Regulation of vascular smooth muscle cells and mesenchymal stem cells by mechanical strain

Vascular smooth muscle cells (SMCs) populate in the media of the blood vessel, and play an important role in the control of vasoactivity and the remodeling of the vessel wall. Blood vessels are constantly subjected to hemodynamic stresses, and the pulsatile nature of the blood flow results in a cyclic mechanical strain in the vessel walls. Accumulating evidence in the past two decades indicates that mechanical strain regulates vascular SMC phenotype, function and matrix remodeling. Bone marrow mesenchymal stem cell (MSC) is a potential cell source for vascular regeneration therapy, and may be used to generate SMCs to construct tissue-engineered vascular grafts for blood vessel replacements. In this review, we will focus on the effects of mechanical strain on SMCs and MSCs, e.g., cell phenotype, cell morphology, cytoskeleton organization, gene expression, signal transduction and receptor activation. We will compare the responses of SMCs and MSCs to equiaxial strain, uniaxial strain and mechanical strain in three-dimensional culture. Understanding the hemodynamic regulation of SMC and MSC functions will provide a basis for the development of new vascular therapies and for the construction of tissue-engineered vascular grafts.     

Measurements of mouse pulmonary artery biomechanics

BACKGROUND: Robust techniques for characterizing the biomechanical properties of mouse pulmonary arteries will permit exciting gene-level hypotheses regarding pulmonary vascular disease to be tested in genetically engineered animals. In this paper, we present the first measurements of the biomechanical properties of mouse pulmonary arteries. METHOD OF APPROACH: In an isolated vessel perfusion system, transmural pressure, internal diameter and wall thickness were measured during inflation and deflation of mouse pulmonary arteries over low (5-40 mmHg) and high (10-120 mmHg) pressure ranges representing physiological pressures in the pulmonary and systemic circulations, respectively. RESULTS: During inflation, circumferential stress versus strain showed the nonlinear "J"-shape typical of arteries. Hudetz's incremental elastic modulus ranged from 27 +/- 13 kPa (n = 7) during low-pressure inflation to 2,700 +/- 1,700 kPa (n = 9) during high-pressure inflation. The low and high-pressure testing protocols yielded quantitatively indistinguishable stress-strain and modulus-strain results. Histology performed to assess the state of the tissue after mechanical testing showed intact medial and adventitial architecture with some loss of endothelium, suggesting that smooth muscle cell contractile strength could also be measured with these techniques. CONCLUSIONS: The measurement techniques described demonstrate the feasibility of quantifying mouse pulmonary artery biomechanical properties. Stress-strain behavior and incremental modulus values are presented for normal, healthy arteries over a wide pressure range. These techniques will be useful for investigations into biomechanical abnormalities in pulmonary vascular disease.     

Functional tissue engineering: the role of biomechanics

"Tissue engineering" uses implanted cells, scaffolds, DNA, protein, and/or protein fragments to replace or repair injured or diseased tissues and organs. Despite its early success, tissue engineers have faced challenges in repairing or replacing tissues that serve a predominantly biomechanical function. An evolving discipline called "functional tissue engineering" (FTE) seeks to address these challenges. In this paper, the authors present principles of functional tissue engineering that should be addressed when engineering repairs and replacements for load-bearing structures. First, in vivo stress/strain histories need to be measured for a variety of activities. These in vivo data provide mechanical thresholds that tissue repairs/replacements will likely encounter after surgery. Second, the mechanical properties of the native tissues must be established for subfailure and failure conditions. These "baseline data" provide parameters within the expected thresholds for different in vivo activities and beyond these levels if safety factors are to be incorporated. Third, a subset of these mechanical properties must be selected and prioritized. This subset is important, given that the mechanical properties of the designs are not expected to completely duplicate the properties of the native tissues. Fourth, standards must be set when evaluating the repairs/replacements after surgery so as to determine, "how good is good enough?" Some aspects of the repair outcome may be inferior, but other mechanical characteristics of the repairs and replacements might be suitable. New and improved methods must also be developed for assessing the function of engineered tissues. Fifth, the effects of physical factors on cellular activity must be determined in engineered tissues. Knowing these signals may shorten the iterations required to replace a tissue successfully and direct cellular activity and phenotype toward a desired end goal. Finally, to effect a better repair outcome, cell-matrix implants may benefit from being mechanically stimulated using in vitro "bioreactors" prior to implantation. Increasing evidence suggests that mechanical stress, as well as other physical factors, may significantly increase the biosynthetic activity of cells in bioartificial matrices. Incorporating each of these principles of functional tissue engineering should result in safer and more efficacious repairs and replacements for the surgeon and patient.     

