Mechanical and dimensional adaptation of rat aorta to hypertension

Myocardial Bridge and Atherosclerosis, an Intimal Relationship

PURPOSE OF REVIEW

This review investigates the relationship between myocardial bridges (MBs), intimal thickening in coronary arteries, and Atherosclerotic cardiovascular disease. It focuses on the role of mechanical forces, such as circumferential strain, in arterial wall remodeling and aims to clarify how MBs affect coronary artery pathology.

REVIEW FINDINGS

MBs have been identified as influential in modulating coronary artery intimal thickness, demonstrating a protective effect against thickening within the MB segment and an increase in thickness proximal to the MB. This is attributed to changes in mechanical stress and hemodynamics. Research involving arterial hypertension models and vein graft disease has underscored the importance of circumferential strain in vascular remodeling and intimal hyperplasia. Understanding the complex dynamics between MBs, mechanical strain, and vascular remodeling is crucial for advancing our knowledge of coronary artery disease mechanisms. This could lead to improved management strategies for cardiovascular diseases, highlighting the need for further research into MB-related vascular changes.

Renal denervation restores biomechanics of carotid arteries in a rat model of hypertension

The prevalence of hypertension increases with aging and is associated with increased arterial stiffness. Resistant hypertension is presented when drug treatments fail to regulate a sustained increased blood pressure. Given that the mechanisms between the sympathetic nervous system and the kidney play an important role in blood regulation, renal denervation (RDN) has emerged as a therapeutic potential in resistant hypertension. In this study, we investigated the effects of RDN on the biomechanical response and microstructure of elastic arteries. Common carotid arteries (CCA) excised from 3-month, 8-month, and 8-month denervated rats were subjected to biaxial extension-inflation test. Our results showed that hypertension developed in the 8-month-old rats. The sustained elevated blood pressure resulted in arterial remodeling which was manifested as a significant stress increase in both axial and circumferential directions after 8 months. RDN had a favorable impact on CCAs with a restoration of stresses in values similar to control arteries at 3 months. After biomechanical testing, arteries were imaged under a multi-photon microscope to identify microstructural changes in extracellular matrix (ECM). Quantification of multi-photon images showed no significant alterations of the main ECM components, elastic and collagen fibers, indicating that arteries remained intact after RDN. Regardless of the experimental group, our microstructural analysis of the multi-photon images revealed that reorientation of the collagen fibers might be the main microstructural mechanism taking place during pressurization with their straightening happening during axial stretching.

A biochemomechanical model of collagen turnover in arterial adaptations to hemodynamic loading

The production, removal, and remodeling of fibrillar collagen is fundamental to mechanical homeostasis in arteries, including dynamic morphological and microstructural changes that occur in response to sustained changes in blood flow and pressure under physiological conditions. These dynamic processes involve complex, coupled biological, chemical, and mechanical mechanisms that are not completely understood. Nevertheless, recent simulations using constrained mixture models with phenomenologically motivated constitutive relations have proven able to predict salient features of the progression of certain vascular adaptations as well as disease processes. Collagen turnover is modeled, in part, via stress-dependent changes in collagen half-life, typically within the range of 10-70 days. By contrast, in this work we introduce a biochemomechanical approach to model the cellular synthesis of procollagen as well as its transition from an intermediate state of assembled microfibrils to mature cross-linked fibers, with mechano-regulated removal. The resulting model can simulate temporal changes in geometry, composition, and stress during early vascular adaptation (weeks to months) for modest changes in blood flow or pressure. It is shown that these simulations capture salient features from data presented in the literature from different animal models.

Renal denervation restores biomechanics of carotid arteries in a rat model of hypertension

The prevalence of hypertension increases with aging and is associated with increased arterial stiffness. Resistant hypertension is presented when drug treatments fail to regulate a sustained increased blood pressure. Given that the mechanisms between the sympathetic nervous system and the kidney play an important role in blood regulation, renal denervation (RDN) has emerged as a therapeutic potential in resistant hypertension. In this study, we investigated the effects of RDN on the biomechanical response and microstructure of elastic arteries. Common carotid arteries (CCA) were excised from 3-, 8- and 8-month-old denervated rats, and subjected to biaxial extension-inflation test. Our results showed that hypertension developed in the 8-month-old rats. The sustained elevated blood pressure resulted in arterial remodeling which was manifested as a significant stress increase in both axial and circumferential directions after 8 months. RDN had a favorable impact on CCAs with a restoration of stresses in values similar to control arteries at 3 months. After biomechanical testing, arteries were imaged under a multi-photon microscope to identify microstructural changes in extracellular matrix (ECM). Quantification of multi-photon images showed no significant alterations of the main ECM components, elastic and collagen fibers, indicating that arteries remained intact after RDN. Regardless of the experimental group, our microstructural analysis of the multi-photon images revealed that reorientation of the collagen fibers might be the main microstructural mechanism taking place during pressurization with their straightening happening during axial stretching.

A Biochemomechanical Model of Collagen Turnover in Arterial Adaptations to Hemodynamic Loading

The production, removal, and remodeling of fibrillar collagen is fundamental to arterial homeostasis, including dynamic morphological and microstructural changes that occur in response to sustained changes in blood flow and pressure under physiological conditions. These dynamic processes involve complex, coupled biological, chemical, and mechanical mechanisms that are not completely understood. Nevertheless, recent simulations using constrained mixture models with phenomenologically motivated constitutive relations have demonstrated a capability to predict salient features of the progression of certain vascular adaptations and disease processes. Collagen turnover is modeled, in part, via stress-dependent changes in collagen half-life, typically taken within the range of 10â€"70 days. By contrast, in this work we introduce a biochemomechanical approach to model the cellular synthesis of procollagen as well as its transition from an intermediate state of assembled microfibrils to mature cross-linked fibers, with mechano-regulated removal. The resulting model can simulate temporal changes in geometry, composition, and stress during early vascular adaptation (weeks to months) for modest changes in blood flow or pressure. It is shown that these simulations capture salient features from data presented in the literature from different animal models.

