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

G2Φnet: Relating genotype and biomechanical phenotype of tissues with deep learning

Many genetic mutations adversely affect the structure and function of load-bearing soft tissues, with clinical sequelae often responsible for disability or death. Parallel advances in genetics and histomechanical characterization provide significant insight into these conditions, but there remains a pressing need to integrate such information. We present a novel genotype-to-biomechanical phenotype neural network (G2Φnet) for characterizing and classifying biomechanical properties of soft tissues, which serve as important functional readouts of tissue health or disease. We illustrate the utility of our approach by inferring the nonlinear, genotype-dependent constitutive behavior of the aorta for four mouse models involving defects or deficiencies in extracellular constituents. We show that G2Φnet can infer the biomechanical response while simultaneously ascribing the associated genotype by utilizing limited, noisy, and unstructured experimental data. More broadly, G2Φnet provides a powerful method and a paradigm shift for correlating genotype and biomechanical phenotype quantitatively, promising a better understanding of their interplay in biological tissues.

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.

Critical Pressure of Intramural Delamination in Aortic Dissection

Computational models of aortic dissection can examine mechanisms by which this potentially lethal condition develops and propagates. We present results from phase-field finite element simulations that are motivated by a classical but seldom repeated experiment. Initial simulations agreed qualitatively and quantitatively with data, yet because of the complexity of the problem it was difficult to discern trends. Simplified analytical models were used to gain further insight. Together, simplified and phase-field models reveal power-law-based relationships between the pressure that initiates an intramural tear and key geometric and mechanical factors-insult surface area, wall stiffness, and tearing energy. The degree of axial stretch and luminal pressure similarly influence the pressure of tearing, which was ~88 kPa for healthy and diseased human aortas having sub-millimeter-sized initial insults, but lower for larger tear sizes. Finally, simulations show that the direction a tear propagates is influenced by focal regions of weakening or strengthening, which can drive the tear towards the lumen (dissection) or adventitia (rupture). Additional data on human aortas having different predisposing disease conditions will be needed to extend these results further, but the present findings show that physiologic pressures can propagate initial medial defects into delaminations that can serve as precursors to dissection.

The Contribution of Glycosaminoglycans/Proteoglycans to Aortic Mechanics in Health and Disease: A Critical Review

While elastin and collagen have received a lot of attention as major contributors to aortic biomechanics, glycosaminoglycans (GAGs) and proteoglycans (PGs) recently emerged as additional key players whose roles must be better elucidated if one hopes to predict aortic ruptures caused by aneurysms and dissections more reliably. GAGs are highly negatively charged polysaccharide molecules that exist in the extracellular matrix (ECM) of the arterial wall. In this critical review, we summarize the current understanding of the contributions of GAGs/PGs to the biomechanics of the normal aortic wall, as well as in the case of aortic diseases such as aneurysms and dissections. Specifically, we describe the fundamental swelling behavior of GAGs/PGs and discuss their contributions to residual stresses and aortic stiffness, thereby highlighting the importance of taking these polyanionic molecules into account in mathematical and numerical models of the aorta. We suggest specific lines of investigation to further the acquisition of experimental data to complement simulations and solidify our current understanding. We underscore different potential roles of GAGs/PGs in thoracic aortic aneurysm (TAAD) and abdominal aortic aneurysm (AAA). Namely, we report findings according to which the accumulation of GAGs/PGs in TAAD causes stress concentrations which may be sufficient to initiate and propagate delamination. On the other hand, there seems to be no clear indication of a relationship between the marked reduction in GAG/PG content and the stiffening and weakening of the aortic wall in AAA.

Bio-chemo-mechanics of the thoracic aorta

The pathophysiology of thoracic aortic aneurysm and dissection is poorly understood, despite high mortality. An evidence review was conducted to examine the biomechanical, chemical and genetic factors involved in thoracic aortic pathology. The composition of connective tissue and smooth muscle cells can mediate important mechanical properties that allow the thoracic aorta to withstand and transmit pressures. Genetic syndromes can affect connective tissue and signalling proteins that interrupt smooth muscle function, leading to tissue failure. There are complex interplaying factors that maintain thoracic aortic function in health and are disrupted in disease, signifying an area for extensive research.

Adventitial remodeling protects against aortic rupture following late smooth muscle-specific disruption of TGFβ signaling

Altered signaling through transforming growth factor-beta (TGFβ) increases the risk of aortic dissection in patients, which has been confirmed in mouse models. It is well known that altered TGFβ signaling affects matrix turnover, but there has not been a careful examination of associated changes in structure-function relations. In this paper, we present new findings on the rupture potential of the aortic wall following late postnatal smooth muscle cell (SMC)-specific disruption of type I and II TGFβ receptors in a mouse model with demonstrated dissection susceptibility. Using a combination of custom computer-controlled biaxial tests and quantitative histology and immunohistochemistry, we found that loss of TGFβ signaling in SMCs compromises medial properties but induces compensatory changes in the adventitia that preserve wall strength above that which is needed to resist in vivo values of wall stress. These findings emphasize the different structural defects that lead to aortic dissection and rupture - compromised medial integrity and insufficient adventitial strength, respectively. Relative differences in these two defects, in an individual subject at a particular time, likely reflects the considerable phenotypic diversity that is common in clinical presentations of thoracic aortic dissection and rupture. There is, therefore, a need to move beyond examinations of bulk biological assays and wall properties to cell- and layer-specific studies that delineate pathologic and compensatory changes in wall biology and composition, and thus the structural integrity of the aortic wall that can dictate differences between life and death.

