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

Impact of Cyclic Strain on Elastin Synthesis in a 3D Human Myometrial Culture Model

The synthesis and assembly of mature, organized elastic fibers remains a limitation to the clinical use of many engineered tissue replacements. There is a critical need for a more in-depth understanding of elastogenesis regulation for the advancement of methods to induce and guide production of elastic matrix structures in engineered tissues that meet the structural and functional requirements of native tissue. The dramatic increase in elastic fibers through normal pregnancy has led us to explore the potential role of mechanical stretch in combination with pregnancy levels of the steroid hormones 17β-estradiol and progesterone on elastic fiber production by human uterine myometrial smooth muscle cells in a three-dimensional (3D) culture model. Opposed to a single strain regimen, we sought to better understand how the amplitude and frequency parameters of cyclic strain influence elastic fiber production in these myometrial tissue constructs (MTC). Mechanical stretch was applied to MTC at a range of strain amplitudes (5%, 10%, and 15% at 0.5 Hz frequency) and frequencies (0.1 Hz, 0.5 Hz, 1 Hz, and constant 0 Hz at 10% amplitude), with and without pregnancy-level hormones, for 6 days. MTC were assessed for cell proliferation, matrix elastin protein content, and expression of the main elastic fiber genes, tropoelastin (ELN) and fibrillin-1 (FBN1). Significant increases in elastin protein and ELN and FBN1 mRNA were produced from samples subjected to a 0.5 Hz, 10% strain regimen, as well as samples stretched at higher amplitude (15%, 0.5 Hz) and higher frequency (1 Hz, 10%); however, no significant effects because of third-trimester mimetic hormone treatment were determined. These results establish that a minimum level of strain is required to stimulate the synthesis of elastic fiber components in our culture model and show this response can be similarly enhanced by increasing either the amplitude or frequency parameter of applied strain. Further, our results demonstrate strain alone is sufficient to stimulate elastic fiber production and suggest hormones may not be a significant factor in regulating elastin synthesis. This 3D culture model will provide a useful tool to further investigate mechanisms underlying pregnancy-induced de novo elastic fiber synthesis and assembly by uterine smooth muscle cells.

Drivers of vascular growth and remodeling: A computational framework to promote benign adaptation in the Ross procedure

In the sixties, Dr Donald Ross designed a surgical solution for young patients with aortic valve disease by using the patients' own pulmonary valve. The Ross procedure is the only aortic valve replacement technique that can restore long-term survival and preserve quality of life. The main failure mode of the Ross procedure is wall dilatation, potentially leading to valve regurgitation and leakage. Dilatation occurs due to the inability of the pulmonary autograft to adapt to the sudden increase in loading when exposing to aortic pressures. Previous experimental data has shown that a permanent external support wrapped around the artery can prevent the acute dilatation of the arterial wall. However, the textile support leads to stress-shielding phenomena due to the loss of mechanical wall compliance. We present a pragmatic and modular computational framework of arterial growth and remodeling predicting the long-term outcomes of cardiovascular tissue adaptation, with and without textile wrapping. The model integrates mean, systolic and diastolic pressures and assumes the resulting wall stresses to drive the biological remodeling rules. Rather than a single mean pressure or stress deviation from the homeostatic state, we demonstrate that only pulsatile stresses can predict available experimental results. Therefore, we suggest that a biodegradable external support could induce benign remodeling in the Ross procedure. Indeed, a biodegradable textile wrapped around the autograft fulfills the trade-off between prevention of acute dilatation on the one hand and recovery of arterial wall compliance on the other hand. After further validation, the computational framework can set the basis for the development of an actual biodegradable external support for the Ross procedure with optimized polymer mechanical properties and degradation behavior.

A Multiscale Approach for Predicting Certain Effects of Hand-Transmitted Vibration on Finger Arteries

Prolonged exposure to strong hand-arm vibrations can lead to vascular disorders such as Vibration White Finger (VWF). We modeled the onset of this peripheral vascular disease in two steps. The first consists in assessing the reduction in shearing forces exerted by the blood on the walls of the arteries (Wall Shear Stress—WSS) during exposure to vibrations. An acute but repeated reduction in WSS can lead to arterial stenosis characteristic of VWF. The second step is devoted to using a numerical mechano-biological model to predict this stenosis as a function of WSS. WSS is reduced by a factor of 3 during exposure to vibration of 40 m·s−2. This reduction is independent of the frequency of excitation between 31 Hz and 400 Hz. WSS decreases logarithmically when the amplitude of the vibration increases. The mechano-biological model simulated arterial stenosis of 30% for an employee exposed for 4 h a day for 10 years. This model also highlighted the chronic accumulation of matrix metalloproteinase 2. By considering daily exposure and the vibratory level, we can calculate the degree of stenosis, thus that of the disease for chronic exposure to vibrations.