Computational analysis of the role of mechanosensitive Notch signaling in arterial adaptation to hypertension

Arteries grow and remodel in response to mechanical stimuli. Hypertension, for example, results in arterial wall thickening. Cell-cell Notch signaling between vascular smooth muscle cells (VSMCs) is known to be involved in this process, but the underlying mechanisms are still unclear. Here, we investigated whether Notch mechanosensitivity to strain may regulate arterial thickening in hypertension. We developed a multiscale computational framework by coupling a finite element model of arterial mechanics, including residual stress, to an agent-based model of mechanosensitive Notch signaling, to predict VSMC phenotypes as an indicator of growth and remodeling. Our simulations revealed that the sensitivity of Notch to strain at mean blood pressure may be a key mediator of arterial thickening in hypertensive arteries. Further simulations showed that loss of residual stress can have synergistic effects with hypertension, and that changes in the expression of Notch receptors, but not Jagged ligands, may be used to control arterial growth and remodeling and to intensify or counteract hypertensive thickening. Overall, we identify Notch mechanosensitivity as a potential mediator of vascular adaptation, and we present a computational framework that can facilitate the testing of new therapeutic and regenerative strategies.

Numerical simulations of the nonsymmetric growth and remodeling of arteries under axial twisting

Blood vessels are often subjected to axial twisting during body movement or surgery. Sustained twisting may lead to blood vessel growth and remodeling, however, it remains unclear how the extracellular matrix in the blood vessels remodel under sustained axial twisting. This study aimed to develop a computational model to simulate stress-induced growth and remodeling (G&R) of thin-walled blood vessels under axial twisting. Cylindrical vessels were subjected to a step increase in axial torque while the axial stretch and lumen pressure remained constant. The vessel walls were modeled based on the constrained mixture theory given as microstructure-based discrete fiber families with isotropic matrix structure models. Simulation results demonstrated that in response to a constant twist angle loading, arterial wall thickness, mass, and twisting torque gradually increase towards a new steady state. However, the stress and mass decrease in one diagonal fiber family while increasing in the other diagonal fiber family before reaching plateaus. A novel finding was that the two helical collagen fiber families showed different growth rates and patterns during remodeling, driven by the different fiber stresses generated by the twisting, and led to non-symmetric material properties. This study sheds new light on arterial wall remodeling under axial twisting.

Through the cleared aorta: three-dimensional characterization of mechanical behaviors of rat thoracic aorta under intraluminal pressurization using optical clearing method

The media of aortic wall is characterized by altering layers of elastin and smooth muscle cells (SMCs), along with collagen fibers in both layers, and plays a central role in functional and pathological remodeling such as hypertension and atherosclerosis. Because the arterial function is linked closely to the arterial wall internal structure, it is essential to investigate the alteration of the arterial microstructure during macroscopic deformation to understand cardiovascular pathologies. The present study adopted a tissue clearing method in three-dimensional mechanical characterization of rat thoracic aorta, and successfully observed changes in the structure of each of the three primary components of the aorta under intraluminal pressurization while maintaining tissue mechanical integrity and flexibility. Layers of elastic fibers and SMCs deformed greater on the intimal side than those on the adventitial side. Furthermore, there was a structural agreement in the alignment angle between SMC nuclei and elastic fibers on their intimal side, but not on the adventitial side. This is the first study that changes in the microstructure of three primary components of the aorta were visualized and evaluated through the aorta. The method established here would also be useful to understand tissue mechanics of other load-bearing soft tissues.

Tissue engineered vascular grafts transform into autologous neovessels capable of native function and growth

Background

Tissue-engineered vascular grafts (TEVGs) have the potential to advance the surgical management of infants and children requiring congenital heart surgery by creating functional vascular conduits with growth capacity.

Methods

Herein, we used an integrative computational-experimental approach to elucidate the natural history of neovessel formation in a large animal preclinical model; combining an in vitro accelerated degradation study with mechanical testing, large animal implantation studies with in vivo imaging and histology, and data-informed computational growth and remodeling models.

Results

Our findings demonstrate that the structural integrity of the polymeric scaffold is lost over the first 26 weeks in vivo, while polymeric fragments persist for up to 52 weeks. Our models predict that early neotissue accumulation is driven primarily by inflammatory processes in response to the implanted polymeric scaffold, but that turnover becomes progressively mechano-mediated as the scaffold degrades. Using a lamb model, we confirm that early neotissue formation results primarily from the foreign body reaction induced by the scaffold, resulting in an early period of dynamic remodeling characterized by transient TEVG narrowing. As the scaffold degrades, mechano-mediated neotissue remodeling becomes dominant around 26 weeks. After the scaffold degrades completely, the resulting neovessel undergoes growth and remodeling that mimicks native vessel behavior, including biological growth capacity, further supported by fluid-structure interaction simulations providing detailed hemodynamic and wall stress information.

Conclusions

These findings provide insights into TEVG remodeling, and have important implications for clinical use and future development of TEVGs for children with congenital heart disease.

What do cells regulate in soft tissues on short time scales?

Cells within living soft biological tissues seem to promote the maintenance of a mechanical state within a defined range near a so-called set-point. This mechanobiological process is often referred to as mechanical homeostasis. During this process, cells interact with the fibers of the surrounding extracellular matrix (ECM). It remains poorly understood, however, what individual cells actually regulate during these interactions, and how these micromechanical regulations are translated to the tissue-level to lead to what we observe as biomaterial properties. Herein, we examine this question by a combination of experiments, theoretical analysis, and computational modeling. We demonstrate that on short time scales (hours) - during which deposition and degradation of ECM fibers can largely be neglected - cells appear to not regulate the stress / strain in the ECM or their own shape, but rather only the contractile forces that they exert on the surrounding ECM. STATEMENT OF SIGNIFICANCE: Cells in soft biological tissues sense and regulate the mechanical state of the extracellular matrix to ensure structural integrity and functionality. This so-called mechanical homeostasis plays an important role in the natural history of various diseases such as aneurysms in the cardiovascular system or cancer. Yet, it remains poorly understood to date which target quantity cells regulate on the mircroscale and how it translates to the macroscale. In this paper, we combine experiments, computer simulations, and theoretical analysis to compare different hypotheses about this target quantity. This allows us to identify a likely candidate for it at least on short time scales and in the simplified environment of tissue equivalents.

