Prevention of mechanical stretch-induced endothelial and smooth muscle cell injury in experimental vein grafts

Anti-Inflammatory and Anti-Thrombogenic Properties of Arterial Elastic Laminae

Elastic laminae, an elastin-based, layered extracellular matrix structure in the media of arteries, can inhibit leukocyte adhesion and vascular smooth muscle cell proliferation and migration, exhibiting anti-inflammatory and anti-thrombogenic properties. These properties prevent inflammatory and thrombogenic activities in the arterial media, constituting a mechanism for the maintenance of the structural integrity of the arterial wall in vascular disorders. The biological basis for these properties is the elastin-induced activation of inhibitory signaling pathways, involving the inhibitory cell receptor signal regulatory protein α (SIRPα) and Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1). The activation of these molecules causes deactivation of cell adhesion- and proliferation-regulatory signaling mechanisms. Given such anti-inflammatory and anti-thrombogenic properties, elastic laminae and elastin-based materials have potential for use in vascular reconstruction.

Biodegradable external wrapping promotes favorable adaptation in an ovine vein graft model

Vein grafts, the most commonly used conduits in multi-vessel coronary artery bypass grafting surgery, have high intermediate- and long-term failure rates. The abrupt and marked increase in hemodynamic loads on the vein graft is a known contributor to failure. Recent computational modeling suggests that veins can more successfully adapt to an increase in mechanical load if the rate of loading is gradual. Applying an external wrap or support at the time of surgery is one way to reduce the transmural load, and this approach has improved performance relative to an unsupported vein graft in several animal studies. Yet, a clinical trial in humans has shown benefits and drawbacks, and mechanisms by which an external wrap affects vein graft adaptation remain unknown. This study aims to elucidate such mechanisms using a multimodal experimental and computational data collection pipeline. We quantify morphometry using magnetic resonance imaging, mechanics using biaxial testing, hemodynamics using computational fluid dynamics, structure using histology, and transcriptional changes using bulk RNA-sequencing in an ovine carotid-jugular interposition vein graft model, without and with an external biodegradable wrap that allows loads to increase gradually. We show that a biodegradable external wrap promotes luminal uniformity, physiological wall shear stress, and a consistent vein graft phenotype, namely, it prevents over-distension, over-thickening, intimal hyperplasia, and inflammation, and it preserves mechanotransduction. These mechanobiological insights into vein graft adaptation in the presence of an external support can inform computational growth and remodeling models of external support and facilitate design and manufacturing of next-generation external wrapping devices. STATEMENT OF SIGNIFICANCE: External mechanical support is emerging as a promising technology to prevent vein graft failure following coronary bypass graft surgery. While variants of this technology are currently under investigation in clinical trials, the fundamental mechanisms of adaptation remain poorly understood. We employ an ovine carotid-jugular interposition vein graft model, with and without an external biodegradable wrap to provide mechanical support, and probe vein graft adaptation using a multimodal experimental and computational data collection pipeline. We quantify morphometry using magnetic resonance imaging, mechanics using biaxial testing, fluid flow using computational fluid dynamics, vascular composition and structure using histology, and transcriptional changes using bulk RNA sequencing. We show that the wrap mitigates vein graft failure by promoting multiple adaptive mechanisms (across biological scales).

High stretch induces endothelial dysfunction accompanied by oxidative stress and actin remodeling in human saphenous vein endothelial cells

The rate of the remodeling of the arterialized saphenous vein conduit limits the outcomes of coronary artery bypass graft surgery (CABG), which may be influenced by endothelial dysfunction. We tested the hypothesis that high stretch (HS) induces human saphenous vein endothelial cell (hSVEC) dysfunction and examined candidate underlying mechanisms. Our results showed that in vitro HS reduces NO bioavailability, increases inflammatory adhesion molecule expression (E-selectin and VCAM1) and THP-1 cell adhesion. HS decreases F-actin in hSVECs, but not in human arterial endothelial cells, and is accompanied by G-actin and cofilin’s nuclear shuttling and increased reactive oxidative species (ROS). Pre-treatment with the broad-acting antioxidant N-acetylcysteine (NAC) supported this observation and diminished stretch-induced actin remodeling and inflammatory adhesive molecule expression. Altogether, we provide evidence that increased oxidative stress and actin cytoskeleton remodeling play a role in HS-induced saphenous vein endothelial cell dysfunction, which may contribute to predisposing saphenous vein graft to failure.

