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

Significance of Dynamic Axial Stretching on Estimating Biomechanical Behavior and Properties of the Human Ascending Aorta

Contrary to most vessels, the ascending thoracic aorta (ATA) not only distends but also elongates in the axial direction. The purpose of this study is to investigate the biomechanical behavior of the ascending thoracic aorta (ATA) in response to dynamic axial stretching during the cardiac cycle. In addition, the implications of neglecting this dynamic axial stretching when estimating the constitutive model parameters of the ATA are investigated. The investigations were performed through in silico simulations by assuming a Gasser-Ogden-Holzapfel (GOH) constitutive model representative of ATA tissue material. The GOH model parameters were obtained from biaxial tests performed on four human ATA tissues in a previous study. Pressure-diameter curves were simulated as synthetic data to assess the effect of neglecting dynamic axial stretching on estimating constitutive model parameters. Our findings reveal a significant increase in axial stress (~ 16%) and stored strain energy (~ 18%) in the vessel when dynamic axial stretching is considered, as opposed to assuming a fixed axial stretch. All but one artery showed increased volume compliance while considering a dynamic axial stretching condition. Furthermore, we observe a notable difference in the estimated constitutive model parameters when dynamic axial stretching of the ATA is neglected, compared to the ground truth model parameters. These results underscore the critical importance of accounting for axial deformations when conducting in vivo biomechanical characterization of the ascending thoracic aorta.

Cellular stiffness sensing through talin 1 in tissue mechanical homeostasis

Tissue mechanical properties are determined mainly by the extracellular matrix (ECM) and actively maintained by resident cells. Despite its broad importance to biology and medicine, tissue mechanical homeostasis remains poorly understood. To explore cell-mediated control of tissue stiffness, we developed mutations in the mechanosensitive protein talin 1 to alter cellular sensing of ECM. Mutation of a mechanosensitive site between talin 1 rod-domain helix bundles R1 and R2 increased cell spreading and tension exertion on compliant substrates. These mutations promote binding of the ARP2/3 complex subunit ARPC5L, which mediates the change in substrate stiffness sensing. Ascending aortas from mice bearing these mutations showed less fibrillar collagen, reduced axial stiffness, and lower rupture pressure. Together, these results demonstrate that cellular stiffness sensing contributes to ECM mechanics, directly supporting the mechanical homeostasis hypothesis and identifying a mechanosensitive interaction within talin that contributes to this mechanism.

Remodeling of Murine Branch Pulmonary Arteries Under Chronic Hypoxia and Short-Term Normoxic Recovery

Chronic hypoxia plays a central role in diverse pulmonary pathologies, but its effects on longitudinal changes in the biomechanical behavior of proximal pulmonary arteries remain poorly understood. Similarly, effects of normoxic recovery have not been well studied. Here, we report hypoxia-induced changes in composition, vasoactivity, and passive biaxial mechanics in the main branch pulmonary artery of male C57BL/6J mice exposed to 10% FiO2 for 1, 2, or 3 weeks. We observed significant changes in extracellular matrix, and consequently wall mechanics, as early as 1 week of hypoxia. While circumferential stress and stiffness returned toward normal values by 2-3 weeks of hypoxia, area fractions of cytoplasm and thin collagen fibers did not return toward normal until after 1 week of normoxic recovery. By contrast, elastic energy storage and overall distensibility remained reduced after 3 weeks of hypoxia as well as following 1 week of normoxic recovery. While smooth muscle and endothelial cell responses were attenuated under hypoxia, smooth muscle but not endothelial cell responses recovered following 1 week of subsequent normoxia. Collectively, these data suggest that homeostatic processes were unable to preserve or restore overall function, at least over a brief period of normoxic recovery. Longitudinal changes are critical in understanding large pulmonary artery remodeling under hypoxia, and its reversal, and will inform predictive models of vascular adaptation.

Multiscale insights into postnatal aortic development

Despite its vital importance for establishing proper cardiovascular function, the process through which the vasculature develops and matures postnatally remains poorly understood. From a clinical perspective, an ability to mechanistically model the developmental time course in arteries and veins, as well as to predict how various pathologies and therapeutic interventions alter the affected vessels, promises to improve treatment strategies and long-term clinical outcomes, particularly in pediatric patients suffering from congenital heart defects. In the present study, we conducted a multiscale investigation into the postnatal development of the murine thoracic aorta, examining key allometric relations as well as relationships between in vivo mechanical stresses, collagen and elastin expression, and the gradual accumulation of load-bearing constituents within the aortic wall. Our findings suggest that the production of fibrillar collagens in the developing aorta associates strongly with the ratio of circumferential stresses between systole and diastole, hence emphasizing the importance of a pulsatile mechanobiological stimulus. Moreover, rates of collagen turnover and elastic fiber compaction can be inferred directly by synthesizing transcriptional data and quantitative histological measurements of evolving collagen and elastin content. Consistent with previous studies, we also observed that wall shear stresses acting on the aorta are similar at birth and in maturity, supporting the hypothesis that at least some stress targets are established early in development and maintained thereafter, thus providing a possible homeostatic basis to guide future experiments and inform future predictive modeling.