A Loss of Nuclear-Cytoskeletal Interactions in Vascular Smooth Muscle Cell Differentiation Induced by a Micro-Grooved Collagen Substrate Enabling the Modeling of an In Vivo Cell Arrangement

Vascular smooth muscle cells (VSMCs) remodel vascular walls actively owing to mechanical cues and dedifferentiate to the synthetic phenotype from contractile phenotype in pathological conditions. It is crucial to clarify the mechanisms behind the VSMC phenotypic transition for elucidating their role in the vascular adaptation and repair and for designing engineered tissues. We recently developed novel micro-grooved collagen substrates with “wavy wrinkle” grooves to induce cell–substrate adhesion, morphological polarization, and a tissue-like cell arrangement with cytoskeletal rearrangements similar to those in vascular tissue in vivo. We found that cultivation with this micro-grooved collagen significantly induced VSMC contractile differentiation. Nonetheless, the detailed mechanism underlying the promotion of such VSMC differentiation by micro-grooved collagen has not been clarified yet. Here, we investigated the detailed mechanism of the cell arrangement into a tissue and contractile-differentiation improvement by our micro-grooved collagen substrates in terms of nuclear–cytoskeletal interactions that possibly affect the nuclear mechanotransduction involved in the activation of transcription factors. We found that VSMCs on micro-grooved collagen manifested significant cell arrangement into a tissue and nucleus slimming with a volume reduction in response to the remodeling of the actin cytoskeleton, with consequent inhibition of nuclear shuttling of a transcriptional coactivator, Yes-associated protein (YAP), and improved contractile differentiation. Furthermore, VSMC nuclei rarely deformed during macroscopic cell stretching and featured a loss of nesprin-1–mediated nuclear–cytoskeletal interactions. These results indicate that our micro-grooved collagen induces a cell alignment mimicking in vivo VSMC tissue and promotes contractile differentiation. In such processes of contractile differentiation, mechanical interaction between the nucleus and actin cytoskeleton may diminish to prevent a nuclear disturbance from the excess mechanical stress that might be essential for maintaining vascular functions.

Vascular Mechanobiology: Homeostasis, Adaptation, and Disease

Cells of the vascular wall are exquisitely sensitive to changes in their mechanical environment. In healthy vessels, mechanical forces regulate signaling and gene expression to direct the remodeling needed for the vessel wall to maintain optimal function. Major diseases of arteries involve maladaptive remodeling with compromised or lost homeostatic mechanisms. Whereas homeostasis invokes negative feedback loops at multiple scales to mediate mechanobiological stability, disease progression often occurs via positive feedback that generates mechanobiological instabilities. In this review, we focus on the cell biology, wall mechanics, and regulatory pathways associated with arterial health and how changes in these processes lead to disease. We discuss how positive feedback loops arise via biomechanical and biochemical means. We conclude that inflammation plays a central role in overriding homeostatic pathways and suggest future directions for addressing therapeutic needs.

Structural and functional remodeling of the female Apoe-/- mouse aorta due to chronic cigarette smoke exposure

Despite a decline in popularity over the past several decades, cigarette smoking remains a leading cause of cardiovascular morbidity and mortality. Yet, the effects of cigarette smoking on vascular structure and function are largely unknown. To evaluate changes in the mechanical properties of the aorta that occur with chronic smoking, we exposed female apolipoprotein E-deficient mice to mainstream cigarette smoke daily for 24 wk, with room air as control. By the time of euthanasia, cigarette-exposed mice had lower body mass but experienced larger systolic/diastolic blood pressure when compared with controls. Smoking was associated with significant wall thickening, reduced axial stretch, and circumferential material softening of the aorta. Although this contributed to maintaining intrinsic tissue stiffness at control levels despite larger pressure loads, the structural stiffness became significantly larger. Furthermore, the aorta from cigarette-exposed mice exhibited decreased ability to store elastic energy and augment diastolic blood flow. Histological analysis revealed a region-dependent increase in the cross-sectional area due to smoking. Increased smooth muscle and extracellular matrix content led to medial thickening in the ascending aorta, whereas collagen deposition increased the thickness of the descending thoracic and abdominal aorta. Atherosclerotic lesions were larger in exposed vessels and featured a necrotic core overlaid by a thinned fibrous cap and macrophage infiltration, consistent with a vulnerable phenotype. Collectively, our data indicate that cigarette smoking decreases the mechanical functionality of the aorta, inflicts morphometric alterations to distinct segments of the aorta, and accelerates the progression of atherosclerosis.NEW & NOTEWORTHY We studied the effects of chronic cigarette smoking on the structure and function of the aorta in a mouse model of nose-only aerosol inhalation. Our data indicated that exposure to cigarette smoke impairs vascular function by reducing the ability of the aorta to store elastic energy and by decreasing aortic distensibility. Combined with a more vulnerable atherosclerotic phenotype, these findings reveal the biomechanical mechanisms that support the development of cardiovascular disease due to cigarette smoking.

Mechanical homeostasis in tissue equivalents: a review

There is substantial evidence that growth and remodeling of load bearing soft biological tissues is to a large extent controlled by mechanical factors. Mechanical homeostasis, which describes the natural tendency of such tissues to establish, maintain, or restore a preferred mechanical state, is thought to be one mechanism by which such control is achieved across multiple scales. Yet, many questions remain regarding what promotes or prevents homeostasis. Tissue equivalents, such as collagen gels seeded with living cells, have become an important tool to address these open questions under well-defined, though limited, conditions. This article briefly reviews the current state of research in this area. It summarizes, categorizes, and compares experimental observations from the literature that focus on the development of tension in tissue equivalents. It focuses primarily on uniaxial and biaxial experimental studies, which are well-suited for quantifying interactions between mechanics and biology. The article concludes with a brief discussion of key questions for future research in this field.

