PD-L1 Immune Checkpoint Targeted Photoactivable Liposomes (iTPALs) Prime the Stroma of Pancreatic Tumors and Promote Self-Delivery

Desmoplasia in pancreatic ductal adenocarcinoma (PDAC) limits the penetration and efficacy of therapies. It has been previously shown that photodynamic priming (PDP) using EGFR targeted photoactivable multi-inhibitor liposomes remediates desmoplasia in PDAC and doubles overall survival. Here, bifunctional PD-L1 immune checkpoint targeted photoactivable liposomes (iTPALs) that mediate both PDP and PD-L1 blockade are presented. iTPALs also improve phototoxicity in PDAC cells and induce immunogenic cell death. PDP using iTPALs reduces collagen density, thereby promoting self-delivery by 5.4-fold in collagen hydrogels, and by 2.4-fold in syngeneic CT1BA5 murine PDAC tumors. PDP also reduces tumor fibroblast content by 39.4%. Importantly, iTPALs also block the PD-1/PD-L1 immune checkpoint more efficiently than free α-PD-L1 antibodies. Only a single sub-curative priming dose using iTPALs provides 54.1% tumor growth inhibition and prolongs overall survival in mice by 42.9%. Overall survival directly correlates with the extent of tumor iTPAL self-delivery following PDP (Pearson's r = 0.670, p = 0.034), while no relationship is found for sham non-specific IgG constructs activated with light. When applied over multiple cycles, as is typical for immune checkpoint therapy, PDP using iTPALs promises to offer durable tumor growth delay and significant survival benefit in PDAC patients, especially when used to promote self-delivery of integrated chemo-immunotherapy regimens.

Effect of Aging, Sex, and Gene (Fbln5) on Arterial Stiffness of Mice: 20 Weeks Adult Fbln5-knockout Mice Have Older Arteries than 100 Weeks Wild-Type Mice

The arterial stiffening is a strong independent predictor of cardiovascular risk and has been used to characterize the biological age of arteries ('arterial age'). Here we revealed that the Fbln5 gene knockout (Fbln5 ^-/- ) significantly increases the arterial stiffening for both male and female mice. We also showed that the arterial stiffening increases with natural aging, but the stiffening effect of Fbln5 ^-/- is much more severe than aging. The arterial stiffening of 20 weeks old mice with Fbln5 ^-/- is much higher than that at 100 weeks in wild-type (Fbln5 ^+/+ ) mice, which indicates that 20 weeks mice (equivalent to ∼26 years old humans) with Fbln5 ^-/- have older arteries than 100 weeks wild-type mice (equivalent to ∼77 years humans). Histological microstructure changes of elastic fibers in the arterial tissue elucidate the underlying mechanism of the increase of arterial stiffening due to Fbln5-knockout and aging. These findings provide new insights to reverse 'arterial age' due to abnormal mutations of Fbln5 gene and natural aging. This work is based on a total of 128 biaxial testing samples of mouse arteries and our recently developed unified-fiber-distribution (UFD) model. The UFD model considers the fibers in the arterial tissue as a unified distribution, which is more physically consistent with the real fiber distribution of arterial tissues than the popular fiber-family-based models (e.g., the well-know Gasser-Ogden-Holzapfel [GOH] model) that separate the fiber distribution into several fiber families. Thus, the UFD model achieves better accuracies with less material parameters. To our best knowledge, the UFD model is the only existing accurate model that could capture the property/stiffness differences between different groups of the experimental data discussed here.

