John T. Favreau

Murine ultrasound imaging for circumferential strain analyses in the angiotensin II abdominal aortic aneurysm model.

OBJECTIVE The underlying causes of abdominal aortic aneurysms (AAAs) remain obscure, although research tools such as the angiotensin II (Ang II) apolipoprotein E-deficient (apoE(-/-)) mouse model have aided investigations. Longitudinal imaging and determination of biomechanical forces in this small-scale model have been difficult. We hypothesized that high-frequency ultrasound biomicroscopy combined with speckle-tracking analytical strategies can be used to define the role of circumferential mechanical strain in AAA formation in the Ang II/apoE(-/-) mouse model of AAAs. We simultaneously examined dietary perturbations that might impact the biomechanical properties of the aortic wall, hypothesizing that the generalized inflammatory phenotype associated with diet-induced obesity would be associated with accelerated loss of circumferential strain and aneurysmal aortic degeneration. METHODS Receiving either a 60 kcal% fat Western diet or standard 10 kcal% fat normal chow, Ang II-treated apoE(-/-) mice (n = 34) underwent sequential aortic duplex ultrasound scan imaging (Vevo 2100 System; VisualSonics, Toronto, Ontario, Canada) of their entire aorta. Circumferential strains were calculated using speckle-tracking algorithms and a custom MatLab analysis. RESULTS Decreased strains in all aortic locations after just 3 days of Ang II treatment were observed, and this effect progressed during the 4-week observation period. Anatomic segments along the aorta impacted wall strain (baseline highest in ascending aorta; P < .05), whereas diet did not. At 2 and 4 weeks, there was the largest progressive decrease in strain in the paravisceral/supraceliac aorta (P < .05), which was the segment most likely to be involved in aneurysm formation in this model. CONCLUSIONS In the Ang II/apoE(-/-) aneurysm model, the aorta significantly stiffens (with decreased strain) shortly after Ang II infusion, and this progressively continues through the next 4 weeks. High-fat feeding did not have an impact on wall strain. Delineation of biomechanical factors and AAA morphology via duplex scan and speckle-tracking algorithms in mouse models should accelerate insights into human AAAs.

A Simplified Murine Intimal Hyperplasia Model Founded on a Focal Carotid Stenosis

Murine models offer a powerful tool for unraveling the mechanisms of intimal hyperplasia and vascular remodeling, although their technical complexity increases experimental variability and limits widespread application. We describe a simple and clinically relevant mouse model of arterial intimal hyperplasia and remodeling. Focal left carotid artery (LCA) stenosis was created by placing 9-0 nylon suture around the artery using an external 35-gauge mandrel needle (middle or distal location), which was then removed. The effect of adjunctive diet-induced obesity was defined. Flowmetry, wall strain analyses, biomicroscopy, and histology were completed. LCA blood flow sharply decreased by ∼85%, followed by a responsive right carotid artery increase of ∼71%. Circumferential strain decreased by ∼2.1% proximal to the stenosis in both dietary groups. At 28 days, morphologic adaptations included proximal LCA intimal hyperplasia, which was exacerbated by diet-induced obesity. The proximal and distal LCA underwent outward and negative inward remodeling, respectively, in the mid-focal stenosis (remodeling indexes, 1.10 and 0.53). A simple, defined common carotid focal stenosis yields reproducible murine intimal hyperplasia and substantial differentials in arterial wall adaptations. This model offers a tool for investigating mechanisms of hemodynamically driven intimal hyperplasia and arterial wall remodeling.

Delivering stem cells to the healthy heart on biological sutures: effects on regional mechanical function.

Current cardiac cell therapies cannot effectively target and retain cells in a specific area of the heart. Cell‐seeded biological sutures were previously developed to overcome this limitation, demonstrating targeted delivery with > 60% cell retention. In this study, both cell‐seeded and non‐seeded fibrin‐based biological sutures were implanted into normal functioning rat hearts to determine the effects on mechanical function and fibrotic response. Human mesenchymal stem cells (hMSCs) were used based on previous work and established cardioprotective effects. Non‐seeded or hMSC‐seeded sutures were implanted into healthy athymic rat hearts. Before cell seeding, hMSCs were passively loaded with quantum dot nanoparticles. One week after implantation, regional stroke work index and systolic area of contraction (SAC) were evaluated on the epicardial surface above the suture. Cell delivery and retention were confirmed by quantum dot tracking, and the fibrotic tissue area was evaluated. Non‐seeded biological sutures decreased SAC near the suture from 0.20 ± 0.01 measured in sham hearts to 0.08 ± 0.02, whereas hMSC‐seeded biological sutures dampened the decrease in SAC (0.15 ± 0.02). Non‐seeded sutures also displayed a small amount of fibrosis around the sutures (1.0 ± 0.1 mm2). Sutures seeded with hMSCs displayed a significant reduction in fibrosis (0.5 ± 0.1 mm2, p < 0.001), with quantum dot‐labelled hMSCs found along the suture track. These results show that the addition of hMSCs attenuates the fibrotic response observed with non‐seeded sutures, leading to improved regional mechanics of the implantation region. Copyright

