Mechanosensing through talin 1 contributes to tissue mechanical homeostasis.

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

A Fast, Robust Method for Quantitative Assessment of Collagen Fibril Architecture from Transmission Electron Micrographs.

Collagen is the most abundant protein in mammals; it exhibits a hierarchical organization and provides structural support to a wide range of soft tissues, including blood vessels. The architecture of collagen fibrils dictates vascular stiffness and strength, and changes therein can contribute to disease progression. While transmission electron microscopy (TEM) is routinely used to examine collagen fibrils under normal and pathological conditions, computational tools that enable fast and minimally subjective quantitative assessment remain lacking. In the present study, we describe a novel semi-automated image processing and statistical modeling pipeline for segmenting individual collagen fibrils from TEM images and quantifying key metrics of interest, including fibril cross-sectional area and aspect ratio. For validation, we show first-of-their-kind illustrative results for adventitial collagen in the thoracic aorta from three different mouse models.

Effects of Age, Sex, and Extracellular Matrix Integrity on Aortic Dilatation and Rupture in a Mouse Model of Marfan Syndrome.

BACKGROUND: Transmural failure of the aorta is responsible for substantial morbidity and mortality; it occurs when mechanical stress exceeds strength. The aortic root and ascending aorta are susceptible to dissection and rupture in Marfan syndrome, a connective tissue disorder characterized by a progressive reduction in elastic fiber integrity. Whereas competent elastic fibers endow the aorta with compliance and resilience, cross-linked collagen fibers confer stiffness and strength. We hypothesized that postnatal reductions in matrix cross-linking increase aortopathy when turnover rates are high. METHODS: We combined ex vivo biaxial mechanical testing with multimodality histological examinations to quantify expected age- and sex-dependent structural vulnerability of the ascending aorta in Fbn1C1041G/+ Marfan versus wild-type mice without and with 4-week exposures to ß-aminopropionitrile, an inhibitor of lysyl oxidase-mediated cross-linking of newly synthesized elastic and collagen fibers. RESULTS: We found a strong ß-aminopropionitrile-associated sexual dimorphism in aortic dilatation in Marfan mice and aortic rupture in wild-type mice, with dilatation correlating with compromised elastic fiber integrity and rupture correlating with compromised collagen fibril organization. A lower incidence of rupture of ß-aminopropionitrile-exposed Marfan aortas associated with increased lysyl oxidase, suggesting a compensatory remodeling of collagen that slows disease progression in the otherwise compromised Fbn1C1041G/+ aorta. CONCLUSIONS: Collagen fiber structure and function in the Marfan aorta are augmented, in part, by increased lysyl oxidase in female and especially male mice, which improves structural integrity, particularly via fibrils in the adventitia. Preserving or promoting collagen cross-linking may represent a therapeutic target for an otherwise vulnerable aorta.

TGFß-2 haploinsufficiency causes early death in mice with Marfan syndrome.

To assess the contribution of individual TGF-ß isoforms to aortopathy in Marfan syndrome (MFS), we quantified the survival and phenotypes of mice with a combined fibrillin1 (the gene defective in MFS) hypomorphic mutation and a TGF-ß1, 2, or 3 heterozygous null mutation. The loss of TGF-ß2, and only TGF-ß2, resulted in 80% of the double mutant animals dying earlier, by postnatal day 20, than MFS only mice. Death was not from thoracic aortic rupture, as observed in MFS mice, but was associated with hyperplastic aortic valve leaflets, aortic regurgitation, enlarged aortic root, increased heart weight, and impaired lung alveolar septation. Thus, there appears to be a relationship between loss of fibrillin1 and TGF-ß2 in the postnatal development of the heart, aorta and lungs.

A fast, robust method for quantitative assessment of collagen fibril architecture from transmission electron micrographs.

Collagen is the most abundant protein in mammals; it exhibits a hierarchical organization and provides structural support to a wide range of soft tissues, including blood vessels. The architecture of collagen fibrils dictates vascular stiffness and strength, and changes therein can contribute to disease progression. While transmission electron microscopy (TEM) is routinely used to examine collagen fibrils under normal and pathological conditions, computational tools that enable fast and minimally subjective quantitative assessment remain lacking. In the present study, we describe a novel semi-automated image processing and statistical modeling pipeline for segmenting individual collagen fibrils from TEM images and quantifying key metrics of interest, including fibril cross-sectional area and aspect ratio. For validation, we show illustrative results for adventitial collagen in the thoracic aorta from three different mouse models.