Central Artery Hemodynamics in Angiotensin II-Induced Hypertension and Effects of Anesthesia

Systemic hypertension is a strong risk factor for cardiovascular, neurovascular, and renovascular diseases. Central artery stiffness is both an initiator and indicator of hypertension, thus revealing a critical relationship between the wall mechanics and hemodynamics. Mice have emerged as a critical animal model for studying effects of hypertension and much has been learned. Regardless of the specific mouse model, data on changes in cardiac function and hemodynamics are necessarily measured under anesthesia. Here, we present a new experimental-computational workflow to estimate awake cardiovascular conditions from anesthetized data, which was then used to quantify effects of chronic angiotensin II-induced hypertension relative to normotension in wild-type mice. We found that isoflurane anesthesia had a greater impact on depressing hemodynamics in angiotensin II-infused mice than in controls, which led to unexpected results when comparing anesthetized results between the two groups of mice. Through comparison of the awake simulations, however, in vivo relevant effects of angiotensin II-infusion on global and regional vascular structure, properties, and hemodynamics were found to be qualitatively consistent with expectations. Specifically, we found an increased in vivo vascular stiffness in the descending thoracic aorta and suprarenal abdominal aorta, leading to increases in pulse pressure in the distal aorta. These insights allow characterization of the impact of regionally varying vascular remodeling on hemodynamics and mouse-to-mouse variations due to induced hypertension.

Mechanosensing through talin 1 contributes to tissue mechanical homeostasis

It is widely believed that tissue mechanical properties, determined mainly by the extracellular matrix (ECM), are actively maintained. However, despite its broad importance to biology and medicine, tissue mechanical homeostasis is poorly understood. To explore this hypothesis, we developed mutations in the mechanosensitive protein talin1 that alter cellular sensing of ECM stiffness. Mutation of a novel mechanosensitive site between talin1 rod domain helix bundles 1 and 2 (R1 and R2) shifted cellular stiffness sensing curves, enabling cells to spread and exert tension on compliant substrates. Opening of the R1-R2 interface promotes binding of the ARP2/3 complex subunit ARPC5L, which mediates the altered stiffness sensing. Ascending aortas from mice bearing these mutations show increased compliance, less fibrillar collagen, and rupture at lower pressure. Together, these results demonstrate that cellular stiffness sensing regulates ECM mechanical properties. These data thus directly support the mechanical homeostasis hypothesis and identify a novel mechanosensitive interaction within talin that contributes to this mechanism.

Constituent-based quasi-linear viscoelasticity: a revised quasi-linear modelling framework to capture nonlinear viscoelasticity in arteries

Arteries exhibit fully nonlinear viscoelastic behaviours (i.e. both elastically and viscously nonlinear). While elastically nonlinear arterial models are well established, effective mathematical descriptions of nonlinear viscoelasticity are lacking. Quasi-linear viscoelasticity (QLV) offers a convenient way to mathematically describe viscoelasticity, but its viscous linearity assumption is unsuitable for whole-wall vascular applications. Conversely, application of fully nonlinear viscoelastic models, involving deformation-dependent viscous parameters, to experimental data is impractical and often reduces to identifying specific solutions for each tested loading condition. The present study aims to address this limitation: By applying QLV theory at the wall constituent rather than at the whole-wall level, the deformation-dependent relative contribution of the constituents allows to capture nonlinear viscoelasticity with a unique set of deformation-independent model parameters. Five murine common carotid arteries were subjected to a protocol of quasi-static and harmonic, pseudo-physiological biaxial loading conditions to characterise their viscoelastic behaviour. The arterial wall was modelled as a constrained mixture of an isotropic elastin matrix and four families of collagen fibres. Constituent-based QLV was implemented by assigning different relaxation functions to collagen- and elastin-borne parts of the wall stress. Nonlinearity in viscoelasticity was assessed via the pressure dependency of the dynamic-to-quasi-static stiffness ratio. The experimentally measured ratio increased with pressure, from 1.03 [Formula: see text] 0.03 (mean [Formula: see text] standard deviation) at 80-40 mmHg to 1.58 [Formula: see text] 0.22 at 160-120 mmHg. Constituent-based QLV captured well this trend by attributing the wall viscosity predominantly to collagen fibres, whose recruitment starts at physiological pressures. In conclusion, constituent-based QLV offers a practical and effective solution to model arterial viscoelasticity.

The extracellular matrix mechanics in the vasculature

Mechanical stimuli from the extracellular matrix (ECM) modulate vascular differentiation, morphogenesis and dysfunction of the vasculature. With innovation in measurements, we can better characterize vascular microenvironment mechanics in health and disease. Recent advances in material sciences and stem cell biology enable us to accurately recapitulate the complex and dynamic ECM mechanical microenvironment for in vitro studies. These biomimetic approaches help us understand the signaling pathways in disease pathologies, identify therapeutic targets, build tissue replacement and activate tissue regeneration. This Review analyzes how ECM mechanics regulate vascular homeostasis and dysfunction. We highlight approaches to examine ECM mechanics at tissue and cellular levels, focusing on how mechanical interactions between cells and the ECM regulate vascular phenotype, especially under certain pathological conditions. Finally, we explore the development of biomaterials to emulate, measure and alter the physical microenvironment of pathological ECM to understand cell-ECM mechanical interactions toward the development of therapeutics.

Biomechanical Characterisation of Thoracic Ascending Aorta with Preserved Pre-Stresses

Mechanical properties of an aneurysmatic thoracic aorta are potential markers of future growth and remodelling and can help to estimate the risk of rupture. Aortic geometries obtained from routine medical imaging do not display wall stress distribution and mechanical properties. Mechanical properties for a given vessel may be determined from medical images at different physiological pressures using inverse finite element analysis. However, without considering pre-stresses, the estimation of mechanical properties will lack accuracy. In the present paper, we propose and evaluate a mechanical parameter identification technique, which recovers pre-stresses by determining the zero-pressure configuration of the aortic geometry. We first validated the method on a cylindrical geometry and subsequently applied it to a realistic aortic geometry. The verification of the assessed parameters was performed using synthetically generated reference data for both geometries. The method was able to estimate the true mechanical properties with an accuracy ranging from 98% to 99%.