Investigation of Pathophysiological Aspects of Aortic Growth, Remodeling, and Failure Using a Discrete-Fiber Microstructural Model

Aortic aneurysms are inherently unpredictable. One can never be sure whether any given aneurysm may rupture or dissect. Clinically, the criteria for surgical intervention are based on size and growth rate, but it remains difficult to identify a high-risk aneurysm, which may require intervention before the cutoff criteria, versus an aneurysm than can be treated safely by more conservative measures. In this work, we created a computational microstructural model of a medial lamellar unit (MLU) incorporating (1) growth and remodeling laws applied directly to discrete, individual fibers, (2) separate but interacting fiber networks for collagen, elastin, and smooth muscle, (3) active and passive smooth-muscle cell mechanics, and (4) failure mechanics for all three fiber types. The MLU model was then used to study different pathologies and microstructural anomalies that may play a role in vascular growth and failure. Our model recapitulated many aspects of arterial remodeling under hypertension with no underlying genetic syndrome including remodeling dynamics, tissue mechanics, and failure. Syndromic effects (smooth muscle cell (SMC) dysfunction or elastin fragmentation) drastically changed the simulated remodeling process, tissue behavior, and tissue strength. Different underlying pathologies were able to produce similarly dilatated vessels with different failure properties, providing a partial explanation for the imperfect nature of aneurysm size as a predictor of outcome.

A mechanical approach to understanding the impact of the nematode Anguillicoloides crassus on the European eel swimbladder

One of the most detrimental factors in the drastic decline of the critically endangered European eel (Anguilla anguilla) was the inadvertent introduction of the invasive nematode Anguillicoloides crassus Infection primarily affects the swimbladder, a gas-filled organ that enables the eel to control its depth in the water. A reduction in swimbladder function may be fatal for eel undergoing their spawning migration to the Sargasso Sea, a journey of over 5000 km. Although the physiological damage caused by this invasive parasite is well studied through the use of quantifiable gross pathological indices, providing a good measure of the swimbladder health status, they cannot separate the role of mechanical and morphological damage. Our study examined the appropriateness of three commonly used indices as a measure of mechanical damage by performing uniaxial tensile tests on swimbladder specimens obtained from an infected eel population. When the test results were compared with the gross pathological indices it was found that thickness correlated most strongly with mechanical damage, both confirming and, more importantly, explaining the counterintuitive findings of earlier work. In a damaged swimbladder, the immune response leads to a trade-off; increasing wall thickness raises the pressure required for organ rupture but decreases strength. The results indicate that for moderate infection the mechanical integrity of the swimbladder can be maintained. For severe infection, however, a reduction in mechanical integrity may reach a tipping point, thereby affecting the successful completion of their oceanic migration.

Evaluation of the Stress-Growth Hypothesis in Saphenous Vein Perfusion Culture

The great saphenous vein (GSV) has served as a coronary artery bypass graft (CABG) conduit for over 50 years. Despite prevalent use, first-year failure rates remain high compared to arterial autograft options. Amongst other factors, vein graft failure can be attributed to material and mechanical mismatching that lead to apoptosis, inflammation, and intimal-medial hyperplasia. Through the implementation of the continuum mechanical-based theory of "stress-mediated growth and remodeling," we hypothesize that the mechanical properties of porcine GSV grafts can be favorably tuned for CABG applications prior to implantation using a prolonged but gradual transition from venous to arterial loading conditions in an inflammatory and thrombogenic deficient environment. To test this hypothesis, we used a hemodynamic-mimetic perfusion bioreactor to guide remodeling through stepwise incremental changes in pressure and flow over the course of 21-day cultures. Biaxial mechanical testing of vessels pre- and post-remodeling was performed, with results fit to structurally-motivated constitutive models using non-parametric bootstrapping. The theory of "small-on-large" was used to describe appropriate stiffness moduli, while histology and viability assays confirmed microstructural adaptations and vessel viability. Results suggest that stepwise transition from venous-to-arterial conditions results in a partial restoration of circumferential stretch and circumferential, but not axial, stress through vessel dilation and wall thickening in a primarily outward remodeling process. These remodeled tissues also exhibited decreased mechanical isotropy and circumferential, but not axial, stiffening. In contrast, only increases in axial stiffness were observed using culture under venous perfusion conditions and those tissues experienced moderate intimal resorption.

Pregnancy-induced Cardiovascular Pathologies: Importance of Structural Components and Lipids

Pregnancy leads to adaptations for maternal and fetal energy needs. The cardiovascular system bears the brunt of the adaptations as the heart and vessels enable nutrient supply to maternal organs facilitated by the placenta to the fetus. The components of the cardiovascular system are critical in the balance between maternal homeostatic and fetus driven homeorhetic regulation. Since lipids intersect maternal cardiovascular function and fetal needs with growth and in stress, factors affecting lipid deposition and mobilization impact risk outcomes. Here, the cardiovascular components and functional derangements associated with cardiovascular pathology in pregnancy, vis-à-vis lipid deposition, mobilization and maternal and/or cardiac and fetal energy needs are detailed. Most reports on the components and associated pathology in pregnancy, are on derangements affecting the extracellular matrix and epicardial fat, followed by the endothelium, vascular smooth muscle, pericytes and myocytes. Targeted studies on all cardiovascular components and pathological outcomes in pregnancy will enhance targeted interventions.

A Two-Dimensional Model of Hypertension-Induced Arterial Remodeling With Account for Stress Interaction Between Elastin and Collagen

We propose a novel structure-based two-dimensional (2D) mathematical model of hypertension-induced arterial remodeling. The model is built in the framework of the constrained mixture theory and global growth approach, utilizing a recently proposed structure-based constitutive model of arterial tissue that accounts for the individual natural configurations of and stress interaction between elastin and collagen. The basic novel predictive result is that provided remodeling causes a change in the elastin/collagen mass fraction ratio, it leads to a structural reorganization of collagen that manifests as an altered fiber undulation and a change in direction of the helically oriented fibers in the tissue natural state. Results obtained from the illustrative simulations for a porcine renal artery show that when remodeling is complete the collagen reorganization might have significant effects on the initial arterial geometry and mechanical properties of the arterial tissue. The proposed model has potential to describe and advance mechanistic understanding of adaptive arterial remodeling, promote the continual refinement of mathematical models of arterial remodeling, and provide motivation for new avenues of experimental investigation.