Fabricating Organized Elastin in Vascular Grafts

Surgically bypassing or replacing a severely damaged artery using a biodegradable synthetic vascular graft is a promising treatment that allows for the remodeling and regeneration of the graft to form a neoartery. Elastin-based structures, such as elastic fibers, elastic lamellae, and laminae, are key functional components in the arterial extracellular matrix. In this review, we identify the lack of elastin in vascular grafts as a key factor that prevents their long-term success. We further summarize advances in vascular tissue engineering that are focused on either de novo production of organized elastin or incorporation of elastin-based biomaterials within vascular grafts to mitigate failure and enhance enduring in vivo performance.

Gradual loading ameliorates maladaptation in computational simulations of vein graft growth and remodelling

Vein graft failure is a prevalent problem in vascular surgeries, including bypass grafting and arteriovenous fistula procedures in which veins are subjected to severe changes in pressure and flow. Animal and clinical studies provide significant insight, but understanding the complex underlying coupled mechanisms can be advanced using computational models. Towards this end, we propose a new model of venous growth and remodelling (G&R) based on a constrained mixture theory. First, we identify constitutive relations and parameters that enable venous adaptations to moderate perturbations in haemodynamics. We then fix these relations and parameters, and subject the vein to a range of combined loads (pressure and flow), from moderate to severe, and identify plausible mechanisms of adaptation versus maladaptation. We also explore the beneficial effects of gradual increases in load on adaptation. A gradual change in flow over 3 days plus an initial step change in pressure results in fewer maladaptations compared with step changes in both flow and pressure, or even a gradual change in pressure and flow over 3 days. A gradual change in flow and pressure over 8 days also enabled a successful venous adaptation for loads as severe as the arterial loads. Optimization is used to accelerate parameter estimation and the proposed framework is general enough to provide a good starting point for parameter estimations in G&R simulations.

Decellularization of Human Internal Mammary Artery: Biomechanical Properties and Histopathological Evaluation

This study undertook to create small-diameter vascular grafts and assess their structure and mechanical properties to withstand arterial implantation. Twenty samples of intact human internal mammary arteries (IMAs) were collected and decellularized using detergent-based methods. To evaluate residual cellular and extracellular matrix (ECM) components, histological analysis was performed. Moreover, collagen typing and ECM structure were analyzed by Picrosirius red and Movat's pentachrome staining. Scanning electron microscopy was also applied to assess microarchitecture of both endothelial and adventitial surfaces of native and decellularized arterial samples. Furthermore, mechanical tests were performed to evaluate the rigidity and suture strength of the arteries. Human IMAs were completely decellularized in all three segments (proximal, middle, and distal). ECM proteins such as collagen and elastic fibers were efficiently preserved and no structural distortion in intima, media, and adventitial surfaces was observed. The parameters of the mechanical tests revealed no significant differences in the mechanical properties of decellularized arteries in comparison to native arteries with considerable strength, suture retention, and stress relaxation (Young's modulus [MPa] = 0.22 ± 0.023 [native] and 0.22 ± 0.015 [acellular]; and suture strength 0.56 ± 0.19 [native] vs. 0.56 ± 0.12 [acellular], respectively). Decellularized IMA represents a potential arterial scaffold as an alternative to autologous grafts for future arterial bypass surgeries. By this technique, microarchitecture and mechanical integrity of decellularized arteries were considerably similar to native arteries. The goal of this study was to introduce an efficient method for complete decellularization of human IMA and evaluate the ECM and biomechanical properties.

Biomaterial-Based Approaches to Address Vein Graft and Hemodialysis Access Failures

Veins used as grafts in heart bypass or as access points in hemodialysis exhibit high failure rates, thereby causing significant morbidity and mortality for patients. Interventional or revisional surgeries required to correct these failures have been met with limited success and exorbitant costs, particularly for the US Centers for Medicare & Medicaid Services. Vein stenosis or occlusion leading to failure is primarily the result of neointimal hyperplasia. Systemic therapies have achieved little long-term success, indicating the need for more localized, sustained, biomaterial-based solutions. Numerous studies have demonstrated the ability of external stents to reduce neointimal hyperplasia. However, successful results from animal models have failed to translate to the clinic thus far, and no external stent is currently approved for use in the US to prevent vein graft or hemodialysis access failures. This review discusses current progress in the field, design considerations, and future perspectives for biomaterial-based external stents. More comparative studies iteratively modulating biomaterial and biomaterial-drug approaches are critical in addressing mechanistic knowledge gaps associated with external stent application to the arteriovenous environment. Addressing these gaps will ultimately lead to more viable solutions that prevent vein graft and hemodialysis access failures.