Development of a scoring function for comparing simulated and experimental tumor spheroids

Progress continues in the field of cancer biology, yet much remains to be unveiled regarding the mechanisms of cancer invasion. In particular, complex biophysical mechanisms enable a tumor to remodel the surrounding extracellular matrix (ECM), allowing cells to invade alone or collectively. Tumor spheroids cultured in collagen represent a simplified, reproducible 3D model system, which is sufficiently complex to recapitulate the evolving organization of cells and interaction with the ECM that occur during invasion. Recent experimental approaches enable high resolution imaging and quantification of the internal structure of invading tumor spheroids. Concurrently, computational modeling enables simulations of complex multicellular aggregates based on first principles. The comparison between real and simulated spheroids represents a way to fully exploit both data sources, but remains a challenge. We hypothesize that comparing any two spheroids requires first the extraction of basic features from the raw data, and second the definition of key metrics to match such features. Here, we present a novel method to compare spatial features of spheroids in 3D. To do so, we define and extract features from spheroid point cloud data, which we simulated using Cells in Silico (CiS), a high-performance framework for large-scale tissue modeling previously developed by us. We then define metrics to compare features between individual spheroids, and combine all metrics into an overall deviation score. Finally, we use our features to compare experimental data on invading spheroids in increasing collagen densities. We propose that our approach represents the basis for defining improved metrics to compare large 3D data sets. Moving forward, this approach will enable the detailed analysis of spheroids of any origin, one application of which is informing in silico spheroids based on their in vitro counterparts. This will enable both basic and applied researchers to close the loop between modeling and experiments in cancer research.

Fluorescence Lifetime Imaging Microscopy (FLIM) reveals spatial-metabolic changes in 3D breast cancer spheroids

Cancer cells are mechanically sensitive to physical properties of the microenvironment, which can affect downstream signaling to promote malignancy, in part through the modulation of metabolic pathways. Fluorescence Lifetime Imaging Microscopy (FLIM) can be used to measure the fluorescence lifetime of endogenous fluorophores, such as the metabolic co-factors NAD(P)H and FAD, in live samples. We used multiphoton FLIM to investigate the changes in cellular metabolism of 3D breast spheroids derived from MCF-10A and MD-MB-231 cell lines embedded in collagen with varying densities (1 vs. 4 mg/ml) over time (Day 0 vs. Day 3). MCF-10A spheroids demonstrated spatial gradients, with the cells closest to the spheroid edge exhibiting FLIM changes consistent with a shift towards oxidative phosphorylation (OXPHOS) while the spheroid core had changes consistent with a shift towards glycolysis. The MDA-MB-231 spheroids had a large shift consistent with increased OXPHOS with a more pronounced change at the higher collagen concentration. The MDA-MB-231 spheroids invaded into the collagen gel over time and cells that traveled the farthest had the largest changes consistent with a shift towards OXPHOS. Overall, these results suggest that the cells in contact with the extracellular matrix (ECM) and those that migrated the farthest had changes consistent with a metabolic shift towards OXPHOS. More generally, these results demonstrate the ability of multiphoton FLIM to characterize how spheroids metabolism and spatial metabolic gradients are modified by physical properties of the 3D ECM.

Biomechanical Properties of Mouse Carotid Arteries With Diet-Induced Metabolic Syndrome and Aging

Metabolic syndrome increases the risk of cardiovascular diseases. Arteries gradually stiffen with aging; however, it can be worsened by the presence of conditions associated with metabolic syndrome. In this study, we investigated the combined effects of diet-induced metabolic syndrome and aging on the biomechanical properties of mouse common carotid arteries (CCA). Male mice at 2 months of age were fed a normal or a high fat and high sucrose (HFHS) diet for 2 (young group), 8 (adult group) and 18–20 (old group) months. CCAs were excised and subjected to in vitro biaxial inflation-extension tests and the Cauchy stress-stretch relationships were determined in both the circumferential and longitudinal directions. The elastic energy storage of CCAs was obtained using a four-fiber family constitutive model, while the material stiffness in the circumferential and longitudinal directions was computed. Our study showed that aging is a dominant factor affecting arterial remodeling in the adult and old mice, to a similar extent, with stiffening manifested with a significantly reduced capability of energy storage by ∼50% (p < 0.05) and decreases in material stiffness and stress (p < 0.05), regardless of diet. On the other hand, high fat high sucrose diet resulted in an accelerated arterial remodeling in the young group at pre-diabetic stage by affecting the circumferential material stiffness and stress (p < 0.05), which was eventually overshadowed by aging progression. These findings have important implications on the effects of metabolic syndrome on elastic arteries in the younger populations.