Homeostatic, Non-Canonical Role of Macrophage Elastase in Vascular Integrity.

BACKGROUND: Matrix metalloproteinase (MMP)-12 is highly expressed in abdominal aortic aneurysms and its elastolytic function has been implicated in the pathogenesis. This concept is challenged, however, by conflicting data. Here, we sought to revisit the role of MMP-12 in abdominal aortic aneurysm. METHODS: Apoe-/- and Mmp12-/-/Apoe-/- mice were infused with Ang II (angiotensin). Expression of neutrophil extracellular traps (NETs) markers and complement component 3 (C3) levels were evaluated by immunostaining in aortas of surviving animals. Plasma complement components were analyzed by immunoassay. The effects of a complement inhibitor, IgG-FH1-5 (factor H-immunoglobulin G), and macrophage-specific MMP-12 deficiency on adverse aortic remodeling and death from rupture in Ang II-infused mice were determined. RESULTS: Unexpectedly, death from aortic rupture was significantly higher in Mmp12-/-/Apoe-/- mice. This associated with more neutrophils, citrullinated histone H3 and neutrophil elastase, markers of NETs, and C3 levels in Mmp12-/- aortas. These findings were recapitulated in additional models of abdominal aortic aneurysm. MMP-12 deficiency also led to more pronounced elastic laminae degradation and reduced collagen integrity. Higher plasma C5a in Mmp12-/- mice pointed to complement overactivation. Treatment with IgG-FH1-5 decreased aortic wall NETosis and reduced adverse aortic remodeling and death from rupture in Ang II-infused Mmp12-/- mice. Finally, macrophage-specific MMP-12 deficiency recapitulated the effects of global MMP-12 deficiency on complement deposition and NETosis, as well as adverse aortic remodeling and death from rupture in Ang II-infused mice. CONCLUSIONS: An MMP-12 deficiency/complement activation/NETosis pathway compromises aortic integrity, which predisposes to adverse vascular remodeling and abdominal aortic aneurysm rupture. Considering these new findings, the role of macrophage MMP-12 in vascular homeostasis demands re-evaluation of MMP-12 function in diverse settings.

Evolving Mural Defects, Dilatation, and Biomechanical Dysfunction in Angiotensin II-Induced Thoracic Aortopathies.

BACKGROUND: Thoracic aortopathy associates with extracellular matrix remodeling and altered biomechanical properties. We sought to quantify the natural history of thoracic aortopathy in a common mouse model and to correlate measures of wall remodeling such as aortic dilatation or localized mural defects with evolving microstructural composition and biomechanical properties of the wall. METHODS: We combined a high-resolution multimodality imaging approach (panoramic digital image correlation and optical coherence tomography) with histopathologic examinations and biaxial mechanical testing to correlate spatially, for the first time, macroscopic mural defects and medial degeneration within the ascending aorta with local changes in aortic wall composition and mechanical properties. RESULTS: Findings revealed strong correlations between local decreases in elastic energy storage and increases in circumferential material stiffness with increasing proximal aortic diameter and especially mural defect size. Mural defects tended to exhibit a pronounced biomechanical dysfunction that is driven by an altered organization of collagen and elastic fibers. CONCLUSIONS: While aneurysmal dilatation is often observed within particular segments of the aorta, dissection and rupture initiate as highly localized mechanical failures. We show that wall composition and material properties are compromised in regions of local mural defects, which further increases the dilatation and overall structural vulnerability of the wall. Identification of therapies focused on promoting robust collagen accumulation may protect the wall from these vulnerabilities and limit the incidence of dissection and rupture.

Uncertainty quantification in subject-specific estimation of local vessel mechanical properties.