Lengthwise regional mechanics of the human aneurysmal ascending thoracic aorta

The prognosis of patients undergoing emergency endovascular repair of ascending thoracic aortic aneurysm (ATAA) depends on defect location, with root disease bearing worse outcomes than proximal or distal aortopathy. We speculate that a spatial gradient in aneurysmal tissue mechanics through the length of the ascending thoracic aorta may fuel noted survival discrepancies. To this end, we performed planar biaxial testing on 153 root, proximal, and distal segments of ATAA samples collected from 80 patients receiving elective open surgical repair. Following data averaging via surface fitting-based interpolation of strain-controlled protocols, we combined in-vitro and in-vivo measurements of loads and geometry to resolve inflation-extension kinematics and evaluate mechanical metrics of stress, stiffness, and energy at consistent deformation levels. Representative (averaged) experimental data and simulated in-vivo conditions revealed significantly larger biaxial stiffness at the root compared to either proximal or distal tissues, which persisted as the entire aorta stiffened during aging. Advancing age further reduced biaxial stretch and energy storage, a measure of aortic function, across all ATAA segments. Importantly, age emerged as a stronger predictor of tissue mechanics in ATAA disease than either bicuspid aortic valve or connective tissue disorders. Besides strengthening the general understanding of aneurysmal disease, our findings provide specifications to customize the design of stent-grafts for the treatment of ATAA disease. Optimization of deployment and interaction of novel endovascular devices with the local native environment is expected to carry significant potential for improving clinical outcomes. STATEMENT OF SIGNIFICANCE: Elucidating the lengthwise regional mechanics of ascending thoracic aortic aneurysms (ATAAs) is critical for the design of endovascular devices tailored to the ascending aorta. Stent-grafts provide a less invasive alternative to support the long-term survival of ATAA patients ineligible for open surgical repair. In this study, we developed a numerical framework that combines semi-inverse constitutive and forward modeling with in-vitro and in-vivo data to extract mechanical descriptors of ATAA tissue behavior at physiologically meaningful deformation. Moving distally from the aortic root to the first ascending aortic branch, we observed a progressive decline in biaxial stiffness. Furthermore, we showed that aging leads to reduced aortic function and is a stronger predictor of mechanics than either valve morphology or underlying syndromic disorder.

Tri-layered constitutive modelling unveils functional differences between the pig ascending and lower thoracic aorta

The arterial wall's tri-layered macroscopic and layer-specific microscopic structure determine its mechanical properties, which vary at different arterial locations. Combining layer-specific mechanical data and tri-layered modelling, this study aimed to characterise functional differences between the pig ascending (AA) and lower thoracic aorta (LTA). AA and LTA segments were obtained for n=9 pigs. For each location, circumferentially and axially oriented intact wall and isolated layer strips were tested uniaxially and the layer-specific mechanical response modelled using a hyperelastic strain energy function. Then, layer-specific constitutive relations and intact wall mechanical data were combined to develop a tri-layered model of an AA and LTA cylindrical vessel, accounting for the layer-specific residual stresses. AA and LTA behaviours were then characterised for in vivo pressure ranges while stretched axially to in vivo length. The media dominated the AA response, bearing>2/3 of the circumferential load both at physiological (100 mmHg) and hypertensive pressures (160 mmHg). The LTA media bore most of the circumferential load at physiological pressure only (57±7% at 100 mmHg), while adventitia and media load bearings were comparable at 160 mmHg. Furthermore, increased axial elongation affected the media/adventitia load-bearing only at the LTA. The pig AA and LTA presented strong functional differences, likely reflecting their different roles in the circulation. The media-dominated compliant and anisotropic AA stores large amounts of elastic energy in response to both circumferential and axial deformations, which maximises diastolic recoiling function. This function is reduced at the LTA, where the adventitia shields the artery against supra-physiological circumferential and axial loads.

Sclerostin ablation prevents aortic valve stenosis in mice

The objective of this study was to test the hypothesis that targeting sclerostin would accelerate the progression of aortic valve stenosis. Sclerostin (mouse gene, Sost) is a secreted glycoprotein that acts as a potent regulator of bone remodeling. Antibody therapy targeting sclerostin is approved for osteoporosis but results from a stage III clinical trial showed multiple off-target cardiovascular effects. Wild-type (WT, Sost+/+) and Sost-gene knockout-expression (Null, Sost-/-) mice were generated and maintained to 12 mo of age on a high-cholesterol diet to induce aortic valve stenosis. Mice were examined by echocardiography, histology, and RNAseq. Immortalized valve interstitial cells were developed from each genotype for in vitro studies. Null mice developed a bone overgrowth phenotype, similar to patients with sclerosteosis. Surprisingly, however, WT mice developed hemodynamic signs of aortic valve stenosis, whereas Null mice were unchanged. WT mice had thicker aortic valve leaflets and higher amounts of α-smooth muscle actin, a marker myofibroblast activation and dystrophic calcification, with very little evidence of Runx2 expression, a marker of osteogenic calcification. RNAseq analysis of aortic roots indicated the HOX family of transcription factors was significantly upregulated in Null mice, and valve interstitial cells from Null animals were enriched with Hoxa1, Hoxb2, and Hoxd3 subtypes with downregulated Hoxa7. In addition, Null valve interstitial cells were shown to be less contractile than their WT counterparts. Contrary to our hypothesis, sclerostin targeting prevented hallmarks of aortic valve stenosis and indicates that targeted antibody treatments for osteoporosis may be beneficial for these patients regarding aortic stenosis.NEW & NOTEWORTHY We have found that genetic ablation of the Sost gene (protein: sclerostin) prevents aortic valve stenosis in aged, Western diet mice. This is a new role for sclerostin in the cardiovascular system. To the knowledge of the authors, this is one of the first studies directly manipulating sclerostin in a cardiovascular disease model and the first to specifically study the aortic valve. We also provide a potential new role for Hox genes in cardiovascular disease, noting pan-Hox upregulation in the aortic roots of sclerostin genetic knockouts. The role of Hox genes in postnatal cardiovascular health and disease is another burgeoning field of study to which this article contributes.

