarenal aorta (avg: -17.9%, max: -13.5%, min: -21.4%). For minimally hyperthermic conditions (38 °C), vessel areas decreased between baseline and dobutamine at the carotid (avg: -19.6%, max: -15.5%, min: -19.3%) and suprarenal aorta (avg: -24.2%, max: -17.4%, min: -17.3%); whereas, only the minimum vessel area decreased for the iliac artery (min: -14.4%). Maximum cyclic strain increased between baseline and dobutamine at the iliac artery for both conditions and at the suprarenal aorta at hyperthermic conditions.Conclusions: At hypothermic conditions, the vessel area response to dobutamine is diminished compared to hyperthermic conditions where the vessel area response mimics normothermic dobutamine conditions. The varied response emphasizes the need to monitor and control body temperature during medical conditions or treatments that may be accompanied by hypothermia, especially when vasoactive agents are used.

High-frequency murine ultrasound provides enhanced metrics of BAPN-induced AAA growth

An abdominal aortic aneurysm (AAA), defined as a pathological expansion of the largest artery in the abdomen, is a common vascular disease that frequently leads to death if rupture occurs. Once diagnosed, clinicians typically evaluate the rupture risk based on maximum diameter of the aneurysm, a limited metric that is not accurate for all patients. In this study, we worked to evaluate additional distinguishing factors between growing and stable murine aneurysms toward the aim of eventually improving clinical rupture risk assessment. With the use of a relatively new mouse model that combines surgical application of topical elastase to cause initial aortic expansion and a lysyl oxidase inhibitor, β-aminopropionitrile (BAPN), in the drinking water, we were able to create large AAAs that expanded over 28 days. We further sought to develop and demonstrate applications of advanced imaging approaches, including four-dimensional ultrasound (4DUS), to evaluate alternative geometric and biomechanical parameters between 1) growing AAAs, 2) stable AAAs, and 3) nonaneurysmal control mice. Our study confirmed the reproducibility of this murine model and found reduced circumferential strain values, greater tortuosity, and increased elastin degradation in mice with aneurysms. We also found that expanding murine AAAs had increased peak wall stress and surface area per length compared with stable aneurysms. The results from this work provide clear growth patterns associated with BAPN-elastase murine aneurysms and demonstrate the capabilities of high-frequency ultrasound. These data could help lay the groundwork for improving insight into clinical prediction of AAA expansion.NEW & NOTEWORTHY This work characterizes a relatively new murine model of abdominal aortic aneurysms (AAAs) by quantifying vascular strain, stress, and geometry. Furthermore, Green-Lagrange strain was calculated with a novel mapping approach using four-dimensional ultrasound. We also compared growing and stable AAAs, finding peak wall stress and surface area per length to be most indicative of growth. In all AAAs, strain and elastin health declined, whereas tortuosity increased.

The Effect of Pentagalloyl Glucose on the Wall Mechanics and Inflammatory Activity of Rat Abdominal Aortic Aneurysms

An abdominal aortic aneurysm (AAA) is a permanent localized expansion of the abdominal aorta with mortality rate of up to 90% after rupture. AAA growth is a process of vascular degeneration accompanied by a reduction in wall strength and an increase in inflammatory activity. It is unclear whether this process can be intervened to attenuate AAA growth, and hence, it is of great clinical interest to develop a technique that can stabilize the AAA. The objective of this work is to develop a protocol for future studies to evaluate the effects of drug-based therapies on the mechanics and inflammation in rodent models of AAA. The scope of the study is limited to the use of pentagalloyl glucose (PGG) for aneurysm treatment in the calcium chloride rat AAA model. Peak wall stress (PWS) and matrix metalloproteinase (MMP) activity, which are the biomechanical and biological markers of AAA growth and rupture, were evaluated over 4 weeks in untreated and treated (with PGG) groups. The AAA specimens were mechanically characterized by planar biaxial tensile testing and the data fitted to a five-parameter nonlinear, hyperelastic, anisotropic Holzapfel–Gasser–Ogden (HGO) material model, which was used to perform finite element analysis (FEA) to evaluate PWS. Our results demonstrated that there was a reduction in PWS between pre- and post-AAA induction FEA models in the treatment group compared to the untreated group using either animal-specific or average material properties. However, this reduction was not statistically significant. Conversely, there was a statistically significant reduction in MMP-activated fluorescent signal between pre- and post-AAA induction models in the treated group compared to the untreated group. Therefore, the primary contribution of this work is the quantification of the stabilizing effects of PGG using biomechanical and biological markers of AAA, thus indicating that PGG could be part of a new clinical treatment strategy that will require further investigation.

