The response of endothelial cells to fluid shear stress using a co-culture model of the arterial wall

Simulation of LDL permeation into multilayer wall of a coronary bifurcation using WSS-dependent model: effects of hemorheology

Atherosclerosis, due to the permeation of low-density lipoprotein (LDL) particles into the arterial wall, is one of the most common and deadly diseases in today's world. Due to its importance, numerous studies have been conducted on the factors affecting this disease. In this study, using numerical simulation, the effects of Wall Shear Stress (WSS), non-Newtonian behavior of blood, different values of hematocrit and blood pressure on LDL permeation into the arterial wall layers are investigated in a 4-layer wall model of a coronary bifurcation. To obtain the velocity and concentration fields in the fluid domain, the Navier-Stokes, Brinkman, and mass transfer equations are numerically solved in the lumen and wall layers. Results show that it is important to consider the effects of WSS on transport properties of endothelium layer in bifurcations and this leads to completely different concentration profiles compared to the constant properties model. Our computations show that a giant accumulation of LDL in the intima layer of the outer wall of the left anterior descending artery, especially in low WSS regions, may lead to atherosclerosis. It is also, necessary to consider the non-Newtonian behavior of blood in bifurcations due to its direct effect on WSS. A pressure-induced increase in the half-width of leaky junctions may be responsible for the higher risk of atherosclerosis in hypertension. In addition, it is shown that the dominant mechanism in LDL permeation into the wall is convection, and also, hypertension increases the effect of mass transfer by convection mechanism more than the diffusion mechanism. Furthermore, our results are consistent with various clinical studies.

A Compressed Collagen Construct for Studying Endothelial-Smooth Muscle Cell Interaction Under High Shear Stress

The coculture of vascular endothelial cells (ECs) on collagen gels containing smooth muscle cells (SMCs) has been carried out to investigate cellular interactions associated with blood vessel pathophysiology under wall shear stress (WSS) conditions. However, due to a lack of gel stiffness, the previous collagen gel coculture constructs are difficult to use for pathologic higher WSS conditions. Here, we newly constructed a coculture model with centrifugally compressed cell-collagen combined construct (C6), which withstands higher WSS conditions. The elastic modulus of C6 was approximately 6 times higher than that of the uncompressed collagen construct. The level of α-smooth muscle actin, a contractile SMC phenotype marker observed in healthy arteries, was elevated in C6 compared with that of the uncompressed construct, and further increased by exposure to a physiological level WSS of 2 Pa, but not by a pathological level of 20 Pa. WSS conditions of 2 and 20 Pa also induced different expression ratios of matrix metalloproteinases and their inhibitors in the C6 coculture model but did not in monocultured ECs and SMCs. The C6 coculture model will be a powerful tool to investigate interactions between ECs and SMCs under pathologically high WSS conditions.

3D Bioprinted Multicellular Vascular Models

3D bioprinting is an emerging additive manufacturing technique to fabricate constructs for human disease modeling. However, current cell-laden bioinks lack sufficient biocompatibility, printability, and structural stability needed to translate this technology to preclinical and clinical trials. Here, a new class of nanoengineered hydrogel-based cell-laden bioinks is introduced, that can be printed into 3D, anatomically accurate, multicellular blood vessels to recapitulate both the physical and chemical microenvironments of native human vasculature. A remarkably unique characteristic of this bioink is that regardless of cell density, it demonstrates a high printability and ability to protect encapsulated cells against high shear forces in the bioprinting process. 3D bioprinted cells maintain a healthy phenotype and remain viable for nearly one-month post-fabrication. Leveraging these properties, the nanoengineered bioink is printed into 3D cylindrical blood vessels, consisting of living co-culture of endothelial cells and vascular smooth muscle cells, providing the opportunity to model vascular function and pathophysiology. Upon cytokine stimulation and blood perfusion, this 3D bioprinted vessel is able to recapitulate thromboinflammatory responses observed only in advanced in vitro preclinical models or in vivo. Therefore, this 3D bioprinted vessel provides a potential tool to understand vascular disease pathophysiology and assess therapeutics, toxins, or other chemicals.

A novel human arterial wall-on-a-chip to study endothelial inflammation and vascular smooth muscle cell migration in early atherosclerosis

Mechanistic understanding of atherosclerosis is largely hampered by the lack of a suitable in vitro human arterial model that recapitulates the arterial wall structure, and the interplay between different cell types and the surrounding extracellular matrix (ECM). This work introduces a novel microfluidic endothelial cell (EC)-smooth muscle cell (SMC) 3D co-culture platform that replicates the structural and biological aspects of the human arterial wall for modeling early atherosclerosis. Using a modified surface tension-based ECM patterning method, we established a well-defined intima-media-like structure, and identified an ECM composition (collagen I and Matrigel mixture) that retains the SMCs in a quiescent and aligned state, characteristic of a healthy artery. Endothelial stimulation with cytokines (IL-1β and TNFα) and oxidized low-density lipoprotein (oxLDL) was performed on-chip to study various early atherogenic events including endothelial inflammation (ICAM-1 expression), EC/SMC oxLDL uptake, SMC migration, and monocyte-EC adhesion. As a proof-of-concept for drug screening applications, we demonstrated the atheroprotective effects of vitamin D (1,25(OH)2D3) and metformin in mitigating cytokine-induced monocyte-EC adhesion and SMC migration. Overall, the developed arterial wall model facilitates quantitative and multi-factorial studies of EC and SMC phenotype in an atherogenic environment, and can be readily used as a platform technology to reconstitute multi-layered ECM tissue biointerfaces.

