Inadequate reinforcement of transmedial disruptions at branch points subtends aortic aneurysm formation in apolipoprotein-E-deficient mice

Intimomedial tears of the aorta heal by smooth muscle cell-mediated fibrosis without atherosclerosis

BACKGROUNDDisease of the aorta varies from atherosclerosis to aneurysms, with complications including rupture, dissection, and poorly characterized limited tears. We studied limited tears without any mural hematoma, termed intimomedial tears, to gain insight into aortic vulnerability to excessive wall stresses. Our premise is that minimal injuries in aortas with sufficient medial resilience to prevent tear progression correspond to initial mechanisms leading to complete structural failure in aortas with significantly compromised medial resilience.METHODSIntimomedial tears were macroscopically identified in 9 of 108 ascending aortas after surgery and analyzed by histology and immunofluorescence confocal microscopy.RESULTSNonhemorrhagic, nonatheromatous tears correlated with advanced aneurysmal disease and most lacked distinctive symptoms or radiological signs. Tears traversed the intima and part of the subjacent media, while the resultant defects were partially or completely filled with neointima characterized by differentiated smooth muscle cells, scattered leukocytes, dense fibrosis, and absent elastic laminae despite tropoelastin synthesis. Healed lesions contained organized fibrin at tear edges without evidence of plasma and erythrocyte extravasation or lipid accumulation.CONCLUSIONThese findings suggest a multiphasic model of aortic wall failure in which primary lesions of intimomedial tears either heal if the media is sufficiently resilient or progress as dissection or rupture by medial delamination and tear completion, respectively. Moreover, mural incorporation of thrombus and cellular responses to injury, two historically important concepts in atheroma pathogenesis, contribute to vessel wall repair with adequate conduit function, but even together are not sufficient to induce atherosclerosis.FUNDINGNIH (R01-HL146723, R01-HL168473) and Yale Department of Surgery.

Next-generation sequencing of prolidase gene identifies novel and common variants associated with low prolidase in coronary artery ectasia

BACKGROUND

Decreased collagen biosynthesis and increased collagenolysis can cause ectasia progression in the arterial walls. Prolidase is a key enzyme in collagen synthesis; a decrease in prolidase activity or level may decrease collagen biosynthesis, which may contribute to ectasia formation. Considering that, the variations in PEPD gene encoding prolidase enzyme were evaluated by analyzing next-generation sequencing (NGS) for the first time together with known risk factors in coronary artery ectasia (CAE) patients.

METHODS

Molecular analysis of the PEPD gene was performed on genomic DNA by NGS in 76 CAE patients and 76 controls. The serum levels of prolidase were measured by the sandwich-ELISA technique.

RESULTS

Serum prolidase levels were significantly lower in CAE group compared to control group, and it was significantly lower in males than females in both groups (p < 0.001). On the other hand, elevated prolidase levels were observed in CAE patients in the presence of diabetes (p < 0.001), hypertension (p < 0.05) and hyperlipidemia (p < 0.05). Logistic regression analysis demonstrated that the low prolidase level (p < 0.001), hypertension (p < 0.02) and hyperlipidemia (p < 0.012) were significantly associated with increased CAE risk. We identified four missense mutations in the PEPD gene, namely G296S, T266A, P365L and S134C (novel) that could be associated with CAE. The pathogenicity of these mutations was predicted to be "damaging" for G296S, S134C and P365L, but "benign" for T266A. We also identified a novel 5'UTR variation (Chr19:34012748 G>A) in one patient who had a low prolidase level. In addition, rs17570 and rs1061338 common variations of the PEPD gene were associated with low prolidase levels in CAE patients, while rs17569 variation was associated with high prolidase levels in both CAE and controls (p < 0.05).

CONCLUSIONS

Our findings indicate that the low serum prolidase levels observed in CAE patients is significantly associated with PEPD gene variations. It was concluded that low serum prolidase level and associated PEPD mutations may be potential biomarkers for the diagnosis of CAE.

mTOR inhibition prevents angiotensin II-induced aortic rupture and pseudoaneurysm but promotes dissection in Apoe-deficient mice