Computational Fluid Dynamics of Vascular Disease in Animal Models

Recent applications of computational fluid dynamics (CFD) applied to the cardiovascular system have demonstrated its power in investigating the impact of hemodynamics on disease initiation, progression, and treatment outcomes. Flow metrics such as pressure distributions, wall shear stresses (WSS), and blood velocity profiles can be quantified to provide insight into observed pathologies, assist with surgical planning, or even predict disease progression. While numerous studies have performed simulations on clinical human patient data, it often lacks prediagnosis information and can be subject to large intersubject variability, limiting the generalizability of findings. Thus, animal models are often used to identify and manipulate specific factors contributing to vascular disease because they provide a more controlled environment. In this review, we explore the use of CFD in animal models in recent studies to investigate the initiating mechanisms, progression, and intervention effects of various vascular diseases. The first section provides a brief overview of the CFD theory and tools that are commonly used to study blood flow. The following sections are separated by anatomical region, with the abdominal, thoracic, and cerebral areas specifically highlighted. We discuss the associated benefits and obstacles to performing CFD modeling in each location. Finally, we highlight animal CFD studies focusing on common surgical treatments, including arteriovenous fistulas (AVF) and pulmonary artery grafts. The studies included in this review demonstrate the value of combining CFD with animal imaging and should encourage further research to optimize and expand upon these techniques for the study of vascular disease.

Rise and fall of elastic fibers from development to aging. Consequences on arterial structure-function and therapeutical perspectives

In the arteries of vertebrates, evolution has given rise to resilient macromolecular structures, elastin and elastic fibers, capable of sustaining an elevated blood pressure and smoothening the discontinuous blood flow and pressure generated by the heart. Elastic fibers are produced only during development and childhood, before being progressively degraded by mechanical stress and enzymatic activities during adulthood and aging. During this period, arterial elastic fiber calcification and loading of lipids also occur, all of these events conducting to arteriosclerosis. This leads to a progressive dysfunction of the large elastic arteries inducing elevated blood pressure as well as altered hemodynamics and organ perfusion, which induce more global malfunctions of the body during normal aging. Additionally, some arterial conditions occur more frequently with advancing age, such as atherosclerosis or aneurysms, which are called age-related diseases or pathological aging. The physiological or pathological degradation of elastic fibers and function of elastic arteries seemed to be rather inevitable over time. However, during the recent years, different molecules - including several ATP-dependent potassium channel openers, such as minoxidil - have been shown to re-induce elastin production and elastic fiber assembly, leading to improvements in the arterial structure and function or in organ perfusion. This review summarizes the changes in the arterial elastic fibers and structure from development until aging, and presents some of the potential pharmacotherapies leading to elastic fiber neosynthesis and arterial function improvement.

Long noncoding RNA GAS5 induces abdominal aortic aneurysm formation by promoting smooth muscle apoptosis

Objective: Long noncoding RNAs (lncRNAs) may serve as specific targets for the treatment of abdominal aortic aneurysms (AAAs). LncRNA GAS5, functionally associated with smooth muscle cell (SMC) apoptosis and proliferation, is likely involved in AAA formation, but the exact role of GAS5 in AAA is unknown. We thus explored the contribution of GAS5 to SMC-regulated AAA formation and its underlying mechanisms.

Methods: Human specimens were used to verify the diverse expression of GAS5 in normal and AAA tissues. The angiotensin II (Ang II)-induced AAA model in ApoE-/- mice and the CaCl₂-induced AAA model in wild-type C57BL/6 mice were used. RNA pull-down and luciferase reporter gene assays were performed in human aortic SMCs to detect the interaction between GAS5 and its downstream targets of protein or microRNA (miR).

Results: GAS5 expression was significantly upregulated in human AAA specimens and two murine AAA models compared to human normal aortas and murine sham-operated controls. GAS5 overexpression induced SMC apoptosis and repressed its proliferation, thereby promoting AAA formation in two murine AAA models. Y-box-binding protein 1 (YBX1) was identified as a direct target of GAS5 while it also formed a positive feedback loop with GAS5 to regulate the downstream target p21. Furthermore, GAS5 acted as a miR-21 sponge to release phosphatase and tensin homolog from repression, which blocked the activation and phosphorylation of Akt to inhibit proliferation and promote apoptosis in SMCs.

Conclusion: The LncRNA GAS5 contributes to SMC survival during AAA formation. Thus, GAS5 might serve as a novel target against AAA.

