Shear stress protects against endothelial regulation of vascular smooth muscle cell migration in a coculture system

Inferior Vena Cava Filter Retrieval: Simple to Complex

Retrievable inferior vena cava filters were designed to provide mechanical prophylaxis from pulmonary embolism with the intent for retrieval once no longer indicated, and have been utilized at a high rate since their introduction. Unfortunately, retrieval rates have historically lagged behind, in part due to significant rates of failed standard retrieval techniques for filters with prolonged dwell time. Refinement in advanced retrieval techniques has now allowed (in experienced centers) for safe removal of filters previously considered irretrievable. An individualized approach is necessary for each potential advanced filter retrieval to determine appropriate course of action. This review will emphasize complex filter retrieval techniques amidst a larger discussion of inferior vena cava filters and their management.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Effects of shear stress on vascular endothelial functions in atherosclerosis and potential therapeutic approaches

Different blood flow patterns in the arteries can alter the adaptive phenotype of vascular endothelial cells (ECs), thereby affecting the functions of ECs and are directly associated with the occurrence of lesions in the early stages of atherosclerosis (AS). Atherosclerotic plaques are commonly found at curved or bifurcated arteries, where the blood flow pattern is dominated by oscillating shear stress (OSS). OSS can induce ECs to transform into pro-inflammatory phenotypes, increase cellular inflammation, oxidative stress response, mitochondrial dysfunction, metabolic abnormalities and endothelial permeability, thereby promoting the progression of AS. On the other hand, the straight artery has a stable laminar shear stress (LSS), which promotes the transformation of ECs into an anti-inflammatory phenotype, improves endothelial cell function, thereby inhibits atherosclerotic progression. ECs have the ability to actively sense, integrate, and convert mechanical stimuli by shear stress into biochemical signals that further induces intracellular changes (such as the opening and closing of ion channels, activation and transcription of signaling pathways). Here we not only outline the relationship between functions of vascular ECs and different forms of fluid shear stress in AS, but also aim to provide new solutions for potential atherosclerotic therapies targeting intracellular mechanical transductions.

A Review of Functional Analysis of Endothelial Cells in Flow Chambers

The vascular endothelial cells constitute the innermost layer. The cells are exposed to mechanical stress by the flow, causing them to express their functions. To elucidate the functions, methods involving seeding endothelial cells as a layer in a chamber were studied. The chambers are known as parallel plate, T-chamber, step, cone plate, and stretch. The stimulated functions or signals from endothelial cells by flows are extensively connected to other outer layers of arteries or organs. The coculture layer was developed in a chamber to investigate the interaction between smooth muscle cells in the middle layer of the blood vessel wall in vascular physiology and pathology. Additionally, the microfabrication technology used to create a chamber for a microfluidic device involves both mechanical and chemical stimulation of cells to show their dynamics in in vivo microenvironments. The purpose of this study is to summarize the blood flow (flow inducing) for the functions connecting to endothelial cells and blood vessels, and to find directions for future chamber and device developments for further understanding and application of vascular functions. The relationship between chamber design flow, cell layers, and microfluidics was studied.

Novel elasticity measurements reveal C. elegans cuticle stiffens with age and in a long-lived mutant

Changes in biomechanical properties have profound impacts on human health. C. elegans might serve as a model for studying the molecular genetics of mammalian tissue decline. Previously, we found that collagens are required for insulin signaling mutants' long lifespan and that overexpression of specific collagens extends wild-type lifespan. However, whether these effects on lifespan are due to mechanical changes during aging has not yet been established. Here, we have developed two novel methods to study the cuticle: we measure mechanical properties of live animals using osmotic shock, and we directly perform the tensile test on isolated cuticles using microfluidic technology. Using these tools, we find that the cuticle, not the muscle, is responsible for changes in the "stretchiness" of C. elegans, and that cuticle stiffness is highly nonlinear and anisotropic. We also found that collagen mutations alter the integrity of the cuticle by significantly changing the elasticity. In addition, aging stiffens the cuticle under mechanical loads beyond the cuticle's healthy stretched state. Measurements of elasticity showed that long-lived daf-2 mutants were considerably better at preventing progressive mechanical changes with age. These tests of C. elegans biophysical properties suggest that the cuticle is responsible for their resilience.

miR-25-5p regulates endothelial progenitor cell differentiation in response to shear stress through targeting ABCA1