Aortic dissection and rupture are triggered by decreased vascular wall strength and/or increased mechanical loads. We investigated the role of mTOR signaling in aortopathy using a well-described model of angiotensin II–induced dissection, aneurysm, or rupture of the suprarenal abdominal aorta in Apoe-deficient mice. Although not widely appreciated, nonlethal hemorrhagic lesions present as pseudoaneurysms without significant dissection in this model. Angiotensin II–induced aortic tears result in free rupture, contained rupture with subadventitial hematoma (forming pseudoaneurysms), dilatation, or healing, while the media invariably thickens regardless of mural tears. Medial thickening results from smooth muscle cell hypertrophy and extracellular matrix accumulation, including matricellular proteins. Angiotensin II activates mTOR signaling in vascular wall cells, and inhibition of mTOR signaling by rapamycin prevents aortic rupture but promotes dissection. Decreased aortic rupture correlates with decreased inflammation and metalloproteinase expression, whereas extensive dissection correlates with induction of matricellular proteins that modulate adhesion of vascular cells. Thus, mTOR activation in vascular wall cells determines whether aortic tears progress to dissection or rupture. Previous mechanistic studies of aortic aneurysm and dissection by angiotensin II in Apoe-deficient mice should be reinterpreted as clinically relevant to pseudoaneurysms, and mTOR inhibition for aortic disease should be explored with caution.

Photobiomodulation and estrogen stabilize mitochondrial membrane potential in angiotensin-II challenged porcine aortic smooth muscle cells

Rupture of Abdominal aortic aneurysm (AAA) is among the 15 leading causes of death after age 65. Using high frequency ultrasound, we showed that photobiomodulation (PBM) prevents formation and progression of AAA in the angiotensin-II (Ang-II)-infused, apolipoprotein-e-deficient mouse model. In the current study we report that while challenge of porcine aortic Smooth Muscle Cells (SMCs) with Ang-II (1 μM) resulted in a marked decay in mitochondrial membrane potential (MitMP) vs non-challenged cells, treatment with PBM (continuous diode laser, 780 nm, 6.7 mW/cm² , 5 minutes, 2 J/cm² ) or pre-incubation with estrogen (50 nM, 1 hour) significantly attenuated this deterioration in MitMP. We also report that PBM and estrogen markedly affected porcine aortic SMC contraction and modified mitochondrial dispersion reflecting important influence on SMC function. These studies provide strong evidence of the important underlying role of mitochondria in the preventive effect of PBM on formation and progression of AAA and its reduced incidence and delayed onset in women.

Multimodality Imaging-Based Characterization of Regional Material Properties in a Murine Model of Aortic Dissection

Chronic infusion of angiotensin-II in atheroprone (ApoE-/-) mice provides a reproducible model of dissection in the suprarenal abdominal aorta, often with a false lumen and intramural thrombus that thickens the wall. Such lesions exhibit complex morphologies, with different regions characterized by localized changes in wall composition, microstructure, and properties. We sought to quantify the multiaxial mechanical properties of murine dissecting aneurysm samples by combining in vitro extension-distension data with full-field multimodality measurements of wall strain and thickness to inform an inverse material characterization using the virtual fields method. A key advance is the use of a digital volume correlation approach that allows for characterization of properties not only along and around the lesion, but also across its wall. Specifically, deformations are measured at the adventitial surface by tracking motions of a speckle pattern using a custom panoramic digital image correlation technique while deformations throughout the wall and thrombus are inferred from optical coherence tomography. These measurements are registered and combined in 3D to reconstruct the reference geometry and compute the 3D finite strain fields in response to pressurization. Results reveal dramatic regional variations in material stiffness and strain energy, which reflect local changes in constituent area fractions obtained from histology but emphasize the complexity of lesion morphology and damage within the dissected wall. This is the first point-wise biomechanical characterization of such complex, heterogeneous arterial segments. Because matrix remodeling is critical to the formation and growth of these lesions, we submit that quantification of regional material properties will increase the understanding of pathological mechanical mechanisms underlying aortic dissection.

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.

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.

Molecular and cellular insights into the pathogenesis of coronary artery ectasia

Coronary artery ectasia describes a local or diffuse dilatation of the epicardial coronary arteries. This review summarizes the molecular and cellular mechanisms involved in the pathogenesis of coronary artery ectasia. Better identification of the pathophysiologic steps will shed light into the clinical significance and may have direct implications for the management strategies of this disease. Additionally, understanding the underlying etiology may help to improve treatment modalities specific to coronary artery ectasia.