Effects of Iliac Stenosis on Abdominal Aortic Aneurysm Formation in Mice and Humans

Reduced lower-limb blood flow has been shown to lead to asymmetrical abdominal aortic aneurysms (AAAs) but the mechanism of action is not fully understood. Therefore, small animal ultrasound (Vevo2100, FUJIFILM VisualSonics) was used to longitudinally study mice that underwent standard porcine pancreatic elastase (PPE) infusion (n = 5), and PPE infusion with modified 20% iliac artery stenosis in the left (n = 4) and right (n = 5) iliac arteries. Human AAA computed tomography images were obtained from patients with normal (n = 9) or stenosed left (n = 2), right (n = 1), and bilateral (n = 1) iliac arteries. We observed rapid early growth and rightward expansion (8/9 mice) in the modified PPE groups (p < 0.05), leading to slightly larger and asymmetric AAAs compared to the standard PPE group. Further examination showed a significant increase in TGFβ1 (p < 0.05) and cellular infiltration (p < 0.05) in the modified PPE group versus standard PPE mice. Congruent, yet variable, observations were made in human AAA patients with reduced iliac outflow compared to those with normal iliac outflow. Our results suggest that arterial stenosis at the time of aneurysm induction leads to faster AAA growth with aneurysm asymmetry and increased vascular inflammation after 8 weeks, indicating that moderate iliac stenosis may have upstream effects on AAA progression.

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.

Development and growth trends in angiotensin II-induced murine dissecting abdominal aortic aneurysms

Abdominal aortic aneurysms are pathological dilations that can suddenly rupture, causing more than 15,000 deaths in the U.S. annually. Current treatment focuses on observation until an aneurysm's size warrants surgical intervention. Thus, there is a need for therapeutic intervention to inhibit growth of smaller aneurysms. An experimental aneurysm model that infuses angiotensin II into apolipoprotein E‐deficient mice is widely used to investigate underlying pathological mechanisms and potential therapeutics, but this model has two caveats: (1) aneurysms do not always form, and (2) aneurysm severity and growth is inconsistent among animals. Here we use high‐frequency ultrasound to collect data from angiotensin II‐induced aneurysms to develop prediction models of both aneurysm formation and growth. Baseline measurements of aortic diameter, volume/length, and strain were used with animal mass and age in a quadratic discriminant analysis and logistic regression to build two statistical models to predict disease status. Longitudinal ultrasound data were also acquired from mice with aneurysms to quantify aneurysm diameter, circumferential strain, blood flow velocity, aneurysm volume/length, and thrombus and open‐false lumen volumes over 28 days. Measurements taken at aneurysm diagnosis were used with branching artery information to produce a multiple linear regression model to predict final aneurysm volume/length. All three statistical models could be useful in future aneurysm therapeutic studies to better delineate the effects of preventative and suppressive treatments from normal variations in the angiotensin II aneurysm model.

Strain Mapping From Four-Dimensional Ultrasound Reveals Complex Remodeling in Dissecting Murine Abdominal Aortic Aneurysms

Current in vivo abdominal aortic aneurysm (AAA) imaging approaches tend to focus on maximum diameter but do not measure three-dimensional (3D) vascular deformation or strain. Complex vessel geometries, heterogeneous wall compositions, and surrounding structures can all influence aortic strain. Improved understanding of complex aortic kinematics has the potential to increase our ability to predict aneurysm expansion and eventual rupture. Here, we describe a method that combines four-dimensional (4D) ultrasound and direct deformation estimation to compute in vivo 3D Green-Lagrange strain in murine angiotensin II-induced suprarenal dissecting aortic aneurysms, a commonly used small animal model. We compared heterogeneous patterns of the maximum, first-component 3D Green-Lagrange strain with vessel composition from mice with varying AAA morphologies. Intramural thrombus and focal breakage in the medial elastin significantly reduced aortic strain. Interestingly, a dissection that was not detected with high-frequency ultrasound also experienced reduced strain, suggesting medial elastin breakage that was later confirmed via histology. These results suggest that in vivo measurements of 3D strain can provide improved insight into aneurysm disease progression. While further work is needed with both preclinical animal models and human imaging studies, this initial murine study indicates that vessel strain should be considered when developing an improved metric for predicting aneurysm growth and rupture.

Ultrasound Imaging of the Thoracic and Abdominal Aorta in Mice to Determine Aneurysm Dimensions

Contemporary high-resolution ultrasound instruments have sufficient resolution to facilitate the measurement of mouse aortas. These instruments have been widely used to measure aortic dimensions in mouse models of aortic aneurysms. Aortic aneurysms are defined as permanent dilations of the aorta, which occur most frequently in the ascending and abdominal regions. Sequential measurements of aortic dimensions by ultrasound are the principal approach for assessing the development and progression of aortic aneurysms in vivo. Although many reported studies used ultrasound imaging to measure aortic diameters as a primary endpoint, there are confounding factors, such as probe position and cardiac cycle, that may impact the accuracy of data acquisition, analysis, and interpretation. The purpose of this protocol is to provide a practical guide on the use of ultrasound to measure the aortic diameter in a reliable and reproducible manner. This protocol introduces the preparation of mice and instruments, the acquisition of appropriate ultrasound images, and data analysis.

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.

Anatomical location, sex, and age influence murine arterial circumferential cyclic strain before and during dobutamine infusion

Background

One of the primary biomechanical factors influencing arterial health is their deformation across the cardiac cycle, or cyclic strain, which is often associated with arterial stiffness. Deleterious changes in the cardiovascular system, e.g., increased arterial stiffness, can remain undetected until the system is challenged, such as under a cardiac stressor like dobutamine.