The importance of flow shear stress (SS) on the differentiation of endothelial progenitor cells (EPCs) has been demonstrated in various studies. Cholesterol retention and microRNA regulation have been also proposed as relevant factors involved in this process, though evidence regarding their regulatory roles in the differentiation of EPCs is currently lacking. In the present study on high shear stress (HSS)-induced differentiation of EPCs, we investigated the importance of ATP-binding cassette transporter 1 (ABCA1), an important regulator in cholesterol efflux, and miR-25-5p, a potential regulator of endothelial reconstruction. We first revealed an inverse correlation between miR-25-5p and ABCA1 expression levels in EPCs under HSS treatment; their direct interaction was subsequently validated by a dual-luciferase reporter assay. Further studies using flow cytometry and quantitative polymerase chain reaction demonstrated that both miR-25-5p overexpression and ABCA1 inhibition led to elevated levels of specific markers of endothelial cells, with concomitant downregulation of smooth muscle cell markers. Finally, knockdown of ABCA1 in EPCs significantly promoted tube formation, which confirmed our conjecture. Our current results suggest that miR-25-5p might regulate the differentiation of EPCs partially through targeting ABCA1, and such a mechanism might account for HSS-induced differentiation of EPCs.

Platelet-derived microvesicles induce calcium oscillations and promote VSMC migration via TRPV4

Rationale: Abnormal migration of vascular smooth muscle cells (VSMCs) from the media to the interior is a critical process during the intimal restenosis caused by vascular injury. Here, we determined the role of platelet-derived microvesicles (PMVs) released by activated platelets in VSMC migration. Methods: A percutaneous transluminal angioplasty balloon dilatation catheter was used to establish vascular intimal injury. Collagen I was used to activate PMVs, mimicking collagen exposure during intimal injury. To determine the effects of PMVs on VSMC migration in vitro, scratch wound healing assays were performed. Fluorescence resonance energy transfer was used to detect variations of calcium dynamics in VSMCs. Results: Morphological results showed that neointimal hyperplasia was markedly increased after balloon injury of the carotid artery in rats, and the main component was VSMCs. PMVs significantly promoted single cell migration and wound closure in vitro. Fluorescence resonance energy transfer revealed that PMVs induced temporal and dynamic calcium oscillations in the cytoplasms of VSMCs. The influx of extracellular calcium, but not calcium from intracellular stores, was involved in the process described above. The channel antagonist GSK219 and specific siRNA revealed that a membrane calcium channel, transient receptor potential vanilloid 4 (TRPV4), participated in the calcium oscillations and VSMC migration induced by PMVs. Conclusions: TRPV4 participated in the calcium oscillations and VSMC migration induced by PMVs. PMVs and the related molecules might be novel therapeutic targets for vascular remodeling during vascular injury.

Endothelial/Epithelial Mesenchymal Transition in Ascending Aortas of Patients With Bicuspid Aortic Valve

Thoracic aortic aneurysm (TAA) is the progressive enlargement of the aorta due to destructive changes in the connective tissue of the aortic wall. Aneurysm development is silent and often first manifested by the drastic events of aortic dissection or rupture. As yet, therapeutic agents that halt or reverse the process of aortic wall deterioration are absent, and the only available therapeutic recommendation is elective prophylactic surgical intervention. Being born with a bicuspid instead of the normal tricuspid aortic valve (TAV) is a major risk factor for developing aneurysm in the ascending aorta later in life. Although the pathophysiology of the increased aneurysm susceptibility is not known, recent studies are suggestive of a transformation of aortic endothelium into a more mesenchymal state i.e., an endothelial-to-mesenchymal transition in these individuals. This process involves the loss of endothelial cell features, resulting in junction instability and enhanced vascular permeability of the ascending aorta that may lay the ground for increased aneurysm susceptibility. This finding differentiates and further emphasizes the specific characteristics of aneurysm development in individuals with a bicuspid aortic valve (BAV). This review discusses the possibility of a developmental fate shared between the aortic endothelium and aortic valves. It further speculates about the impact of aortic endothelium phenotypic shift on aneurysm development in individuals with a BAV and revisits previous studies in the light of the new findings.

Biofabrication of thick vascularized neo-pedicle flaps for reconstructive surgery

Wound chronicity due to intrinsic and extrinsic factors perturbs adequate lesion closure and reestablishment of the protective skin barrier. Immediate and proper care of chronic wounds is necessary for a swift recovery and a reduction of patient vulnerability to infection. Advanced therapies supplemented with standard wound care procedures have been clinically implemented to restore aberrant tissue; however, these treatments are ineffective if local vasculature is too compromised to support minimally-invasive strategies. Autologous "flaps", which are tissues equipped with their own hierarchical vascular supply, can be harvested from one region of the patient and transplanted to the wound where it is reperfused upon microsurgical anastomosis to appropriate recipient vessels. Despite the success of autologous flap transfer, these procedures are extremely invasive, incur obligatory donor-site morbidity, and require sufficient donor-tissue availability, microsurgical expertise, and specialized equipment. 3D-bioprinting modalities, such as extrusion-based bioprinting, can be used to address the clinical constraints of autologous flap transfer, primarily addressing donor-site morbidity and tissue availability. This advancement in regenerative medicine allows the biofabrication of heterogeneous tissue structures with high shape fidelity and spatial resolution to generate biomimetic constructs with the anatomically-precise geometries of native tissue to ensure tissue-specific function. Yet, meaningful progress toward this clinical application has been limited by the lack of vascularization required to meet the nutrient and oxygen demands of clinically relevant tissue volumes. Thus, various criteria for the fabrication of functional tissues with hierarchical, patent vasculature must be considered when implementing 3D-bioprinting technologies for deep, chronic wounds.