Computational simulation of the adaptive capacity of vein grafts in response to increased pressure

Vein maladaptation, leading to poor long-term patency, is a serious clinical problem in patients receiving coronary artery bypass grafts (CABGs) or undergoing related clinical procedures that subject veins to elevated blood flow and pressure. We propose a computational model of venous adaptation to altered pressure based on a constrained mixture theory of growth and remodeling (G&R). We identify constitutive parameters that optimally match biaxial data from a mouse vena cava, then numerically subject the vein to altered pressure conditions and quantify the extent of adaptation for a biologically reasonable set of bounds for G&R parameters. We identify conditions under which a vein graft can adapt optimally and explore physiological constraints that lead to maladaptation. Finally, we test the hypothesis that a gradual, rather than a step, change in pressure will reduce maladaptation. Optimization is used to accelerate parameter identification and numerically evaluate hypotheses of vein remodeling.

Flow reversal promotes intimal thickening in vein grafts

OBJECTIVE

After vascular interventions, unidentified mechanisms disrupt the homeostasis of a focal narrowing to initiate an intimal thickening response. We hypothesize that perturbations in the hemodynamic microenvironment are the initiating event for this disruption of homeostasis and intimal thickening in vein bypass grafts. The objective of this study was to investigate the relation between local flow perturbations and its influence on the vein graft architecture.

METHODS

An external ligature was used to create an 80% focal midgraft stenosis in bilateral rabbit carotid vein grafts. A unilateral distal ligation created a ninefold difference in flow rate between high-flow and low-flow grafts. Ten vein grafts were harvested at 28 days and serially sectioned for morphologic evaluation and vein graft reconstruction. Computational fluid dynamics analyses were performed to examine the hemodynamic environment within these complex flow regions.

RESULTS

The largest intimal thickening occurred exclusively within the region immediately distal to the maximum stenosis in high-flow grafts, which was characterized by persistent flow separation and reversal for the entire cardiac cycle. In regions of low to moderate shear stress (<5 Pa), the typical inverse correlation between intimal thickness and wall shear was observed.

CONCLUSIONS

Regions of vein bypass grafts exposed to persistent flow reversal are most at risk for intimal thickening and loss of lumen.

Biomechanical regulation of vascular smooth muscle cell functions: from in vitro to in vivo understanding

Vascular smooth muscle cells (VSMCs) have critical functions in vascular diseases. Haemodynamic factors are important regulators of VSMC functions in vascular pathophysiology. VSMCs are physiologically active in the three-dimensional matrix and interact with the shear stress sensor of endothelial cells (ECs). The purpose of this review is to illustrate how haemodynamic factors regulate VSMC functions under two-dimensional conditions in vitro or three-dimensional co-culture conditions in vivo. Recent advances show that high shear stress induces VSMC apoptosis through endothelial-released nitric oxide and low shear stress upregulates VSMC proliferation and migration through platelet-derived growth factor released by ECs. This differential regulation emphasizes the need to construct more actual environments for future research on vascular diseases (such as atherosclerosis and hypertension) and cardiovascular tissue engineering.

Effect of decellularization protocol on the mechanical behavior of porcine descending aorta

Enzymatic-detergent decellularization treatments may use a combination of chemical reagents to reduce vascular tissue to sterilized scaffolds, which may be seeded with endothelial cells and implanted with a low risk of rejection. However, these chemicals may alter the mechanical properties of the native tissue and contribute to graft compliance mismatch. Uniaxial tensile data obtained from native and decellularized longitudinal aortic tissue samples was analyzed in terms of engineering stress and fit to a modified form of the Yeoh rubber model. One decellularization protocol used SDS, while the other two used TritonX-100, RNase-A, and DNase-I in combination with EDTA or sodium-deoxycholate. Statistical significance of Yeoh model parameters was determined by paired t-test analysis. The TritonX-100/EDTA and 0.075% SDS treatments resulted in relatively variable mechanical changes and did not effectively lyse VSMCs in aortic tissue. The TritonX-100/sodium-deoxycholate treatment effectively lysed VSMCs and was characterized by less variability in mechanical behavior. The data suggests a TritonX-100/sodium-deoxycholate treatment is a more effective option than TritonX-100/EDTA and SDS treatments for the preparation of aortic xenografts and allografts because it effectively lyses VSMCs and is the least likely treatment, among those considered, to promote a decrease in mechanical compliance.