Purpose

To quantify cyclic strain in mice at different locations along the arterial tree prior to and during dobutamine infusion, while evaluating the effects of sex and age.

Study Type

Control/cohort study.

Animal Model

Twenty C57BL/6 mice; male, female; ∼12 and 24 weeks of age; n = 5 per group.

Field Strength/Sequence

7T; CINE MRI with 12 frames, velocity compensation, and prospective cardiac gating.

Assessment

Prior to and during the infusion of dobutamine, Green–Lagrange circumferential cyclic strain was calculated from perimeter measurements derived from CINE data acquired at the carotid artery, suprarenal and infrarenal abdominal aorta, and iliac artery.

Statistical Tests

Analysis of variance (ANOVA) followed by post‐hoc tests was used to evaluate the influence of dobutamine, anatomical location, sex, and age.

Results

Heart rates did not differ between groups prior to or during dobutamine infusion (P = 0.87 and P = 0.08, respectively). Dobutamine increased cyclic strain in each group. Within a group, increases in strain were similar across arteries. At the suprarenal aorta, strain was reduced in older mice at baseline (young 27.6 > mature 19.3%, P = 0.01) and during dobutamine infusion (young 53.0 > mature 36.2%, P = 0.005). In the infrarenal aorta, the response (dobutamine – baseline) was reduced in older mice (young 21.9 > mature 13.5%, P = 0.04).

Data Conclusion

Dobutamine infusion increases circumferential cyclic strain throughout the arterial tree of mice. This effect is quantifiable using CINE MRI. The results demonstrate that strain prior to and during dobutamine is influenced by anatomical location, sex, and age.

Level of Evidence: 3

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2019;49:69–80.

Osteophytes and fracture calluses share developmental milestones and are diminished by unloading

Osteophytes are a typical radiographic finding during osteoarthritis (OA), but the mechanisms leading to their formation are not well known. Comparatively, fracture calluses have been studied extensively; therefore, drawing comparisons between osteophytes and fracture calluses may lead to a deeper understanding of osteophyte formation. In this study, we compared the time courses of osteophyte and fracture callus formation, and investigated mechanisms contributing to development of these structure. Additionally, we investigated the effect of mechanical unloading on the formation of both fracture calluses and osteophytes. Mice underwent either transverse femoral fracture or non-invasive anterior cruciate ligament rupture. Fracture callus and osteophyte size and ossification were evaluated after 3, 5, 7, 14, 21, or 28 days. Additional mice were subjected to hindlimb unloading after injury for 3, 7, or 14 days. Protease activity and gene expression profiles after injury were evaluated after 3 or 7 days of normal ambulation or hindlimb unloading using in vivo fluorescence reflectance imaging (FRI) and quantitative PCR. We found that fracture callus and osteophyte growth achieved similar developmental milestones, but fracture calluses formed and ossified at earlier time points. Hindlimb unloading ultimately led to a threefold decrease in chondro/osteophyte area, and a twofold decrease in fracture callus area. Unloading was also associated with decreased inflammation and protease activity in injured limbs detected with FRI, particularly following ACL rupture. qPCR analysis revealed disparate cellular responses in fractured femurs and injured joints, suggesting that fracture calluses and osteophytes may form via different inflammatory, anabolic, and catabolic pathways. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:699-710, 2018.

Cardiac and respiratory-gated volumetric murine ultrasound

Current cardiovascular ultrasound mainly employs planar imaging techniques to assess function and physiology. These techniques rely on geometric assumptions, which are dependent on the imaging plane, susceptible to inter-observer variability, and may be inaccurate when studying complex diseases. Here, we developed a gated volumetric murine ultrasound technique to visualize cardiovascular motion with high spatiotemporal resolution and directly evaluate cardiovascular health. Cardiac and respiratory-gated cine loops, acquired at 1000 frames-per-second from sequential positions, were temporally registered to generate a four-dimensional (4D) dataset. We applied this technique to (1) evaluate left ventricular (LV) function from both healthy mice and mice with myocardial infarction and (2) characterize aortic wall strain of angiotensin II-induced dissecting abdominal aortic aneurysms in apolipoprotein E-deficient mice. Combined imaging and processing times for the 4D technique was approximately 2-4 times longer than conventional 2D approaches, but substantially more data is collected with 4D ultrasound and further optimization can be implemented to reduce imaging times. Direct volumetric measurements of 4D cardiac data aligned closely with those obtained from MRI, contrary to conventional methods, which were sensitive to transducer alignment, leading to overestimation or underestimation of estimated LV parameters in infarcted hearts. Green-Lagrange circumferential strain analysis revealed higher strain values proximal and distal to the aneurysm than within the aneurysmal region, consistent with published reports. By eliminating the need for geometrical assumptions, the presented 4D technique can be used to more accurately evaluate cardiac function and aortic pulsatility. Furthermore, this technique allows for the visualization of regional differences that may be overlooked with conventional 2D approaches.