Influence of TGF-β1 expression in endothelial cells on smooth muscle cell phenotypes and MMP production under shear stress in a co-culture model

Recently, our group has contrasted an endothelial cell-smooth muscle cell (EC-SMC) co-culture model with 3D-cultured SMCs and found that SMCs could respond to high shear stress (SS), which has not been explored before. SMCs were not directly exposed to the flow but were under an EC monolayer; therefore, it is necessary to explore the influence of EC on SMC behaviors under high SS for understanding the mechanism of SMC response to various magnitudes of SS. In the present study, TGF-β1 expression in ECs in an EC-SMC co-culture model was suppressed by an siRNA transfection method. Next, phenotypic changes were observed and MMP-2 and -9 productions were measured in SMCs in the co-culture model after 72-h flow exposure to different SS levels. We confirmed that TGF-β1 expression in ECs could influence SMC phenotypic change under SS conditions and that TGF-β1 expression in ECs could also change MMP-2 production but not MMP-9 production in SMCs under SS conditions in the co-culture model. These results could be useful for understanding the mechanisms of SMC response to SS, particularly for understanding signal transduction emanating from ECs.

Mechanobiology in vascular remodeling

Vascular remodeling is a common pathological process in cardiovascular diseases and includes changes in cell proliferation, apoptosis and differentiation as well as vascular homeostasis. Mechanical stresses, such as shear stress and cyclic stretch, play an important role in vascular remodeling. Vascular cells can sense the mechanical factors through cell membrane proteins, cytoskeletons and nuclear envelope proteins to initiate mechanotransduction, which involves intercellular signaling, gene expression, and protein expression to result in functional regulations. Non-coding RNAs, including microRNAs and long non-coding RNAs, are involved in the regulation of vascular remodeling processes. Mechanotransduction triggers a cascade reaction process through a complicated signaling network in cells. High-throughput technologies in combination with functional studies targeting some key hubs and bridging nodes of the network can enable the prioritization of potential targets for subsequent investigations of clinical translation. Vascular mechanobiology, as a new frontier field of biomechanics, searches for principles of stress-growth in vasculature to elucidate how mechanical factors induce biological effects that lead to vascular remodeling, with the goal of understanding the mechanical basis of the pathological mechanism of cardiovascular diseases at the cellular and molecular levels. Vascular mechanobiology will play a unique role in solving the key scientific problems of human physiology and disease, as well as generating important theoretical and clinical results.

Retrievable IVC Filters: Comprehensive Review of Device-related Complications and Advanced Retrieval Techniques

Use of retrievable inferior vena cava filters (RIVCFs) has grown exponentially since their introduction into clinical practice, but many of these devices are not retrieved. Some are not retrieved due to poor clinical follow-up, but other devices remain in situ for extended periods because they present significant technical challenges during retrieval. Because of these and other factors, many of these devices were thus left in place permanently. However, recent data have placed a renewed emphasis on device retrieval due to increased risk of RIVCF-related complications, which are positively correlated with filter dwell time. Development of advanced filter retrieval techniques has had a significant impact on the removal of embedded RIVCFs, permitting retrieval of the majority of devices. The purpose of this article is to present an imaging and data review of the dominant device-related factors that complicate RIVCF retrieval and to describe the relevant advanced retrieval techniques to manage these factors. RIVCF imaging is frequently encountered in daily clinical practice via various imaging modalities. Therefore, diagnostic radiologists can play a vital role in identifying filter-related issues. Familiarity with the context for managing these issues in the interventional suite is essential for improving triage and care of patients with RIVCFs. ^© RSNA, 2017.

Tumacrophage: macrophages transformed into tumor stem-like cells by virulent genetic material from tumor cells

Tumor-associated macrophages are regarded as tumor-enhancers as they have key roles in the subversion of adaptive immunity and in inflammatory circuits that promote tumor progression. Here, we show that cancer cells can subvert macrophages yielding cells that have gained pro-tumor functions. When macrophages isolated from mice or humans are co-cultured with dead cancer cell line cells, induced to undergo apoptosis to mimic chemotherapy, up-regulation of pro-tumor gene expression was identified. Phagocytosis of apoptotic cancer cells by macrophages resulted in their transformation into tumor stem (initiating)-like cells, as indicated by the expression of epithelial markers (e.g., cytokeratin) and stem cell markers (e.g., Oct4) and their capability to differentiate in vitro and self-renew in serum-free media. Moreover, we identified a subset of monocytes/macrophages cells in the blood of cancer (breast, ovarian and colorectal) patients undergoing chemotherapy that harbor tumor transcripts. Our findings uncover a new role for macrophages in tumor development, where they can be transformed into tumor-like cells, potentially by horizontal gene transfer of tumor-derived genes, thus, by taking advantage of chemotherapy, these transformed macrophages promote tumor metastasis by escaping immune surveillance.