Vascular Smooth Muscle Cells Mechanosensitive Regulators and Vascular Remodeling

Blood vessels are subjected to mechanical loads of pressure and flow, inducing smooth muscle circumferential and endothelial shear stresses. The perception and response of vascular tissue and living cells to these stresses and the microenvironment they are exposed to are critical to their function and survival. These mechanical stimuli not only cause morphological changes in cells and vessel walls but also can interfere with biochemical homeostasis, leading to vascular remodeling and dysfunction. However, the mechanisms underlying how these stimuli affect tissue and cellular function, including mechanical stimulation-induced biochemical signaling and mechanical transduction that relies on cytoskeletal integrity, are unclear. This review focuses on signaling pathways that regulate multiple biochemical processes in vascular mesangial smooth muscle cells in response to circumferential stress and are involved in mechanosensitive regulatory molecules in response to mechanotransduction, including ion channels, membrane receptors, integrins, cytoskeletal proteins, nuclear structures, and cascades. Mechanoactivation of these signaling pathways is closely associated with vascular remodeling in physiological or pathophysiological states.

The Phenotypic Responses of Vascular Smooth Muscle Cells Exposed to Mechanical Cues

During the development of atherosclerosis and other vascular diseases, vascular smooth muscle cells (SMCs) located in the intima and media of blood vessels shift from a contractile state towards other phenotypes that differ substantially from differentiated SMCs. In addition, these cells acquire new functions, such as the production of alternative extracellular matrix (ECM) proteins and signal molecules. A similar shift in cell phenotype is observed when SMCs are removed from their native environment and placed in a culture, presumably due to the absence of the physiological signals that maintain and regulate the SMC phenotype in the vasculature. The far majority of studies describing SMC functions have been performed under standard culture conditions in which cells adhere to a rigid and static plastic plate. While these studies have contributed to discovering key molecular pathways regulating SMCs, they have a significant limitation: the ECM microenvironment and the mechanical forces transmitted through the matrix to SMCs are generally not considered. Here, we review and discuss the recent literature on how the mechanical forces and derived biochemical signals have been shown to modulate the vascular SMC phenotype and provide new perspectives about their importance.

Coronary artery mechanics induces human saphenous vein remodelling via recruitment of adventitial myofibroblast-like cells mediated by Thrombospondin-1

Rationale: Despite the preferred application of arterial conduits, the greater saphenous vein (SV) remains indispensable for coronary bypass grafting (CABG), especially in multi-vessel coronary artery disease (CAD). The objective of the present work was to address the role of mechanical forces in the activation of maladaptive vein bypass remodeling, a process determining progressive occlusion and recurrence of ischemic heart disease.

Methods: We employed a custom bioreactor to mimic the coronary shear and wall mechanics in human SV vascular conduits and reproduce experimentally the biomechanical conditions of coronary grafting and analyzed vein remodeling process by histology, histochemistry and immunofluorescence. We also subjected vein-derived cells to cyclic uniaxial mechanical stimulation in culture, followed by phenotypic and molecular characterization using RNA and proteomic methods. We finally validated our results in vitro and using a model of SV carotid interposition in pigs.

Results: Exposure to pulsatile flow determined a remodeling process of the vascular wall involving reduction in media thickness. Smooth muscle cells (SMCs) underwent conversion from contractile to synthetic phenotype. A time-dependent increase in proliferating cells expressing mesenchymal (CD44) and early SMC (SM22α) markers, apparently recruited from the SV adventitia, was observed especially in CABG-stimulated vessels. Mechanically stimulated SMCs underwent transition from contractile to synthetic phenotype. MALDI-TOF-based secretome analysis revealed a consistent release of Thrombospondin-1 (TSP-1), a matricellular protein involved in TGF-β-dependent signaling. TSP-1 had a direct chemotactic effect on SV adventitia resident progenitors (SVPs); this effects was inhibited by blocking TSP-1 receptor CD47. The involvement of TSP-1 in adventitial progenitor cells differentiation and graft intima hyperplasia was finally contextualized in the TGF-β-dependent pathway, and validated in a saphenous vein into carotid interposition pig model.