NiCHE Platform: Nature-Inspired Catechol-Conjugated Hyaluronic Acid Environment Platform for Salivary Gland Tissue Engineering

Recently, there has been growing interest in replacing severely damaged salivary glands with artificial salivary gland functional units created in vitro by tissue engineering approaches. Although various materials such as poly(lactic-co-glycolic acid), polylactic acid, poly(glycolic acid), and polyethylene glycol hydrogels have been used as scaffolds for salivary gland tissue engineering, none of them is effective enough to closely recapitulate the branched structural complexity and heterogeneous cell population of native salivary glands. Instead of discovering new biomaterial candidates, we synthesized hyaluronic acid-catechol (HACA) conjugates to establish a versatile hyaluronic acid coating platform named "NiCHE (nature-inspired catechol-conjugated hyaluronic acid environment)" for boosting the salivary gland tissue engineering efficacy of the previously reported biomaterials. By mimicking hyaluronic acid-rich niche in the mesenchyme of embryonic submandibular glands (eSMGs) with NiCHE coating on substrates including polycarbonate membrane, stiff agarose hydrogel, and polycaprolactone scaffold, we observed significantly enhanced cell adhesion, vascular endothelial and progenitor cell proliferation, and branching of in vitro-cultured eSMGs. High mechanical stiffness of the substrate is known to inhibit eSMG growth, but the NiCHE coating significantly reduced such stiffness-induced negative effects, leading to successful differentiation of progenitor cells to functional acinar and myoepithelial cells. These enhancement effects of the NiCHE coating were due to the increased proliferation of vascular endothelial cells via interaction between CD44 and surface-immobilized HAs. As such, our NiCHE coating platform renders any kind of material highly effective for salivary gland tissue culture by mimicking in vivo embryonic mesenchymal HA. Based on our results, we expect the NiCHE coating to expand the range of biomaterial candidates for salivary glands and other branching epithelial organs.

Developing a Clinically Relevant Tissue Engineered Heart Valve-A Review of Current Approaches

Tissue engineered heart valves (TEHVs) have the potential to address the shortcomings of current implants through the combination of cells and bioactive biomaterials that promote growth and proper mechanical function in physiological conditions. The ideal TEHV should be anti-thrombogenic, biocompatible, durable, and resistant to calcification, and should exhibit a physiological hemodynamic profile. In addition, TEHVs may possess the capability to integrate and grow with somatic growth, eliminating the need for multiple surgeries children must undergo. Thus, this review assesses clinically available heart valve prostheses, outlines the design criteria for developing a heart valve, and evaluates three types of biomaterials (decellularized, natural, and synthetic) for tissue engineering heart valves. While significant progress has been made in biomaterials and fabrication techniques, a viable tissue engineered heart valve has yet to be translated into a clinical product. Thus, current strategies and future perspectives are also discussed to facilitate the development of new approaches and considerations for heart valve tissue engineering.

Multiphase modelling of the effect of fluid shear stress on cell yield and distribution in a hollow fibre membrane bioreactor

We present a simplified two-dimensional model of fluid flow, nutrient transport and cell distribution in a hollow fibre membrane bioreactor, with the aim of exploring how fluid flow can be used to control the distribution and yield of a cell population which is sensitive to both fluid shear stress and nutrient concentration. The cells are seeded in a scaffold in a layer on top of the hollow fibre, only partially occupying the extracapillary space. Above this layer is a region of free-flowing fluid which we refer to as the upper fluid layer. The flow in the lumen and upper fluid layer is described by the Stokes equations, whilst the flow in the porous fibre membrane is assumed to follow Darcy’s law. Porous mixture theory is used to model the dynamics of and interactions between the cells, scaffold and fluid in the cell–scaffold construct. The concentration of a limiting nutrient (e.g. oxygen) is governed by an advection–reaction–diffusion equation in each region. Through exploitation of the small aspect ratio of each region and asymptotic analysis, we derive a coupled system of partial differential equations for the cell volume fraction and nutrient concentration. We use this model to investigate the effect of mechanotransduction on the distribution and yield of the cell population, by considering cases in which cell proliferation is either enhanced or limited by fluid shear stress and by varying experimentally controllable parameters such as flow rate and cell–scaffold construct thickness.

In vitro generation of atrioventricular heart valve neoscaffolds

Tissue engineering of cardiovascular structures represents a novel approach to improve clinical strategies in heart valve disease treatment. The aim of this study was to engineer decellularized atrioventricular heart valve neoscaffolds with an intact ultrastructure and to reseed them with umbilical cord-derived endothelial cells under physiological conditions in a bioreactor environment. Mitral (n=38) and tricuspid (n=36) valves were harvested from 40 hearts of German Landrace swine from a selected abattoir. Decellularization of atrioventricular heart valves was achieved by a detergent-based cell extraction protocol. Evaluation of the decellularization method was conducted with light microscopy and quantitative analysis of collagen and elastin content. The presence of residual DNA within the decellularized atrioventricular heart valves was determined with spectrophotometric quantification. The described decellularization regime produced full removal of native cells while maintaining the mechanical stability and the quantitative composition of the atrioventricular heart valve neoscaffolds. The surface of the xenogeneic matrix could be successfully reseeded with in vitro-expanded human umbilical cord-derived endothelial cells under physiological flow conditions. After complete decellularization with the detergent-based protocol described here, physiological reseeding of the xenogeneic neoscaffolds resulted in the formation of a confluent layer of human umbilical cord-derived endothelial cells. These results warrant further research toward the generation of atrioventricular heart valve neoscaffolds on the basis of decellularized xenogeneic tissue.