Anisotropic stiffness and tensional homeostasis induce a natural anisotropy of volumetric growth and remodeling in soft biological tissues

Growth in soft biological tissues in general results in anisotropic changes of the tissue geometry. It remains a key challenge in biomechanics to understand, quantify, and predict this anisotropy. In this paper, we demonstrate that anisotropic tissue stiffness and the well-known mechanism of tensional homeostasis induce a natural anisotropy of the geometric changes resulting from volumetric growth in soft biological tissues. As a rule of thumb, this natural anisotropy makes differential tissue volume elements dilate mainly in the direction(s) of lowest stiffness. This simple principle is shown to explain the experimentally observed growth behavior in a host of different soft biological tissues without relying on any additional heuristic assumptions or quantities (such as ad hoc defined growth tensors).

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.

Comparative mechanics of diverse mammalian carotid arteries

The prevalence of diverse animal models as surrogates for human vascular pathologies necessitate a comprehensive understanding of the differences that exist between species. Comparative passive mechanics are presented here for the common carotid arteries taken from bovine, porcine, ovine, leporine, murine-rat, and murine-mouse specimens. Data is generated using a scalable biaxial mechanical testing device following consistent circumferential (pressure-diameter) and axial (force-length) testing protocols. The structural mechanical response of carotids under equivalent loading, quantified by the deformed inner radius, deformed wall thickness, lumen area compliance and axial force, varies significantly among species but generally follows allometric scaling. Conversely, descriptors of the local mechanical response within the deformed arterial wall, including mean circumferential stress, mid-wall circumferential stretch, and mean axial stress, are relatively consistent across species. Unlike the larger animals studied, the diameter distensibility curves of murine specimens are non-monotonic and have a significantly higher value at 100 mmHg. Taken together, our results provide baseline structural and mechanical information for carotid arteries across a broad range of common animal models.

Mechanical Characterization of the Lamellar Structure of Human Abdominal Aorta in the Development of Atherosclerosis: An Atomic Force Microscopy Study

Atherosclerosis is a major risk factor for cardiovascular disease. However, mechanisms of interaction of atherosclerotic plaque development and local stiffness of the lamellar structure of the arterial wall are not well established. In the current study, the local Young's modulus of the wall and plaque components were determined for three different groups of healthy, mildly diseased and advanced atherosclerotic human abdominal aortas. Histological staining was performed to highlight the atherosclerotic plaque components and lamellar structure of the aortic media, consisting of concentric layers of elastin and interlamellar zones. The force spectroscopy mode of the atomic force microscopy was utilized to determine Young's moduli of aortic wall lamellae and plaque components at the micron level. The high variability of Young's moduli (E) at different locations of the atherosclerotic plaque such as the fibrous cap (E = 15.5± 2.6 kPa), calcification zone (E = 103.7±19.5 kPa), and lipid pool (E = 3.5±1.2 kPa) were observed. Reduction of elastin lamellae stiffness (18.6%), as well as stiffening of interlamellar zones (50%), were detected in the diseased portion of the medial layer of abdominal aortic wall compared to the healthy artery. Additionally, significant differences in the stiffness of both elastin lamellae and interlamellar zones were observed between the diseased wall and disease-free wall in incomplete plaques. Our results elucidate the alternation of the stiffness of different lamellae in the human abdominal aortic wall with atherosclerotic plaque development and may provide new insight on the remodeling of the aortic wall during the progression of atherosclerosis.

Remodeling of the arterial wall: Response to restoration of normal blood flow after flow reduction

BACKGROUND

Although many studies have shown that arteries change diameter in response to chronic change in blood flow (BF), keeping wall shear stress (WSS) at physiologically normal levels, relatively little is known about the effects of flow restoration after flow reduction and also the role of vascular smooth muscle (VSM) during such a remodeling process.

OBJECTIVE

To elucidate the biomechanical responses of the arterial wall to the restoration of normal BF after flow reduction and compare the results with our previous results observed in response to decreased BF alone.

METHODS

Carotid artery BF in the Wistar rat was decreased by ligation and then restored to normal levels by release of the ligation. The effects of BF changes on the biomechanical properties of the carotid arterial wall were determined from measurements of diameters and pressures of excised artery segments.

RESULTS

During BF reduction and restoration, WSS was maintained at physiological levels by changes in the internal diameter. No significant changes in the incremental elastic modulus were found in response to changes in BF. VSM tone was significantly enhanced during the changes in BF.

CONCLUSIONS

Arteries change diameters in response to BF reduction and also flow restoration to normal after flow reduction, keeping WSS at physiologically normal levels. The lack of changes in vascular elasticity suggests that there were no significant changes in major wall constituents, such as elastin and collagen. VSM may play the dominant role in observed arterial remodeling and adaptation.

A lumped parameter model of mechanically mediated acute and long-term adaptations of contractility and geometry in lymphatics for characterization of lymphedema

A primary purpose of the lymphatic system is to transport fluid from peripheral tissues to the central venous system in order to maintain tissue-fluid balance. Failure to perform this task results in lymphedema marked by swelling of the affected limb as well as geometric remodeling and reduced contractility of the affected lymphatic vessels. The mechanical environment has been implicated in the regulation of lymphatic contractility, but it is unknown how changes in the mechanical environment are related to loss of contractile function and remodeling of the tissue. The purpose of this paper was to introduce a new theoretical framework for acute and long-term adaptations of lymphatic vessels to changes in mechanical loading. This theoretical framework combines a simplified version of a published lumped parameter model for lymphangion function and lymph transport, a published microstructurally motivated constitutive model for the active and passive mechanical behavior of isolated rat thoracic ducts, and novel models for acute mechanically mediated vasoreactive adaptations and long-term volumetric growth to simulate changes in muscle contractility and geometry of a single isolated rat thoracic duct in response to a sustained elevation in afterload. The illustrative examples highlight the potential role of the mechanical environment in the acute maintenance of contractility and long-term geometric remodeling, presumably aimed at meeting fluid flow demands while also maintaining mechanical homeostasis. Results demonstrate that contractility may adapt in response to shear stress to meet fluid flow demands and show that pressure-induced long-term geometric remodeling may attenuate these adaptations and reduce fluid flow. The modeling framework and illustrative simulations help suggest relevant experiments that are necessary to accurately quantify and predict the acute and long-term adaptations of lymphangions to altered mechanical loading.