Quantitative estimation of local mechanical properties remains critically important in the ongoing effort to elucidate how blood vessels establish, maintain, or lose mechanical homeostasis. Recent advances based on panoramic digital image correlation (pDIC) have made high-fidelity 3D reconstructions of small-animal (e.g., murine) vessels possible when imaged in a variety of quasi-statically loaded configurations. While we have previously developed and validated inverse modeling approaches to translate pDIC-measured surface deformations into biomechanical metrics of interest, our workflow did not heretofore include a methodology to quantify uncertainties associated with local point estimates of mechanical properties. This limitation has compromised our ability to infer biomechanical properties on a subject-specific basis, such as whether stiffness differs significantly between multiple material locations on the same vessel or whether stiffness differs significantly between multiple vessels at a corresponding material location. In the present study, we have integrated a novel uncertainty quantification and propagation pipeline within our inverse modeling approach, relying on empirical and analytic Bayesian techniques. To demonstrate the approach, we present illustrative results for the ascending thoracic aorta from three mouse models, quantifying uncertainties in constitutive model parameters as well as circumferential and axial tangent stiffness. Our extended workflow not only allows parameter uncertainties to be systematically reported, but also facilitates both subject-specific and group-level statistical analyses of the mechanics of the vessel wall.

Aortic aneurysms and dissections: Unmet needs from physicians and engineers perspectives.

The treatment of aortic disease is complex, requiring cardiothoracic and vascular surgeons to make pre-, post- and intraoperative decisions directly influencing patient survival and well-being. Despite tremendous advancement in vascular surgery and endovascular techniques in the last two decades, along with the abundance of research in the field, many unmet needs and unanswered questions remain. Tight collaboration between engineers and physicians is a keystone in translating new tools, techniques, and devices into practice. Here, we have gathered our perspective, as physicians and engineers, in several pressing issues associated with the diagnosis and treatment of aortic aneurysms and dissection, referring to the current knowledge and practice, signifying unmet needs as well as future directions.

The effect of clinically recommended Evolut sizes on anchorage forces after BASILICA.

Coronary artery obstruction (CAO), a fatal complication of transcatheter aortic valve replacement (TAVR), is commonly found after Valve-in-Valve implantation inside a degenerated bioprosthetic valve. Leaflet laceration (BASILICA technique) has been proposed to prevent CAO and to potentially reduce the risk of leaflet thrombosis. We have previously demonstrated that this technique can reduce the anchorage forces of the TAVR device, which may lead to future complications. In this short communication, we hypothesize that the anchorage force reduction can be minimized by implanting a TAVR with a larger diameter, if two sizes are clinically recommended. We evaluated this hypothesis by employing finite element models of the deployments of the Evolut 26 and 29 mm inside a 27 mm Mitroflow valve, with and without leaflet lacerations. The results show that a laceration substantially decreases the contact area between the Evolut stent and the Mitroflow valve. The larger Evolut has a larger contact area and stronger anchorage forces. Additionally, the anchorage forces are less sensitive to additional lacerations in the larger Evolut (29 case). The results suggest that a larger self-expending device can ensure stronger anchorage and can lower the risk of possible migration, when TAVR is performed in a lacerated bioprosthesis.

Tandem ureteral stents drainage lowers renal pelvis pressure in malignant ureteral obstruction: Experimental and computational models.

Malignant ureteral obstruction may lead to impaired renal function and requires drainage by a percutaneous nephrostomy tube or an internal ureteric stent. Usage of stiff tandem ureteral stents may decrease stent failure rates. In this paper we combined computational and in vitro models to examine the flow in a malignant ureteral obstruction (MUO) managed by 4 methods of drainage: single soft stent, single stiff stent, soft tandem ureteral stents, and stiff tandem ureteral stents. Pressure at the renal pelvis was the primary outcome of the computational and in vitro models. Different drainage modalities were compared using ANCOVA. Results of computational and in vitro models agreed completely. Drainage by stiff tandem ureteral stents provides lower renal pelvis pressure levels compared with single and soft stents (p < 0.001), especially for high levels of external pressure. Usage of stiff tandem ureteral stents may decrease stent-failure rates and postpone the need for percutaneous nephrostomy tube insertion.

Novel Design of Peripheral Infusion Catheter Improves the Kinetics of Intravenous Drug Release.