Regulation of vascular smooth muscle cell phenotype in three-dimensional coculture system by Jagged1-selective Notch3 signaling

The modulation of vascular smooth muscle cell (VSMC) phenotype is an essential element to fabricate engineered conduits of clinical relevance. In vivo, owing to their close proximity, endothelial cells (ECs) play a role in VSMC phenotype switching. Although considerable progress has been made in vascular tissue engineering, significant knowledge gaps exist on how the contractile VSMC phenotype is induced at the conclusion of the tissue fabrication process. The objectives of this study were as follows: (1) to establish ligand presentation modes on transcriptional activation of VSMC-specific genes, (2) to develop a three-dimensional (3D) coculture model using human coronary artery smooth muscle cells (HCASMCs) and human coronary artery endothelial cells (HCAECs) on porous synthetic scaffolds and, (3) to investigate EC-mediated Notch signaling in 3D cultures and the induction of the HCASMC contractile phenotype. Whereas transcriptional activation of VSMC-specific genes was not induced by presenting soluble Jagged1 and Jagged1 bound to protein G beads, a direct link between HCAEC-bound Jagged1 and HCASMC differentiation genes was observed. Our 3D culture results showed that HCASMCs seeded to scaffolds and cultured for up to 16 days readily attached, infiltrated the scaffold, proliferated, and formed dense confluent layers. HCAECs, seeded on top of an HCASMC layer, formed a distinct, separate monolayer with cell-type partitioning, suggesting that HCAEC growth was contact inhibited. While we observed EC monolayer formation with 200,000 HCAECs/scaffold, seeding 400,000 HCAECs/scaffold revealed the formation of cord-like structures akin to angiogenesis. Western blot analyses showed that 3D coculture induced an upregulation of Notch3 receptor in HCASMCs and its ligand Jagged1 in HCAECs. This was accompanied by a corresponding induction of the contractile HCASMC phenotype as demonstrated by increased expression of smooth muscle-α-actin (SM-α-actin) and calponin. Knockdown of Jagged1 with siRNA showed a reduction in SM-α-actin and calponin in cocultures, identifying a link between Jagged1 and the expression of contractile proteins in 3D cocultures. We therefore conclude that the Notch3 signaling pathway is an important regulator of VSMC phenotype and could be targeted when fabricating engineered vascular tissues.

The living aortic valve: From molecules to function

The aortic valve lies in a unique hemodynamic environment, one characterized by a range of stresses (shear stress, bending forces, loading forces and strain) that vary in intensity and direction throughout the cardiac cycle. Yet, despite its changing environment, the aortic valve opens and closes over 100,000 times a day and, in the majority of human beings, will function normally over a lifespan of 70–90 years. Until relatively recently heart valves were considered passive structures that play no active role in the functioning of a valve, or in the maintenance of its integrity and durability. However, through clinical experience and basic research the aortic valve can now be characterized as a living, dynamic organ with the capacity to adapt to its complex mechanical and biomechanical environment through active and passive communication between its constituent parts. The clinical relevance of a living valve substitute in patients requiring aortic valve replacement has been confirmed. This highlights the importance of using tissue engineering to develop heart valve substitutes containing living cells which have the ability to assume the complex functioning of the native valve.

New tools and new biology: recent miniaturized systems for molecular and cellular biology

Recent advances in applied physics and chemistry have led to the development of novel microfluidic systems. Microfluidic systems allow minute amounts of reagents to be processed using μm-scale channels and offer several advantages over conventional analytical devices for use in biological sciences: faster, more accurate and more reproducible analytical performance, reduced cell and reagent consumption, portability, and integration of functional components in a single chip. In this review, we introduce how microfluidics has been applied to biological sciences. We first present an overview of the fabrication of microfluidic systems and describe the distinct technologies available for biological research. We then present examples of microsystems used in biological sciences, focusing on applications in molecular and cellular biology.

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.

Impact of endothelial cells on 3D cultured smooth muscle cells in a biomimetic hydrogel

For the development of vascular tissue engineering, the impact of endothelial cells (ECs) on smooth muscle cell (SMC) spreading, proliferation, and differentiation is explored in the current study using a coculture model. In this coculture model, SMCs were encapsulated in a biomimetic hydrogel based on methacrylated dextran-graft-lysine (Dex-MA-LA) and methacrylamide-modified gelatin (Gel-MA), and exposed to a monolayer of ECs. With EC coculture, SMC proliferation in 3D hydrogel was promoted at initial period, and the formation of denser cellular networks was enhanced. ECs dynamically modulated SMC phenotype by promoting a more contractile SMC phenotype initially (on day 2), indicated by the upregulated expression of contractile genes α-actin, calponin, smooth muscle-myosin heavy chain (SM-MHC), and smoothelin; however, the onset of maximum expressions was delayed by ECs. Full differentiation of SMCs was not obtained even with EC coculture. Higher level of platelet-derived growth factor (PDGF)-BB and latent transforming growth factor (TGF)-β1 were detected in medium of coculture. These biochemical cues together with the physical cue of tensional force within cellular networks may be responsible for the dynamic modulation of SMC phenotype in coculture. Synthesis of elastin was promoted by ECs at transcriptional level. The formation of denser cellular networks and synthesis of elastin suggest that coculture with ECs is a potential method to construct functional vessel media layer in vitro.

Shear stress modulation of IL-1β-induced E-selectin expression in human endothelial cells

BACKGROUND

Endothelial cells (ECs) are continuously exposed to hemodynamic forces imparted by blood flow. While it is known that endothelial behavior can be influenced by cytokine activation or fluid shear, the combined effects of these two independent agonists have yet to be fully elucidated.

METHODOLOGY

We investigated EC response to long-term inflammatory cues under physiologically relevant shear conditions via E-selectin expression where monolayers of human umbilical vein ECs were simultaneously exposed to laminar fluid shear and interleukin-1ß (shear-cytokine activation) in a parallel plate flow chamber.

RESULTS AND CONCLUSION

Naïve ECs exposed to shear-cytokine activation display significantly higher E-selectin expression for up to 24 hr relative to ECs activated in static (static-cytokine). Peak E-selectin expression occurred after 8-12 hr of continuous shear-cytokine activation contrary to the commonly observed 4-6 hr peak expression in ECs exposed to static-cytokine activation. Cells with some history of high shear conditioning exhibited either high or muted E-selectin expression depending on the durations of the shear pre-conditioning and the ensuing shear-cytokine activation. Overall, the presented data suggest that a high laminar shear enhances acute EC response to interleukin-1ß in naïve or shear-conditioned ECs as may be found in the pathological setting of ischemia/reperfusion injury while conferring rapid E-selectin downregulation to protect against chronic inflammation.