Conclusions: Our results provide the evidence of a matricellular mechanism involved in the human vein arterialization process controlled by alterations in tissue mechanics, and open the way to novel potential strategies to block VGD progression based on targeting cell mechanosensing-related effectors.

Mechanotransduction in the Cardiovascular System: From Developmental Origins to Homeostasis and Pathology

With the term ‘mechanotransduction’, it is intended the ability of cells to sense and respond to mechanical forces by activating intracellular signal transduction pathways and the relative phenotypic adaptation. While a known role of mechanical stimuli has been acknowledged for developmental biology processes and morphogenesis in various organs, the response of cells to mechanical cues is now also emerging as a major pathophysiology determinant. Cells of the cardiovascular system are typically exposed to a variety of mechanical stimuli ranging from compression to strain and flow (shear) stress. In addition, these cells can also translate subtle changes in biophysical characteristics of the surrounding matrix, such as the stiffness, into intracellular activation cascades with consequent evolution toward pro-inflammatory/pro-fibrotic phenotypes. Since cellular mechanotransduction has a potential readout on long-lasting modifications of the chromatin, exposure of the cells to mechanically altered environments may have similar persisting consequences to those of metabolic dysfunctions or chronic inflammation. In the present review, we highlight the roles of mechanical forces on the control of cardiovascular formation during embryogenesis, and in the development and pathogenesis of the cardiovascular system.

Resident multipotent vascular stem cells exhibit amplitude dependent strain avoidance similar to that of vascular smooth muscle cells

Atherosclerosis is one of the leading causes of mortality worldwide, and presents as a narrowing or occlusion of the arterial lumen. Interventions to re-open the arterial lumen can result in re-occlusion through intimal hyperplasia. Historically only de-differentiated vascular smooth muscle cells were thought to contribute to intimal hyperplasia. However recent significant evidence suggests that resident medial multipotent vascular stem cells (MVSC) may also play a role. We therefore investigated the strain response of MVSC since these resident cells are also subjected to strain within their native environment. Accordingly, we applied uniaxial 1 Hz cyclic uniaxial tensile strain at three amplitudes around a mean strain of 5%, (4-6%, 2-8% and 0-10%) to either rat MVSC or rat VSMC before their strain response was evaluated. While both cell types strain avoid, the strain avoidant response was greater for MVSC after 24 h, while VSMC strain avoid to a greater degree after 72 h. Additionally, both cell types increase strain avoidance as strain amplitude is increased. Moreover, MVSC and VSMC both demonstrate a strain-induced decrease in cell number, an effect more pronounced for MVSC. These experiments demonstrate for the first time the mechano-sensitivity of MVSC that may influence intimal thickening, and emphasizes the importance of strain amplitude in controlling the response of vascular cells in tissue engineering applications.

Deriving vascular smooth muscle cells from mesenchymal stromal cells: Evolving differentiation strategies and current understanding of their mechanisms

Vascular smooth muscle cells (VSMCs) play essential roles in regulating blood vessel form and function. Regeneration of functional vascular smooth muscle tissue to repair vascular diseases is an area of intense research in tissue engineering and regenerative medicine. For functional vascular smooth muscle tissue regeneration to become a practical therapy over the next decade, the field will need to have access to VSMC sources that are effective, robust and safe. While pluripotent stem cells hold good future promise to this end, more immediate translation is expected to come from approaches that generate functional VSMCs from adult sources of multipotent adipose-derived and bone marrow-derived mesenchymal stromal cells (ASCs and BMSCs). The research to this end is extensive and is dominated by studies relating to classical biochemical signalling molecules used to induce differentiation of ASCs and BMSCs. However, prolonged use of the biochemical induction factors is costly and can cause potential endotoxin contamination in the culture. Over recent years several non-traditional differentiation approaches have been devised to mimic defined aspects of the native micro-environment in which VSMCs reside to contribute to the differentiation of VSMC-like cells from ASCs and BMSCs. In this review, the promises and limitations of several non-traditional culture approaches (e.g., co-culture, biomechanical, and biomaterial stimuli) targeting VSMC differentiation are discussed. The extensive crosstalk between the underlying signalling cascades are delineated and put into a translational context. It is expected that this review will not only provide significant insight into VSMC differentiation strategies for vascular smooth muscle tissue engineering applications, but will also highlight the fundamental importance of engineering the cellular microenvironment on multiple scales (with consideration of different combinatorial pathways) in order to direct cell differentiation fate and obtain cells of a desired and stable phenotype. These strategies may ultimately be applied to different sources of stem cells in the future for a range of biomaterial and tissue engineering disciplines.