Characterization of vascular strain during in-vitro angioplasty with high-resolution ultrasound speckle tracking

Background

Ultrasound elasticity imaging provides biomechanical and elastic properties of vascular tissue, with the potential to distinguish between tissue motion and tissue strain. To validate the ability of ultrasound elasticity imaging to predict structurally defined physical changes in tissue, strain measurement patterns during angioplasty in four bovine carotid artery pathology samples were compared to the measured physical characteristics of the tissue specimens.

Methods

Using computational image-processing techniques, the circumferences of each bovine artery specimen were obtained from ultrasound and pathologic data.

Results

Ultrasound-strain-based and pathology-based arterial circumference measurements were correlated with an R² value of 0.94 (p = 0.03). The experimental elasticity imaging results confirmed the onset of deformation of an angioplasty procedure by indicating a consistent inflection point where vessel fibers were fully unfolded and vessel wall strain initiated.

Conclusion

These results validate the ability of ultrasound elasticity imaging to measure localized mechanical changes in vascular tissue.     

The role of smooth muscle cells in vessel wall pathophysiology and reconstruction using bioactive synthetic polymers

This review summarizes recent trends in the construction of bioartificial vascular replacements, i.e. hybrid grafts containing synthetic polymeric scaffolds and cells. In these advanced replacements, vascular smooth muscle cells (VSMC) should be considered as a physiological component, although it is known that activation of the migration and proliferation of VSMC plays an important role in the onset and development of vascular diseases, and also in restenosis of currently used vascular grafts. Therefore, in novel bioartificial vascular grafts, VSMCs should be kept in quiescent mature contractile phenotype. This can be achieved by (1) appropriate physical and chemical properties of the material, such as its chemical composition, polarity, wettability, surface roughness and topography, electrical charge and conductivity, functionalization with biomolecules and mechanical properties, (2) appropriate cell culture conditions, such as composition of cell culture media and dynamic load, namely cyclic strain, and (3) the presence of a confluent, mature, semipermeable, non-thrombogenic and non-immunogenic endothelial cell (EC) barrier, covering the luminal surface of the graft and separating the VSMCs from the blood. Both VSMCs and ECs can also be differentiated from stem and progenitor cells of various sources. In the case of degradable scaffolds, the material will gradually be removed by the cells and will be replaced by their own new extracellular matrix. Thus, the material component in advanced blood vessel substitutes acts as a temporary scaffold that promotes regeneration of the damaged vascular tissue.     

Finite element analysis of the biomechanical interaction between coronary sinus and proximal anchoring stent in coronary sinus annuloplasty