Engineering airway epithelium

Airway epithelium is constantly presented with injurious signals, yet under healthy circumstances, the epithelium maintains its innate immune barrier and mucociliary elevator function. This suggests that airway epithelium has regenerative potential (I. R. Telford and C. F. Bridgman, 1990). In practice, however, airway regeneration is problematic because of slow turnover and dedifferentiation of epithelium thereby hindering regeneration and increasing time necessary for full maturation and function. Based on the anatomy and biology of the airway epithelium, a variety of tissue engineering tools available could be utilized to overcome the barriers currently seen in airway epithelial generation. This paper describes the structure, function, and repair mechanisms in native epithelium and highlights specific and manipulatable tissue engineering signals that could be of great use in the creation of artificial airway epithelium.

Endothelial cell behaviour within a microfluidic mimic of the flow channels of a modular tissue engineered construct

To study the effect of disturbed flow patterns on endothelial cells, the channels found within a modular tissue engineering construct were reproduced in a microfluidic chip and lined with endothelial cells whose resulting phenotype under flow was assessed using confocal microscopy. Modular tissue engineered constructs formed by the random packing of sub-millimetre, cylindrically shaped, endothelial cell-covered modules into a larger container creates interconnected channels that permit the flow of fluids such as blood. Due to the random packing, the flow path is tortuous and has the potential to create disturbed flow, resulting in an activated endothelium. At an average shear stress of 2.8 dyn cm⁻², endothelial cells within channels of varying geometries showed higher amounts of activation, as evidenced by an increase in ICAM-1 and VCAM-1 levels with respect to static controls. VE-cadherin expression also increased, however, it appeared discontinuous around the perimeter of the cells. An increase in flow (15.6 dyn cm⁻²) was sufficient to reduce ICAM-1 and VCAM-1 expression to a level below that of static controls for many disturbed flow-prone channels that contained branches, curves, expansions and contractions. VE-cadherin expression was also reduced and became discontinuous in all channels, possibly due to paracrine signaling. Other than showing a mild correlation to VE-cadherin, which may be linked through a cAMP-initiated pathway, KLF2 was found to be largely independent of shear stress for this system. To gauge the adhesiveness of the endothelium to leukocytes, THP-1 cells were introduced into flow-conditioned channels and their attachment measured. Relative to static controls, THP-1 adhesion was reduced in straight and bifurcating channels. However, even in the presence of flow, areas where multiple channels converged were found to be the most prone to THP-1 attachment. The microfluidic system enabled a full analysis of the effect of the tortuous flow expected in a modular construct on endothelial cell phenotype.

Endothelial Cell Vascular Smooth Muscle Cell Co-Culture Assay For High Throughput Screening Assays For Discovery of Anti-Angiogenesis Agents and Other Therapeutic Molecules

Drug development for many diseases would be aided greatly by accurate in vitro model systems that replicate key elements of in vivo physiology. The recent development of co-culture systems of endothelial cells and vascular smooth muscle cells can be extended to high throughput systems for the identification of compounds for angiogenesis, vascular repair and hypertension. In this review, the various co-culture systems are reviewed and biological interactions between endothelial cells and vascular smooth muscle cells are discussed. Key considerations in the design of high throughput systems are presented and selected examples are discussed.

Perfusion and characterization of an endothelial cell-seeded modular tissue engineered construct formed in a microfluidic remodeling chamber

Tissue engineered constructs containing tortuous endothelial cell-lined perfusion channels were formed by randomly assembling endothelial cell-seeded submillimeter-sized collagen cylinders (modules) into a microfluidic perfusion chamber. The interconnected void space produced by random module packing created flow channels that were lined with endothelial cells. The effect of perfusion (0.5 mL min(-1), Re( *) = 27.78 and shear stress = 0.16 dyn cm(-2)) through the tortuous channels on construct remodeling and endothelium quiescence was studied. Over time, modules fused at their points of contact and as they contracted, decreased the internal void space, which reduced the overall perfusion through the construct. As compared to static controls, perfusion caused a transient increase in activation (ICAM-1 and VCAM-1 expression) after 1 h followed by a decrease after 24 h. Proliferation (by BrdU) was reduced significantly, while KLF2, which is upregulated with atheroprotective laminar shear stress, was upregulated significantly after 24 h. VE-cadherin became discontinuous and was significantly downregulated after 24 h, which was likely caused by the dismantling of the endothelial cell adherens junctions during remodeling. Collectively, these outcomes suggest that flow through the construct did not drive the endothelial cells towards an inflamed, "atherosclerotic like" disturbed flow pathology.

Smooth muscle cells orchestrate the endothelial cell response to flow and injury

BACKGROUND

Local modulation of vascular mammalian target of rapamycin (mTOR) signaling reduces smooth muscle cell (SMC) proliferation after endovascular interventions but may be associated with endothelial cell (EC) toxicity. The trilaminate vascular architecture juxtaposes ECs and SMCs to enable complex paracrine coregulation but shields SMCs from flow. We hypothesized that flow differentially affects mTOR signaling in ECs and SMCs and that SMCs regulate mTOR in ECs.

METHODS AND RESULTS

SMCs and/or ECs were exposed to coronary artery flow in a perfusion bioreactor. We demonstrated by flow cytometry, immunofluorescence, and immunoblotting that EC expression of phospho-S6 ribosomal protein (p-S6RP), a downstream target of mTOR, was doubled by flow. Conversely, S6RP in SMCs was growth factor but not flow responsive, and SMCs eliminated the flow sensitivity of ECs. Temsirolimus, a sirolimus analog, eliminated the effect of growth factor on SMCs and of flow on ECs, reducing p-S6RP below basal levels and inhibiting endothelial recovery. EC p-S6RP expression in stented porcine arteries confirmed our in vitro findings: Phosphorylation was greatest in ECs farthest from intact SMCs in metal stented arteries and altogether absent after sirolimus stent elution.