Photoplethysmography and ultrasonic-measurement-integrated simulation to clarify the relation between two-dimensional unsteady blood flow field and forward and backward waves in a carotid artery

Understanding the spatiotemporal change in hemodynamics is essential for the basic research of atherosclerosis. The objective of this study was to establish a methodology to clarify the relation between a two-dimensional (2D) unsteady blood flow field and forward and backward propagating waves in a carotid artery. This study utilized photoplethysmography (PPG) for blood pressure measurement and two-dimensional ultrasonic-measurement-integrated (2D-UMI) simulation for flow field analysis. The validity of the methodology was confirmed in an experiment for a carotid artery of a healthy volunteer. Synchronization between the pressure measurement and flow field analysis was achieved with an error of <10 ms. A 2D unsteady blood flow field in the carotid artery was characterized in relation to forward and backward waves. 2D-UMI simulation reproduced the flow field in which the wall shear stress takes a maximum at the time of the backward wave superiority in the systolic phase, whereas 2D ordinary simulation failed to reproduce this feature because of poor reproducibility of velocity distribution. In conclusion, the proposed methodology using PPG and 2D-UMI simulation was shown to be a potential tool to clarify the relation between 2D unsteady blood flow field and the forward and backward waves in a carotid artery.

Mechanical factors direct mouse aortic remodelling during early maturation

Numerous diseases have been linked to genetic mutations that lead to reduced amounts or disorganization of arterial elastic fibres. Previous work has shown that mice with reduced amounts of elastin (Eln+/-) are able to live a normal lifespan through cardiovascular adaptations, including changes in haemodynamic stresses, arterial geometry and arterial wall mechanics. It is not known if the timeline and presence of these adaptations are consistent in other mouse models of elastic fibre disease, such as those caused by the absence of fibulin-5 expression (Fbln5-/-). Adult Fbln5-/- mice have disorganized elastic fibres, decreased arterial compliance and high blood pressure. We examined mechanical behaviour of the aorta in Fbln5-/- mice through early maturation when the elastic fibres are being assembled. We found that the physiologic circumferential stretch, stress and modulus of Fbln5-/- aorta are maintained near wild-type levels. Constitutive modelling suggests that elastin contributions to the total stress are decreased, whereas collagen contributions are increased. Understanding how collagen fibre structure and mechanics compensate for defective elastic fibres to meet the mechanical requirements of the maturing aorta may help to better understand arterial remodelling in human elastinopathies.

Multiphasic stress relaxation response of freshly isolated and cultured vascular smooth muscle cells measured by quasi-in situ tensile test

Vascular smooth muscle cells (SMCs) undergo a phenotypic change from a contractile to a synthetic state under pathological conditions, such as atherogenesis and restenosis. Although the viscoelastic properties of SMCs are of particular interest because of their role in the development of these vascular diseases, the effects of phenotypic changes on their viscoelastic properties are unclear at this stage. We performed the stress relaxation test at constant strain (ε=30%) for the freshly isolated contractile SMCs (FSMCs) and the cultured synthetic SMCs (CSMCs) maintaining in situ cell shape and cytoskeletal integrity. We also investigated the effect of extracellular Ca2+ on their viscoelastic behaviors. FSMCs and CSMCs exhibited multiphasic stress relaxation, which consisted of rapid relaxation, occurring on a time scale of several seconds and several 10 seconds, and slow relaxation occurring on a time scale of 1000 seconds. The estimated elastic modulus of CSMCs was less than one-half that of FSMCs, that was associated with a decreased of amount of actin stress fibers (SFs) during the transition from contractile to synthetic phenotypes. FSMCs showed a conservation of tension with extracellular Ca2+ following rapid stress relaxation. In contrast, CSMCs showed a consecutive decrease in tension independent of Ca2+. This suggests that the decrease in tension in a long time scale may be involved in mechanical remodeling of SFs induced through a Rho-dependent pathway, which is Ca2+-independent and become predominant in the transition from contractile to synthetic phenotypes.

Obesity and carotid artery remodeling

BACKGROUND/OBJECTIVE

The present study tested the hypothesis that obesity-related changes in carotid intima-media thickness (IMT) might represent not only preclinical atherosclerosis but an adaptive remodeling meant to preserve circumferential wall stress (CWS) in altered hemodynamic conditions characterized by body size-dependent increase in stroke volume (SV) and blood pressure (BP).

SUBJECTS/METHODS

Common carotid artery (CCA) luminal diameter (LD), IMT and CWS were measured in three different populations in order to study: (A) cross-sectional associations between SV, BP, anthropometric parameters and CCA LD (266 healthy subjects with wide range of body weight (24-159 kg)); (B) longitudinal associations between CCA LD and 3-year IMT progression rate (ΔIMT; 571 healthy non-obese subjects without increased cardiovascular (CV) risk); (C) the impact of obesity on CCA geometry and CWS (88 obese subjects without CV complications and 88 non-obese subjects matched for gender and age).

RESULTS

CCA LD was independently associated with SV that was determined by body size. In the longitudinal study, baseline LD was an independent determinant of ΔIMT, and ΔIMT of subjects in the highest LD quartile was significantly higher (28±3 μm) as compared with those in the lower quartiles (8±3, 16±4 and 16±3 μm, P=0.001, P<0.05 and P=0.01, respectively). In addition, CCA CWS decreased during the observational period in the highest LD quartile (from 54.2±8.6 to 51.6±7.4 kPa, P<0.0001). As compared with gender- and age-matched lean individuals, obese subjects had highly increased CCA LD and BP (P<0.0001 for both), but only slightly higher CWS (P=0.05) due to a significant increase in IMT (P=0.005 after adjustment for confounders).

CONCLUSIONS

Our findings suggest that in obese subjects, the CCA wall thickens to compensate the luminal enlargement caused by body size-induced increase in SV, and therefore, to normalize the wall stress. CCA diameter in obesity could represent an additional biomarker, depicting the impact of altered hemodynamics on arterial wall.

