Adaptation of endothelial cells to physiologically-modeled, variable shear stress

Hemodynamics Indicates Differences Between Patients With And Without A Stroke Outcome After Left Ventricular Assist Device Implantation

Stroke remains a leading cause of complications and mortality in heart failure patients treated with LVAD circulatory support. Hemodynamics plays a central role in affecting risk and etiology of stroke during LVAD support. Yet, detailed quantitative assessment of hemodynamic variables and their relation to stroke outcomes in patients with an implanted LVAD remains a challenge. We present an in silico hemodynamics analysis in a set of 12 patients on LVAD support; 6 with reported stroke outcomes and 6 without. We conducted patient-specific hemodynamics simulations for models with the LVAD outflow graft reconstructed from cardiac-gated CT images. A pre-implantation baseline flow model was virtually generated for each case by removing the LVAD outflow graft and driving flow from the aortic root. Hemodynamics was characterized using quantitative descriptors for helical flow, vortex generation, and wall shear stress. Our analysis showed higher average values for descriptors of positive helical flow, vortex generation, and wall shear stress, across the 6 cases with stroke outcomes on LVAD support, when compared with cases without stroke. When the descriptors for LVAD-driven flow were compared against estimated baseline flow pre-implantation, extent of positive helicity was higher, and vorticity and wall shear were lower in cases with stroke compared to those without. The study suggests that quantitative analysis of hemodynamics after LVAD implantation; and hemodynamic alterations from a pre-implant flow scenario, can potentially reveal hidden information linked to stroke outcomes during LVAD support. This has broad implications on understanding stroke etiology, LVAD treatment planning, surgical optimization, and efficacy assessment.

Quantitative Assessment of Aortic Hemodynamics for Varying Left Ventricular Assist Device Outflow Graft Angles and Flow Pulsation

Left ventricular assist devices (LVADs) comprise a primary treatment choice for advanced heart failure patients. Treatment with LVAD is commonly associated with complications like stroke and gastro-intestinal (GI) bleeding, which adversely impacts treatment outcomes, and causes fatalities. The etiology and mechanisms of these complications can be linked to the fact that LVAD outflow jet leads to an altered state of hemodynamics in the aorta as compared to baseline flow driven by aortic jet during ventricular systole. Here, we present a framework for quantitative assessment of aortic hemodynamics in LVAD flows realistic human vasculature, with a focus on quantifying the differences between flow driven by LVAD jet and the physiological aortic jet when no LVAD is present. We model hemodynamics in the aortic arch proximal to the LVAD outflow graft, as well as in the abdominal aorta away from the LVAD region. We characterize hemodynamics using quantitative descriptors of flow velocity, stasis, helicity, vorticity and mixing, and wall shear stress. These are used on a set of 27 LVAD scenarios obtained by parametrically varying LVAD outflow graft anastomosis angles, and LVAD flow pulse modulation. Computed descriptors for each of these scenarios are compared against the baseline flow, and a detailed quantitative characterization of the altered state of hemodynamics due to LVAD operation (when compared to baseline aortic flow) is compiled. These are interpreted using a conceptual model for LVAD flow that distinguishes between flow originating from the LVAD outflow jet (and its impingement on the aorta wall), and flow originating from aortic jet during aortic valve opening in normal physiological state.

Staphylococcus-induced proliferative glomerulonephritis and cerebral hemorrhage - fatal complications in a young female with postpartum cardiomyopathy and an implanted left ventricular assist device: a case report and review of the literature