Short peripheral catheters are ubiquitous in today's healthcare environment, enabling effective and direct delivery of fluids and medications intravenously. A commonly associated complication of their use is thrombophlebitis-thrombus formation-involved inflammation of the vein wall. A novel design of a very short peripheral catheter showed promising results in a pig model in reducing the mechanical irritation to the vein wall. Here, the kinetics of drug release through the novel catheter was compared to a standard commercial catheter using experimental and computational models. In a good agreement, in vitro and in silico models reveal the superiority of the novel catheter design with faster washout time, favorable spatial distribution within the vein, and substantially lower wall shear stress. We submit therefore that the novel design has an improved drug removal profile compared to the conventional catheter and can potentially reduce chemical irritation to the vein wall and minimize the risk for thrombophlebitis. CLINICAL RELEVANCE: Short peripheral catheters are ubiquitous in today's healthcare environment, allowing effective and direct delivery of fluids and medications intravenously. It is well known, however, that prolonged exposure to an irritant drug may lead to its absorption in the endothelial layer lining the vein wall, promoting among other, thrombophlebitis that may lead to increased morbidity, delayed treatment, and prolonged hospitalization. There have been multiple calls to consider low infusion rates with various infusion protocols and to place the catheter tip as central as possible to promote faster drug clearance and reduce the potential vessel damage, but the requisite device had not been available, and the short peripheral catheter is still, and for decades, the standard of care. Towards this end, we recently introduced a novel very short peripheral catheter design, and here, we demonstrate using experimental and computational models its favorable spatial and temporal drug-releasing profiles compared with the standard catheter. The clinically potential relevance is underscore both by the more efficient perfusion of IV drugs and lower irritation to the vein wall at the site of injection. Graphical abstract.

Progressive Microstructural Deterioration Dictates Evolving Biomechanical Dysfunction in the Marfan Aorta.

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

Mechanics-driven mechanobiological mechanisms of arterial tortuosity.

Arterial tortuosity manifests in many conditions, including hypertension, genetic mutations predisposing to thoracic aortopathy, and vascular aging. Despite evidence that tortuosity disrupts efficient blood flow and that it may be an important clinical biomarker, underlying mechanisms remain poorly understood but are widely appreciated to be largely biomechanical. Many previous studies suggested that tortuosity may arise via an elastic structural buckling instability, but the novel experimental-computational approach used here suggests that tortuosity arises from mechanosensitive, cell-mediated responses to local aberrations in the microstructural integrity of the arterial wall. In particular, computations informed by multimodality imaging show that aberrations in elastic fiber integrity, collagen alignment, and collagen turnover can lead to a progressive loss of structural stability that entrenches during the development of tortuosity. Interpreted in this way, microstructural defects or irregularities of the arterial wall initiate the condition and hypertension is a confounding factor.

Mechanical Compression Effects on the Secretion of vWF and IL-8 by Cultured Human Vein Endothelium.

Short peripheral catheters are ubiquitous in today's healthcare environment enabling effective delivery of fluids and medications directly into a patient's vasculature. However, complications related to their use, such as short peripheral catheter thrombophlebitis (SPCT), affect up to 80% of hospitalized patients. While indwelling within the vein, the catheters exert prolonged constant pressure upon the endothelium which can trigger inflammation processes. We have developed and studied an in-vitro model of cultured endothelial cells subjected to mechanical compression of modular self-designed weights, and explored their inflammatory response by quantification of two key biomarkers- vWF and IL-8. Evaluation was performed by ELISA immunoassay and processing of vWF-labeled immunofluorescence images. We found that application of weights correspond to 272 Pa yielded increased release of vWF and IL-8 up to 150% and 250% respectively, comparing to the exertion of 136 Pa. Analyses of the immunofluorescence images revealed significantly longer and more extracellular vWF-strings as well as higher intensity stained-pixels in cells exposed to elevated pressures. The release of both factors found to be significantly dependent on the extent of the exerted pressure. The research shed a light on the relationship between induced mechanical compression and the pathogenesis of SPCT. Minimizing, let alone eliminating the contact between the catheter and the vein wall will mitigate the pressure acting on the endothelium, thereby reducing the secretion of inflammatory factors and lessen the incidence of SPCT.

Quantitative T2 mapping for detection and quantification of thrombophlebitis in a rabbit model.