Angiotensin II Infusion Does Not Cause Abdominal Aortic Aneurysms in Apolipoprotein E-Deficient Rats

The apolipoprotein E-deficient (apoE-/-) mouse model has advanced our understanding of cardiovascular disease mechanisms and experimental therapeutics. This spontaneous model recapitulates aspects of human atherosclerosis, and allows for the development of dissecting abdominal aortic aneurysms (AAAs) when combined with angiotensin II. We characterized apoE-/- rats and hypothesized that, similar to mice, they would develop dissecting AAAs. We created rats with a 16-bp deletion of the apoE gene using transcription activator-like effector nucleases. We imaged the suprarenal aorta for 28 days after implantation of miniosmotic pumps that infuse angiotensin II (AngII, 200 ng/kg/min). Blood pressure (BP), serum lipids and lipoproteins, and histology were also analyzed. These rats did not develop pathological aortic dissection, but we did observe a decrease in circumferential cyclic strain, a rise in BP, and microstructural changes in the aortic medial layer. We also measured increased serum lipids with and without administration of a high-fat diet, but did not detect atherosclerotic plaques. Chronic infusion of AngII did not lead to the formation of dissecting AAAs or atherosclerosis in the rats used in this study. While reduced amounts of atherosclerosis may explain this resistance to dissecting aneurysms, further investigation is needed to fully characterize species-specific differences.

Cross-sectional area of the murine aorta linearly increases with increasing core body temperature

PURPOSE

The cardiovascular (CV) system plays a vital role in thermoregulation. To date, the response of core vasculature to increasing core temperature has not been adequately studied in vivo. Our objective was to non-invasively quantify the arterial response in murine models due to increases in body temperature, with a focus on core vessels of the torso and investigate whether responses were dependent on sex or age.

METHODS

Male and female, adult and aged mice were anaesthetised and underwent magnetic resonance imaging (MRI). Data were acquired from the circle of Willis (CoW), heart, infrarenal aorta and peripheral arteries at core temperatures of 35, 36, 37 and 38 °C (±0.2 °C).

RESULTS

Vessels in the CoW did not change. Ejection fraction decreased and cardiac output (CO) increased with increasing temperature in adult female mice. Cross-sectional area of the aorta increased significantly and linearly with temperature for all groups, but at a diminished rate for aged animals (p < 0.01; male and female: adult, 0.019 and 0.024 mm²/°C; aged, 0.017 and 0.011 mm²/°C). Aged male mice had a diminished response in the periphery (% increase in femoral artery area from 35 to 38 °C, male and female: adult, 67 and 65%; aged, 0.1 and 57%).

CONCLUSION

Previously unidentified increases in aortic area due to increasing core temperature are biologically important because they may affect conductive and convective heat transfer. Leveraging non-invasive methodology to quantify sex and age dependent vascular responses due to increasing core temperature could be combined with bioheat modelling in order to improve understanding of thermoregulation.

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.

In Vivo Multiscale and Spatially-Dependent Biomechanics Reveals Differential Strain Transfer Hierarchy in Skeletal Muscle

Biological tissues have a complex hierarchical architecture that spans organ to subcellular scales and comprises interconnected biophysical and biochemical machinery. Mechanotransduction, gene regulation, gene protection, and structure-function relationships in tissues depend on how force and strain are modulated from macro to micro scales, and vice versa. Traditionally, computational and experimental techniques have been used in common model systems (e.g., embryos) and simple strain measures were applied. But the hierarchical transfer of mechanical parameters like strain in mammalian systems is largely unexplored in vivo. Here, we experimentally probed complex strain transfer processes in mammalian skeletal muscle tissue over multiple biological scales using complementary in vivo ultrasound and optical imaging approaches. An iterative hyperelastic warping technique quantified the spatially-dependent strain distributions in tissue, matrix, and subcellular (nuclear) structures, and revealed a surprising increase in strain magnitude and heterogeneity in active muscle as the spatial scale also increased. The multiscale strain heterogeneity indicates tight regulation of mechanical signals to the nuclei of individual cells in active muscle, and an emergent behavior appearing at larger (e.g. tissue) scales characterized by dramatically increased strain complexity.

In vivo photoacoustic lipid imaging in mice using the second near-infrared window

Photoacoustic imaging has emerged as a promising technique to improve preclinical and clinical imaging by providing users with label-free optical contrast of tissue. Here, we present a proof-of-concept study for noninvasive in vivo murine lipid imaging using 1210 nm light to investigate differences in periaortic fat among mice of different gender, genotypes, and maturation. Acquired lipid signals suggest that adult male apoE^-/- mice have greater periaortic fat accumulation compared to adolescent males, apoE^-/- females, and wild-type mice. These results demonstrate the potential of photoacoustic tomography for studying vascular pathophysiology and improving the diagnosis of lipid-based diseases.