Recent clinical studies of the percutaneous transvenous mitral annuloplasty (PTMA) devices have shown a short-term reduction of mitral regurgitation after implantation. However, adverse events associated with the devices such as compression and perforation of vessel branches, device migration and fracture were reported. In this study, a finite element analysis was carried out to investigate the biomechanical interaction between the proximal anchor stent of a PTMA device and the coronary sinus (CS) vessel in three steps including: (i) the stent release and contact with the CS wall, (ii) the axial pull t the stent connector and (iii) the pressure inflation of the vessel wall. To investigate the impact of the material properties of tissues and stents on the interactive responses, the CS vessel was modelled with human and porcine material properties, and the proximal stent was modelled with two different Nitinol materials with one being stiffer than the other. The results indicated that the vessel wall stresses and contact forces imposed by the stents were much higher in the human model than the porcine model. However, the mechanical differences induced by the two stent types were relatively small. The softer stent exhibited a better fatigue safety factor when deployed in the human model than in the porcine model. These results underscored the importance of the CS tissue mechanical properties. Vessel wall stress and stent radial force obtained in the human model were higher than those obtained in the porcine model, which also brought up questions as to the validity of using the porcine model to assess device mechanical function. The quantification of these biomechanical interactions can offer scientific insight into the development and optimisation of the PTMA device design.     

A comparison of biomechanical properties between human and porcine cornea

Due to the difficulty in obtaining human corneas, pig corneas are often substituted as models for cornea research. The purpose of this study is to find the similarities and differences in the biomechanical properties between human and porcine corneas. Uniaxial tests were conducted using an Instron apparatus to determine their tensile strength, stress-strain relationship, and stress-relaxation properties. The tensile strength and stress-strain relation were very similar but significant differences between the two tissues were observed in the stress-relaxation relationship. Under the same stretch ratio lambda=1.5, porcine cornea relaxed much more than human cornea. If tensile strength and the stress-strain relation are the only mechanical factors to be investigated, porcine cornea can be used as a substitute model for human cornea research. However, when stress relaxation is a factor, porcine corneas cannot be used as an appropriate model for human corneas in mechanical property studies. It is very difficult to get enough specimens of human cornea, so we did the experiments for stress-strain relationship at a specific value of strain rate (corresponding to the velocity of loading 10mm/min), and for stress relaxation at a specific stretch ratio lambda=1.5.     

Tissue engineering a blood vessel: Regulation of vascular biology by mechanical stresses

Important to the tissue engineeping of a substitute blood vessel is an understanding of those faators which regulat vascular biology. A major factor in the mechanical environment imposed by the hemodynamics of the vascular system. In this the vascular endothelium play a critical role, and mver the past two deaades much has been learned about the influence of hemodynamics on vascular endothelial biology, to a large degree using cell culture to study the effects of flow and cyclic stretch. In our laboratory, such studies ape low being extended through the development of a model of the arterial wall involving the co-culture of endothelial cells and smomth muscle cells. The development of such a model and its use in the study of endothelial cells and smmooth muscle the evolution of approaaheq to tissue engileeping a blood vessel.     

Blood flow and vascular gene expression: fluid shear stress as a modulator of endothelial phenotype

Vascular endothelium, the cellular monolayer lining the entire cardiovascular system, is exposed to a variety of biochemical and biomechanical stimuli. Fluid shear stresses generated by blood flow in the vasculature can profoundly influence the phenotype of the endothelium by regulating the activity of certain flow-sensitive proteins (for example, enzymes), as well as by modulating gene expression. The finding that specific fluid mechanical forces can alter endothelial structure and function has provided a framework for a mechanistic understanding of flow-dependent processes, ranging from vascular remodeling in response to hemodynamic changes, to the initiation and localization of chronic vascular diseases such as atherosclerosis.     