Short peripheral catheter thrombophlebitis (SPCT), a sterile inflammation of the vein wall, is the most common complication associated with short peripheral catheters (SPCs) and affects up to 80% of hospitalized patients receiving IV therapy. Extensive research efforts have been devoted for improvement and optimization of the catheter material, but means for examination of any novel design are limited, inaccurate and require costly comprehensive pre-clinical and clinical trials. Therefore, there is a conclusive need for a reliable quantitative method for evaluation of SPCT, in particular for research purposes examining the thrombophlebitis-related symptoms of any novel catheter design. In this study, we developed for the first time a quantitative MRI based tool for evaluation of SPCT. The extent and severity of SPCT caused by two different commercially available SPCs with known predisposition for thrombophlebitis, were studied in a rabbit model. MRI analysis was consistent with the standardized pathology evaluation and showed remarkable difference in the percent of edema between the experimental groups. These differences were in line with previous studies and provide evidence that this type of analysis may be useful for future assessment of SPCT in vivo. As a non-invasive method, it may constitute a cost effective solution for examination of new catheters and other medical devices, thereby reducing the need for animal sacrifice.

The effect of pathologic venous valve on neighboring valves: fluid-structure interactions modeling.

Understanding the hemodynamics surrounding the venous valve environment is of a great importance for prosthetic valves design. The present study aims to evaluate the effect of leaflets' stiffening process on the venous valve hemodynamics, valve's failure on the next proximal valve hemodynamics and valve's failure in a secondary daughter vein on the healthy valve hemodynamics in the main vein when both of these valves are distal to a venous junction. Fully coupled, two-way fluid-structure interaction computational models were developed and employed. The sinus pocket region experiences the lowest fluid shear stress, and the base region of the sinus side of the leaflet experiences the highest tissue stress. The leaflets' stiffening increases the tissue stress the valve is experiencing in a very low fluid shear region. A similar effect occurs with the proximal healthy valve as a consequence of the distal valve's failure and with the mother vein valve as a consequence of daughter vein valve's failure. Understanding the described mechanisms may be helpful for elucidating the venous valve stiffness-function relationship in nature, the reasons for a retrograde development of reflux and the relationship between venous valves located near venous junctions, and for designing better prosthetic valves and for improving their positioning.

Modelling catheter-vein biomechanical interactions during an intravenous procedure.

A reliable intravenous (IV) access into the upper extremity veins requires the insertion of a temporary short peripheral catheter (SPC). This so common procedure is, however, associated with a risk of developing short peripheral catheter thrombophlebitis (SPCT) which causes distress and potentially prolongs patient hospitalization. We have developed and studied a biomechanical SPC-vein computational model during an IV procedure, and explored the biomechanical effects of repeated IV episodes on onset and reoccurrences of SPCT. The model was used to determine the effects of different insertion techniques as well as inter-patient biological variability on the catheter-vein wall contact pressures and wall deformations. We found that the maximal pressure exerted upon the vein wall was inhomogeneously distributed, and that the bending region was exposed to significantly greater pressures and deformations. The maximal exerted contact pressure on the inner vein's wall was 2938 Pa. The maximal extent of the SPC penetration into the vein wall reached 3.6 µm, which corresponds to approximately 100% of the average height of the inner layer, suggesting local squashing of endothelial cells at the contact site. The modelling describes a potential biomechanical damage pathway that can explain the reoccurrence of SPCT.

High accuracy differential pressure measurements using fluid-filled catheters - A feasibility study in compliant tubes.

High accuracy differential pressure measurements are required in various biomedical and medical applications, such as in fluid-dynamic test systems, or in the cath-lab. Differential pressure measurements using fluid-filled catheters are relatively inexpensive, yet may be subjected to common mode pressure errors (CMP), which can significantly reduce the measurement accuracy. Recently, a novel correction method for high accuracy differential pressure measurements was presented, and was shown to effectively remove CMP distortions from measurements acquired in rigid tubes. The purpose of the present study was to test the feasibility of this correction method inside compliant tubes, which effectively simulate arteries. Two tubes with varying compliance were tested under dynamic flow and pressure conditions to cover the physiological range of radial distensibility in coronary arteries. A third, compliant model, with a 70% stenosis severity was additionally tested. Differential pressure measurements were acquired over a 3 cm tube length using a fluid-filled double-lumen catheter, and were corrected using the proposed CMP correction method. Validation of the corrected differential pressure signals was performed by comparison to differential pressure recordings taken via a direct connection to the compliant tubes, and by comparison to predicted differential pressure readings of matching fluid-structure interaction (FSI) computational simulations. The results show excellent agreement between the experimentally acquired and computationally determined differential pressure signals. This validates the application of the CMP correction method in compliant tubes of the physiological range for up to intermediate size stenosis severity of 70%.