Compromised Cardiopulmonary Function in Fibulin-5 Deficient Mice

Competent elastic fibers are critical to the function of the lung and right circulation. Murine models of elastopathies can aid in understanding the functional roles of the elastin and elastin-associated glycoproteins that constitute elastic fibers. Here, we quantify together lung and pulmonary arterial structure, function, and mechanics with right heart function in a mouse model deficient in the elastin-associated glycoprotein fibulin-5. Differences emerged as a function of genotype, sex, and arterial region. Specifically, functional studies revealed increased lung compliance in fibulin-5 deficiency consistent with a histologically observed increased alveolar disruption. Biaxial mechanical tests revealed that the primary branch pulmonary arteries exhibit decreased elastic energy storage capacity and wall stress despite only modest differences in circumferential and axial material stiffness in the fibulin-5 deficient mice. Histological quantifications confirm a lower elastic fiber content in the fibulin-5 deficient pulmonary arteries, with fragmented elastic laminae in the outer part of the wall - likely the reason for reduced energy storage. Ultrasound measurements confirm sex differences in compromised right ventricular function in the fibulin-5 deficient mice. These results reveal compromised right heart function, but opposite effects of elastic fiber dysfunction on the lung parenchyma (significantly increased compliance) and pulmonary arteries (trend toward decreased distensibility), and call for further probing of ventilation-perfusion relationships in pulmonary pathologies. Amongst many other models, fibulin-5 deficient mice can contribute to our understanding of the complex roles of elastin in pulmonary health and disease.

Quantification of the heterogeneous effect of static and dynamic perivascular structures on patient-specific local aortic wall mechanics using inverse finite element modeling and DENSE MRI

Recent studies have highlighted the relevance of perivascular interactions on aortic wall mechanics. Most of the approaches assume static perivascular structures; however, the beating heart dynamically displaces the neighboring aorta. We develop a model to account for the effect of periaortic interactions due to static and dynamic structures by prescribing a moving elastic foundation boundary condition (EFBC) embedded into an inverse finite element algorithm using in vivo displacements from 2D displacement encoding with stimulated echoes (DENSE) MRI as target data. We applied this method at three different locations of interest, the distal aortic arch (DAA), descending thoracic aorta (DTA), and infrarenal abdominal aorta (IAA) for a total of 27 cases in healthy humans. The model reproduces the target diastole-to-systole deformation and bulk displacement of the aortic wall with median displacement errors below 0.5mm. The EFBC showed good agreement with the location of anatomical features and was consistent among individuals of similar characteristics. Results show that an energy source acting on the adventitia is required to reproduce the displacements measured at the vicinity of the heart, but not at the abdomen. The average adventitial load as a percentage of the luminal pulse-pressure was found to increase with age and to decrease along the descending aorta, from 61% at the DAA to 37% at the DTA, and 30% at the IAA. This approach offers a patient-specific method to estimate in vivo adventitial loads and aortic wall stiffness, which can bring a better understanding of normal and pathological in vivo aortic function.

Evaluation of early bilateral ovariectomy in mice as a model of left heart disease

Post-menopausal women tend to have worse cardiovascular outcomes in a manner that is associated with osteoporosis severity. In this study we performed the first evaluation of the left ventricle and aortic valve phenotype of ovariectomized mice aged on Western diet to one year. Disease was monitored in vivo using echocardiography and dual x-ray absorptiometry imaging and ex vivo using quantitative histological and immunostaining analysis. Mice had decreased bone mineral density in response to ovariectomy and increased fat mass in response to Western diet. Ovariectomized mice had a significantly increased left ventricle mass compared to control animals, absent of fibrosis. There was a slight increase in aortic valve peak velocity but no change in mean pressure gradient across the valve in the ovariectomy group. There was no evidence of leaflet hypertrophy, fibrosis, calcification, or protein markers of dystrophic or osteogenic calcification. This model of ovariectomy may present a novel method of studying left ventricle hypertrophy in female populations but does not have a phenotype for study of aortic stenosis. This is particularly useful as it does not require genetic manipulation or drug treatment and more faithfully mimics the aging, high-cholesterol diet, and post-menopausal osteoporosis many female patients experience potentially resulting in a more translatable disease model.

Patient-Specific Inverse Modeling of In Vivo Cardiovascular Mechanics with Medical Image-Derived Kinematics as Input Data: Concepts, Methods, and Applications

Inverse modeling approaches in cardiovascular medicine are a collection of methodologies that can provide non-invasive patient-specific estimations of tissue properties, mechanical loads, and other mechanics-based risk factors using medical imaging as inputs. Its incorporation into clinical practice has the potential to improve diagnosis and treatment planning with low associated risks and costs. These methods have become available for medical applications mainly due to the continuing development of image-based kinematic techniques, the maturity of the associated theories describing cardiovascular function, and recent progress in computer science, modeling, and simulation engineering. Inverse method applications are multidisciplinary, requiring tailored solutions to the available clinical data, pathology of interest, and available computational resources. Herein, we review biomechanical modeling and simulation principles, methods of solving inverse problems, and techniques for image-based kinematic analysis. In the final section, the major advances in inverse modeling of human cardiovascular mechanics since its early development in the early 2000s are reviewed with emphasis on method-specific descriptions, results, and conclusions. We draw selected studies on healthy and diseased hearts, aortas, and pulmonary arteries achieved through the incorporation of tissue mechanics, hemodynamics, and fluid-structure interaction methods paired with patient-specific data acquired with medical imaging in inverse modeling approaches.