Functional Effects of Delivering Human Mesenchymal Stem Cell-Seeded Biological Sutures to an Infarcted Heart

Abstract Stem cell therapy has the potential to improve cardiac function after myocardial infarction (MI); however, existing methods to deliver cells to the myocardium, including intramyocardial injection, suffer from low engraftment rates. In this study, we used a rat model of acute MI to assess the effects of human mesenchymal stem cell (hMSC)-seeded fibrin biological sutures on cardiac function at 1 week after implant. Biological sutures were seeded with quantum dot (Qdot)-loaded hMSCs for 24 h before implantation. At 1 week postinfarct, the heart was imaged to assess mechanical function in the infarct region. Regional parameters assessed were regional stroke work (RSW) and systolic area of contraction (SAC) and global parameters derived from the pressure waveform. MI (n = 6) significantly decreased RSW (0.026 ± 0.011) and SAC (0.022 ± 0.015) when compared with sham operation (RSW: 0.141 ± 0.009; SAC: 0.166 ± 0.005, n = 6) (p < 0.05). The delivery of unseeded biological sutures to the infarcted hearts did not change regional mechanical function compared with the infarcted hearts (RSW: 0.032 ± 0.004, SAC: 0.037 ± 0.008, n = 6). The delivery of hMSC-seeded sutures exerted a trend toward increase of regional mechanical function compared with the infarcted heart (RSW: 0.057 ± 0.011; SAC: 0.051 ± 0.014, n = 6). Global function showed no significant differences between any group (p > 0.05); however, there was a trend toward improved function with the addition of either unseeded or seeded biological suture. Histology demonstrated that Qdot-loaded hMSCs remained present in the infarcted myocardium after 1 week. Analysis of serial sections of Massons trichrome staining revealed that the greatest infarct size was in the infarct group (7.0% ± 2.2%), where unseeded (3.8% ± 0.6%) and hMSC-seeded (3.7% ± 0.8%) suture groups maintained similar infarct sizes. Furthermore, the remaining suture area was significantly decreased in the unseeded group compared with that in the hMSC-seeded group (p < 0.05). This study demonstrated that hMSC-seeded biological sutures are a method to deliver cells to the infarcted myocardium and have treatment potential.

Effects of cyclic stretch on three-dimensional vascular smooth muscle cell rings

Vascular grafts are used to repair, replace, or bypass diseased arteries, and there is a growing need for tissue-engineered blood vessels (TEBVs) as replacement grafts. However, TEBV utility is often limited by underdeveloped mechanical integrity. Thus, the purpose of this research was to design, manufacture, and validate a cyclic circumferential stretch bioreactor to mechanically stimulate engineered vascular tissue and improve tensile strength. The bioreactor consists of a closed cam-syringe-tubing system that forces fluid into the tubing with each rotation of the cam, thereby distending and relaxing the tubing. Various-sized cams were fabricated to vary the stretch magnitude. Self-assembled human SMC rings were cultured for six days and then placed on the tubing in the bioreactor. Circumferential stretch applied to both the tubing and SMC tissue ring wall were measured using a high speed camera and speckle tracking program. We observed a 31-56% decrease in peak strain values between the tubing and the tissue ring regions; this variation may reflect differences in wall thickness and modulus in the tissue rings. To assess the effects of cyclic distension, 7-day-old SMC rings were cultured dynamically for 7 days and exposed to 0, 5%, 7.5%, 10%, or 15% cyclic stretch (1 Hz). Stretched rings exhibited a reduction in UTS and MTM compared to unstretched control samples.

Reduction in Mechanical Wall Strain Precedes Intimal Hyperplasia Formation in a Murine Model of Arterial Occlusive Disease

Coronary artery disease and peripheral artery disease remain a significant source of mortality and vascular morbidity in the United States; both affecting over 14 million Americans.[1] Although a number of both open and endovascular procedures are available for treating occlusive lesions, post-procedure intimal hyperplasia (IH) and pathological wall adaptation in treated arteries cause further need for treatment. As on average 50% of patients receiving these treatments must receive further vascular intervention to prevent the continued expansion of IH into the vessel lumen, there is a need to improve our understanding of the underlying causes of IH formation.[2]Copyright

A novel method for recording wall strain in a murine vein graft model

The failure of venous grafts that are placed in the arterial circulation is a significant clinical problem that should be addressed by researchers. One factor that is believed to play a role in causing these failures is changes in the wall strain environment. In this study we present a novel approach to recording wall strain in a murine model and demonstrate our ability to differentiate strains between arteries and veins. Extending to human conditions in the near future, this method can be utilized to elucidate the role of vessel wall strains in vein graft failure.