Cells and stimuli in small-caliber blood vessel tissue engineering

The absence of successful solutions in treatments of small-caliber vessel diseases led to the Vascular Tissue Engineering approach to develop functional nonimmunogenic tissue engineered blood vessels. In this context, the choice of cells to be seeded and the microenvironment conditioning are pivotal. Biochemical and biomechanical stimuli seem to activate physiological regulatory pathways that induce the production of molecules and proteins stimulating stem cell differentiation toward vascular lineage and reproducing natural cross-talks among vascular cells to improve the maturation of tissue engineered blood vessels. Thus, this review focuses on (1) available cell sources, and (2) biochemical and biomechanical stimuli, with the final aim to obtain the long-term stability of the endothelium and mechanical properties suitable for withstanding physiological load.

Impact of cyclic stretch on induced elastogenesis within collagenous conduits

In vitro tissue engineering of vascular conduits requires a synergy between several external factors, including biochemical supplementation and mechanotranductive stimulation. The goal of this study was to improve adult human vascular smooth muscle cell orientation and elastic matrix synthesis within 3D tubular collagen gel constructs. We used a combination of elastogenic factors (EFs) previously tested in our lab, along with cyclic circumferential strains at low amplitude (2.5%) delivered at a range of frequencies (0.5, 1.5, and 3 Hz). After 21 days of culture, the constructs were analyzed for elastic matrix outcomes, activity of matrix metalloproteinases (MMPs)-2 and -9, cell densities and phenotype, and mechanical properties of constructs. While cell densities remained unaffected by the addition of stretch, contractile phenotypic markers were elevated in all stretched constructs relative to control. Constructs cultured with EFs stretched at 1.5 Hz exhibited the maximum elastin mRNA expression and total matrix elastin (over sixfold vs. the static EFs control). MMP-2 content was comparable in all treatment conditions, but MMP-9 levels were elevated at the higher frequencies (1.5 and 3 Hz). Minimal circumferential orientation was achieved and the mechanical properties remained comparable among the treatment conditions. Overall, constructs treated with EFs and stretched at 1.5 Hz exhibited the most elastogenic outcomes.

Periostin links mechanical strain to inflammation in abdominal aortic aneurysm

AIMS

Abdominal aortic aneurysms (AAAs) are characterized by chronic inflammation, which contributes to the pathological remodeling of the extracellular matrix. Although mechanical stress has been suggested to promote inflammation in AAA, the molecular mechanism remains uncertain. Periostin is a matricellular protein known to respond to mechanical strain. The aim of this study was to elucidate the role of periostin in mechanotransduction in the pathogenesis of AAA.

METHODS AND RESULTS

We found significant increases in periostin protein levels in the walls of human AAA specimens. Tissue localization of periostin was associated with inflammatory cell infiltration and destruction of elastic fibers. We examined whether mechanical strain could stimulate periostin expression in cultured rat vascular smooth muscle cells. Cells subjected to 20% uniaxial cyclic strains showed significant increases in periostin protein expression, focal adhesion kinase (FAK) activation, and secretions of monocyte chemoattractant protein-1 (MCP-1) and the active form of matrix metalloproteinase (MMP)-2. These changes were largely abolished by a periostin-neutralizing antibody and by the FAK inhibitor, PF573228. Interestingly, inhibition of either periostin or FAK caused suppression of the other, indicating a positive feedback loop. In human AAA tissues in ex vivo culture, MCP-1 secretion was dramatically suppressed by PF573228. Moreover, in vivo, periaortic application of recombinant periostin in mice led to FAK activation and MCP-1 upregulation in the aortic walls, which resulted in marked cellular infiltration.