Vascular smooth muscle cell functional contractility depends on extracellular mechanical properties

Vascular smooth muscle cells' primary function is to maintain vascular homeostasis through active contraction and relaxation. In diseases such as hypertension and atherosclerosis, this function is inhibited concurrent to changes in the mechanical environment surrounding vascular smooth muscle cells. It is well established that cell function and extracellular mechanics are interconnected; variations in substrate modulus affect cell migration, proliferation, and differentiation. To date, it is unknown how the evolving extracellular mechanical environment of vascular smooth muscle cells affects their contractile function. Here, we have built upon previous vascular muscular thin film technology to develop a variable-modulus vascular muscular thin film that measures vascular tissue functional contractility on substrates with a range of pathological and physiological moduli. Using this modified vascular muscular thin film, we found that vascular smooth muscle cells generated greater stress on substrates with higher moduli compared to substrates with lower moduli. We then measured protein markers typically thought to indicate a contractile phenotype in vascular smooth muscle cells and found that phenotype is unaffected by substrate modulus. These data suggest that mechanical properties of vascular smooth muscle cells' extracellular environment directly influence their functional behavior and do so without inducing phenotype switching.

Remodeling of left circumflex coronary arterial tree in pacing-induced heart failure

Congestive heart failure (CHF) is a very serious heart disease that manifests an imbalance between left ventricle supply and demand. Although the mechanical demand of the failing heart has been well characterized, the systematic remodeling of the entire coronary arterial tree that constitutes the supply of the myocardium is lacking. We hypothesize that the well-known increase in ventricle wall stress during CHF causes coronary vascular rarefaction to increase the vascular flow resistance, which in turn compromises the perfusion of the heart. Morphometric (diameters, length, and numbers) data of the swine left circumflex (LCx) arterial tree were measured in both CHF (n = 6) and control (n = 6) groups, from which a computer reconstruction of the entire LCx tree was implemented down to the capillary level to enable a hemodynamic analysis of coronary circulation. The vascular flow resistance was increased by ∼75% due to a significant decrease of vessel numbers (∼45%) and diameters in the first capillary segments (∼10%) of the LCx arterial tree after 3-4 wk of pacing. The structural remodeling significantly changed the wall shear stress in vessel segments of the entire LCx arterial tree of CHF animals. This study enhances our knowledge of coronary arterial tree remodeling in heart failure, which provides a deeper understanding of the deterioration of supply-demand relation in left ventricle.

A method for incorporating three-dimensional residual stretches/stresses into patient-specific finite element simulations of arteries

The existence of residual stresses in human arteries has long been shown experimentally. Researchers have also demonstrated that residual stresses have a significant effect on the distribution of physiological stresses within arterial tissues, and hence on their development, e.g., stress-modulated remodeling. Through progress in medical imaging, image analysis and finite element (FE) meshing tools it is now possible to construct in vivo patient-specific geometries and thus to study specific, clinically relevant problems in arterial mechanics via FE simulations. Classical continuum mechanics and FE methods assume that constitutive models and the corresponding simulations start from unloaded, stress-free reference configurations while the boundary-value problem of interest represents a loaded geometry and includes residual stresses. We present a pragmatic methodology to simultaneously account for both (i) the three-dimensional (3-D) residual stress distributions in the arterial tissue layers, and (ii) the equilibrium of the in vivo patient-specific geometry with the known boundary conditions. We base our methodology on analytically determined residual stress distributions (Holzapfel and Ogden, 2010, J. R. Soc. Interface 7, 787-799) and calibrate it using data on residual deformations (Holzapfel et al., 2007, Ann. Biomed. Eng. 35, 530-545). We demonstrate our methodology on three patient-specific FE simulations calibrated using experimental data. All data employed here are generated from human tissues - both the aorta and thrombus, and their respective layers - including the geometries determined from magnetic resonance images, and material properties and 3-D residual stretches determined from mechanical experiments. We study the effect of 3-D residual stresses on the distribution of physiological stresses in the aortic layers (intima, media, adventitia) and the layers of the intraluminal thrombus (luminal, medial, abluminal) by comparing three types of FE simulations: (i) conventional calculations; (ii) calculations accounting only for prestresses; (iii) calculations including both 3-D residual stresses and prestresses. Our results show that including residual stresses in patient-specific simulations of arterial tissues significantly impacts both the global (organ-level) deformations and the stress distributions within the arterial tissue (and its layers). Our method produces circumferential Cauchy stress distributions that are more uniform through the tissue thickness (i.e., smaller stress gradients in the local radial directions) compared to both the conventional and prestressing calculations. Such methods, combined with appropriate experimental data, aim at increasing the accuracy of classical FE analyses for patient-specific studies in computational biomechanics and may lead to increased clinical application of simulation tools.

A constitutive modeling interpretation of the relationship among carotid artery stiffness, blood pressure, and age in hypertensive subjects

Aging has a profound influence on arterial wall structure and function. We have previously reported the relationship among pulse wave velocity, age, and blood pressure in hypertensive subjects. In the present study, we aimed for a quantitative interpretation of the observed changes in wall behavior with age using a constitutive modeling approach. We implemented a model of arterial wall biomechanics and fitted this to the group-averaged pressure-area ( P-A) relationship of the “young” subgroup of our study population. Using this model as our take-off point, we assessed which parameters had to be changed to let the model describe the “old” subgroup’s P-A relationship. We allowed elastin stiffness and collagen recruitment parameters to vary and adjusted residual stress parameters according to published age-related changes. We required wall stress to be homogeneously distributed over the arterial wall and assumed wall stress normalization with age by keeping average “old” wall stress at the “young” level. Additionally, we required axial force to remain constant over the cardiac cycle. Our simulations showed an age-related shift in pressure-load bearing from elastin to collagen, caused by a decrease in elastin stiffness and a considerable increase in collagen recruitment. Correspondingly, simulated diameter and wall thickness increased by about 20 and 17%, respectively. The latter compared well with a measured thickness increase of 21%. We conclude that the physiologically realistic changes in constitutive properties we found under physiological constraints with respect to wall stress could well explain the influence of aging in the stiffness-pressure-age pattern observed.