Murine ultrasound-guided transabdominal para-aortic injections of self-assembling type I collagen oligomers

Abdominal aortic aneurysms (AAAs) represent a potentially life-threatening condition that predominantly affects the infrarenal aorta. Several preclinical murine models that mimic the human condition have been developed and are now widely used to investigate AAA pathogenesis. Cell- or pharmaceutical-based therapeutics designed to prevent AAA expansion are currently being evaluated with these animal models, but more minimally invasive strategies for delivery could improve their clinical translation. The purpose of this study was to investigate the use of self-assembling type I collagen oligomers as an injectable therapeutic delivery vehicle in mice. Here we show the success and reliability of a para-aortic, ultrasound-guided technique for injecting quickly-polymerizing collagen oligomer solutions into mice to form a collagen-fibril matrix at body temperature. A commonly used infrarenal mouse AAA model was used to determine the target location of these collagen injections. Ultrasound-guided, closed-abdominal injections supported consistent delivery of collagen to the area surrounding the infrarenal abdominal aorta halfway between the right renal artery and aortic trifurcation into the iliac and tail arteries. This minimally invasive approach yielded outcomes similar to open-abdominal injections into the same region. Histological analysis on tissue removed on day 14 post-operatively showed minimal in vivo degradation of the self-assembled fibrillar collagen and the majority of implants experienced minimal inflammation and cell invasion, further confirming this material's potential as a method for delivering therapeutics. Finally, we showed that the typical length and position of this infrarenal AAA model was statistically similar to the length and targeted location of the injected collagen, increasing its feasibility as a localized therapeutic delivery vehicle. Future preclinical and clinical studies are needed to determine if specific therapeutics incorporated into the self-assembling type I collagen matrix described here can be delivered near the aorta and locally limit AAA expansion.

Cyclic Strain and Hypertension Increase Osteopontin Expression in the Aorta

Hypertension has a direct impact on vascular hypertrophy and is a known risk factor for the development of atherosclerosis. Osteopontin (OPN) has emerged as an important protein mediator of inflammation and remodeling of large arteries. However, its role and mechanism of regulation in the setting of hypertension is still unknown. Our objectives for this study were therefore to investigate the role of OPN in hypertension-induced vascular remodeling and inflammation. OPN Knockout (KO) and wild type (WT) mice were made hypertensive with angiotensin II (Ang II) infusion for seven days. We observed that OPN KO aortas were protected against Ang II-induced medial hypertrophy and inflammation, despite comparable increases in systolic blood pressure (SBP) in both groups. OPN expression was increased in WT aortas from hypertensive mice (induced by either Ang II or norepinephrine). OPN expression was increased in aortic smooth muscle cells (SMCs) subjected to cyclic mechanical strain suggesting that mechanical deformation of the aortic wall is responsible in part for the increased OPN expression induced by hypertension. Finally, we utilized hypertensive transgenic smooth muscle cell-specific catalase overexpressing (Tg^SMC-Cat) mice to determine the role of H₂O₂ in mediating hypertension-induced increases in OPN expression. We also found that the hypertension-induced increase in OPN expression was inhibited in transgenic smooth muscle cell-specific catalase overexpressing (Tg^SMC-Cat) mice, suggesting that H₂O₂, plays a vital role in mediating the hypertension-induced increase in OPN expression. Taken together, these results define a potentially important role for OPN in the pathophysiology of hypertension.

A Methodology for the Derivation of Unloaded Abdominal Aortic Aneurysm Geometry With Experimental Validation

In this work, we present a novel method for the derivation of the unloaded geometry of an abdominal aortic aneurysm (AAA) from a pressurized geometry in turn obtained by 3D reconstruction of computed tomography (CT) images. The approach was experimentally validated with an aneurysm phantom loaded with gauge pressures of 80, 120, and 140 mm Hg. The unloaded phantom geometries estimated from these pressurized states were compared to the actual unloaded phantom geometry, resulting in mean nodal surface distances of up to 3.9% of the maximum aneurysm diameter. An in-silico verification was also performed using a patient-specific AAA mesh, resulting in maximum nodal surface distances of 8 μm after running the algorithm for eight iterations. The methodology was then applied to 12 patient-specific AAA for which their corresponding unloaded geometries were generated in 5-8 iterations. The wall mechanics resulting from finite element analysis of the pressurized (CT image-based) and unloaded geometries were compared to quantify the relative importance of using an unloaded geometry for AAA biomechanics. The pressurized AAA models underestimate peak wall stress (quantified by the first principal stress component) on average by 15% compared to the unloaded AAA models. The validation and application of the method, readily compatible with any finite element solver, underscores the importance of generating the unloaded AAA volume mesh prior to using wall stress as a biomechanical marker for rupture risk assessment.