A novel jamming phase diagram links tumor invasion to non-equilibrium phase separation

It is well established that the early malignant tumor invades surrounding extracellular matrix (ECM) in a manner that depends upon material properties of constituent cells, surrounding ECM, and their interactions. Recent studies have established the capacity of the invading tumor spheroids to evolve into coexistent solid-like, fluid-like, and gas-like phases. Using breast cancer cell lines invading into engineered ECM, here we show that the spheroid interior develops spatial and temporal heterogeneities in material phase which, depending upon cell type and matrix density, ultimately result in a variety of phase separation patterns at the invasive front. Using a computational approach, we further show that these patterns are captured by a novel jamming phase diagram. We suggest that non-equilibrium phase separation based upon jamming and unjamming transitions may provide a unifying physical picture to describe cellular migratory dynamics within, and invasion from, a tumor.

Comparative Study of Human and Murine Aortic Biomechanics and Hemodynamics in Vascular Aging

Introduction: Aging has many effects on the cardiovascular system, including changes in structure (aortic composition, and thus stiffening) and function (increased proximal blood pressure, and thus cardiac afterload). Mouse models are often used to gain insight into vascular aging and mechanisms of disease as they allow invasive assessments that are impractical in humans. Translation of results from murine models to humans can be limited, however, due to species-specific anatomical, biomechanical, and hemodynamic differences. In this study, we built fluid-solid-interaction (FSI) models of the aorta, informed by biomechanical and imaging data, to compare wall mechanics and hemodynamics in humans and mice at two equivalent ages: young and older adults. Methods: For the humans, 3-D computational models were created using wall property data from the literature as well as patient-specific magnetic resonance imaging (MRI) and non-invasive hemodynamic data; for the mice, comparable models were created using population-based properties and hemodynamics as well as subject-specific anatomies. Global aortic hemodynamics and wall stiffness were compared between humans and mice across age groups. Results: For young adult subjects, we found differences between species in pulse pressure amplification, compliance and resistance distribution, and aortic stiffness gradient. We also found differences in response to aging between species. Generally, the human spatial gradients of stiffness and pulse pressure across the aorta diminished with age, while they increased for the mice. Conclusion: These results highlight key differences in vascular aging between human and mice, and it is important to acknowledge these when using mouse models for cardiovascular research.

Mechanics of ascending aortas from TGFβ-1, -2, -3 haploinsufficient mice and elastase-induced aortopathy

Transforming growth factor-beta (TGFβ-1, -2, -3) ligands act through a common receptor complex yet each is expressed in a unique and overlapping fashion throughout development. TGFβ plays a role in extra-cellular matrix composition with mutations to genes encoding TGFβ and TGFβ signaling molecules contributing to diverse and deadly thoracic aortopathies common in Loeys-Dietz syndrome (LDS). In this investigation, we studied the TGFβ ligand-specific mechanical phenotype of ascending thoracic aortas (ATA) taken from 4-to-6 months-old Tgfb1^+/-, Tgfb2^+/-, and Tgfb3^+/- mice, their wild-type (WT) controls, and an elastase infusion model representative of severe elastolysis. Heterozygous mice were studied at an age without dilation to elucidate potential pre-aortopathic mechanical cues. Our findings indicate that ATAs from Tgfb2^+/- mice demonstrated significant wall thickening, a corresponding decrease in biaxial stress, decreased biaxial stiffness, and a decrease in stored energy. These results were unlike the pathological elastase model where decreases in biaxial stretch were found along with increases in diameter, biaxial stress, and biaxial stiffness. ATAs from Tgfb1^+/- and Tgfb3^+/-, on the other hand, had few mechanical differences when compared to wild-type controls. Although aortopathy generally occurs later in development, our findings reveal that in 4-to-6 month-old animals, only Tgfb2^+/- mice demonstrate a significant phenotype that fails to model ubiquitous elastolysis.