Effects of shear stress on endothelial cells: go with the flow

Haemodynamic forces influence the functional properties of vascular endothelium. Endothelial cells (ECs) have a variety of receptors, which sense flow and transmit mechanical signals through mechanosensitive signalling pathways to recipient molecules that lead to phenotypic and functional changes. Arterial architecture varies greatly exhibiting bifurcations, branch points and curved regions, which are exposed to various flow patterns. Clinical studies showed that atherosclerotic plaques develop preferentially at arterial branches and curvatures, that is in the regions exposed to disturbed flow and shear stress. In the atheroprone regions, the endothelium has a proinflammatory phenotype associated with low nitric oxide production, reduced barrier function and increased proadhesive, procoagulant and proproliferative properties. Atheroresistant regions are exposed to laminar flow and high shear stress that induce prosurvival antioxidant signals and maintain the quiescent phenotype in ECs. Indeed, various flow patterns contribute to phenotypic and functional heterogeneity of arterial endothelium whose response to proatherogenic stimuli is differentiated. This may explain the preferential development of endothelial dysfunction in arterial sites with disturbed flow.

Immunomodulatory polymeric scaffold enhances extracellular matrix production in cell co-cultures under dynamic mechanical stimulation

Despite the importance of immune cells in regulating the wound healing process following injury, there are few examples of synthetic biomaterials that have the capacity to push the body's immune cells toward pro-regeneration phenotypes, and fewer still that are designed with the intention of achieving this immunomodulatory character. While monocytes and their derived macrophages have been recognized as important contributors to tissue remodeling in vivo, this is primarily believed to be due to their ability to regulate other cell types. The ability of monocytes and macrophages to generate tissue products themselves, however, is currently not well appreciated within the field of tissue regeneration. Furthermore, while monocytes/macrophages are found in remodeling tissue that is subjected to mechanical loading, the effect this biomechanical strain on monocytes/macrophages and their ability to regulate tissue-specific cellular activity has not been understood due to the complexity of the many factors involved in the in vivo setting, hence necessitating the use of controlled in vitro culture platforms to investigate this phenomenon. In this study, human monocytes were co-cultured with human coronary artery smooth muscle cells (VSMCs) on a tubular (3mm ID) degradable polyurethane scaffold, with a unique combination of non-ionic polar, hydrophobic and ionic chemistry (D-PHI). The goal was to determine if such a synthetic matrix could be used in a co-culture system along with dynamic biomechanical stimulus (10% circumferential strain, 1Hz) conditions in order to direct monocytes to enhance tissue generation, and to better comprehend the different ways in which monocytes/macrophages may contribute to new tissue production. Mechanical strain and monocyte co-culture had a complementary and non-mitigating effect on VSMC growth. Co-culture samples demonstrated increased deposition of sulphated glycosaminoglycans (GAGs) and elastin, as well as increases in the release of FGF-2, a growth factor that can stimulate VSMC growth, while dynamic culture supported increases in collagen I and III as well as increased mechanical properties (elastic modulus, tensile strength) vs. static controls. Macrophage polarization toward an M1 state was not promoted by the biomaterial or culture conditions tested. Monocytes/macrophages cultured on D-PHI were also shown to produce vascular extracellular matrix components, including collagen I, collagen III, elastin, and GAGs. This study highlights the use of synthetic biomaterials having immunomodulatory character in order to promote cell and tissue growth when used in tissue engineering strategies, and identifies ECM deposition by monocytes/macrophages as an unexpected source of this new tissue.

STATEMENT OF SIGNIFICANCE

The ability of biomaterials to regulate macrophage activation towards a wound healing phenotype has recently been shown to support positive tissue regeneration. However, the ability of immunomodulatory biomaterials to harness monocyte/macrophage activity to support tissue engineering strategies in vitro holds enormous potential that has yet to be investigated. This study used a monocyte co-culture on a degradable polyurethane (D-PHI) to regulate the response of VSMCs in combination with biomechanical strain in a vascular tissue engineering context. Results demonstrate that immunomodulatory biomaterials, such as D-PHI, that support a desirable macrophage activation state can be combined with biomechanical strain to augment vascular tissue production in vitro, in part due to the novel and unexpected contribution of monocytes/macrophages themselves producing vascular ECM proteins.