CONCLUSIONS

The mTOR pathway is activated in ECs in response to luminal flow. SMCs inhibit this flow-induced stimulation of endothelial mTOR pathway. Thus, we now define a novel external stimulus regulating phosphorylation of S6RP and another level of EC-SMC crosstalk. These interactions may explain the impact of local antiproliferative delivery that targets SMC proliferation and suggest that future stents integrate design influences on flow and drug effects on their molecular targets.

Cellular and molecular regulation of spiral artery remodelling: lessons from the cardiovascular field

A number of important changes take place in the maternal uterine vasculature during the first few weeks of pregnancy resulting in increased blood flow to the intervillous space. Vascular endothelial and smooth muscle cells are lost from the spiral arteries and are replaced by fetal trophoblast cells. Failure of the vessels to remodel sufficiently is a common feature of pregnancy pathologies such as early pregnancy loss, intrauterine growth restriction and pre-eclampsia. There is evidence to suggest that some vascular changes occur prior to trophoblast invasion, however, in the absence of trophoblasts remodelling of the spiral arteries is reduced. Until recently our knowledge of these events has been obtained from immunohistochemical studies which, although extremely useful, can give little insight into the mechanisms involved. With the development of more complex in vitro models a picture of events at a cellular and molecular level is beginning to emerge, although some caution is required in extrapolating to the in vivo situation. Trophoblasts synthesise and release a plethora of cytokines and growth factors including members of the tumour necrosis factor family. Studies suggest that these factors may be important in regulating the remodelling process by inducing both endothelial and vascular smooth muscle cell apoptosis. In addition, it is evident from studies in other vascular beds that the structure of the vessel is influenced by factors such as flow, changes in the composition of the extracellular matrix, the phenotype of the vascular cells and the local immune cell environment. It is the aim of this review to present our current knowledge of the mechanisms involved in spiral artery remodelling and explore other possible pathways and cellular interactions that may be involved, informed by studies in the cardiovascular field.

Smooth muscle cell phenotype alters cocultured endothelial cell response to biomaterial-pretreated leukocytes

Model in vitro culturing systems were developed to analyze roles of biomaterial-induced leukocyte activation on endothelial cell (EC) and smooth muscle cell (SMC) phenotype, and their crosstalk. Isolated monocytes or neutrophils were pretreated with model biomaterial beads and applied directly to "more secretory" (cultured in media containing 5% fetal bovine serum) or forced contractile (serum and growth factor starved) human aortic SMCs (HASMCs), or to the human aortic EC (HAEC) surface of HAEC/HASMC cocultures (HASMC phenotype varied to be "more or less secretory") for 5 or 24 h of static culture. Surface expression of proinflammatory [ICAM-1, VCAM-1, E-selectin], procoagulant (tissue factor), and anticoagulant (thrombomodulin) markers, as well as HAEC proliferation, were assessed by flow cytometry. Incubation of HAEC with biomaterial-pretreated monocytes (and neutrophils to lesser degree) suppressed HAEC proliferation and induced a proinflammatory/procoagulant HAEC phenotype. This HAEC phenotype was amplified in coculture with "more secretory" HASMCs and subdued in coculture with "less secretory" HASMCs. Direct incubation of biomaterial-pretreated monocytes or neutrophils with "more secretory" HASMCs further increased HASMC ICAM-1 and tissue factor expression. Direct incubation of biomaterial-pretreated monocytes or neutrophils with forced contractile HASMCs upregulated ICAM-1, VCAM-1, and tissue factor expression above the presence of serum-containing media alone.

Computational modeling of coupled blood-wall mass transport of LDL: effects of local wall shear stress

The work herein represents a novel approach for the modeling of low-density lipoprotein (LDL) transport from the artery lumen into the arterial wall, taking into account the effects of local wall shear stress (WSS) on the endothelial cell layer and its pathways of volume and solute flux. We have simulated LDL transport in an axisymmetric representation of a stenosed coronary artery, where the endothelium is represented by a three-pore model that takes into account the contributions of the vesicular pathway, normal junctions, and leaky junctions also employing the local WSS to yield the overall volume and solute flux. The fraction of leaky junctions is calculated as a function of the local WSS based on published experimental data and is used in conjunction with the pore theory to determine the transport properties of this pathway. We have found elevated levels of solute flux at low shear stress regions because of the presence of a larger number of leaky junctions compared with high shear stress regions. Accordingly, we were able to observe high LDL concentrations in the arterial wall in these low shear stress regions despite increased filtration velocity, indicating that the increase in filtration velocity is not sufficient for the convective removal of LDL.

Ultrasound-induced angiogenic response in endothelial cells

Mechanical forces are known to affect endothelial cell (EC) function and can promote the formation of mature, muscular arterioles and arteries (arteriogenesis). The present study explores the possible angiogenic role of ultrasonic irradiation on EC phenotype using an in-vitro approach. Therapeutic ultrasound (TUS) stimulation at 1-MHz frequency was applied to bovine aortic endothelial cells (BAECs) in 2-D monolayer cultures and 3-D spheroid cultures. An angiogenic EC phenotype was characterized by the proliferation rate, migration, sprouting and Flk-1 expression in response to ultrasound stimulation. Irradiation lasting as long as 30 min caused a down-regulation and redistribution of Flk-1, increased EC proliferation rates and enhanced migration and sprouting in the 3-D spheroid cultures. The ultrasound-mediated EC stimulation in monolayers may be attributed to stable cavitation and micro-streaming, which are induced by pulsating microbubbles near the EC surface. Three-dimensional EC spheroid cultures surrounded by a highly viscous gel phase also exhibited ultrasound-induced angiogenic characteristics, although microbubbles may not participate in this response because of the impeding medium. The described in-vitro influence of low-intensity ultrasound on angiogenic EC phenotype has implications for TUS as a safe and controlled noninvasive stimulus for vascular regeneration.