Epithelial layer unjamming shifts energy metabolism toward glycolysis

In development of an embryo, healing of a wound, or progression of a carcinoma, a requisite event is collective epithelial cellular migration. For example, cells at the advancing front of a wound edge tend to migrate collectively, elongate substantially, and exert tractions more forcefully compared with cells many ranks behind. With regards to energy metabolism, striking spatial gradients have recently been reported in the wounded epithelium, as well as in the tumor, but within the wounded cell layer little is known about the link between mechanical events and underlying energy metabolism. Using the advancing confluent monolayer of MDCKII cells as a model system, here we report at single cell resolution the evolving spatiotemporal fields of cell migration speeds, cell shapes, and traction forces measured simultaneously with fields of multiple indices of cellular energy metabolism. Compared with the epithelial layer that is unwounded, which is non-migratory, solid-like and jammed, the leading edge of the advancing cell layer is shown to become progressively more migratory, fluid-like, and unjammed. In doing so the cytoplasmic redox ratio becomes progressively smaller, the NADH lifetime becomes progressively shorter, and the mitochondrial membrane potential and glucose uptake become progressively larger. These observations indicate that a metabolic shift toward glycolysis accompanies collective cellular migration but show, further, that this shift occurs throughout the cell layer, even in regions where associated changes in cell shapes, traction forces, and migration velocities have yet to penetrate. In characterizing the wound healing process these morphological, mechanical, and metabolic observations, taken on a cell-by-cell basis, comprise the most comprehensive set of biophysical data yet reported. Together, these data suggest the novel hypothesis that the unjammed phase evolved to accommodate fluid-like migratory dynamics during episodes of tissue wound healing, development, and plasticity, but is more energetically expensive compared with the jammed phase, which evolved to maintain a solid-like non-migratory state that is more energetically economical.

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.

Sex-dependent differences in central artery haemodynamics in normal and fibulin-5 deficient mice: implications for ageing

Mouse models provide unique opportunities to study vascular disease, but they demand increased experimental and computational resolution. We describe a workflow for combining in vivo and in vitro biomechanical data to build mouse-specific computational models of the central vasculature including regional variations in biaxial wall stiffness, thickness and perivascular support. These fluid-solid interaction models are informed by micro-computed tomography imaging and in vivo ultrasound and pressure measurements, and include mouse-specific inflow and outflow boundary conditions. Hence, the model can capture three-dimensional unsteady flows and pulse wave characteristics. The utility of this experimental-computational approach is illustrated by comparing central artery biomechanics in adult wild-type and fibulin-5 deficient mice, a model of early vascular ageing. Findings are also examined as a function of sex. Computational results compare well with measurements and data available in the literature and suggest that pulse wave velocity, a spatially integrated measure of arterial stiffness, does not reflect well the presence of regional differences in stiffening, particularly those manifested in male versus female mice. Modelling results are also useful for comparing quantities that are difficult to measure or infer experimentally, including local pulse pressures at the renal arteries and characteristics of the peripheral vascular bed that may differ with disease.

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

Considerable insight into effectors of cardiovascular function can be gleaned from controlled studies on mice, especially given the diverse models that are available. Toward this end, however, there is a need for consistent and complementary methods of in vivo and in vitro data analysis, synthesis, and interpretation. The overall objective of this study is twofold. First, we present new semi-automated methods to quantify in vivo measurements of vascular function in anesthetized mice as well as new approaches to synthesize these data with those from in vitro biaxial mechanical characterizations. Second, we contrast regional differences in biomechanical behaviors along the central vasculature by combining biaxial strains measured in vivo with data on the unloaded geometry and biaxial material properties measured in vitro. Results support the hypothesis that the healthy ascending aorta stores significant elastic energy during systole, which is available to work on the heart and blood during diastole, particularly during periods of physical exertion, and further suggest that perivascular tethering allows arteries to work at lower values of wall stress and material stiffness than often assumed. The numerous measurements of vascular function and properties provided herein can also serve as reference values for normal wild-type male and female mice, to which values for myriad genetic, surgical, and pharmacological models can be compared in future studies.