Nuclear envelope proteins Nesprin2 and LaminA regulate proliferation and apoptosis of vascular endothelial cells in response to shear stress

The dysfunction of vascular endothelial cells (ECs) influenced by flow shear stress is crucial for vascular remodeling. However, the roles of nuclear envelope (NE) proteins in shear stress-induced EC dysfunction are still unknown. Our results indicated that, compared with normal shear stress (NSS), low shear stress (LowSS) suppressed the expression of two types of NE proteins, Nesprin2 and LaminA, and increased the proliferation and apoptosis of ECs. Targeted small interfering RNA (siRNA) and gene overexpression plasmid transfection revealed that Nesprin2 and LaminA participate in the regulation of EC proliferation and apoptosis. A protein/DNA array was further used to detect the activation of transcription factors in ECs following transfection with target siRNAs and overexpression plasmids. The regulation of AP-2 and TFIID mediated by Nesprin2 and the activation of Stat-1, Stat-3, Stat-5 and Stat-6 by LaminA were verified under shear stress. Furthermore, using Ingenuity Pathway Analysis software and real-time RT-PCR, the effects of Nesprin2 or LaminA on the downstream target genes of AP-2, TFIID, and Stat-1, Stat-3, Stat-5 and Stat-6, respectively, were investigated under LowSS. Our study has revealed that NE proteins are novel mechano-sensitive molecules in ECs. LowSS suppresses the expression of Nesprin2 and LaminA, which may subsequently modulate the activation of important transcription factors and eventually lead to EC dysfunction.

Shear stress-initiated signaling and its regulation of endothelial function

Atherosclerosis develops preferentially at branches and curvatures of the arterial tree, where blood flow pattern is disturbed rather than being laminar, and wall shear stress has an irregular distribution without defined directions. The endothelium in the atherosusceptible regions, in comparison to that in atheroresistant regions, shows activation of proproliferative and proinflammatory gene expressions, reduced production of nitric oxide (NO), increased leukocyte adhesion, and permeability, as well as other atheroprone phenotypes. Differences in gene expressions and cell phenotypes have been detected in endothelia residing in native atherosusceptible and atheroresistant regions of the arteries, or in arteries from animal models with artificial creation of disturbed flow. Similar results have also been shown in in vitro systems that apply controlled shear stresses with or without clear directions to cultured endothelial cells in fluid-dynamically designed flow-loading devices. The available evidence indicates that the coordination of multiple signaling networks, rather than individual separate pathways, links the mechanical signals to specific genetic circuitries in orchestrating the mechanoresponsive networks to evoke comprehensive genetic and functional responses.

Wall shear stress effects on endothelial-endothelial and endothelial-smooth muscle cell interactions in tissue engineered models of the vascular wall

Vascular functions are affected by wall shear stresses (WSS) applied on the endothelial cells (EC), as well as by the interactions of the EC with the adjacent smooth muscle cells (SMC). The present study was designed to investigate the effects of WSS on the endothelial interactions with its surroundings. For this purpose we developed and constructed two co-culture models of EC and SMC, and compared their response to that of a single monolayer of cultured EC. In one co-culture model the EC were cultured on the SMC, whereas in the other model the EC and SMC were cultured on the opposite sides of a membrane. We studied EC-matrix interactions through focal adhesion kinase morphology, EC-EC interactions through VE-Cadherin expression and morphology, and EC-SMC interactions through the expression of Cx43 and Cx37. In the absence of WSS the SMC presence reduced EC-EC connectivity but produced EC-SMC connections using both connexins. The exposure to WSS produced discontinuity in the EC-EC connections, with a weaker effect in the co-culture models. In the EC monolayer, WSS exposure (12 and 4 dyne/cm² for 30 min) increased the EC-EC interaction using both connexins. WSS exposure of 12 dyne/cm² did not affect the EC-SMC interactions, whereas WSS of 4 dyne/cm² elevated the amount of Cx43 and reduced the amount of Cx37, with a different magnitude between the models. The reduced endothelium connectivity suggests that the presence of SMC reduces the sealing properties of the endothelium, showing a more inflammatory phenotype while the distance between the two cell types reduced their interactions. These results demonstrate that EC-SMC interactions affect EC phenotype and change the EC response to WSS. Furthermore, the interactions formed between the EC and SMC demonstrate that the 1-side model can simulate better the arterioles, while the 2-side model provides better simulation of larger arteries.

Influence of mesenchymal stem cells on the response of endothelial cells to laminar flow and shear stress

The interactions between endothelial cells (ECs) and smooth muscle cells (SMCs) in a complex hemodynamic environment play an important role in the control of blood vessel function. Since autologous SMCs are not readily available for the tissue engineering of a blood vessel substitute, a substitute for SMCs, such as human adult bone marrow-derived mesenchymal stem cells (MSCs), is needed. The objective of this study was to use a three-dimensional coculture model of the blood vessel wall, comprised of ECs and MSCs, to determine how the presence of MSCs affects EC function. Two vascular coculture models with an EC monolayer were created using type I collagen. All models were exposed to steady laminar flow with a shear stress of 15 dyn/cm(2) for up to 48 h. ECs in both the MSC and SMC coculture models expressed up-regulated EC-specific markers compared to the EC-only control model. The most dramatic difference observed between the two coculture models was in the experiments assessing monocyte adhesion. Here, fewer monocytes bound after laminar shear compared to static conditions; however, the number of bound monocytes was much lower for the EC-MSC coculture model than the EC-SMC coculture model for both static and shear conditions. These results suggest the feasibility of developing a tissue-engineered blood vessel substitute using MSCs as a substitute for SMCs.