Harmonic Distortion of Blood Pressure Waveform as a Measure of Arterial Stiffness

Aging and disease alter the composition and elastic properties of the aortic wall resulting in shape changes in blood pressure waveform (BPW). Here, we propose a new index, harmonic distortion (HD), to characterize BPW and its relationship with other in vitro and in vivo measures. Using a Fourier transform of the BPW, HD is calculated as the ratio of energy above the fundamental frequency to that at the fundamental frequency. Male mice fed either a normal diet (ND) or a high fat, high sucrose (HFHS) diet for 2–10 months were used to study BPWs in diet-induced metabolic syndrome. BPWs were recorded for 20 s hourly for 24 h, using radiotelemetry. Pulse wave velocity (PWV), an in vivo measure of arterial stiffness, was measured in the abdominal aorta via ultrasound sonography. Common carotid arteries were excised from a subset of mice to determine the tangent modulus using biaxial tension-inflation test. Over a 24-h period, both HD and systolic blood pressure (SBP) show a large variability, however HD linearly decreases with increasing SBP. HD is also linearly related to tangent modulus and PWV with slopes significantly different between the two diet groups. Overall, our study suggests that HD is sensitive to changes in blood pressure and arterial stiffness and has a potential to be used as a noninvasive measure of arterial stiffness in aging and disease.

Aortic stiffness is lower when perivascular adipose tissue (PVAT) is included: a novel ex vivo mechanics study

Perivascular adipose tissue (PVAT) is increasingly recognized as an essential layer of the functional vasculature, being responsible for producing vasoactive substances and assisting arterial stress relaxation. Here we test the hypothesis that PVAT reduces aortic stiffness. Our model was the thoracic aorta of the male Sprague Dawley rat. Uniaxial mechanical tests for three groups of tissue were performed: aorta +PVAT (+PVAT), aorta - PVAT (-PVAT), and isolated PVAT (PVAT only). The output of the mechanical test is reported in the form of a Cauchy stress-stretch curve. This work presents a novel, physiologically relevant approach to measure mechanical stiffness ex vivo in isolated PVAT. Low-stress stiffness (), high-stress stiffness (), and the stress corresponding to a stretch of 1.2 () were measured as metrics of distensibility. The low-stress stiffness was largest in the -PVAT samples and smallest in PVAT only samples. Both the high-stress stiffness and the stress at 1.2 stretch were significantly higher in -PVAT samples when compared to +PVAT samples. Taken together these results suggest that -PVAT samples are stiffer (less distensible) both at low stress (not significant) as well as at high stress (significant) when compared to +PVAT samples. These conclusions are supported by the results of the continuum mechanics material model we also used to interpret the same experimental data. Thus, tissue stiffness is significantly lower when considering PVAT as part of the aortic wall. As such, PVAT should be considered as a target for improving vascular function in diseases with elevated aortic stiffness, including hypertension.

Progressive Microstructural Deterioration Dictates Evolving Biomechanical Dysfunction in the Marfan Aorta

Medial deterioration leading to thoracic aortic aneurysms arises from multiple causes, chief among them mutations to the gene that encodes fibrillin-1 and leads to Marfan syndrome. Fibrillin-1 microfibrils associate with elastin to form elastic fibers, which are essential structural, functional, and instructional components of the normal aortic wall. Compromised elastic fibers adversely impact overall structural integrity and alter smooth muscle cell phenotype. Despite significant progress in characterizing clinical, histopathological, and mechanical aspects of fibrillin-1 related aortopathies, a direct correlation between the progression of microstructural defects and the associated mechanical properties that dictate aortic functionality remains wanting. In this paper, age-matched wild-type, Fbn1 C1041G/+, and Fbn1 mgR/mgR mouse models were selected to represent three stages of increasing severity of the Marfan aortic phenotype. Ex vivo multiphoton imaging and biaxial mechanical testing of the ascending and descending thoracic aorta under physiological loading conditions demonstrated that elastic fiber defects, collagen fiber remodeling, and cell reorganization increase with increasing dilatation. Three-dimensional microstructural characterization further revealed radial patterns of medial degeneration that become more uniform with increasing dilatation while correlating strongly with increased circumferential material stiffness and decreased elastic energy storage, both of which comprise aortic functionality.

Mechanisms of Hypoxia-Induced Pulmonary Arterial Stiffening in Mice Revealed by a Functional Genetics Assay of Structural, Functional, and Transcriptomic Data

Hypoxia adversely affects the pulmonary circulation of mammals, including vasoconstriction leading to elevated pulmonary arterial pressures. The clinical importance of changes in the structure and function of the large, elastic pulmonary arteries is gaining increased attention, particularly regarding impact in multiple chronic cardiopulmonary conditions. We establish a multi-disciplinary workflow to understand better transcriptional, microstructural, and functional changes of the pulmonary artery in response to sustained hypoxia and how these changes inter-relate. We exposed adult male C57BL/6J mice to normoxic or hypoxic (FiO₂ 10%) conditions. Excised pulmonary arteries were profiled transcriptionally using single cell RNA sequencing, imaged with multiphoton microscopy to determine microstructural features under in vivo relevant multiaxial loading, and phenotyped biomechanically to quantify associated changes in material stiffness and vasoactive capacity. Pulmonary arteries of hypoxic mice exhibited an increased material stiffness that was likely due to collagen remodeling rather than excessive deposition (fibrosis), a change in smooth muscle cell phenotype reflected by decreased contractility and altered orientation aligning these cells in the same direction as the remodeled collagen fibers, endothelial proliferation likely representing endothelial-to-mesenchymal transitioning, and a network of cell-type specific transcriptomic changes that drove these changes. These many changes resulted in a system-level increase in pulmonary arterial pulse wave velocity, which may drive a positive feedback loop exacerbating all changes. These findings demonstrate the power of a multi-scale genetic-functional assay. They also highlight the need for systems-level analyses to determine which of the many changes are clinically significant and may be potential therapeutic targets.