Effects of cyclic strain and growth factors on vascular smooth muscle cell responses

Under physiological and pathological conditions, vascular smooth muscle cells (SMC) are exposed to different biochemical factors and biomechanical forces. Previous studies pertaining to SMC responses have not investigated the effects of both factors on SMCs. Thus, in our research we investigated the combined effects of growth factors like Bfgf (basic fibroblast growth factor), TGF-β (transforming growth factor β) and PDGF (platelet-derived growth factor) along with physiological cyclic strain on SMC responses. Physiological cyclic strain (10% strain) significantly reduced SMC proliferation compared to static controls while addition of growth factors bFGF, TGF-β or PDGF-AB had a positive influence on SMC growth compared to strain alone. Microarray analysis of SMCs exposed to these growth factors and cyclic strain showed that several bioactive genes (vascular endothelial growth factor, epidermal growth factor receptor, etc.) were altered upon exposure. Further work involving biochemical and pathological cyclic strain stimulation will help us better understand the role of cyclic strain and growth factors in vascular functions and development of vascular disorders.

Apoptosis, cell proliferation and modulation of cyclin-dependent kinase inhibitor p21(cip1) in vascular remodelling during vein arterialization in the rat

Neo-intima development and atherosclerosis limit long-term vein graft use for revascularization of ischaemic tissues. Using a rat model, which is technically less challenging than smaller rodents, we provide evidence that the temporal morphological, cellular, and key molecular events during vein arterialization resemble the human vein graft adaptation. Right jugular vein was surgically connected to carotid artery and observed up to 90 days. Morphometry demonstrated gradual thickening of the medial layer and important formation of neo-intima with deposition of smooth muscle cells (SMC) in the subendothelial layer from day 7 onwards. Transmission electron microscopy showed that SMCs switch from the contractile to synthetic phenotype on day 3 and new elastic lamellae formation occurs from day 7 onwards. Apoptosis markedly increased on day 1, while alpha-actin immunostaining for SMC almost disappeared by day 3. On day 7, cell proliferation reached the highest level and cellular density gradually increased until day 90. The relative magnitude of cellular changes was higher in the intima vs. the media layer (100 vs. 2 times respectively). Cyclin-dependent kinase inhibitors (CDKIs) p27(Kip1) and p16(INKA) remained unchanged, whereas p21(Cip1) was gradually downregulated, reaching the lowest levels by day 7 until day 90. Taken together, these data indicate for the first time that p21(Cip1) is the main CDKI protein modulated during the arterialization process the rat model of vein arterialization that may be useful to identify and validate new targets and interventions to improve the long-term patency of vein grafts.

Dimensional analysis of human saphenous vein grafts: Implications for external mesh support

OBJECTIVE

Constrictive external mesh support of vein grafts was shown to mitigate intimal hyperplasia in animal experiments. To determine the degree of constriction required for the elimination of dimensional irregularities in clinically used vein grafts, a detailed anatomic study of human saphenous veins was conducted.

METHODS

In 200 consecutive patients having coronary artery bypass grafting, harvested saphenous veins (length 34.4 +/- 10.8 cm) were analyzed regarding diameter irregularities, side branch distribution, and microstructure.

RESULTS

The mean outer diameter of surgically distended saphenous veins was 4.2 +/- 0.6 mm (men, 4.3 +/- 0.6 mm vs women, 3.9 +/- 0.5 mm; P < .0001). Although the outer diameter significantly decreased over the initial 18 cm (-7.6%; P < .0001), the overall increase between malleolus and thigh was not significant (+11.2%). Smaller-diameter veins (<3.5 mm) had more pronounced diameter fluctuations than larger veins (31.8% +/- 11.0% vs 21.2% +/- 8.8%; P < .0001), with more than 71% of all veins showing caliber changes of more than 20%. There was 1 side branch every 5.4 +/- 4.3 cm, with a significantly higher incidence between 20 and 32 cm from the malleolus (P < .0001 to distal, P < .0004 to proximal). Generally, women had more side branches than men (0.30 +/- 0.15 cm(-1) vs 0.25 +/- 0.12 cm(-1); P = .0190). Thick-walled veins (565.7 +/- 138.4 mum) had a significantly higher number of large side branches (P < .0001), and thin-walled veins (398.7 +/- 123.2 mum) had significantly more small side branches (P < .0001). Pronounced intimal thickening ("cushions") was found in 28% of vessels (119.8 +/- 28.0 mum vs 40.1 +/- 18.2 mum; P < .0001).