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.

Induction of thoracic aortic remodeling by endothelial-specific deletion of microRNA-21 in mice

MicroRNAs (miRs) are known to have an important role in modulating vascular biology. MiR21 was found to be involved in the pathogenesis of proliferative vascular disease. The role of miR21 in endothelial cells (ECs) has well studied in vitro, but the study in vivo remains to be elucidated. In this study, miR21 endothelial-specific knockout mice were generated by Cre/LoxP system. Compared with wild-type mice, the miR21 deletion in ECs resulted in structural and functional remodeling of aorta significantly, such as diastolic pressure dropping, maximal tension depression, endothelium-dependent relaxation impairment, an increase of opening angles and wall-thickness/inner diameter ratio, and compliance decrease, in the miR21 endothelial-specific knockout mice. Furthermore, the miR21 deletion in ECs induced down-regulation of collagen I, collagen III and elastin mRNA and proteins, as well as up-regulation of Smad7 and down-regulation of Smad2/5 in the aorta of miR21 endothelial-specific knockout mice. CTGF and downstream MMP/TIMP changes were also identified to mediate vascular remodeling. The results showed that miR21 is identified as a critical molecule to modulate vascular remodeling, which will help to understand the role of miR21 in vascular biology and the pathogenesis of vascular diseases.

Development of an endothelial-smooth muscle cell coculture model using phenotype-controlled smooth muscle cells

A coculture of endothelial cells (ECs) and smooth muscle cells (SMCs), which mimics cellular interactions appearing in vivo, has been performed in studies on the relationship between atherogenesis and fluid shear stress conditions. Although healthy arteries in vivo consist of contractile phenotype SMCs, cultured cells used in many studies normally exhibit a synthetic phenotype. Here, we developed an EC-SMC coculture model to investigate the interactions between ECs and contractile SMCs, and examined the effect of shear stress applied to ECs on SMC phenotypes. Cultured human umbilical artery SMCs were differentiated into contractile states by arresting cell growth using a serum-free medium. Western blotting confirmed that SMC expression of contractile protein markers, α-smooth muscle actin (SMA) and calponin, increased to levels similar to those observed in arterial cells. After coculturing contractile SMCs with ECs separated by a collagen gel layer, the expression of α-SMA decreased under static conditions, indicating that the SMC phenotype tended to be synthetic by coculturing with ECs, but shear stress applied to cocultured ECs maintained the level of α-SMA expression in SMCs. The coculture model constructed in the present study will be a useful tool to investigate interactions between ECs and contractile SMCs under shear conditions.

PDGF-BB and TGF-{beta}1 on cross-talk between endothelial and smooth muscle cells in vascular remodeling induced by low shear stress

Shear stress, especially low shear stress (LowSS), plays an important role in vascular remodeling during atherosclerosis. Endothelial cells (ECs), which are directly exposed to shear stress, convert mechanical stimuli into intracellular signals and interact with the underlying vascular smooth muscle cells (VSMCs). The interactions between ECs and VSMCs modulate the LowSS-induced vascular remodeling. With the use of proteomic analysis, the protein profiles of rat aorta cultured under LowSS (5 dyn/cm(2)) and normal shear stress (15 dyn/cm(2)) were compared. By using Ingenuity Pathway Analysis to identify protein-protein association, a network was disclosed that involves two secretary molecules, PDGF-BB and TGF-β1, and three other linked proteins, lamin A, lysyl oxidase, and ERK 1/2. The roles of this network in cellular communication, migration, and proliferation were further studied in vitro by a cocultured parallel-plate flow chamber system. LowSS up-regulated migration and proliferation of ECs and VSMCs, increased productions of PDGF-BB and TGF-β1, enhanced expressions of lysyl oxidase and phospho-ERK1/2, and decreased Lamin A in ECs and VSMCs. These changes induced by LowSS were confirmed by using PDGF-BB recombinant protein, siRNA, and neutralizing antibody. TGF-β1 had similar influences on ECs as PDGF-BB, but not on VSMCs. Our results suggest that ECs convert the LowSS stimuli into up-regulations of PDGF-BB and TGF-β1, but these two factors play different roles in LowSS-induced vascular remodeling. PDGF-BB is involved in the paracrine control of VSMCs by ECs, whereas TGF-β1 participates in the feedback control from VSMCs to ECs.