2-Photon Characterization of Optical Proteolytic Beacons for Imaging Changes in Matrix-Metalloprotease Activity in a Mouse Model of Aneurysm

Abdominal aortic aneurysm is a multifactorial disease that is a leading cause of death in developed countries. Matrix-metalloproteases (MMPs) are part of the disease process, however, assessing their role in disease initiation and progression has been difficult and animal models have become essential. Combining Förster resonance energy transfer (FRET) proteolytic beacons activated in the presence of MMPs with 2-photon microscopy allows for a novel method of evaluating MMP activity within the extracellular matrix (ECM). Single and 2-photon spectra for proteolytic beacons were determined in vitro. Ex vivo experiments using the apolipoprotein E knockout angiotensin II-infused mouse model of aneurysm imaged ECM architecture simultaneously with the MMP-activated FRET beacons. 2-photon spectra of the two-color proteolytic beacons showed peaks for the individual fluorophores that enable imaging of MMP activity through proteolytic cleavage. Ex vivo imaging of the beacons within the ECM revealed both microstructure and MMP activity. 2-photon imaging of the beacons in aneurysmal tissue showed an increase in proteolytic cleavage within the ECM (p<0.001), thus indicating an increase in MMP activity. Our data suggest that FRET-based proteolytic beacons show promise in assessing MMP activity within the ECM and will therefore allow future studies to identify the heterogeneous distribution of simultaneous ECM remodeling and protease activity in aneurysmal disease.

Comparison of Traditional and Integrated Digital Anesthetic Vaporizers

Recent efforts have focused on mitigating anesthetic gas emissions during laboratory animal experiments. A recently developed, digitally controlled, integrated digital vaporizer (IDV) using a syringe pump has been designed to use and administer anesthetic gas to mice and rats more efficiently. The entire IDV system can be placed on a laboratory bench, requires fewer charcoal filters to act as passive scavengers when used at a low gas flow rate, and does not need compressed gas to operate, a requirement for traditional passive systems. The objective of this study was to compare isoflurane usage between a traditional vaporizer (TdV) and an IDV system at both the same settings and those recommended by the manufacturer. We used 10 C57BL/6 male mice and administered isoflurane through either nose cones or tracheal tubes connected to a pulsatile ventilator. The results showed that isoflurane usage is highly dependent on the flow rate of the carrier gas, but the IDV system was more precise and handled low flow rates (150 mL/min) better than did the TdV system. We observed only slight differences in heart rate, respiration rate, core body temperature, time to loss of the righting reflex, and recovery time between group averages for both systems when set to manufacturer-recommended settings. Although observed decreased levels of waste anesthetic gas at low flow rates are expected from the IDV system, further work is needed to assess environmental anesthetic gas levels and exposure to laboratory personnel.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

Telemetric Blood Pressure Assessment in Angiotensin II-Infused ApoE-/- Mice: 28 Day Natural History and Comparison to Tail-Cuff Measurements

Abdominal aortic aneurysm (AAA) is a disease of the aortic wall, which can progress to catastrophic rupture. Assessment of mechanical characteristics of AAA, such as aortic distensibility, may provide important insights to help identify at-risk patients and understand disease progression. While the majority of studies on this topic have focused on retrospective patient data, recent studies have used mouse models of AAA to prospectively evaluate the evolution of aortic mechanics. Quantification of aortic distensibility requires accurate measurement of arterial blood pressure, particularly pulse pressure, which is challenging to perform accurately in murine models. We hypothesized that volume/pressure tail-cuff measurements of arterial pulse pressure in anesthetized mice would have sufficient accuracy to enable calculations of aortic distensibility with minimal error. Telemetry devices and osmotic mini-pumps filled with saline or angiotensin-II were surgically implanted in male apolipoprotein-E deficient (ApoE^-/-) mice. Blood pressure in the aortic arch was measured continuously via telemetry. In addition, simultaneous blood pressure measurements with a volume/pressure tail-cuff system were performed under anesthesia at specific intervals to assess agreement between techniques. Compared to controls, mice infused with angiotensin-II had an overall statistically significant increase in systolic pressure, with no overall difference in pulse pressure; however, pulse pressure did increase significantly with time. Systolic measurements agreed well between telemetry and tail-cuff (coefficient of variation = 10%), but agreement of pulse pressure was weak (20%). In fact, group-averaged pulse pressure from telemetry was a better predictor of a subject’s pulse pressure on a given day than a simultaneous tail-cuff measurement. Furthermore, these approximations introduced acceptable errors (15.1 ± 12.8%) into the calculation of aortic distensibility. Contrary to our hypothesis, we conclude that tail-cuff measures of arterial pulse pressure have limited accuracy. Future studies of aneurysm mechanics using the ApoE^-/-/angiotensin-II model would be better in assuming pulse pressure profiles consistent with our telemetry findings instead of attempting to measure pulse pressure in individual mice.