Recent Advances in Biomechanical Characterization of Thoracic Aortic Aneurysms

Thoracic aortic aneurysm (TAA) is a focal enlargement of the thoracic aorta, but the etiology of this disease is not fully understood. Previous work suggests that various genetic syndromes, congenital defects such as bicuspid aortic valve, hypertension, and age are associated with TAA formation. Though occurrence of TAAs is rare, they can be life-threatening when dissection or rupture occurs. Prevention of these adverse events often requires surgical intervention through full aortic root replacement or implantation of endovascular stent grafts. Currently, aneurysm diameters and expansion rates are used to determine if intervention is warranted. Unfortunately, this approach oversimplifies the complex aortopathy. Improving treatment of TAAs will likely require an increased understanding of the biological and biomechanical factors contributing to the disease. Past studies have substantially contributed to our knowledge of TAAs using various ex vivo, in vivo, and computational methods to biomechanically characterize the thoracic aorta. However, any singular approach typically focuses on only material properties of the aortic wall, intra-aneurysmal hemodynamics, or in vivo vessel dynamics, neglecting combinatorial factors that influence aneurysm development and progression. In this review, we briefly summarize the current understanding of TAA causes, treatment, and progression, before discussing recent advances in biomechanical studies of TAAs and possible future directions. We identify the need for comprehensive approaches that combine multiple characterization methods to study the mechanisms contributing to focal weakening and rupture. We hope this summary and analysis will inspire future studies leading to improved prediction of thoracic aneurysm progression and rupture, improving patient diagnoses and outcomes.

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.

Central artery stiffness and thoracic aortopathy

Thoracic aortopathy, especially aneurysm, dissection, and rupture, is responsible for significant morbidity and mortality. Uncontrolled hypertension and aging are primary risk factors for such conditions, and they contribute to an increase in the mechanical stress on the wall and an increase in its structural vulnerability, respectively. Select genetic mutations also predispose to these lethal conditions, and the collection of known mutations suggests that dysfunctional mechanosensing and mechanoregulation of the extracellular matrix may contribute to pathogenesis and disease progression. In the absence of a well-accepted pharmacotherapy, nonsurgical treatments tend to focus on reducing the mechanical loading on the aorta, particularly via the use of antihypertensive medications and recommendations to avoid strenuous exercises such as weight lifting. In this brief review, we discuss the important effects of central artery stiffening on global hemodynamics and, in particular, on the increase in pulse pressure that acts on the proximal thoracic aorta. We consider Marfan syndrome as an illustrative aortopathy but discuss other conditions leading to thoracic aortic aneurysm and dissection. We highlight the importance of phenotyping the aorta biomechanically, not just clinically, and emphasize the utility of mouse models in elucidating molecular and mechanical mechanisms of disease. Notwithstanding the widely recognized role of central artery stiffening in driving end-organ disease, we suggest that there is similarly a need to consider its key role in thoracic aortopathy.

A Uniaxial Testing Approach for Consistent Failure in Vascular Tissues

Although uniaxial tensile testing is commonly used to evaluate failure properties of vascular tissue, there is no established protocol for specimen shape or gripping method. Large percentages of specimens are reported to fail near the clamp and can potentially confound the studies, or, if discarded will result in sample waste. The objective of this study is to identify sample geometry and clamping conditions that can achieve consistent failure in the midregion of small arterial specimens, even for vessels from older individuals. Failure location was assessed in 17 dogbone specimens from human cerebral and sheep carotid arteries using soft inserts. For comparison with commonly used protocols, an additional 22 rectangular samples were tested using either sandpaper or foam tape inserts. Midsample failure was achieved in 94% of the dogbone specimens, while only 14% of the rectangular samples failed in the midregion, the other 86% failing close to the clamps. Additionally, we found midregion failure was more likely to be abrupt, caused by cracking or necking. In contrast, clamp failure was more likely to be gradual and included a delamination mode not seen in midregion failure. Hence, this work provides an approach that can be used to obtain consistent midspecimen failure, avoiding confounding clamp-related artifacts. Furthermore, with consistent midregion failure, studies can be designed to image the failure process in small vascular samples providing valuable quantitative information about changes to collagen and elastin structure during the failure process.