Background: The continuous-flow left ventricular assist device (CF-LVAD) is used to save the lives of patients in the final stage of congestive heart failure, replacing the pump function of the left ventricle. Although quality of life increases significantly, CF-LVAD-related complications might prove fatal, as in the case presented in this paper.Methods: A 20-year-old female, during her second pregnancy, presented with signs of heart failure. Emergency caesarean section was necessary to save the baby, but peripartum cardiomyopathy developed in the mother. The use of an implantable cardioverter-defibrillator (ICD) was necessary 5 years later. As the clinical progression was unfavorable under medical treatment, with the patient reaching INTERMACS (Interagency Registry for Mechanically Assisted Circulatory Support) Profile 1 (refractory cardiogenic shock), the treatment of choice was the implantation of a CF-LVAD.Results: After 3 years of follow-up (at the age of 28), the patient presented with a positive hemoculture for Staphylococcus aureus. Prolonged antibiotic therapy and attentive follow-up was prescribed. Although an effective antiplatelet and anticoagulant treatment was applied, and despite therapeutic values of prothrombin time and international normalized ratio (INR), the patient died as result of a fatal cerebral hemorrhage. The autopsy also revealed septic emboli, disseminated intravascular coagulation, and focal proliferative glomerulonephritis.Conclusions: Although the benefits of CF-LVAD are significant, bleeding episodes can be severe and LVAD-associated infection can trigger glomerular injury and increase mortality.

Targeting vascular inflammation through emerging methods and drug carriers

Acute inflammation is a common dangerous component of pathogenesis of many prevalent conditions with high morbidity and mortality including sepsis, thrombosis, acute respiratory distress syndrome (ARDS), COVID-19, myocardial and cerebral ischemia-reperfusion, infection, and trauma. Inflammatory changes of the vasculature and blood mediate the course and outcome of the pathology in the tissue site of insult, remote organs and systemically. Endothelial cells lining the luminal surface of the vasculature play the key regulatory functions in the body, distinct under normal vs. pathological conditions. In theory, pharmacological interventions in the endothelial cells might enable therapeutic correction of the overzealous damaging pro-inflammatory and pro-thrombotic changes in the vasculature. However, current agents and drug delivery systems (DDS) have inadequate pharmacokinetics and lack the spatiotemporal precision of vascular delivery in the context of acute inflammation. To attain this level of precision, many groups design DDS targeted to specific endothelial surface determinants. These DDS are able to provide specificity for desired tissues, organs, cells, and sub-cellular compartments needed for a particular intervention. We provide a brief overview of endothelial determinants, design of DDS targeted to these molecules, their performance in experimental models with focus on animal studies and appraisal of emerging new approaches. Particular attention is paid to challenges and perspectives of targeted therapeutics and nanomedicine for advanced management of acute inflammation.

Impact of physiological loads of arterial wall on nucleus deformation in endothelial cells: A computational study

INTRODUCTION

Computational modeling can enhance the understanding of cell mechanics. To achieve this, finite element models of endothelial cells were proposed with shapes mimicking their natural state inside the endothelium within the cardiovascular system. Implementing the recently proposed bendo-tensegrity concept, these models consider flexural (buckling) as well as tensional/compressional behavior of microtubules and also incorporate the waviness of intermediate filaments.

MATERIALS AND METHODS

Four different models were created (flat and domed hexagons, both regular and elongated in the direction of blood flow) and loaded by biaxial deformation, blood pressure, and shear load from blood flow - natural physiological conditions of the arterial endothelium - aiming to investigate the "in situ" mechanical response of the cell.

RESULTS

The impact of individual components of loads on the nucleus deformation (more specifically on the first principal strain) potentially influencing mechanotransduction was investigated and the role of the cytoskeleton and its constituents in the mechanical response of the endothelial cell was assessed. The results show (i) the impact of pulsating blood pressure on cyclic deformations of the nucleus, which increase substantially with decreasing axial pre-stretch of the cell, (ii) the importance of relatively low shear stresses in the cell response and nucleus deformation.

CONCLUSION

Not only the pulsatile blood pressure but also the wall shear stress may induce significant deformation of the nucleus and thus trigger remodelation processes in endothelial cells.