A hidden structural vulnerability in the thrombospondin-2 deficient aorta increases the propensity to intramural delamination

Mice lacking thrombospondin-2 (TSP2) represent an animal model of impaired collagen fibrillogenesis. Collagen constitutes ~1/3 of the wall of the normal murine descending thoracic aorta (DTA) and is thought to confer mechanical strength at high pressures. Microstructural analysis of the DTA from TSP2-null mice revealed irregular and disorganized collagen fibrils in the adventitia and at the interface between the media and adventitia. Yet, biaxial mechanical tests performed under physiologic loading conditions showed that most mechanical metrics, including stress and stiffness, were not different between mutant and control DTAs at 20- and 40-weeks of age, thus suggesting that the absence of TSP2 is well compensated under normal conditions. A detailed bilayered analysis of the wall mechanics predicted, however, that the adventitia of TSP2-null DTAs fails to engage at high pressures, which could render the media vulnerable to mechanical damage. Failure tests confirmed that the pressure at which the DTA ruptures is significantly lower in 20-week-old TSP2-null mice compared to age-matched controls (640±37 vs. 1120±45mmHg). Moreover, half of the 20-week-old and all 40-week-old mutant DTAs failed by delamination, not rupture. This delamination occurred at the interface between the media and the adventitia, with separation planes often observed at ~45 degrees with respect to the circumferential/axial directions. Combined with the observed microstructural anomalies, our theoretical-experimental biomechanical results suggest that TSP2-null DTAs are more susceptible to material failure when exposed to high pressures and this vulnerability may result from a reduced resistance to shear loading at the medial/adventitial border.

Incidence, severity, mortality, and confounding factors for dissecting AAA detection in angiotensin II-infused mice: a meta-analysis

AIMS

While angiotensin II-infused mice are the most popular model for preclinical aneurysm research, representative data on incidence, severity, and mortality of dissecting abdominal aortic aneurysms (AAAs) have never been established, and the influence of confounding factors is unknown.

METHODS AND RESULTS

We performed a meta-analysis including 194 manuscripts representing 1679 saline-infused, 4729 non-treated angiotensin II-infused, and 4057 treated angiotensin II-infused mice. Incidence (60%) and mortality (20%) rates are reported overall as well as for grade I (22%), grade II (26%), grade III (29%), and grade IV (24%) aneurysms. Dissecting AAA incidence was significantly (P < 0.05) influenced by sex, age, genetic background, infusion time, and dose of angiotensin II. Mortality was influenced by sex, genetic background, and dose, but not by age or infusion time. Surprisingly, both incidence and mortality were significantly different (P < 0.05) when comparing angiotensin II-infused mice in descriptive studies (56% incidence and 19% mortality) with angiotensin II-infused mice that served as control animals in treatment studies designed to either enhance (35% incidence and 13% mortality) or reduce (73% incidence and 25% mortality) dissecting AAA formation. After stratification to account for confounding factors (selection bias), the observed effect was still present for incidence, but not for mortality. Possible underlying causes are detection bias (non-uniform definition for detection and quantification of dissecting AAA in mice) or publication bias (studies with negative results, related to incidence in the control group, not being published).

CONCLUSIONS

Our data provide a new meta-analysis-based reference for incidence and mortality of dissecting AAA in angiotensin II-infused mice, and indicate that treatment studies using this mouse model should be interpreted with caution.

An animal-specific FSI model of the abdominal aorta in anesthetized mice

Recent research has revealed that angiotensin II-induced abdominal aortic aneurysm in mice can be related to medial ruptures occurring in the vicinity of abdominal side branches. Nevertheless a thorough understanding of the biomechanics near abdominal side branches in mice is lacking. In the current work we present a mouse-specific fluid-structure interaction (FSI) model of the abdominal aorta in ApoE(-/-) mice that incorporates in vivo stresses. The aortic geometry was based on contrast-enhanced in vivo micro-CT images, while aortic flow boundary conditions and material model parameters were based on in vivo high-frequency ultrasound. Flow waveforms predicted by FSI simulations corresponded better to in vivo measurements than those from CFD simulations. Peak-systolic principal stresses at the inner and outer aortic wall were locally increased caudal to the celiac and left lateral to the celiac and mesenteric arteries. Interestingly, these were also the locations at which a tear in the tunica media had been observed in previous work on angiotensin II-infused mice. Our preliminary results therefore suggest that local biomechanics play an important role in the pathophysiology of branch-related ruptures in angiotensin-II infused mice. More elaborate follow-up research is needed to demonstrate the role of biomechanics and mechanobiology in a longitudinal setting.