Development of ligament tissue biodegradable devices: A review

This bibliographic review is focused on ligament tissue rehabilitation, its anatomy–physiology, and, mainly, on the dimensioning considerations of a composite material solution. The suture strength is problematic during the tissue recovering, implying reduction of mobility for several months. However, early postoperative active mobilization may enable a faster and more effective recovering of tissue biomechanical functions. As the risk of tendon rupture becomes a significant concern, a repair technique must be used to withstand the tensile forces generated by active mobilization. However, to avoid stress shielding effect on ligament tissue, an augmentation device must be designed on stiffness basis, that preferably will decrease. Absorbable biocomposite reinforcements have been used to allow early postoperative active mobilization and avoid the shortcomings of current repair solutions. Tensile strength decrease of the repair, during the initial inflammatory phase, is expected, derived from oedema and tendon degradation. In the fibroblastic phase, stiffness and strength will increase, which will stabilize during the remodeling phase.The reinforcement should be able to carry the dynamic load due to locomotion with a mechanical behavior similar to the undamaged natural tissue, during all rehabilitation process. Moreover, the degradation rate must also be compatible with the ligament tissue recovering. The selection and combination of different biodegradable materials, in order to make the biocomposite reinforcement functionally compatible to the damaged sutured tissue, in terms of mechanical properties and degradation rate, is a major step on the design process. Modelling techniques allow pre-clinical evaluation of the reinforcement functional compatibility, and the optimization by comparison of different composite solutions in terms of biomechanical behavior.     

Compressive mechanical properties of atherosclerotic plaques—Indentation test to characterise the local anisotropic behaviour

Accurate material models and associated parameters of atherosclerotic plaques are crucial for reliable biomechanical plaque prediction models. These biomechanical models have the potential to increase our understanding of plaque progression and failure, possibly improving risk assessment of plaque rupture, which is the main cause of ischaemic strokes and myocardial infarction. However, experimental biomechanical data on atherosclerotic plaque tissue is scarce and shows a high variability. In addition, most of the biomechanical models assume isotropic behaviour of plaque tissue, which is a general over-simplification. This review discusses the past and the current literature that focus on mechanical properties of plaque derived from compression experiments, using unconfined compression, micro-indentation or nano-indentation. Results will be discussed and the techniques will be mutually compared. Thereafter, an in-house developed indentation method combined with an inverse finite element method is introduced, allowing analysis of the local anisotropic mechanical properties of atherosclerotic plaques. The advantages and limitations of this method will be evaluated and compared to other methods reported in literature.     

Extra-cellular matrix in vascular networks

The vascular network is a series of linked conduits of blood vessels composed of the endothelium, a monolayer of cells that adorn the vessel lumen and surrounding layer(s) of mesenchymal cells (vascular smooth muscle, pericytes and fibroblasts). In addition to providing structural support, the mesenchymal cells are essential for vessel contractility. The extracellular matrix is a major constituent of blood vessels and provides a framework in which these various cell types are attached and embedded. The composition and organization of vascular extracellular matrix is primarily controlled by the mesenchymal cells, and is also responsible for the mechanical properties of the vessel wall, forming complex networks of structural proteins which are highly regulated. The extracellular matrix also plays a central role in cellular adhesion, differentiation and proliferation. This review examines the cellular and extracellular matrix components of vessels, with specific emphasis on the regulation of collagen type I and implications in vascular disease.     

Micropatterned cell sheets with defined cell and extracellular matrix orientation exhibit anisotropic mechanical properties

For an arterial replacement graft to be effective, it must possess the appropriate strength in order to withstand long-term hemodynamic stress without failure, yet be compliant enough that the mismatch between the stiffness of the graft and the native vessel wall is minimized. The native vessel wall is a structurally complex tissue characterized by circumferentially oriented collagen fibers/cells and lamellar elastin. Besides the biochemical composition, the functional properties of the wall, including stiffness, depend critically on the structural organization. Therefore, it will be crucial to develop methods of producing tissues with defined structures in order to more closely mimic the properties of a native vessel. To this end, we sought to generate cell sheets that have specific ECM/cell organization using micropatterned polydimethylsiloxane (PDMS) substrates to guide cell organization and tissue growth. The patterns consisted of large arrays of alternating grooves and ridges. Adult bovine aortic smooth muscle cells cultured on these substrates in the presence of ascorbic acid produced ECM-rich sheets several cell layers thick in which both the cells and ECM exhibited strong alignment in the direction of the micropattern. Moreover, mechanical testing revealed that the sheets exhibited mechanical anisotropy similar to that of native vessels with both the stiffness and strength being significantly larger in the direction of alignment, demonstrating that the microscale control of ECM organization results in functional changes in macroscale material behavior.     