In Vitro Flow Chamber Design for the Study of Endothelial Cell (Patho)Physiology

In the native vasculature, flowing blood produces a frictional force on vessel walls that affects endothelial cell function and phenotype. In the arterial system, the vasculature's local geometry directly influences variations in flow profiles and shear stress magnitudes. Straight arterial sections with pulsatile shear stress have been shown to promote an athero-protective endothelial phenotype. Conversely, areas with more complex geometry, such as arterial bifurcations and branch points with disturbed flow patterns and lower, oscillatory shear stress, typically lead to endothelial dysfunction and the pathogenesis of cardiovascular diseases. Many studies have investigated the regulation of endothelial responses to various shear stress environments. Importantly, the accurate in vitro simulation of in vivo hemodynamics is critical to the deeper understanding of mechanotransduction through the proper design and use of flow chamber devices. In this review, we describe several flow chamber apparatuses and their fluid mechanics design parameters, including parallel-plate flow chambers, cone-and-plate devices, and microfluidic devices. In addition, chamber-specific design criteria and relevant equations are defined in detail for the accurate simulation of shear stress environments to study endothelial cell responses.

High Output Heart Failure in Multiple Myeloma: Pathogenetic Considerations

Simple Summary

Multiple myeloma is a plasma cell disorder that accounts for around 10% of all haematological malignancies. This neoplasia is often associated with a significant prevalence of cardiovascular complications resulting from several factors, unrelated and/or related to the disease. Among cardiovascular complications, the high output heart failure is of great importance as it is related to a worse prognosis for patients. It is important to point out that, despite the availability of more and more numerous and effective drugs, myeloma remains an incurable disease, with frequent relapses and several treatment lines, with the need, therefore, for a careful evaluation of patients, especially from a cardiological point of view. For this reason, we are proposing a comprehensive overview of different pathogenetic mechanisms responsible for high output heart failure in multiple myeloma, including artero-venous shunts, enhanced angiogenesis, glutamminolysis, hyperammonemia and hemorheological alterations, with the belief that a multidisciplinary approach, in clinical evaluation is critical for the optimal management of the patient.

Abstract

The high output heart failure is a clinical condition in which the systemic congestion is associated to a high output state, and it can be observed in a non-negligible percentage of hematological diseases, particularly in multiple myeloma, a condition in which the risk of adverse cardiovascular events may increase, with a worse prognosis for patients. For this reason, though an accurate literature search, we provided in this review a complete overview of different pathogenetic mechanisms responsible for high output heart failure in multiple myeloma. Indeed, this clinical finding is present in the 8% of multiple myeloma patients, and it may be caused by artero-venous shunts, enhanced angiogenesis, glutamminolysis, hyperammonemia and hemorheological alterations with increase in plasma viscosity. The high output heart failure in multiple myeloma is associated with significant morbidity and mortality, emphasizing the need for a multidisciplinary approach.

Emerging Piezo1 signaling in inflammation and atherosclerosis; a potential therapeutic target

Purpose of Review: Atherosclerosis is the principal cause of cardiovascular diseases (CVDs) which are the major cause of death worldwide. Mechanical force plays an essential role in cardiovascular health and disease. To bring the awareness of mechanosensitive Piezo1 role in atherosclerosis and its therapeutic potentials we review recent literature to highlight its involvement in various mechanisms of the disease. Recent Findings: Recent studies reported Piezo1 channel as a sensor, and transducer of various mechanical forces into biochemical signals, which affect various cellular activities such as proliferation, migration, apoptosis and vascular remodeling including immune/inflammatory mechanisms fundamental phenomenon in atherogenesis. Summary: Numerous evidences suggest Piezo1 as a player in different mechanisms of cell biology, including immune/inflammatory and other cellular mechanisms correlated with atherosclerosis. This review discusses mechanistic insight about this matter and highlights the drugability and therapeutic potentials consistent with emerging functions Piezo1 in various mechanisms of atherosclerosis. Based on the recent works, we suggest Piezo1 as potential therapeutic target and a valid candidate for future research. Therefore, a deeper exploration of Piezo1 biology and translation towards the clinic will be a novel strategy for treating atherosclerosis and other CVDs.