CONCLUSION

Although the preferential location of side branches may be addressed by the deliberate discarding of infragenicular vein segments, a diameter constriction of 27% on average would eliminate diameter irregularities in 98% of vein grafts.

Human saphenous vein organ culture under controlled hemodynamic conditions

INTRODUCTION

Saphenous vein grafting is still widely used to revascularize ischemic myocardium. The effectiveness of this procedure is limited by neointima formation and accelerated atherosclerosis, which frequently leads to graft occlusion. A better understanding of this process is important to clarify the mechanisms of vein graft disease and to aid in the formulation of strategies for prevention and/or therapeutics.

OBJECTIVE

To develop an ex vivo flow system that allows for controlled hemodynamics in order to mimic arterial and venous conditions.

METHODS

Human saphenous veins were cultured either under venous (flow: 5 ml/min) or arterial hemodynamic conditions (flow: 50 ml/min, pressure: 80 mmHg) for 1-, 2- and 4-day periods. Cell viability, cell density and apoptosis were compared before and after these intervals using MTT, Hoeschst 33258 stain, and TUNEL assays, respectively.

RESULTS

Fresh excised tissue segments were well preserved prior to the study. Hoechst 33258 and MTT stains showed progressive losses in cell density and cell viability in veins cultured under arterial hemodynamic conditions from 1 to 4 days, while no alterations were observed in veins cultured under venous conditions. Although the cell density from 1-day cultured veins under arterial conditions was similar to that of freshly excised veins, the TUNEL assay indicated that most of these cells were undergoing apoptosis.

CONCLUSION

The results observed resemble the events taking place during early in vivo arterial-vein grafting and provide evidence that an ex vivo perfusion system may be useful for the identification of new therapeutic targets that ameliorate vein graft remodeling and increase graft patency over time.

Constrictive external nitinol meshes inhibit vein graft intimal hyperplasia in nonhuman primates

OBJECTIVE

External mesh support of vein grafts has been shown to mitigate the formation of intimal hyperplasia. The aim of the present study was to address the issue of optimal mesh size in a nonhuman primate model that mimics the dimensional mismatch typically encountered between clinical vein grafts and their target arteries.

METHODS

The effect of mesh size on intimal hyperplasia and endothelial preservation was assessed in bilateral femoral interposition grafts in Chacma baboons (n(Sigma) = 32/n = 8 per mesh size). No mesh support (group I) was compared with external nitinol meshes at three different sizes: loose fitting (group II), 25% diameter constricting (group III), and 50% diameter constricting (group IV). Mesh sizes were seen not only in isolation but also against the background of anastomotic size mismatch at implantation, expressed as quotient of cross-sectional area of host artery to vein graft (Q(C)).

RESULTS

Significant amounts of intimal hyperplasia were found in group I (Q(C) median 0.20; intimal hyperplasia 6 weeks = 1.63 +/- 0.34 mm(2); intimal hyperplasia 12 weeks = 1.73 +/- 0.5 mm(2)) and group II (Q(C) median 0.25; intimal hyperplasia 6 weeks = 1.96 +/- 1.64 mm(2); intimal hyperplasia 12 weeks = 2.88 +/- 1.69 mm(2)). In contrast, group III (Q(C) median 0.45; intimal hyperplasia 6 weeks = 0.08 +/- 0.13 mm(2); intimal hyperplasia 12 weeks = 0.18 +/- 0.32 mm(2)) and IV (Q(C) median 1.16; intimal hyperplasia 6 weeks = 0.02 +/- 0.03 mm(2); intimal hyperplasia 12 weeks = 0.11 +/- 0.10 mm(2)) showed dramatically suppressed intimal hyperplasia (P < .01) at both time points. Endothelial integrity was only preserved in group IV (P < .05). There were no significant differences in vascularization and inflammation in either interlayer or intergroup comparisons.