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.

Segmental aortic stiffening contributes to experimental abdominal aortic aneurysm development

BACKGROUND

Stiffening of the aortic wall is a phenomenon consistently observed in age and in abdominal aortic aneurysm (AAA). However, its role in AAA pathophysiology is largely undefined.

METHODS AND RESULTS

Using an established murine elastase-induced AAA model, we demonstrate that segmental aortic stiffening precedes aneurysm growth. Finite-element analysis reveals that early stiffening of the aneurysm-prone aortic segment leads to axial (longitudinal) wall stress generated by cyclic (systolic) tethering of adjacent, more compliant wall segments. Interventional stiffening of AAA-adjacent aortic segments (via external application of surgical adhesive) significantly reduces aneurysm growth. These changes correlate with the reduced segmental stiffness of the AAA-prone aorta (attributable to equalized stiffness in adjacent segments), reduced axial wall stress, decreased production of reactive oxygen species, attenuated elastin breakdown, and decreased expression of inflammatory cytokines and macrophage infiltration, and attenuated apoptosis within the aortic wall, as well. Cyclic pressurization of segmentally stiffened aortic segments ex vivo increases the expression of genes related to inflammation and extracellular matrix remodeling. Finally, human ultrasound studies reveal that aging, a significant AAA risk factor, is accompanied by segmental infrarenal aortic stiffening.

CONCLUSIONS

The present study introduces the novel concept of segmental aortic stiffening as an early pathomechanism generating aortic wall stress and triggering aneurysmal growth, thereby delineating potential underlying molecular mechanisms and therapeutic targets. In addition, monitoring segmental aortic stiffening may aid the identification of patients at risk for AAA.

Heat shock protein 90 inhibition by 17-DMAG attenuates abdominal aortic aneurysm formation in mice

Abdominal aortic aneurysm (AAA) is a common degenerative vascular disease whose pathogenesis is associated with activation of multiple signaling pathways including Jun NH2-terminal kinases (JNK) and NF-κB. It is now well recognized that these pathways are chaperoned by the heat shock protein 90 (Hsp90), suggesting that inhibition of Hsp90 may be a novel strategy for inhibiting AAAs. The aim of this study is to investigate whether inhibition of Hsp90 by 17-DMAG (17-dimethyl-aminothylamino-17-demethoxy-geldanamycin) attenuates ANG II-induced AAA formation in mice, and, if so, to elucidate the mechanisms involved. Apolipoprotein E-null mice were infused with ANG II to induce AAA formation and simultaneously treated by intraperitoneal injection with either vehicle or 17-DMAG for 4 wk. ANG II infusion induced AAA formation in 80% of mice, which was accompanied by increased matrix metalloproteinase (MMP) activity, enhanced tissue inflammation, oxidative stress, and neovascularization. Importantly, these effects were inhibited by 17-DMAG treatment. Mechanistically, we showed that 17-DMAG prevented the formation and progression of AAA through its inhibitory effects on diverse biological pathways including 1) by blocking ANG II-induced phosphorylation of ERK1/2 and JNK that are critically involved in the regulation of MMPs in vascular smooth muscle cells, 2) by inhibiting IκB kinase expression and expression of MCP-1, and 3) by attenuating ANG II-stimulated angiogenic processes critical to AAA formation. Our results demonstrate that inhibition of Hsp90 by 17-DMAG effectively attenuates ANG II-induced AAA formation by simultaneously inhibiting vascular inflammation, extracellular matrix degradation, and angiogenesis, which are critical in the formation and progression of AAAs.

miR-24 limits aortic vascular inflammation and murine abdominal aneurysm development

Identification and treatment of abdominal aortic aneurysm (AAA) remain among the most prominent challenges in vascular medicine. MicroRNAs (miRNAs) are crucial regulators of cardiovascular pathology and represent intriguing targets to limit AAA expansion. Here we show, by using two established murine models of AAA disease along with human aortic tissue and plasma analysis, that miR-24 is a key regulator of vascular inflammation and AAA pathology. In vivo and in vitro studies reveal chitinase 3-like 1 (Chi3l1) to be a major target and effector under the control of miR-24, regulating cytokine synthesis in macrophages as well as their survival, promoting aortic smooth muscle cell migration and cytokine production, and stimulating adhesion molecule expression in vascular endothelial cells. We further show that modulation of miR-24 alters AAA progression in animal models, and that miR-24 and CHI3L1 represent novel plasma biomarkers of AAA disease progression in humans.

Abdominal aortic aneurysm (AAA) is a potentially fatal and often asymptomatic disease whose causes remain unclear. Here the authors show that a microRNA, miR-24, and its target, the glycoprotein chitinase 3-like 1, represent key regulators of AAA development.