Valvular endothelial cells and the mechanoregulation of valvular pathology

Endothelial cells are critical mediators of haemodynamic forces and as such are important foci for initiation of vascular pathology. Valvular leaflets are also lined with endothelial cells, though a similar role in mechanosensing has not been demonstrated. Recent evidence has shown that valvular endothelial cells respond morphologically to shear stress, and several studies have implicated valvular endothelial dysfunction in the pathogenesis of disease. This review seeks to combine what is known about vascular and valvular haemodynamics, endothelial response to mechanical stimuli and the pathogenesis of valvular diseases to form a hypothesis as to how mechanical stimuli can initiate valvular endothelial dysfunction and disease progression. From this analysis, it appears that inflow surface-related bacterial/thrombotic vegetative endocarditis is a high shear-driven endothelial denudation phenomenon, while the outflow surface with its related calcific/atherosclerotic degeneration is a low/oscillatory shear-driven endothelial activation phenomenon. Further understanding of these mechanisms may help lead to earlier diagnostic tools and therapeutic strategies.

Mechanopathobiology of Atherogenesis: A Review

Cardiovascular disease is the number one cause of mortality in the United States. Atherosclerosis, the primary etiology of cardiovascular disease is hypothesized to be a time-dependent response to arterial injury. Although risk factors for atherosclerosis are systemic in nature, certain arteries (e.g., coronary arteries) are more susceptible to plaque formation than others. The heterogeneous distribution of atherosclerosis in the vasculature is thought to be related to biomechanical factors. A review of the relevant pathological features of atherogenesis and how physiologically-consistent mechanical stimuli can impact those processes supports this notion. However, specific investigations geared toward finding the mechanistic link between mechanical stimuli and early atherogenic processes are required to differentiate those stimuli that facilitate and those that inhibit atherogenesis. Such knowledge is required for intelligent direction in the search for potential targets for clinical intervention.

Endothelial connexin 37, connexin 40, and connexin 43 respond uniquely to substrate and shear stress

Endothelial connexins have been linked to atherosclerosis and hypertension; however, little is know about their sensitivity to stimuli and individual functions. This study investigates the responses of endothelial connexin 37, connexin 40, and connexin 43 (Cx37, Cx40, and Cx43) to shear stress and substrate. Human endothelial cells were seeded on adsorbed collagen or a collagen gel containing smooth muscle cells and exposed to static or laminar shear stress. Connexin mRNA, protein, and gap junction communication were examined. Endothelial monolayers were treated with connexin-specific short interfering RNA (siRNA) and evaluated for communication, proliferation, and morphology under static and shear stress. Results show differential responses of Cx37, Cx40, and Cx43 to substrate and shear stress with reduced communication after shear exposure. RNA interference of individual connexins resulted in expression change of nontarget connexins, which suggests linked expression. Gap junction communication under static conditions is reduced following Cx43 siRNA treatment. Endothelial cells are more elongated with RNA interference (RNAi) targeting Cx40. In conclusion, endothelial connexins demonstrated novel sensitivity to mechanical environment and substrate. Individual isotypes show differential responses and RNAi knockdown provides new insight into connexin function and potential roles in the vasculature.

Complimentary endothelial cell/smooth muscle cell co-culture systems with alternate smooth muscle cell phenotypes

Development of in vitro models of native and injured vasculature is crucial for better understanding altered wound healing in disease, device implantation, or tissue engineering. Conditions were optimized using polyethyleneteraphalate transwell filters for human aortic endothelial cell (HAEC)/smooth muscle cell (HASMC) co-cultures with divergent HASMC phenotypes ('more or less secretory') while maintaining quiescent HAECs. Resulting HASMC phenotype was studied at 48 and 72 h following co-culture initiation, and compared to serum and growth factor starved monocultured 'forced contractile' HASMCs. Forced contractile HASMCs demonstrated organized alpha-smooth muscle actin filaments, minimal interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1) secretion, and low intracellular cell adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and tissue factor expression. Organization of alpha-smooth muscle actin was lost in 'more secretory' HASMCs in co-culture with HAECs, and IL-8 and MCP-1 secretion, as well as ICAM-1, VCAM-1, and tissue factor expression were significantly upregulated at both time points. Alternately, 'less secretory' HASMCs in co-culture with HAECs showed similar characteristics to forced contractile HASMCs at the 48 h time point, while by the 72 h time point they behaved similarly to 'more secretory' HASMCs. These co-culture systems could be useful in better understanding vascular healing, however there remain time constraint considerations for maintaining culture integrity/cell phenotype.

Preclinical testing of tissue-engineered heart valves re-endothelialized under simulated physiological conditions

BACKGROUND

The in vivo regeneration capacity of decellularized heart valve grafts is still controversial. The aim of this study was to evaluate function, morphological changes, and cellular composition of decellularized versus re-endothelialized ovine pulmonary valves (PV) after implantation into lambs for 1 or 3 months.

METHODS AND RESULTS

PV (n=21) were decellularized using detergents. Twelve PV were repopulated with autologous jugular veins endothelial cells (ECs) in a dynamic pulsatile bioreactor under simulated physiological conditions. Morphological evaluation before implantation included histological stainings (H&E, Movat-pentachrome, von-Kossa, DAPI), immunostainings (anti-perlecan, anti-eNOS, anti-procollagen-I, anti-SM-alpha-actin), electron microscopy (EM), and DNA extraction. Decellularization led to cell-free scaffolds with preserved extracellular matrix (ECM) including basement membrane. Reseeded PV (n=5) were completely covered with ECs expressing endothelial nitric oxide synthase (eNOS) and von Willebrand factor (vWF). The function of orthotopically implanted decellularized and re-endothelialized PV (n=7, each) was analyzed after 1 and 3 months by echocardiography and revealed no differences in competence between both groups. A confluent EC monolayer expressing eNOS/vWF was only found in re-endothelialized PV but not in decellularized PV, whereas the valve matrices were comparable repopulated with interstitial cells expressing SM-alpha-actin and procollagen-I. More thrombotic and neointima formations were observed in decellularized PV. No signs of calcification were detected in both PV types.

CONCLUSIONS

In vitro re-endothelialization of detergent-decellularized valves with autologous ECs under simulated physiological conditions significantly improves total EC valve coverage 3 months after implantation, whereas the valve repopulation with interstitial cells in vivo occurs most likely by cell migration inside the scaffold.