Involvement of tenascin-C and PG-M/versican in flexor tenosynovial pathology of idiopathic carpal tunnel syndrome

Increased intra-carpal-tunnel pressure due to swelling of the flexor tenosynovium is the most probable pathological mechanism of idiopathic carpal tunnel syndrome (CTS). To clarify the role of tenascin-C and PG-M/versican, which have often been found to be involved in tissue remodeling and vascular stenosis in the pathogenesis of CTS, we histologically and biochemically examined the production of extracellular matrix in the flexor tenosynovium from 40 idiopathic CTS patients. Tenascin-C was temporarily expressed in the vessel wall, synovial lining and fibrous tissue, with expression regulated differently in each tissue. Tenascin-C expression by vessels correlated with disease duration and appeared to be involved in vascular lesion pathology. Morphometric analysis showed that tenascin-C expression by small arteries is correlated with PG-M/versican expression in surrounding connective tissue. PG-M/versican was also present at the neointima of severely narrowed vessels. Although tenascin-C expression by synovial lining and connective tissue shows marked regional variation and seems inconsistent, in vitro examination suggested that tenascin-C production by these tissues is regulated in response to mechanical strain on the flexor tenosynovium.     

What do we know about growth of vessel elements of secondary xylem in woody plants?

Despite extensive knowledge about vessel element growth and the determination of the axial course of vessels, these processes are still not fully understood. They are usually explained as resulting primarily from hormonal regulation in stems. This review focuses on an increasingly discussed aspect – mechanical conditions in the vascular cambium. Mechanical conditions in cambial tissue are important for the growth of vessel elements, as well as other cambial derivatives. In relation to the type of stress acting on cambial cells (compressive versus tensile stress) we: (i) discuss the shape of the enlarging vessel elements observed in anatomical sections; (ii) present hypotheses regarding the location of intrusive growth of vessel elements and cambial initials; (iii) explain the relationship between the growth of vessel elements and fibres; and (iv) consider the effect of mechanical stress in determining the course of a vessel. We also highlight the relationship between mechanical stress and transport of the most extensively studied plant hormone – auxin. We conclude that the integration of a biomechanical factor with the commonly acknowledged hormonal regulation could significantly enhance the analysis of the formation of vessel elements as well as entire vessels, which transport water and minerals in numerous plant species.     

The osteocyte as a wiring transmission system

The mechanism of transduction of mechanical strains into biological signals remains one of the more baffling problems of skeletal homeostasis. The updated literature ascribes to osteocytes the function of sensing the strains induced into the bone matrix by mechanical stresses. Whether the osteocytes perform such function by themselves or they are helped by other cells is also unknown. Indeed TEM investigations carried out in our laboratory pointed out the existence of a functional syncytium among all the cells of the osteogenic lineage (COL; stromal cells, osteoblasts or bone lining cells, osteocytes). On the basis of this finding, we suggested that COL may reciprocally modulate their function not only by volume transmission (paracrine and autocrine stimulation) but also by wiring transmission, namely in a neuronal like manner. Thanks to their location, osteocytes should theoretically be the first cells of COL functional syncytium to sense mechanical strains, whereas stromal cells should be the first to be activated by hormonal molecules diffusing across the endothelial lining. Since PTH and Estrogen receptors have also been localized on osteocytes, and considering that such hormones have been suggested to modulate the sensitivity to strain of the bone mechanosensor, we suggested that the osteocyte syncytium may constitute the microscopic bone structure that sense both mechanical strain and biochemical factors and, at any moment, after having combined the two types of stimuli, issues the appropriate signals to the other bone cells by volume and/or wiring-transmission. Stromal cells, on the other hand, besides transmitting signals from vascular endothelium to bone cells, may control the differentiation and then direct the course of the osteoblasts around the vascular framework.     