CONCLUSION

By using an animal model that addressed the clinical phenomenon of diameter discrepancy between vein graft and bypassed artery, we could demonstrate that suppression of intimal hyperplasia required constrictive mesh sizes.

Does PGA external stenting reduce compliance mismatch in venous grafts?

Background

Autogenous vein grafting is widely used in regular bypassing procedures. Due to its mismatch with the host artery in both mechanical property and geometry, the graft often over expands under high arterial blood pressure and forms a step-depth where eddy flow develops, thus causing restenosis, fibrous graft wall, etc. External stents, such as sheaths being used to cuff the graft, have been introduced to eliminate these mismatches and increase the patency. Although histological and immunochemical studies have shown some positive effects of the external stent, the mechanical mismatch under the protection of an external stent remains poorly analyzed.

Methods

In this study, the jugular veins taken from hypercholesterolemic rabbits were transplanted into the carotid arteries, and non-woven polyglycolic acid (PGA) fabric was used to fabricate the external stents to study the effect of the biodegradable external stent. Eight weeks after the operation, the grafts were harvested to perform mechanical tests and histological examinations. An arc tangent function was suggested to describe the relationship between pressure and cross-sectional area to analyse the compliance of the graft.

Results

The results from the mechanical tests indicated that grafts either with or without external stents displayed large compliance in the low-pressure range and were almost inextensible in the high-pressure range. This was very different from the behavior of the arteries or veins in vivo. The data from histological tests showed that, with external stents, collagen fibers were more compact, whilst those in the graft without protection were looser and thicker. No elastic fiber was found in either kind of grafts. Furthermore, grafts without protection were over-expanded which resulted in much bigger cross-sectional areas.

Conclusion

The PGA external extent contributes little to the reduction of the mechanical mismatch between the graft and its host artery while remodeling develops. For the geometric mismatch, it reduces the cross-section area, therefore matching with the host artery much better. Although there are some positive effects, conclusively the PGA is not an ideal material for external stent.

Controlled Release of Basic Fibroblast Growth Factor From Gelatin Hydrogel Sheet Improves Structural and Physiological Properties of Vein Graft in Rat

Objectives— Autologous vein grafts are still widely used, but their long-term patency is suboptimal. The objective of the current study was to determine whether wrapping a vein graft in gelatin hydrogel sheet incorporating basic fibroblast growth factor improves their mechanical and physiological properties.

Methods and Results— Autologous femoral vein was interposed into the abdominal aorta in rats. The rats were divided into 3 groups: nontreated grafts (group A), grafts wrapped in basic fibroblast growth factor-free gelatin hydrogel sheet (group B), and grafts wrapped in basic fibroblast growth factor-impregnated gelatin hydrogel sheet (group C). On day 1, endothelial desquamation was observed in group A, and the media in groups A and B were disrupted, staining positive in the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay. In contrast, the media in group C remained intact and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling–negative, associated with activation of MAPK. Graft dilation was significantly inhibited in groups B and C compared with group A, with those in group C showing the smallest degree of neointimal proliferation. At 8 weeks grafts in group C developed neointima with homogeneous elastic laminae, presence of rigid neoadventitia that displayed neovascularity, and the highest blood flow velocity.

Conclusions— Wrapping vein grafts in basic fibroblast growth factor- impregnated gelatin hydrogel sheet improved their structural and physiological properties, and might therefore also improve long-term patency.

Negative regulation of vascular smooth muscle cell migration by blood shear stress

Vortex blood flow with reduced blood shear stress in a vein graft has been hypothesized to promote smooth muscle cell (SMC) migration and intimal hyperplasia, pathological events leading to vein graft restenosis. To demonstrate that blood shear stress regulates these processes, we developed a modified vein graft model where the SMC response to reduced vortex blood flow was compared with that of control vein grafts. Vortex blood flow induced SMC migration and neointimal hyperplasia in control vein grafts, whereas reduction of vortex blood flow in the modified vein graft strongly suppressed these effects. A venous polymer implant with known fluid shear stress was employed to clarify the molecular mechanism of shear-dependent SMC migration in vivo. In the polymer implant, the phosphorylation of extracellular signal-regulated kinase (ERK1/2) and myosin light chain kinase (MLCK), found primarily in SMCs, increased from day 3 to day 5 and returned toward the control level from day 5 to day 10, with the peak phosphorylation associated with the maximal speed of SMC migration. Treatment with PD-98059 (an inhibitor specific to the ERK1/2 activator MEK1/2) significantly suppressed the phosphorylation of MLCK, suggesting a role for ERK1/2 in regulating the activity of MLCK. Treatment with PD-98059 or ML-7 (an inhibitor specific to MLCK) reduced shear stress-dependent SMC migration, resulting in an SMC distribution independent of fluid shear stress. These results suggest that fluid shear stress regulates SMC migration via the mediation of ERK1/2 and MLCK.