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

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

Biaxial Stretch Improves Elastic Fiber Maturation, Collagen Arrangement, and Mechanical Properties in Engineered Arteries

Tissue-engineered blood vessels (TEVs) are typically produced using the pulsatile, uniaxial circumferential stretch to mechanically condition and strengthen the arterial grafts. Despite improvements in the mechanical integrity of TEVs after uniaxial conditioning, these tissues fail to achieve critical properties of native arteries such as matrix content, collagen fiber orientation, and mechanical strength. As a result, uniaxially loaded TEVs can result in mechanical failure, thrombus, or stenosis on implantation. In planar tissue equivalents such as artificial skin, biaxial loading has been shown to improve matrix production and mechanical properties. To date however, multiaxial loading has not been examined as a means to improve mechanical and biochemical properties of TEVs during culture. Therefore, we developed a novel bioreactor that utilizes both circumferential and axial stretch that more closely simulates loading conditions in native arteries, and we examined the suture strength, matrix production, fiber orientation, and cell proliferation. After 3 months of biaxial loading, TEVs developed a formation of mature elastic fibers that consisted of elastin cores and microfibril sheaths. Furthermore, the distinctive features of collagen undulation and crimp in the biaxial TEVs were absent in both uniaxial and static TEVs. Relative to the uniaxially loaded TEVs, tissues that underwent biaxial loading remodeled and realigned collagen fibers toward a more physiologic, native-like organization. The biaxial TEVs also showed increased mechanical strength (suture retention load of 303 ± 14.53 g, with a wall thickness of 0.76 ± 0.028 mm) and increased compliance. The increase in compliance was due to combinatorial effects of mature elastic fibers, undulated collagen fibers, and collagen matrix orientation. In conclusion, biaxial stretching is a potential means to regenerate TEVs with improved matrix production, collagen organization, and mechanical properties.

Reduced Biaxial Contractility in the Descending Thoracic Aorta of Fibulin-5 Deficient Mice

The precise role of smooth muscle cell contractility in elastic arteries remains unclear, but accumulating evidence suggests that smooth muscle dysfunction plays an important role in the development of thoracic aortic aneurysms and dissections (TAADs). Given the increasing availability of mouse models of these conditions, there is a special opportunity to study roles of contractility ex vivo in intact vessels subjected to different mechanical loads. In parallel, of course, there is a similar need to study smooth muscle contractility in models that do not predispose to TAADs, particularly in cases where disease might be expected. Multiple mouse models having compromised glycoproteins that normally associate with elastin to form medial elastic fibers present with TAADs, yet those with fibulin-5 deficiency do not. In this paper, we show that deletion of the fibulin-5 gene results in a significantly diminished contractility of the thoracic aorta in response to potassium loading despite otherwise preserved characteristic active behaviors, including axial force generation and rates of contraction and relaxation. Interestingly, this diminished response manifests around an altered passive state that is defined primarily by a reduced in vivo axial stretch. Given this significant coupling between passive and active properties, a lack of significant changes in passive material stiffness may help to offset the diminished contractility and thereby protect the wall from detrimental mechanosensing and its sequelae.

Pharmacologically Improved Contractility Protects Against Aortic Dissection in Mice With Disrupted Transforming Growth Factor-β Signaling Despite Compromised Extracellular Matrix Properties

OBJECTIVE

Transforming growth factor-beta is a pleiotropic cytokine having diverse roles in vascular morphogenesis, homeostasis, and pathogenesis. Altered activity of and signaling through transforming growth factor-beta has been implicated in thoracic aortic aneurysms and dissections, conditions characterized by a reduced structural integrity of the wall that associates with altered biomechanics and mechanobiology. We quantify and contrast the passive and active biaxial biomechanical properties of the ascending and proximal descending thoracic aorta in a mouse model of altered transforming growth factor-beta signaling, with and without treatment with rapamycin.