Neointimal hyperplasia and calcification in medium sized arteries in adult patients with chronic kidney disease

The nature of arterial changes resulting in cardiovascular events and dialysis vascular access failures in adult predialysis patients is not well known. This study examined intimal changes, calcium deposition, and consequent stiffness in brachial and radial arteries of adult CKD patients. Ten brachial-artery and seven radial-artery specimens were obtained during fistula creation from nine predialysis and eight dialysis-dependent, nondiabetic patients; and age-gender matched controls undergoing coronary bypass grafts (6 radial) or kidney donation (6 renal). Arterial stiffness was measured at baseline. Vessel histology, morphometric analysis of intima-media, and direct quantification of calcium load was performed using standard techniques. Both predialysis and dialysis patients demonstrated significant arterial intimal hyperplasia with intima:media ratio higher than controls (0.13 ± 0.12 vs. 0.02 ± 0.05, p = 0.01). Calcium deposition was demonstrated on histology and the calcium content in patients was higher than controls (34.68 ± 26.86 vs. 10.95 ± 9.18 μg/μg, p = 0.003). The blood vessel calcium content correlated with arterial stiffness (r = 0.64, p = 0.018). This study for the first time describes, and suggests mechanistic linkage between, intimal hyperplasia, pathological calcium deposition, and increased functional arterial stiffness in dialysis and predialysis patients. Our research could serve as a unique window into the in vivo status of the uremic vasculature impacting fistula maturation and cardiovascular disease.

Implementation and Validation of Aortic Remodeling in Hypertensive Rats

A computational framework was implemented and validated to better understand the hypertensive artery remodeling in both geometric dimensions and material properties. Integrating the stress-modulated remodeling equations into commercial finite element codes allows a better control and visualization of local mechanical parameters. Both arterial thickening and stiffening effects were captured and visualized. An adaptive material remodeling strategy combined with the element birth and death techniques for the geometrical growth were implemented. The numerically predicted remodeling results in terms of the wall thickness, inner diameter, and the ratio of elastin to collagen content of the artery were compared with and fine-tuned by the experimental data from a documented rat model. The influence of time constant on the material remodeling was also evaluated and discussed. In addition, the geometrical growth and material remodeling were isolated to better understand the contributions of each element to the arterial remodeling and their coupling effect. Finally, this framework was applied to an image-based 3D artery generated from computer tomography to demonstrate its heterogeneous remodeling process. Results suggested that hypertension induced arterial remodeling are quite heterogeneous due to both nonlinear geometry and material adaptation process. The developed computational model provided more insights into the evolutions of morphology and material of the artery, which could complement the discrete experimental data for improving the clinical management of hypertension. The proposed framework could also be extended to study other types of stress-driven tissue remodeling including in-stent restenosis and grafting.

Biomechanical factors as triggers of vascular growth

Haemodynamic factors influence all forms of vascular growth (vasculogenesis, angiogenesis, arteriogenesis). Because of its prominent role in atherosclerosis, shear stress has gained particular attention, but other factors such as circumferential stretch are equally important to maintain the integrity and to (re)model the vascular network. While these haemodynamic forces are crucial determinants of the appearance and the structure of the vasculature, they are in turn subjected to structural changes in the blood vessels, such as an increased arterial stiffness in chronic arterial hypertension and ageing. This results in an interplay between the various forces (biomechanical forces) and the involved vascular elements. Although many molecular mediators of biomechanical forces still need to be identified, there is plenty of evidence for the causal role of these forces in vascular growth processes, which will be summarized in this review. In addition, we will discuss the effects of concomitant diseases and disorders on these processes by altering either the biomechanics or their transduction into biological signals. Particularly endothelial dysfunction, diabetes, hypercholesterolaemia, and age affect mechanosensing and -transduction of flow signals, thereby underpinning their influence on cardiovascular health. Finally, current approaches to modify biomechanical forces to therapeutically modulate vascular growth in humans will be described.

Carotid circumferential wall stress homeostasis in early remodeling: theoretical approach and clinical application

PURPOSE

To assess the influence of cardiovascular risk factors on arterial wall growth and the remodeling process.

METHODS

In a theoretical part, we used a well-established relationship linking the rate of thickening of the arterial wall to the circumferential wall stress (CWS) increase. In a clinical part, we measured the intima-media thickness (IMT) in 166 subjects with increased cardiovascular risk score but no treatment for hypertension or hypercholesterolemia, no diabetes, and no cardiovascular disease. Far wall IMT and lumen diameter were measured along the right carotid artery by high-resolution ultrasonography and computerized image analysis.

RESULTS

A decreasing linear relationship between IMT and CWS was deduced from the theoretical model, implying that an increase in CWS would result in an IMT increase, and that the higher the IMT-CWS slope, the higher the thickening response. Subjects with advanced age, renal insufficiency, high 10-year Framingham risk, carotid atherosclerosis, and advanced atherosclerosis at other sites had sharper IMT-CWS slope (p < 0.05), in agreement with the homeostasis of CWS hypothesis.

CONCLUSIONS

The IMT increase responding to a CWS increase was greater in high-risk patients.

Computational simulations of hemodynamic changes within thoracic, coronary, and cerebral arteries following early wall remodeling in response to distal aortic coarctation

Mounting evidence suggests that the pulsatile character of blood pressure and flow within large arteries plays a particularly important role as a mechano-biological stimulus for wall growth and remodeling. Nevertheless, understanding better the highly coupled interactions between evolving wall geometry, structure, and properties and the hemodynamics will require significantly more experimental data. Computational fluid-solid-growth models promise to aid in the design and interpretation of such experiments and to identify candidate mechanobiological mechanisms for the observed arterial adaptations. Motivated by recent aortic coarctation models in animals, we used a computational fluid-solid interaction model to study possible local and systemic effects on the hemodynamics within the thoracic aorta and coronary, carotid, and cerebral arteries due to a distal aortic coarctation and subsequent spatial variations in wall adaptation. In particular, we studied an initial stage of acute cardiac compensation (i.e., maintenance of cardiac output) followed by early arterial wall remodeling (i.e., spatially varying wall thickening and stiffening). Results suggested, for example, that while coarctation increased both the mean and pulse pressure in the proximal vessels, the locations nearest to the coarctation experienced the greatest changes in pulse pressure. In addition, after introducing a spatially varying wall adaptation, pressure, left ventricular work, and wave speed all increased. Finally, vessel wall strain similarly experienced spatial variations consistent with the degree of vascular wall adaptation.