Comparison of morphometric, structural, mechanical, and physiologic characteristics of human superficial femoral and popliteal arteries

Peripheral arterial disease differentially affects the superficial femoral (SFA) and the popliteal (PA) arteries, but their morphometric, structural, mechanical, and physiologic differences are poorly understood. SFAs and PAs from 125 human subjects (age 13-92, average 52±17 years) were compared in terms of radii, wall thickness, and opening angles. Structure and vascular disease were quantified using histology, mechanical properties were determined with planar biaxial extension, and constitutive modeling was used to calculate the physiologic stress-stretch state, elastic energy, and the circumferential physiologic stiffness. SFAs had larger radii than PAs, and both segments widened with age. Young SFAs were 5% thicker, but in old subjects the PAs were thicker. Circumferential (SFA: 96→193°, PA: 105→139°) and longitudinal (SFA: 139→306°, PA: 133→320°) opening angles increased with age in both segments. PAs were more diseased than SFAs and had 11% thicker intima. With age, intimal thickness increased 8.5-fold, but medial thickness remained unchanged (620μm) in both arteries. SFAs had 30% more elastin than the PAs, and its density decreased ~50% with age. SFAs were more compliant than PAs circumferentially, but there was no difference longitudinally. Physiologic circumferential stress and stiffness were 21% and 11% higher in the SFA than in the PA across all ages. The stored elastic energy decreased with age (SFA: 1.4→0.4kPa, PA: 2.5→0.3kPa). While the SFA and PA demonstrate appreciable differences, most of them are due to vascular disease. When pathology is the same, so are the mechanical properties, but not the physiologic characteristics that remain distinct due to geometrical differences.

Mechanical, structural, and physiologic differences in human elastic and muscular arteries of different ages: Comparison of the descending thoracic aorta to the superficial femoral artery

Elastic and muscular arteries differ in structure, function, and mechanical properties, and may adapt differently to aging. We compared the descending thoracic aortas (TA) and the superficial femoral arteries (SFA) of 27 tissue donors (average 41±18 years, range 13-73 years) using planar biaxial testing, constitutive modeling, and bidirectional histology. Both TAs and SFAs increased in size with age, with the outer radius increasing more than the inner radius, but the TAs thickened 6-fold and widened 3-fold faster than the SFAs. The circumferential opening angle did not change in the TA, but increased 2.4-fold in the SFA. Young TAs were relatively isotropic, but the anisotropy increased with age due to longitudinal stiffening. SFAs were 51% more compliant longitudinally irrespective of age. Older TAs and SFAs were stiffer, but the SFA stiffened 5.6-fold faster circumferentially than the TA. Physiologic stresses decreased with age in both arteries, with greater changes occurring longitudinally. TAs had larger circumferential, but smaller longitudinal stresses than the SFAs, larger cardiac cycle stretch, 36% lower circumferential stiffness, and 8-fold more elastic energy available for pulsation. TAs contained elastin sheets separated by smooth muscle cells (SMCs), collagen, and glycosaminoglycans, while the SFAs had SMCs, collagen, and longitudinal elastic fibers. With age, densities of elastin and SMCs decreased, collagen remained constant due to medial thickening, and the glycosaminoglycans increased. Elastic and muscular arteries demonstrate different morphological, mechanical, physiologic, and structural characteristics and adapt differently to aging. While the aortas remodel to preserve the Windkessel function, the SFAs maintain higher longitudinal compliance.

Genetic ablation of serotonin receptor 2B improves aortic valve hemodynamics of Notch1 heterozygous mice in a high-cholesterol diet model

Calcific aortic valve disease (CAVD) is a deadly disease that is rising in prevalence due to population aging. While the disease is complex and poorly understood, one well-documented driver of valvulopathy is serotonin agonism. Both serotonin overexpression, as seen with carcinoid tumors and drug-related agonism, such as with Fenfluramine use, are linked with various diseases of the valves. Thus, the objective of this study was to determine if genetic ablation or pharmacological antagonism of the 5-HT_2B serotonin receptor (gene: Htr2b) could improve the hemodynamic and histological progression of calcific aortic valve disease. Htr2b mutant mice were crossed with Notch1^+/- mice, an established small animal model of CAVD, to determine if genetic ablation affects CAVD progression. To assess the effect of pharmacological inhibition on CAVD progression, Notch1^+/- mice were treated with the 5-HT_2B receptor antagonist SB204741. Mice were analyzed using echocardiography, histology, immunofluorescence, and real-time quantitative polymerase chain reaction. Htr2b mutant mice showed lower aortic valve peak velocity and mean pressure gradient–classical hemodynamic indicators of aortic valve stenosis–without concurrent left ventricle change. 5-HT_2B receptor antagonism, however, did not affect hemodynamic progression. Leaflet thickness, collagen density, and CAVD-associated transcriptional markers were not significantly different in any group. This study reveals that genetic ablation of Htr2b attenuates hemodynamic development of CAVD in the Notch1^+/- mice, but pharmacological antagonism may require high doses or long-term treatment to slow progression.