Engineering a 3D human intracranial aneurysm model using liquid-assisted injection molding and tuned hydrogels

Physiologically relevant intracranial aneurysm (IA) models are crucially required to facilitate testing treatment options for IA. Herein, we report the development of a new in vitro tissue-engineered platform, which recapitulates the microenvironment, structure, and cellular complexity of native human IA. A new modified liquid-assisted injection molding technique was developed to fabricate a three-dimensional hollow IA model with clinically relevant IA dimensions within a mechanically tuned Gelatin Methacryloyl (GelMA) hydrogel. An endothelium lining was created inside the IA model by culturing human umbilical vein endothelial cells over pre-cultured human brain vascular smooth muscle cells. These cellularized IA models were subjected to medium perfusion at flow rates between 6.3 and 15.75 mL/min for inducing biomimetic vessel wall shear stress (10-25 dyn/cm²) to the cells for ten days. Both cell types maintained their secretome profiles and showed more than 96% viability, demonstrating the biocompatibility of the hydrogel during perfusion cell culture at such flow rates. Based on the characterized viscoelastic properties of the GelMA hydrogel and with the aid of a fluid-structure interaction model, the capability of the IA model in predicting the response of the IA to different fluid flow profiles was mathematically shown. With physiologically relevant behavior, our developed in vitro human IA model could allow researchers to better understand the pathophysiology and treatment of IA. STATEMENT OF SIGNIFICANCE: A three-dimensional intracranial aneurysm (IA) tissue model recapitulating the microenvironment, structure, and cellular complexity of native human IA was developed. • An endothelium lining was created inside the IA model over pre-cultured human brain vascular smooth muscle cells over at least 10-day successful culture. • The cells maintained their secretome profiles, demonstrating the biocompatibility of hydrogel during a long-term perfusion cell culture. • The IA model showed its capability in predicting the response of IA to different fluid flow profiles. • The cells in the vessel region behaved differently from cells in the aneurysm region due to alteration in hemodynamic shear stress. • The IA model could allow researchers to better understand the pathophysiology and treatment options of IA.

Endothelial-Derived Extracellular Vesicles Induce Cerebrovascular Dysfunction in Inflammation

Blood-brain barrier (BBB) dysfunction is a key hallmark in the pathology of many neuroinflammatory disorders. Extracellular vesicles (EVs) are lipid membrane-enclosed carriers of molecular cargo that are involved in cell-to-cell communication. Circulating endothelial EVs are increased in the plasma of patients with neurological disorders, and immune cell-derived EVs are known to modulate cerebrovascular functions. However, little is known about whether brain endothelial cell (BEC)-derived EVs themselves contribute to BBB dysfunction. Human cerebral microvascular cells (hCMEC/D3) were treated with TNFα and IFNy, and the EVs were isolated and characterised. The effect of EVs on BBB transendothelial resistance (TEER) and leukocyte adhesion in hCMEC/D3 cells was measured by electric substrate cell-substrate impedance sensing and the flow-based T-cell adhesion assay. EV-induced molecular changes in recipient hCMEC/D3 cells were analysed by RT-qPCR and Western blotting. A stimulation of naïve hCMEC/D3 cells with small EVs (sEVs) reduced the TEER and increased the shear-resistant T-cell adhesion. The levels of microRNA-155, VCAM1 and ICAM1 were increased in sEV-treated hCMEC/D3 cells. Blocking the expression of VCAM1, but not of ICAM1, prevented sEV-mediated T-cell adhesion to brain endothelia. These results suggest that sEVs derived from inflamed BECs promote cerebrovascular dysfunction. These findings may provide new insights into the mechanisms involving neuroinflammatory disorders.