APPROACH AND RESULTS

Postnatal disruption of the gene (Tgfbr2) that codes the type II transforming growth factor-beta receptor compromises vessel-level contractility and elasticity. Daily treatment with rapamycin, a mechanistic target of rapamycin inhibitor that protects against aortic dissection in these mice, largely preserves or restores the contractile function while the passive properties remain compromised. Importantly, this increased smooth muscle contractility protects an otherwise vulnerable aortic wall from pressure-induced intramural delaminations in vitro.

CONCLUSIONS

Notwithstanding the protection afforded by rapamycin in vivo and in vitro, the residual mechanical dysfunctionality suggests a need for caution if rapamycin is to be considered as a potential therapeutic. There is a need for in vivo evaluations in cases of increased hemodynamic loading, including hypertension or extreme exercise, which could unduly stress a structurally vulnerable aortic wall. Given these promising early results, however, such studies are clearly warranted.

Decreased elastic energy storage, not increased material stiffness, characterizes central artery dysfunction in fibulin-5 deficiency independent of sex

Central artery stiffness has emerged over the past 15 years as a clinically significant indicator of cardiovascular function and initiator of disease. Loss of elastic fiber integrity is one of the primary contributors to increased arterial stiffening in aging, hypertension, and related conditions. Elastic fibers consist of an elastin core and multiple glycoproteins; hence defects in any of these constituents can adversely affect arterial wall mechanics. In this paper, we focus on mechanical consequences of the loss of fibulin-5, an elastin-associated glycoprotein involved in elastogenesis. Specifically, we compared the biaxial mechanical properties of five central arteries-the ascending thoracic aorta, descending thoracic aorta, suprarenal abdominal aorta, infrarenal abdominal aorta, and common carotid artery-from male and female wild-type and fibulin-5 deficient mice. Results revealed that, independent of sex, all five regions in the fibulin-5 deficient mice manifested a marked increase in structural stiffness but also a marked decrease in elastic energy storage and typically an increase in energy dissipation, with all differences being most dramatic in the ascending and abdominal aortas. Given that the primary function of large arteries is to store elastic energy during systole and to use this energy during diastole to work on the blood, fibulin-5 deficiency results in a widespread diminishment of central artery function that can have significant effects on hemodynamics and cardiac function.

An Experimental-Computational Study of Catheter Induced Alterations in Pulse Wave Velocity in Anesthetized Mice

Computational methods for solving problems of fluid dynamics and fluid-solid-interactions have advanced to the point that they enable reliable estimates of many hemodynamic quantities, including those important for studying vascular mechanobiology or designing medical devices. In this paper, we use a customized version of the open source code SimVascular to develop a computational model of central artery hemodynamics in anesthetized mice that is informed with experimental data on regional geometries, blood flows and pressures, and biaxial wall properties. After validating a baseline model against available data, we then use the model to investigate the effects of commercially available catheters on the very parameters that they are designed to measure, namely, murine blood pressure and (pressure) pulse wave velocity (PWV). We found that a combination of two small profile catheters designed to measure pressure simultaneously in the ascending aorta and femoral artery increased the PWV due to an overall increase in pressure within the arterial system. Conversely, a larger profile dual-sensor pressure catheter inserted through a carotid artery into the descending thoracic aorta decreased the PWV due to an overall decrease in pressure. In both cases, similar reductions in cardiac output were observed due to increased peripheral vascular resistance. As might be expected, therefore, invasive transducers can alter the very quantities that are designed to measure, yet advanced computational models offer a unique method to evaluate or augment such measurements.