Normal shear stress and vascular smooth muscle cells modulate migration of endothelial cells through histone deacetylase 6 activation and tubulin acetylation

Endothelial cells (ECs) line the innermost of the blood vessel wall and are constantly subjected to shear stress imposed by blood flow. ECs were also influenced by the neighboring vascular smooth muscle cells (VSMCs). The bidirectional communication between ECs and VSMCs modulates vascular homeostasis. In this study, the involvement of histone deacetylase 6 (HDAC6) in modulating migration of ECs co-cultured with VSMCs by the normal level of laminar shear stress (NSS) was investigated. ECs was either cultured alone or co-cultured with VSMCs under static conditions or subjected to NSS of 15 dyne/cm2 by using a parallel-plate co-culture flow chamber system. It was demonstrated that both NSS and VSMCs could increase EC migration. The migration level of ECs co-cultured with VSMCs under NSS was not higher than that under the static condition. The process of EC migration regulated by VSMCs and NSS was associated with the increased expression of HDAC6 and low level of acetylated tubulin. The increase in HDAC6 expression was accompanied by a time-dependent decrease in the acetylation of tubulin in ECs co-cultured with VSMCs. Inhibition of the HDAC6 by siRNA or tributyrin, an inhibitor of HDACs, induced a parallel alteration in the migration and the acetylated tubulin of ECs co-cultured with VSMCs. It was observed by immunofluorescence staining that the acetylated tubulin was distributed mostly around the cell nucleus in ECs co-cultured with VSMCs. The results suggest that the NSS may display a protective function on the vascular homeostasis by modulating EC migration to a normal level in a VSMC-dependent manner. This modulation process involves the down-regulation of acetylated tubulin which results from increased HDAC6 activity in ECs.

Live cell imaging of mechanotransduction

Mechanical forces play important roles in the regulation of cellular functions, including polarization, migration and stem cell differentiation. Tremendous advancement in our understanding of mechanotransduction has been achieved with the recent development of imaging technologies and molecular biosensors. In particular, genetically encoded biosensors based on fluorescence resonance energy transfer (FRET) technology have been widely developed and applied in the field of mechanobiology. In this article, we will provide an overview of the recent progress of FRET application in mechanobiology, specifically mechanotransduction. We first introduce fluorescent proteins and FRET technology. We then discuss the mechanotransduction processes in different cells including stem cells, with a special emphasis on the important signalling molecules involved in mechanotransduction. Finally, we discuss methods that can allow the integration of simultaneous FRET imaging and mechanical stimulation to trigger signalling transduction. In summary, FRET technology has provided a powerful tool for the study of mechanotransduction to advance our systematic understanding of the molecular mechanisms by which cells respond to mechanical stimulation.

Shear stress and vascular smooth muscle cells promote endothelial differentiation of endothelial progenitor cells via activation of Akt

BACKGROUND

The structural and functional integrity of endothelium plays a pivotal role in the pathogenesis of cardiovascular diseases. Endothelial progenitor cells (EPCs) is a population of pluripotent cells, which have a crucial role in maintaining the integrity of endothelium, but the mechanisms regulating the differentiation of EPCs have not been understood completely. In this study, the effects of shear stress and vascular smooth muscle cells (VSMCs) on the differentiation of EPCs, as well as the role of Akt were examined in a coculture model.

METHODS

Human EPCs separated from cord blood were subjected to shear stress by using a parallel-plate coculture flow chamber. The expression of CD133, CD34, CD31 and von Willebrand factor of EPCs were assessed by a flow cytometry. Activation of Akt was detected by Western blot.

FINDINGS

Both shear stress and VSMCs increase the expression of endothelial markers, CD31 and von Willebrand factor (vWF), simultaneously, decrease progenitor markers, CD133 and CD34, of EPCs. However, the effect of shear stress on EPC differentiation was much more significant when VMSCs were presented. Akt was activated in EPCs by shear stress, VSMCs or both. When the activation of Akt was blocked with its inhibitor during EPCs exposed to shear stress, CD133 and CD34 expressions were upregulated and correspondingly CD31 and vWF expressions were downregulated significantly.

INTERPRETATIONS

The results demonstrate that shear stress and VSMCs promote endothelial differentiation of EPCs via activation of Akt, which provide a new insight to clinical application on the regeneration of the vascular endothelium.