Effects of uniaxial cyclic strain on adipose-derived stem cell morphology,proliferation, and differentiation

Cells and tissues in vivo are subjected to various forms of mechanical forces that are essential to their normal development and functions. The arterial blood vessel wall is continuously exposed to mechanical stresses such as pressure, strain, and shear due to the pulsatile nature of blood flow. Vascular smooth muscle cells (SMCs) populate the media of blood vessels and play important roles in the control of vasoactivity and the remodeling of the vessel wall. It is well documented that the phenotype and functions of vascular SMCs are not only regulated by chemical factors such as transforming growth factor-β₁ (TGF-β₁), but also by mechanical factors such as uniaxial strain. The purpose of our study was to explore the effects of TGF-β₁ alone or in combination with uniaxial cyclic strain on adipose-derived stem cell (ASC) morphology, proliferation, and differentiation. Low passage ASCs were stimulated with 10% strain at 1 Hz for 7 days, with or without TGF-β₁. Cyclic strain inhibited proliferation, and caused alignment of the cells and of the F-actin cytoskeleton perpendicular to the direction of strain. Strain alone resulted in a decrease in the expression of early SMC markers α-SMA and h ₁-calponin. While the response of SMCs and other progenitor cells such as bone marrow stromal cells to mechanical forces has been extensively studied, the roles of these forces on ASCs remain unexplored. This work advances our understanding of the mechanical regulation of ASCs. Presented in part at the third annual meeting of the International Fat Applied Technology Society (IFATS), September 10–13, 2005, Omni Charlottesville Hotel, Charlottesville, VA, USA.     

Integrative Modeling of Small Artery Structure and Function Uncovers Critical Parameters for Diameter Regulation

Organ perfusion is regulated by vasoactivity and structural adaptation of small arteries and arterioles. These resistance vessels are sensitive to pressure, flow and a range of vasoactive stimuli. Several strongly interacting control loops exist. As an example, the myogenic response to a change of pressure influences the endothelial shear stress, thereby altering the contribution of shear-dependent dilation to the vascular tone. In addition, acute responses change the stimulus for structural adaptation and vice versa. Such control loops are able to maintain resistance vessels in a functional and stable state, characterized by regulated wall stress, shear stress, matched active and passive biomechanics and presence of vascular reserve. In this modeling study, four adaptation processes are identified that together with biomechanical properties effectuate such integrated regulation: control of tone, smooth muscle cell length adaptation, eutrophic matrix rearrangement and trophic responses. Their combined action maintains arteries in their optimal state, ready to cope with new challenges, allowing continuous long-term vasoregulation. The exclusion of any of these processes results in a poorly regulated state and in some cases instability of vascular structure.     

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.     

Cardiovascular stent design and vessel stresses: a finite element analysis

Intravascular stents of various designs are currently in use to restore patency in atherosclerotic coronary arteries and it has been found that different stents have different in-stent restenosis rates. It has been hypothesized that the level of vascular injury caused to a vessel by a stent determines the level of restenosis. Computational studies may be used to investigate the mechanical behaviour of stents and to determine the biomechanical interaction between the stent and the artery in a stenting procedure. In this paper, we test the hypothesis that two different stent designs will provoke different levels of stress within an atherosclerotic artery and hence cause different levels of vascular injury. The stents analysed using the finite-element method were the S7 (Medtronic AVE) and the NIR (Boston Scientific) stent designs. An analysis of the arterial wall stresses in the stented arteries indicates that the modular S7 stent design causes lower stress to an atherosclerotic vessel with a localized stenotic lesion compared to the slotted tube NIR design. These results correlate with observed clinical restenosis rates, which have found higher restenosis rates in the NIR compared with the S7 stent design. Therefore, the testing methodology outlined here is proposed as a pre-clinical testing tool, which could be used to compare and contrast existing stent designs and to develop novel stent designs.