Local versus global mechanical effects of intramural swelling in carotid arteries

Glycosaminoglycans (GAGs) are increasingly thought to play important roles in arterial mechanics and mechanobiology. We recently suggested that these highly negatively charged molecules, well known for their important contributions to cartilage mechanics, can pressurize intralamellar units in elastic arteries via a localized swelling process and thereby impact both smooth muscle mechanosensing and structural integrity. In this paper, we report osmotic loading experiments on murine common carotid arteries that revealed different degrees and extents of transmural swelling. Overall geometry changed significantly with exposure to hypo-osmotic solutions, as expected, yet mean pressure-outer diameter behaviors remained largely the same. Histological analyses revealed further that the swelling was not always distributed uniformly despite being confined primarily to the media. This unexpected finding guided a theoretical study of effects of different distributions of swelling on the wall stress. Results suggested that intramural swelling can introduce highly localized changes in the wall mechanics that could induce differential mechanobiological responses across the wall. There is, therefore, a need to focus on local, not global, mechanics when examining issues such as swelling-induced mechanosensing.

Tgfbr2 disruption in postnatal smooth muscle impairs aortic wall homeostasis

TGF-β is essential for vascular development; however, excess TGF-β signaling promotes thoracic aortic aneurysm and dissection in multiple disorders, including Marfan syndrome. Since the pathology of TGF-β overactivity manifests primarily within the arterial media, it is widely assumed that suppression of TGF-β signaling in vascular smooth muscle cells will ameliorate aortic disease. We tested this hypothesis by conditional inactivation of Tgfbr2, which encodes the TGF-β type II receptor, in smooth muscle cells of postweanling mice. Surprisingly, the thoracic aorta rapidly thickened, dilated, and dissected in these animals. Tgfbr2 disruption predictably decreased canonical Smad signaling, but unexpectedly increased MAPK signaling. Type II receptor-independent effects of TGF-β and pathological responses by nonrecombined smooth muscle cells were excluded by serologic neutralization. Aortic disease was caused by a perturbed contractile apparatus in medial cells and growth factor production by adventitial cells, both of which resulted in maladaptive paracrine interactions between the vessel wall compartments. Treatment with rapamycin restored a quiescent smooth muscle phenotype and prevented dissection. Tgfbr2 disruption in smooth muscle cells also accelerated aneurysm growth in a murine model of Marfan syndrome. Our data indicate that basal TGF-β signaling in smooth muscle promotes postnatal aortic wall homeostasis and impedes disease progression.

A microstructurally motivated model of arterial wall mechanics with mechanobiological implications

Through mechanobiological control of the extracellular matrix, and hence local stiffness, smooth muscle cells of the media and fibroblasts of the adventitia play important roles in arterial homeostasis, including adaptations to altered hemodynamics, injury, and disease. We present a new approach to model arterial wall mechanics that seeks to define better the mechanical environments of the media and adventitia while avoiding the common prescription of a traction-free reference configuration. Specifically, we employ the concept of constituent-specific deposition stretches from the growth and remodeling literature and define a homeostatic state at physiologic pressure and axial stretch that serves as a convenient biologically and clinically relevant reference configuration. Information from histology and multiphoton imaging is then used to prescribe structurally motivated constitutive relations for a bi-layered model of the wall. The utility of this approach is demonstrated by describing in vitro measured biaxial pressure-diameter and axial force-length responses of murine carotid arteries and predicting the associated intact and radially cut traction-free configurations. The latter provides a unique validation while confirming that this constrained mixture approach naturally recovers estimates of residual stresses, which are fundamental to wall mechanics, without the usual need to prescribe an opening angle that is only defined conveniently on cylindrical geometries and cannot be measured in vivo. Among other findings, the model suggests that medial and adventitial stresses can be nearly uniform at physiologic loads, albeit at separate levels, and that the adventitia bears increasingly more load at supra-physiologic pressures while protecting the media from excessive stresses.