Compromised mechanical homeostasis in arterial aging and associated cardiovascular consequences

Aging leads to central artery stiffening and associated hemodynamic sequelae. Because healthy arteries exhibit differential geometry, composition, and mechanical behaviors along the central vasculature, we sought to determine whether wall structure and mechanical function differ across five vascular regions-the ascending and descending thoracic aorta, suprarenal and infrarenal abdominal aorta, and common carotid artery-in 20 versus 100-week-old male wild-type mice. Notwithstanding generally consistent changes across these regions, including a marked thickening of the arterial wall, diminished in vivo axial stretch, and loss of elastic energy storage capacity, the degree of changes tended to be slightly greater in abdominal than in thoracic or carotid vessels. Likely due to the long half-life of vascular elastin, most mechanical changes in the arterial wall resulted largely from a distributed increase in collagen, including thicker fibers in the media, and localized increases in glycosaminoglycans. Changes within the central arteries associated with significant increases in central pulse pressure and adverse changes in the left ventricle, including increased cardiac mass and decreased diastolic function. Given the similar half-life of vascular elastin in mice and humans but very different life-spans, there are important differences in the aging of central vessels across these species. Nevertheless, the common finding of aberrant matrix remodeling contributing to a compromised mechanical homeostasis suggests that studies of central artery aging in the mouse can provide insight into mechanisms and treatment strategies for the many adverse effects of vascular aging in humans.

Computational predictions of damage propagation preceding dissection of ascending thoracic aortic aneurysms

Dissections of ascending thoracic aortic aneurysms (ATAAs) cause significant morbidity and mortality worldwide. They occur when a tear in the intima-media of the aorta permits the penetration of the blood and the subsequent delamination and separation of the wall in 2 layers, forming a false channel. To predict computationally the risk of tear formation, stress analyses should be performed layer-specifically and they should consider internal or residual stresses that exist in the tissue. In the present paper, we propose a novel layer-specific damage model based on the constrained mixture theory, which intrinsically takes into account these internal stresses and can predict appropriately the tear formation. The model is implemented in finite-element commercial software Abaqus coupled with user material subroutine. Its capability is tested by applying it to the simulation of different exemplary situations, going from in vitro bulge inflation experiments on aortic samples to in vivo overpressurizing of patient-specific ATAAs. The simulations reveal that damage correctly starts from the intimal layer (luminal side) and propagates across the media as a tear but never hits the adventitia. This scenario is typically the first stage of development of an acute dissection, which is predicted for pressures of about 2.5 times the diastolic pressure by the model after calibrating the parameters against experimental data performed on collected ATAA samples. Further validations on a larger cohort of patients should hopefully confirm the potential of the model in predicting patient-specific damage evolution and possible risk of dissection during aneurysm growth for clinical applications.

Comparison of 10 murine models reveals a distinct biomechanical phenotype in thoracic aortic aneurysms

Thoracic aortic aneurysms are life-threatening lesions that afflict young and old individuals alike. They frequently associate with genetic mutations and are characterized by reduced elastic fibre integrity, dysfunctional smooth muscle cells, improperly remodelled collagen and pooled mucoid material. There is a pressing need to understand better the compromised structural integrity of the aorta that results from these genetic mutations and renders the wall vulnerable to dilatation, dissection or rupture. In this paper, we compare the biaxial mechanical properties of the ascending aorta from 10 murine models: wild-type controls, acute elastase-treated, and eight models with genetic mutations affecting extracellular matrix proteins, transmembrane receptors, cytoskeletal proteins, or intracellular signalling molecules. Collectively, our data for these diverse mouse models suggest that reduced mechanical functionality, as indicated by a decreased elastic energy storage capability or reduced distensibility, does not predispose to aneurysms. Rather, despite normal or lower than normal circumferential and axial wall stresses, it appears that intramural cells in the ascending aorta of mice prone to aneurysms are unable to maintain or restore the intrinsic circumferential material stiffness, which may render the wall biomechanically vulnerable to continued dilatation and possible rupture. This finding is consistent with an underlying dysfunctional mechanosensing or mechanoregulation of the extracellular matrix, which normally endows the wall with both appropriate compliance and sufficient strength.

Biomechanical Phenotyping of the Murine Aorta: What Is the Best Control?

The availability of diverse mouse models is revealing increasingly greater information on arterial mechanics, including homeostatic adaptations and pathologic maladaptations to genetic, pharmacological, and surgical manipulations. Fundamental to understanding such biomechanical changes, however, is reliable information on appropriate control vessels. In this paper, we contrast 15 different geometrical and mechanical metrics of biaxial wall mechanics for the ascending aorta across seven different types of possible control mice. We show that there is a comforting similarity across these multiple controls for most, though not all, metrics. In particular, three potential controls, namely, noninduced conditional mice, exhibit higher values of distensibility, an important clinical metric of structural stiffness, and two of these potential controls also have higher values of intrinsic circumferential material stiffness. There is motivation, therefore, to understand better the biomechanical changes that can arise with noninduced Cre-lox or similar approaches for generating mutations conditionally. In cases of germline mutations generated by breeding heterozygous +/- mice, however, the resulting homozygous +/+ mice tend to exhibit properties similar to traditional (C57BL/6) controls.