Endothelial injury induces vascular smooth muscle cell proliferation in highly localized regions of a direct contact co-culture system

Though previous studies have indicated a relationship between the proliferation of endothelial cells and vascular smooth muscle cells (VSMCs) in co-culture, the results have been contradictory and the signaling mechanism poorly understood. In this transmembrane co-culture study, VSMCs and endothelial cells were grown to confluence on opposite sides of a microporous membrane to mimic the intima/media border of vessels. The endothelial layer was injured, and then cultured for 3 days, resulting in partial re-endothelialization. VSMC proliferation across from the injured/partially recovered endothelial region was significantly higher than across from the de-endothelialized region (a sevenfold increase) and the uninjured region (a threefold increase). ELISA indicated that PDGF, which was undetectable in uninjured co-culture and homotypic controls, increased after injury and the addition of a piperazinyl-quinazoline carboxamide PDGF receptor inhibitor blocked VSMC proliferation across from the injured/partially recovered region. We conclude that co-culture signaling initiated by endothelial cell injury locally stimulates VSMC proliferation and that this signaling could be mediated by PDGF-BB.

Role of Rac and Rho-GDI alpha in the frequency-dependent expression of h1-calponin in vascular smooth muscle cells under cyclic mechanical strain

Phenotype transformation of vascular smooth muscle cells (VSMCs) has been reported to be directly influenced by the frequency of mechanical strain. This study explored the effects of different frequencies of mechanical strain on expression of phenotype marker h1-calponin and the possible mechanism. VSMCs were subjected to cyclic strains of 10% elongation at 1 and 2 Hz for 24 h by using a Flexercell strain unit. The protein expression of h1-calponin was assessed by Western blotting and the possible protein kinases involved were evaluated by their specific inhibitor or targeted siRNA 'knock-down.' The results showed that cyclic strains modulated the expressions of h1-calponin, phospho-p38, Rac and Rho-guanine nucleotide dissociation inhibitor alpha (Rho-GDI alpha) in nonlinear frequency-dependent manners. This nonlinear frequency-dependent change of h1-calponin expression could be blocked by a specific p38 inhibitor, SB202190. The changed expression of phospho-p38 induced by the frequencies of cyclic strain was reversed by targeted siRNA 'knock-down' of Rac, while enhanced by targeted siRNA 'knock-down' of Rho-GDI alpha. These results suggest that the frequency-dependent expression of h1-calponin under cyclic strain is mediated at least partly by the regulation of Rac and Rho-GDI alpha expression on the activation of p38 pathway.

Frequency-Dependent Phenotype Modulation of Vascular Smooth Muscle Cells under Cyclic Mechanical Strain

Phenotype transformation of vascular smooth muscle cells (VSMCs) is known to be modulated by mechanical strain. The present study was designed to investigate how different frequencies of mechanical strain affected VSMC phenotype. VSMCs were subjected to the strains of 10% elongation at 0, 0.5, 1 and 2 Hz for 24 h using a Flexercell strain unit. VSMC phenotype was assessed by cell morphology, measurement of two-dimensional cell area, Western blotting for protein and RT-PCR for mRNA expression of differentiation markers. Possible protein kinases involved were evaluated by Western blotting with their specific antibodies. The strains at certain frequencies could induce a contractile morphology in VSMC with almost perpendicular alignment to the strain direction. The strains also regulated protein and mRNA expression of several differentiation markers, as well as the activation of extracellular signal-regulated kinases (ERKs), p38 MAP kinase and protein kinase B (Akt) in a frequency-dependent manner. Furthermore, the inhibition of the p38 pathway could block the frequency-induced phenotype modulation of VSMCs, but not inhibition of ERK or Akt pathways. These results indicate that the frequency of cyclic strain can result in the differentiated phenotype of VSMCs, and it is mediated at least partly by the activation of the p38 pathway.

Coculture with endothelial cells enhances vascular smooth muscle cell adhesion and spreading via activation of β1-integrin and phosphatidylinositol 3-kinase/Akt

The interactions between endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) play significant roles in the homeostasis of the blood vessel during vascular remodeling. Cell adhesion and spreading are an essential process for VSMC migration, survival and proliferation in the events of vascular physiology and pathophysiology. However, effects of ECs on adhesion and spreading of VSMCs have not been characterized yet. Here, the interaction of ECs and VSMCs on adhesion and spreading of VSMCs were investigated by using a coculture system. The results showed that VSMCs cocultured with ECs exhibited a significant increase in the number of adherent and spreading cells, and much more mRNA (twofold, P<0.01) and protein (threefold, P<0.05) expression of beta(1)-integrin comparing to the control, i.e., VSMCs cultured alone. Furthermore, the enhanced functional activity of beta(1)-integrin expression was confirmed by FACS. A beta(1)-integrin blocking antibody (P5D2) could inhibit the EC-induced VSMC adhesion and spreading. It was demonstrated that in correspondence with enhanced cell adhesion, ECs also prompted focal adhesion complex assembly and stress fiber formation of VSMCs. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway was more pronouncedly activated in response to VSMC attachment. Our results for the first time show that coculture with ECs enhances VSMC adhesion and spreading by up-regulating beta(1)-integrin expression and activating the PI3K/Akt pathway, suggesting that the interaction between ECs and VSMCs serves an important role in vascular homeostasis and remodeling.