Mechanical cell injury

The tissues of the body are continually subjected to mechanical stimulation by external forces, such as gravity, and internally generated forces, such as the pumping of blood or muscle contraction. Within a physiological range, the forces elicit adaptive responses acutely (to rapidly alter function) and chronically (to remodel tissue structure to optimize load-bearing capabilities). When the forces exceed certain thresholds, injury results. To understand the mechanisms of mechanical injury at the cellular level, we must analyze the structural response of the cell to various modes of deformation and examine the biological consequences of the structural alterations caused by the trauma. This chapter reviews the mechanics of cell membrane deformation and failure. Evidence for the strain-rate-dependent, transient disruption of cell membranes, or mechanoporation, is presented for a variety of cell types. The complex interactions between the structural damage and the biological sequelae are illustrated using clinically relevant forms of cell injury. Finally, novel therapeutic approaches targeting membrane integrity are described.

Neointima formation on vascular elastic laminae and collagen matrices scaffolds implanted in the rat aortae

Synthetic polymers, including polytetrafluoroethylene and Dacron, and biomatrix proteins, including collagen and fibrin, have been used for the construction of vascular substitutes. However, these materials induce inflammatory reactions, contributing to thrombosis, smooth muscle cell (SMC) proliferation, and neointima formation, processes leading to the failure of vascular substitutes. Thus, a pressing issue in vascular reconstruction is to construct vascular substitutes with surface materials that are inflammation-resistant. Here, we demonstrate that the vascular elastic laminae exhibit such a property. Aortic specimens from donor rats were treated with 0.1M NaOH for various times, resulting in elastic lamina-collagen matrix scaffolds with and without the basal lamina. Matrix scaffolds were implanted into the host aorta with three different surface materials, including the elastic lamina, basal lamina, and adventitial collagen, and observed for leukocyte adhesion, endothelial migration, cell proliferation, and neointimal formation on these surfaces. It was found that the elastic lamina was associated with significantly lower leukocyte adhesion, BrdU incorporation, and neointima formation than the basal lamina and adventitial collagen, while the migration of endothelial cells was comparable on all three surfaces. The adventitial collagen matrix was associated with leukocyte infiltration from blood and subsequent SMC migration from the host aorta, whereas the elastic laminae were resistant to such processes. The morphology of the implanted elastic laminae appeared normal at all times. These observations suggest that the vascular elastic laminae exhibit inflammation-resistant properties and inhibit SMC mitogenic activities compared with collagen-containing matrices and may be considered a potential surface material for vascular reconstruction.

The use of animal models in developing the discipline of cardiovascular tissue engineering: a review

Cardiovascular disease remains one of the major causes of death and disability in the Western world. Tissue engineering offers the prospect of being able to meet the demand for replacement of heart valves, vessels for coronary and lower limb bypass surgery and the generation of cardiac tissue for addition to the diseased heart. In order to test prospective tissue-engineered devices, these constructs must first be proven in animal models before receiving CE marking or FDA approval for a clinical trial. The choice of animal depends on the nature of the tissue-engineered construct being tested. Factors that need to be considered include technical requirements of implanting the construct, availability of the animal, cost and ethical considerations. In this paper, we review the history of animal studies in cardiovascular tissue engineering and the uses of animal tissue as sources for tissue engineering.

Early adaptation of human lower extremity vein grafts: wall stiffness changes accompany geometric remodeling

OBJECTIVE

To quantitatively describe the temporal changes in elastic properties and wall dimensions in lower-extremity vein grafts after implantation.

DESIGN OF STUDY

This is a prospective study of patients (N = 38) undergoing lower extremity bypass grafts (N = 41) with autologous veins. Pulse wave velocity (PWV), luminal diameter, and wall thickness measurements were obtained by duplex ultrasound scan intraoperatively and at 1, 3, and 6 months postoperatively for assessment of graft dimensions and wall stiffness.