Elastin haploinsufficiency in mice has divergent effects on arterial remodeling with aging depending on sex

Elastin is a primary structural protein in the arterial wall that contributes to vascular mechanical properties and degrades with aging. Aging is associated with arterial stiffening and an increase in blood pressure. There is evidence that arterial aging follows different timelines with sex. Our objective was to investigate how elastin content affects arterial remodeling in male and female mice with aging. We used male and female wild-type (Eln^+/+) and elastin heterozygous (Eln^+/-) mice at 6, 12, and 24 mo of age and measured their blood pressure and arterial morphology, wall structure, protein content, circumferential stress, stretch ratio, and stiffness. Two arteries were used with varying contents of elastin: the left common carotid and ascending aorta. We show that Eln^+/- arteries start at a different homeostatic set point for circumferential wall stress, stretch, and material stiffness but show similar increases with aging to Eln^+/+ mice. With aging, structural stiffness is greatly increased, while material stiffness and circumferential stress are only slightly increased, highlighting the importance of maintaining these homeostatic values. Circumferential stretch shows the smallest change with age and may be important for controlling cellular phenotype. Independent sex differences are mostly associated with males being larger than females; however, many of the measured factors show age × sex and/or genotype × sex interactions, indicating that males and females follow different cardiovascular remodeling timelines with aging and are differentially affected by reduced elastin content.NEW & NOTEWORTHY A comprehensive study on arterial mechanical behavior as a function of elastin content, aging, and sex in mice. Elastin haploinsufficient arteries start at a different homeostatic set point for mechanical parameters such as circumferential stress, stretch, and material stiffness. Structural stiffness of the arterial wall greatly increases with aging, as expected, but there are interactions between sex and aging for most of the mechanical parameters that are important to consider in future work.

Aortic remodeling is modest and sex-independent in mice when hypertension is superimposed on aging

OBJECTIVES

Increased central artery stiffness associates with cardiovascular disease. Among other factors, hypertension and aging are strong contributors to central artery stiffening, yet it has been difficult to separate their effects. Herein, we study isolated and combined effects of hypertension and aging on central artery remodeling in multiple mouse models as a function of sex.

METHODS

We biomechanically phenotyped the aorta as a function of two different methods of inducing hypertension [infusion of angiotensin II (AngII) or combining a high salt diet with inhibition of endothelial-derived nitric oxide synthase using L-NAME] in male and female wild-type and fibulin-5 null mice, the latter of which models aspects of aortic aging.

RESULTS

Despite increasing blood pressure similarly, salt + L-NAME led to adaptive and maladaptive remodeling in the abdominal and thoracic aorta, respectively, whereas AngII caused luminal dilatation but little remodeling of the wall. Importantly, effects of aging were more dramatic than those resulting from induced hypertension and, consequently, superimposing hypertension on aging led to modest additional changes in luminal radius and wall thickness, though wall stress and stiffness increased mainly because of the elevated pressure.

CONCLUSION

Our results suggest that effects of hypertension on aortic remodeling are modest when superimposed on aging in mice, largely independent of sex. These findings are consistent with general observations in humans and in spontaneously hypertensive rats, though separated here for the first time in a rodent model characterized by a severe loss of elastic fiber integrity similar to that found in the aged human aorta.

Paradoxical aortic stiffening and subsequent cardiac dysfunction in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is an ultra-rare disorder with devastating sequelae resulting in early death, presently thought to stem primarily from cardiovascular events. We analyse novel longitudinal cardiovascular data from a mouse model of HGPS (Lmna^G609G/G609G) using allometric scaling, biomechanical phenotyping, and advanced computational modelling and show that late-stage diastolic dysfunction, with preserved systolic function, emerges with an increase in the pulse wave velocity and an associated loss of aortic function, independent of sex. Specifically, there is a dramatic late-stage loss of smooth muscle function and cells and an excessive accumulation of proteoglycans along the aorta, which result in a loss of biomechanical function (contractility and elastic energy storage) and a marked structural stiffening despite a distinctly low intrinsic material stiffness that is consistent with the lack of functional lamin A. Importantly, the vascular function appears to arise normally from the low-stress environment of development, only to succumb progressively to pressure-related effects of the lamin A mutation and become extreme in the peri-morbid period. Because the dramatic life-threatening aortic phenotype manifests during the last third of life there may be a therapeutic window in maturity that could alleviate concerns with therapies administered during early periods of arterial development.

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.

Mechanical and structural changes in human thoracic aortas with age

Aortic mechanical and structural characteristics have profound effects on pathophysiology, but many aspects of physiologic stress-stretch state and intramural changes due to aging remain poorly understood in human tissues. While difficult to assess in vivo due to residual stresses and pre-stretch, physiologic stress-stretch characteristics can be calculated using experimentally-measured mechanical properties and constitutive modeling. Mechanical properties of 76 human descending thoracic aortas (TA) from 13 to 78-year-old donors (mean age 51±18 years) were measured using multi-ratio planar biaxial extension. Constitutive parameters were derived for aortas in 7 age groups, and the physiologic stress-stretch state was calculated. Intramural characteristics were quantified from histological images and related to aortic morphometry and mechanics. TA stiffness increased with age, and aortas became more nonlinear and anisotropic. Systolic and diastolic elastic energy available for pulsation decreased with age from 30 to 8 kPa and from 18 to 5 kPa, respectively. Cardiac cycle circumferential stretch dropped from 1.14 to 1.04, and circumferential and longitudinal physiologic stresses decreased with age from 90 to 72 kPa and from 90 to 17 kPa, respectively. Aortic wall thickness and radii increased with age, while the density of elastin in the tunica media decreased. The number of elastic lamellae and circumferential physiologic stress per lamellae unit remained constant with age at 102±10 and 0.85±0.04 kPa, respectively. Characterization of mechanical, physiological, and structural features in human aortas of different ages can help understand aortic pathology, inform the development of animal models that simulate human aging, and assist with designing devices for open and endovascular aortic repairs. STATEMENT OF SIGNIFICANCE: This manuscript describes mechanical and structural changes occurring in human thoracic aortas with age, and presents material parameters for 4 commonly used constitutive models. Presented data can help better understand aortic pathology, inform the development of animal models that simulate human aging, and assist with designing devices for open and endovascular aortic repairs.