CONCLUSION

Our findings indicated that periostin plays an important role in mechanotransduction that maintains inflammation via FAK activation in AAA.

Fetal development, mechanobiology and optimal control processes can improve vascular tissue regeneration in bioreactors: an integrative review

Vascular tissue engineering aims to regenerate blood vessels to replace diseased arteries for cardiovascular patients. With the scaffold-based approach, cells are seeded on a scaffold showing specific properties and are expected to proliferate and self-organize into a functional vascular tissue. Bioreactors can significantly contribute to this objective by providing a suitable environment for the maturation of the tissue engineered blood vessel. It is recognized from the mechanotransduction principles that mechanical stimuli can influence the protein synthesis of the extra-cellular matrix thus leading to maturation and organization of the tissues. Up to date, no bioreactor is especially conceived to take advantage of the mechanobiology and optimize the construct maturation through an advanced control strategy. In this review, experimental strategies in the field of vascular tissue engineering are detailed, and a new approach inspired by fetal development, mechanobiology and optimal control paradigms is proposed. In this new approach, the culture conditions (i.e. flow, circumferential strain, pressure frequency, and others) are supposed to dynamically evolve to match the maturity of vascular constructs and maximize the efficiency of the regeneration process. Moreover, this approach allows the investigation of the mechanisms of growth, remodeling and mechanotransduction during the culture.

Loss of mechanical strain impairs abdominal wall fibroblast proliferation, orientation, and collagen contraction function

BACKGROUND

Laparotomy wound load forces are reduced when dehiscence and incisional hernia formation occur. The purpose of this study was to determine the effects of strain loss on abdominal fascial fibroblast proliferation, orientation, and collagen compaction function.

METHODS

Cultured rat linea alba fibroblasts were subjected to continuous cyclic strain (CS), CS interrupted at 24 or 48 hours followed by culture at rest (IS-24 and IS-48) or were cultured without mechanical strain (NS). Cell number was measured and images analyzed for cell orientation. Fibroblasts from these groups were seeded onto the surface of (FPCL-S) or mixed into (FPCL-M) a collagen gel matrix and gel area was measured over time.

RESULTS

Continuous strain stimulated proliferation when compared with the nonstrained cells. The loss of strain (IS) delayed proliferation compared with CS throughout (P < .05). CS fibroblasts aligned perpendicular to the direction of strain within 12 hours. Within 12 hours of strain loss, IS-48 fibroblasts became significantly less aligned (P < .0001), and seemed similar to the randomly organized NS fibroblasts 48 hours after strain removal. The CS and IS-24 groups demonstrated faster and greater overall FPCL-M compaction than both the IS-48 and NS groups (P < .0002). The CS group contracted the gel faster than the NS group in FPCL-S (P = .029).

CONCLUSION

Mechanical strain rapidly induces a proliferative, morphologic, and functional response in abdominal wall fibroblasts that is dependent on the continued presence of the strain signal and quickly lost when the load force is removed. The loss of wound edge tension that occurs during laparotomy wound separation and hernia formation may contribute to impaired wound healing through loss of a key stimulatory mechanical signal with important implications for abdominal wall reconstruction.

A Novel Multiwell Device to Study Vascular Smooth Muscle Cell Responses Under Cyclic Strain

Vascular smooth muscle cells (VSMCs) are constantly exposed to cyclic stretch in the body, which makes it beneficial to study the effects of cyclic stretch on VSMCs. In this study, we developed a poly(dimethyl siloxane) (PDMS) compact six-well device that can be used to study the combined effect of cyclic strain and various growth factors on cultured VSMCs. Cell adhesion, alignment, and proliferation under 10% or 20% cyclic strain at 1 Hz were studied using this surface-enhanced PDMS device. The combined effects of cyclic strain with either transforming growth factor-β, vascular endothelial growth factor, fibroblast growth factor, or epidermal growth factor on VSMC proliferation was also examined. Results showed that VSMCs adhered well on the surface-enhanced multiwell device and they aligned perpendicularly to the direction of the cyclic strain. Cell proliferation was inhibited by 10% cyclic strain at 1 Hz compared with static control. The mitogenic effects of the growth factor were less potent under either 10% or 20% cyclic strain. With simple modification to accommodate more wells, this device could potentially be a useful tool for more economical, high throughput screening application.