RESULTS

Lower extremity vein grafts showed an increase in PWV (from 16 +/- 1 to 21 +/- 3 cm/s; mean +/- SEM; P =.08), reflecting an increase in wall stiffness (from 1.2 +/- 0.2 to 2.5 +/- 0.7 x 10(6) dynes/cm; P =.02) and wall thickness (from 0.47 +/- 0.03 to 0.61 +/- 0.004 mm; P =.04) over the first 6 months after implantation. Changes in lumen diameter were positively correlated with changes in external graft diameter (P <.01) and negatively correlated with initial lumen diameter (P <.01) but not with changes in the wall thickness.

CONCLUSIONS

These results suggest complex remodeling of vein grafts during the first several months after implantation, with increased wall thickness occurring independent of variable changes in lumen diameter. Simultaneously, a marked increase in wall stiffness over this interval suggests a likely role for collagen deposition.

Permanent wall stretching in porcine coronary and internal mammary arteries

BACKGROUND

Anastomotic connectors may induce substantial arterial wall deformation and, hence, wall injury. We studied arterial wall damage and repair after sustained large longitudinal elongation in the porcine coronary and internal mammary arteries in vivo.

METHODS

A stretch device that elongates a part of the artery by 80% was implanted in 8 pigs. Elongated coronary arteries (n = 14) and internal mammary arteries (n = 15) were examined histologically at either 2 days (4 pigs) or 5 weeks of follow-up (4 pigs).

RESULTS

No mural thrombus was observed at the elongated site. In the coronary artery at 2 days, few and only minor histologic changes were found. At 5 weeks, in two of seven coronary segments, a thin rim of intimal hyperplasia was found, in one case with a maximum thickness of 76 micro m. The internal mammary artery hardly showed any changes.

CONCLUSIONS

Permanent longitudinal elongation by 80% caused little structural changes in the porcine coronary and internal mammary artery wall. Anastomotic connectors that impose relatively large deformations can be safely evaluated in the pig.

Role of blood shear stress in the regulation of vascular smooth muscle cell migration

Smooth muscle cell (SMC) migration from the media to the intima of blood vessels contributes to neointimal formation and atherogenesis. Here, we demonstrate how blood shear stress regulates vascular SMC migration in the encapsulating tissue of a micro-cylinder implanted in the center of the rat vena cava with the micro-cylinder perpendicular to blood flow. In this model, the micro-cylinder was exposed to a laminar flow with a known shear stress field in the leading region and a vortex flow in the trailing region. After surgery, the micro-cylinder was encapsulated by a thrombus-like tissue within one day, followed by SMC migration from the vena cava to the encapsulating tissue from day 3 to 20. SMC migration was time-dependent with a peak migration speed at day 5. At each given time (excluding day 1), blood shear stress exerts an inhibitory effect on SMC migration with significantly suppressed SMC migration in the laminar flow region than in the stagnation, separation, and vortex flow regions. SMCs were relatively parallel to the shear stress direction in high shear stress regions, whereas perpendicular to the shear stress direction in low shear stress regions. These results suggest that blood shear stress plays a role in regulating SMC migration and orientation in this model.

Role of tensile stress and strain in the induction of cell death in experimental vein grafts

Tensile stress and strain are known to induce vascular cell proliferation, a process that is physiologically counterbalanced by cell death. Here we investigate whether tensile stress and strain regulate vascular-cell death by using an end-to-end anastomosed rat vein graft model. In such a model, the circumferential tensile stress in the graft wall was increased by approximately 140 times immediately after surgery compared with that in the venous wall. This change was associated with an increase in the percentage of TUNEL-positive cells at 1, 6, 24, 120, 240, and 720h with two distinct peaks at 1 and 24h (10.1+/-3.5 and 14.4+/-3.2%, respectively) compared with that in control jugular veins (0.4+/-0.5 and 0.5+/-0.5% at 1 and 24h, respectively). When tensile stress and strain in the vein graft wall were reduced by using a biomechanical engineering approach, the rate of cell death was reduced significantly (3.6+/-1.1 and 1.6+/-0.5% at 1 and 24h, respectively). Furthermore, DEVD-CHO, a tetrapeptide aldehyde that inhibits the activity of caspase 3, significantly suppressed this event. These results suggest that a step increase in tensile stress and strain in experimental vein grafts induces rapid cell death, which is possibly mediated by cell death signaling mechanisms.