In vitro re-endothelialization of detergent decellularized heart valves under simulated physiological dynamic conditions

The production of viable biological heart valves is of central interest in tissue engineering (TE). The aim of this study was to generate decellularized heart valves with an intact ultra-structure and to repopulate these with endothelial cells (EC) under simulated physiological conditions. Decellularization of ovine pulmonary valve conduits was performed under agitation in detergents followed by six wash cycles. Viability of EC cultures exposed to washing solution served to prove efficiency of washing. Resulting scaffolds were free of cells with preserved extracellular matrix. Biomechanical standard tension tests demonstrated comparable parameters to native tissue. Luminal surfaces of decellularized valvular grafts were seeded with ovine jugular vein EC in dynamic bioreactors. After rolling culture for 48 h, pulsatile medium circulation with a flow of 0.1 L/min was started. The flow was incremented 0.3 L/min/day up to 2.0 L/min (cycle rate: 60 beats/min), while pH, pO2, pCO2, lactate and glucose were maintained at constant physiological levels. After 7 days, a monolayer of cells covered the inner valve surface, which expressed vWF, indicating an endothelial origin. A complete endothelialization of detergent decellularized scaffold can be achieved under simulated physiological circulation conditions using a dynamic bioreactor system, which allows continuous control of the culture environment.

Bioreactors for tissue mass culture: Design, characterization, and recent advances

This paper reviews reports on three-dimensional mammalian tissue growth in bioreactors and the corresponding mammalian tissue growth requirements. The needs for nutrient and waste removal of several mammalian tissues are reviewed and compared with the environment of many reactors currently in use such as the continuous stirred tank, the hollow fiber, the Couette-Taylor, the airlift, and the rotating-wall reactors developed by NASA. Many studies conclude that oxygen supply appears to be one of the most important factors limiting tissue growth. Various correlations to describe oxygen mass transfer are presented and discussed with the aim to provide some guidance to design, construct, and test reactors for tissue mass culture. To obtain tissue thickness clinically valuable, dimensionless and other types of analysis tend to point out that diffusive transport will have to be matched with an important convection to bring sufficient oxygen molecular flux to the growing cells located within a tissue mass. As learned from solid-state fermentation and hairy root culture, during the growth of large biomass, heterogeneity (i.e., channeling, temperature gradients, non-uniform cell growth, transfer gradients, etc.) can cause some important problems and these should be addressed in tissue engineering as well. Reactors (along with the scaffolds) should be designed to minimize these issues. The role of the uterus, the reactor built by Nature, is examined, and the environment provided to a growing embryo is reported, yielding possible paths for further reactor developments. Finally, the importance of cell seeding methods is also addressed.

Endothelial cell-smooth muscle cell co-culture in a perfusion bioreactor system

Vascular endothelial cells (EC) are exposed to a complex biomechanical environment in vivo and are responsible for relaying important messages to the underlying tissue. EC and smooth muscle cells (SMC) communicate to regulate vascular development and function. In this work, a vascular perfusion bioreactor is used to grow tubular constructs seeded with EC and SMC under pulsatile shear stress in long-term co-culture to study the effects of EC on SMC function. SMC seeded into porous poly(glycolic acid) tubular scaffolds are cultured in the bioreactor for 25 days. Constructs are seeded with EC on day 10 or day 23 creating 2-day (short-term) or 15-day (long-term) EC and SMC co-cultures. Long-term EC-SMC co-culture significantly increases cell proliferation and downregulates collagen and proteoglycan deposition compared to short-term co-culture. After 25 days of culture, 15-day co-culture constructs have a more uniform cell distribution across the construct thickness and SMC express a more contractile phenotype compared to 2-day co-culture constructs. These data demonstrate strong interactions between SMC and EC in the bioreactor under physiologically relevant conditions. Thus, the vascular construct perfusion bioreactor is an important tool to investigate cell-cell and cell-extracellular matrix interactions in vascular cell biology and tissue engineering.

Mechanical, biochemical, and extracellular matrix effects on vascular smooth muscle cell phenotype

The vascular smooth muscle cell (VSMC) is surrounded by a complex extracellular matrix that provides and modulates a variety of biochemical and mechanical cues that guide cell function. Conventional two-dimensional monolayer culture systems recreate only a portion of the cellular environment, and therefore there is increasing interest in developing more physiologically relevant three-dimensional culture systems. This review brings together recent studies on how mechanical, biochemical, and extracellular matrix stimulation can be applied to study VSMC function and how the combination of these factors leads to changes in phenotype. Particular emphasis is placed on in vitro experimental studies in which multiple stimuli are combined, especially in three-dimensional culture systems and in vascular tissue engineering applications. These studies have provided new insight into how VSMC phenotype is controlled, and they have underscored the interdependence of biochemical and mechanical signaling. Future improvements in creating more complex in vitro culture environments will lead to a better understanding of VSMC biology, new treatments for vascular disease, as well as improved blood vessel substitutes.

Intermittent short-duration exposure to low wall shear stress induces intimal thickening in arteries exposed to chronic high shear stress

We sought to determine whether intermittent short-duration exposure to low wall shear stress could induce intimal thickening in arteries chronically exposed to high shear stress. An arteriovenous fistula (AVF) was created between the left common carotid artery and the corresponding external jugular vein in 20 Japanese white male rabbits. After 4 weeks, blood flow was increased 10-fold to 182 +/- 39 ml/min and shear stress was increased to 33.4 +/- 13 dyn/cm(2). The AVF was then occluded for 1 h by finger compression with an 85% reduction in carotid artery blood flow (27 +/- 7 ml/min) and a reduction in wall shear stress to 4.9 +/- 1.7 dyn/cm(2) (P < 0.0001). Release of finger compression restored flow to the AVF and high shear stress to the carotid artery. This procedure was repeated at weekly intervals with a cumulative total of 4 h of low shear stress exposure. Arteries exposed to intermittent low shear stress developed a layer of intimal thickening which consisted of 3-4 layers of smooth muscle cells lined with thin elastic fibers and medial hyperplasia. Control arteries exposed to 8 weeks of continuous high shear had no intimal thickening. Transient exposure to low shear stress upregulated TGF-beta1, MMP-2, -14, and TIMP-2 gene expression while MMP-9 expression was downregulated. We conclude that repeated, intermittent short-duration exposure to low shear stress in the setting of high flow and high shear stress can induce arterial intimal thickening. Short-duration alterations in hemodynamic forces can induce rapid vascular cell message expression, which may effect arterial remodeling. This experiment suggests that a threshold value of 5 dyn/cm(2) may be needed in order to initiate and sustain the intimal thickening response.