Biomechanical phenotyping of central arteries in health and disease: advantages of and methods for murine models

The stiffness and structural integrity of the arterial wall depends primarily on the organization of the extracellular matrix and the cells that fashion and maintain this matrix. Fundamental to the latter is a delicate balance in the continuous production and removal of structural constituents and the mechanical state in which such turnover occurs. Perturbations in this balance due to genetic mutations, altered hemodynamics, or pathological processes result in diverse vascular phenotypes, many of which have yet to be well characterized biomechanically. In this paper, we emphasize the particular need to understand regional variations in the biaxial biomechanical properties of central arteries in health and disease and, in addition, the need for standardization in the associated biaxial testing and quantification. As an example of possible experimental methods, we summarize testing protocols that have evolved in our laboratory over the past 8 years. Moreover, we note advantages of a four fiber family stress-stretch relation for quantifying passive biaxial behaviors, the use of stored energy as a convenient scalar metric of the associated material stiffness, and the utility of appropriate linearizations of the nonlinear, anisotropic relations both for purposes of comparison across laboratories and to inform computational fluid-solid-interaction models. We conclude that, notwithstanding prior advances, there remain many opportunities to advance our understanding of arterial mechanics and mechanobiology, particularly via the diverse genetic, pharmacological, and surgical models that are, or soon will be, available in the mouse.

Inhibition of microRNA-29 enhances elastin levels in cells haploinsufficient for elastin and in bioengineered vessels--brief report

OBJECTIVE

The goal of this study was to determine whether antagonizing microRNA (miR)-29 enhances elastin (ELN) levels in cells and tissues lacking ELN.

METHODS AND RESULTS

miR-29 mimics reduced ELN levels in fibroblasts and smooth muscle cells, whereas miR-29 inhibition increased ELN levels. Antagonism of miR-29 also increased ELN levels in cells from patients haploinsufficient for ELN and in bioengineered human vessels.

CONCLUSION

miR-29 antagonism may promote increased ELN levels during conditions of ELN deficiencies.

Mechanical assessment of elastin integrity in fibrillin-1-deficient carotid arteries: implications for Marfan syndrome

AIMS

Elastin is the primary component of elastic fibres in arteries, which contribute significantly to the structural integrity of the wall. Fibrillin-1 is a microfibrillar glycoprotein that appears to stabilize elastic fibres mechanically and thereby to delay a fatigue-induced loss of function due to long-term repetitive loading. Whereas prior studies have addressed some aspects of ageing-related changes in the overall mechanical properties of arteries in mouse models of Marfan syndrome, we sought to assess for the first time the load-carrying capability of the elastic fibres early in maturity, prior to the development of ageing-related effects, dilatation, or dissection.

METHODS AND RESULTS

We used elastase to degrade elastin in common carotid arteries excised, at 7-9 weeks of age, from a mouse model (mgR/mgR) of Marfan syndrome that expresses fibrillin-1 at 15-25% of normal levels. In vitro biaxial mechanical tests performed before and after exposure to elastase suggested that the elastic fibres exhibited a nearly normal load-bearing capability. Observations from nonlinear optical microscopy suggested further that competent elastic fibres not only contribute to load-bearing, they also increase the undulation of collagen fibres, which endows the normal arterial wall with a more compliant response to pressurization.

CONCLUSION

These findings support the hypothesis that it is an accelerated fatigue-induced damage to or protease-related degradation of initially competent elastic fibres that render arteries in Marfan syndrome increasingly susceptible to dilatation, dissection, and rupture.

On constitutive descriptors of the biaxial mechanical behaviour of human abdominal aorta and aneurysms

The abdominal aorta (AA) in older individuals can develop an aneurysm, which is of increasing concern in our ageing population. The structural integrity of the ageing aortic wall, and hence aneurysm, depends primarily on effective elastin and multiple families of oriented collagen fibres. In this paper, we show that a structurally motivated phenomenological 'four-fibre family' constitutive relation captures the biaxial mechanical behaviour of both the human AA, from ages less than 30 to over 60, and abdominal aortic aneurysms. Moreover, combining the statistical technique known as non-parametric bootstrap with a modal clustering method provides improved confidence intervals for estimated best-fit values of the eight associated constitutive parameters. It is suggested that this constitutive relation captures the well-known loss of structural integrity of elastic fibres owing to ageing and the development of abdominal aneurysms, and that it provides important insight needed to construct growth and remodelling models for aneurysms, which in turn promise to improve our ability to predict disease progression.