H19 is not hypomethylated or upregulated with age or sex in the aortic valves of mice

Epigenetic dysregulation of long noncoding RNA H19 was recently found to be associated with calcific aortic valve disease (CAVD) in humans by repressing NOTCH1 transcription. This finding offers a possible epigenetic explanation for the abundance of cases of CAVD that are not explained by any clear genetic mutation. In this study, we examined the effect of age and sex on epigenetic dysregulation of H19 and subsequent aortic stenosis. Cohorts of littermate, wild-type C57BL/6 mice were studied at developmental ages analogous to human middle age through advanced age. Cardiac and aortic valve function were assessed with M-mode echocardiography and pulsed wave Doppler ultrasound, respectively. Bisulfite sequencing was used to determine methylation-based epigenetic regulation of H19, and RT-PCR was used to determine changes in gene expression profiles. Male mice were found to have higher peak systolic velocities than females, with several of the oldest mice showing signs of early aortic stenosis. The imprinting control region of H19 was not hypomethylated with age, and H19 expression was lower in the aortic valves of older mice than in the youngest group. These results suggest that age-related upregulation of H19 is not observed in murine aortic valves and that other factors may initiate H19-related CAVD in humans.

Effects of longitudinal pre-stretch on the mechanics of human aorta before and after thoracic endovascular aortic repair (TEVAR) in trauma patients

Thoracic endovascular aortic repair (TEVAR) has evolved as a first-line therapy for trauma patients. Most trauma patients are young, and their aortas are compliant and longitudinally pre-stretched. We have developed a method to include longitudinal pre-stretch in computational models of human thoracic aortas of different ages before and after TEVAR. Finite element models were built using computerized tomography angiography data obtained from human subjects in 6 age groups 10-69 years old. Aortic properties were determined with planar biaxial testing, and pre-stretch was simulated using a series of springs. GORE C-Tag stent-graft was computationally deployed in aortas with and without pre-stretch, and the stress-strain fields were compared. Pre-stretch had significant qualitative and quantitative effects on the aortic stress-strain state before and after TEVAR. Before TEVAR, mean intramural aortic stresses with and without pre-stretch decreased with age from 108 kPa and 83 kPa in the youngest age group, to 60 kPa in the oldest age group. TEVAR increased intramural stresses by an average of 73 ± 15 kPa and 48 ± 10 kPa for aortas with and without pre-stretch and produced high stress concentrations near the aortic isthmus. Inclusion of pre-stretch in young aortas increased intramural stresses by 30%, while in > 50-year-old subjects it did not change the results. Computational modeling of aorta-stent-graft interaction that includes pre-stretch can be instrumental for device design and assessment of its long-term performance, and in the future may help more accurately determine the stress-strain characteristics associated with TEVAR complications.

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.

Mechanical stresses associated with flattening of human femoropopliteal artery specimens during planar biaxial testing and their effects on the calculated physiologic stress-stretch state

Planar biaxial testing is commonly used to characterize the mechanical properties of arteries, but stresses associated with specimen flattening during this test are unknown. We quantified flattening effects in human femoropopliteal arteries (FPAs) of different ages and determined how they affect the calculated arterial physiologic stress-stretch state. Human FPAs from 472 tissue donors (age 12-82 years, mean 53 ± 16 years) were tested using planar biaxial extension, and morphometric and mechanical characteristics were used to assess the flattening effects. Constitutive parameters for the invariant-based model were adjusted to account for specimen flattening and used to calculate the physiologic stresses, stretches, axial force, circumferential stiffness, and stored energy for the FPAs in seven age groups. Flattened specimens were overall 12 ± 4% stiffer longitudinally and 19 ± 11% stiffer circumferentially when biaxially tested. Differences between the stress-stretch curves adjusted and non-adjusted for the effects of flattening were relatively constant across all age groups longitudinally, but increased with age circumferentially. In all age groups, these differences were smaller than the intersubject variability. Physiologic stresses, stretches, axial force, circumferential stiffness, and stored energy were all qualitatively and quantitatively similar when calculated with and without the flattening effects. Stresses, stretches, axial force, and stored energy reduced with age, but circumferential stiffness remained relatively constant between 25 and 65 years of age suggesting a homeostatic target of 0.75 ± 0.02 MPa. Flattening effects associated with planar biaxial testing are smaller than the intersubject variability and have little influence on the calculated physiologic stress-stretch state of human FPAs.

Absence of LTBP-3 attenuates the aneurysmal phenotype but not spinal effects on the aorta in Marfan syndrome

Fibrillin-1 is an elastin-associated glycoprotein that contributes to the long-term fatigue resistance of elastic fibers as well as to the bioavailability of transforming growth factor-beta (TGFβ) in arteries. Altered TGFβ bioavailability and/or signaling have been implicated in aneurysm development in Marfan syndrome (MFS), a multi-system condition resulting from mutations to the gene that encodes fibrillin-1. We recently showed that the absence of the latent transforming growth factor-beta binding protein-3 (LTBP-3) in fibrillin-1-deficient mice attenuates the fragmentation of elastic fibers and focal dilatations that are characteristic of aortic root aneurysms in MFS mice, at least to 12 weeks of age. Here, we show further that the absence of LTBP-3 in this MFS mouse model improves the circumferential mechanical properties of the thoracic aorta, which appears to be fundamental in preventing or significantly delaying aneurysm development. Yet, a spinal deformity either remains or is exacerbated in the absence of LTBP-3 and seems to adversely affect the axial mechanical properties of the thoracic aorta, thus decreasing overall vascular function despite the absence of aneurysmal dilatation. Importantly, because of the smaller size of mice lacking LTBP-3, allometric scaling facilitates proper interpretation of aortic dimensions and thus the clinical phenotype. While this study demonstrates that LTBP-3/TGFβ directly affects the biomechanical function of the thoracic aorta, it highlights that spinal deformities in MFS might indirectly and adversely affect the overall aortic phenotype. There is a need, therefore, to consider together the vascular and skeletal effects in this syndromic disease.

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.