Roles of Hemodynamic Forces in Vascular Cell Differentiation

The pulsatile nature of blood flow is a key stimulus for the modulation of vascular cell differentiation. Within the vascular media, physiologic stress is manifested as cyclic strain, while in the lumen, cells are subjected to shear stress. These two respective biomechanical forces influence the phenotype and degree of differentiation or proliferation of smooth muscle cells and endothelial cells within the human vasculature. Elucidation of the effect of these mechanical forces on cellular differentiation has led to a surge of research into this area because of the implications for both the treatment of atherosclerotic disease and the future of vascular tissue engineering. The use of mechanical force to directly control vascular cell differentiation may be utilized as an invaluable engineering tool in the future. However, an understanding of the role of hemodynamics in vascular cell differentiation and proliferation is critical before application can be realized. Thus, this review will provide a current perspective on the latest research and controversy behind the role of hemodynamic forces for vascular cell differentiation and phenotype modulation. Furthermore, this review will illustrate the application of hemodynamic force for vascular tissue engineering and explicate future directions for research.

Shear Stress Inhibits Smooth Muscle Cell–Induced Inflammatory Gene Expression in Endothelial Cells

Objectives— Vascular endothelial cells (ECs) are influenced by shear stress and neighboring smooth muscle cells (SMCs). We investigated the inflammation-relevant gene expression in EC/SMC cocultures under static condition and in response to shear stress.

Materials and Methods— Under static condition, DNA microarrays and reverse-transcription polymerase chain reaction identified 23 inflammation-relevant genes in ECs whose expression was significantly affected by coculture with SMCs, with 18 upregulated and 5 downregulated. Application of shear stress (12 dynes/cm 2 ) to the EC side of the coculture for 6 hours inhibited most of the proinflammatory gene expressions in ECs induced by coculture with SMCs. Inhibition of nuclear factor-κB (NF-κB) activation by the p65-antisense, lactacystin, and N-acetyl-cysteine blocked the coculture-induced EC expression of proinflammatory genes, indicating that the NF-κB binding sites in the promoters of these genes play a significant role in their expression as a result of coculture with SMCs. Chromatin immunoprecipitation assays demonstrated the in vivo regulation of NF-κB recruitment to selected target promoters. Shear stress inhibited the SMC coculture-induced NF-κB activation in ECs and monocytic THP-1 cell adhesion to ECs.

Conclusions— Our findings suggest that shear stress plays an inhibitory role in the proinflammatory gene expression in ECs located in close proximity to SMCs.

Controlling the cellular organization of tissue-engineered cardiac constructs

There are currently no effective treatments to restore the cardiac muscle lost because of ischemia for the millions of people who suffer heart attacks annually. Cell therapy procedures have emerged as novel therapeutic strategies for treatment of heart failure after myocardial infarction but have been hampered by the lack of adequate cell sources of cardiomyocytes and by the inability to integrate cell grafts into cardiac muscle. A cardiac patch composed of organized and functional cardiomyocytes could drastically enhance the efficacy of this important clinical approach. Here, we report our ongoing efforts to develop a bioartificial cardiac muscle capable of synchronized multidirectional contraction within a three-dimensional hydrogel scaffold. Neonatal rat cardiomyocytes, smooth muscle cells, and reconstituted polymeric collagen enriched with growth factors and hormones are used. A bioreactor system is used to impart precise strains onto the developing tissue constructs in vitro. The results demonstrate that cell-mediated collagen compaction is significantly enhanced by strain preconditioning, resulting in a more favorable cellular organization. Furthermore, the results demonstrate that strain stimulation guides cellular orientation in the direction of applied strain (i.e., in the circumferential direction). Hence, we demonstrate the importance of mechanical preconditioning as a means of promoting the in vitro development of engineered cardiac muscle for use with myocardial regeneration therapies.

Vascular Smooth Muscle Cells on Hyaluronic Acid: Culture and Mechanical Characterization of an Engineered Vascular Construct

Esterified hyaluronic acid (HYAFF) is routinely used for clinical tissue-engineering applications such as skin and cartilage. The material is degraded by neotissue formation and degradation products are highly biocompatible. In the present article we investigate the possibility to culture vascular smooth muscle cells on this biodegradable material for the generation of tubular constructs to be used for vascular tissue engineering. We have evaluated cell attachment and growth, and the possibility to obtain a three-dimensional tubular shape culture from flat HYAFF sheets. We also evaluated the mechanical properties of the cell constructs, using a specific testing protocol, and compared them with the properties of segments of porcine coronary artery. Morphology and viability tests demonstrated that vascular cells, either from porcine or human origin, adhere and grow on nonwoven meshes of HYAFF, and that precoating of the material with fibronectin or collagen had a modest effect on cell growth and extracellular matrix production. Cell growth reached a maximum 7 days after seeding. Simple wrapping of flat sheets of nonwoven meshes containing vascular cells around a cylindrical mandrel, and culture under static conditions for 14 days, yielded tubular constructs suitable for mechanical tests. Despite cell colonization, constructs showed lower mechanical resistance as compared with porcine coronary arteries. The material used and the technique developed result in highly cellularized tubular constructs. Whether the mechanical properties may be improved by dynamic culture conditions is worthy of investigation.