Recapitulating monocyte extravasation to the synovium in an organotypic microfluidic model of the articular joint

The synovium of osteoarthritis (OA) patients can be characterized by an abnormal accumulation of macrophages originating from extravasated monocytes. Since targeting monocyte extravasation may represent a promising therapeutic strategy, our aim was to develop an organotypic microfluidic model recapitulating this process. Synovium and cartilage were modeled by hydrogel-embedded OA synovial fibroblasts and articular chondrocytes separated by a synovial fluid channel. The synovium compartment included a perfusable endothelialized channel dedicated to monocyte injection. Monocyte extravasation in response to chemokines and OA synovial fluid was quantified. The efficacy of chemokine receptor antagonists, RS-504393 (CCR2 antagonist) and Cenicriviroc (CCR2/CCR5 antagonist) in inhibiting extravasation was tested pre-incubating monocytes with the antagonists before injection. After designing and fabricating the chip, culture conditions were optimized to achieve an organotypic model including synovial fibroblasts, articular chondrocytes, and a continuous endothelial monolayer expressing intercellular adhesion molecule-1 and vascular cell adhesion molecule-1. A significantly higher number of monocytes extravasated in response to the chemokine mix (p< 0.01) and OA synovial fluid (p< 0.01), compared to a control condition. In both cases, endothelium pre-activation enhanced monocyte extravasation. The simultaneous blocking of CCR2 and CCR5 proved to be more effective (p< 0.001) in inhibiting monocyte extravasation in response to OA synovial fluid than blocking of CCR2 only (p< 0.01). The study of extravasation in the model provided direct evidence that OA synovial fluid induces monocytes to cross the endothelium and invade the synovial compartment. The model can be exploited either to test molecules antagonizing this process or to investigate the effect of extravasated monocytes on synovium and cartilage cells.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Microphysiological models of the central nervous system with fluid flow

There are over 1,000 described neurological and neurodegenerative disorders affecting nearly 100 million Americans - roughly one third of the U.S. population. Collectively, treatment of neurological conditions is estimated to cost $800 billion every year. Lowering this societal burden will require developing better model systems in which to study these diverse disorders. Microphysiological systems are promising tools for modeling healthy and diseased neural tissues to study mechanisms and treatment of neuropathology. One major benefit of microphysiological systems is the ability to incorporate biophysical forces, namely the forces derived from biological fluid flow. Fluid flow in the central nervous system (CNS) is a complex but important element of physiology, and pathologies as diverse as traumatic or ischemic injury, cancer, neurodegenerative disease, and natural aging have all been found to alter flow pathways. In this review, we summarize recent advances in three-dimensional microphysiological systems for studying the biology and therapy of CNS disorders and highlight the ability and growing need to incorporate biological fluid flow in these miniaturized model systems.

Studying the Endothelial Glycocalyx in vitro: What Is Missing?

All human cells are coated by a surface layer of proteoglycans, glycosaminoglycans (GAGs) and plasma proteins, called the glycocalyx. The glycocalyx transmits shear stress to the cytoskeleton of endothelial cells, maintains a selective permeability barrier, and modulates adhesion of blood leukocytes and platelets. Major components of the glycocalyx, including syndecans, heparan sulfate, and hyaluronan, are shed from the endothelial surface layer during conditions including ischaemia and hypoxia, sepsis, atherosclerosis, diabetes, renal disease, and some viral infections. Studying mechanisms of glycocalyx damage in vivo can be challenging due to the complexity of immuno-inflammatory responses which are inextricably involved. Previously, both static as well as perfused in vitro models have studied the glycocalyx, and have reported either imaging data, assessment of barrier function, or interactions of blood components with the endothelial monolayer. To date, no model has simultaneously incorporated all these features at once, however such a model would arguably enhance the study of vasculopathic processes. This review compiles a series of current in vitro models described in the literature that have targeted the glycocalyx layer, their limitations, and potential opportunities for further developments in this field.

Mechanosensation and Mechanotransduction by Lymphatic Endothelial Cells Act as Important Regulators of Lymphatic Development and Function

Our understanding of the function and development of the lymphatic system is expanding rapidly due to the identification of specific molecular markers and the availability of novel genetic approaches. In connection, it has been demonstrated that mechanical forces contribute to the endothelial cell fate commitment and play a critical role in influencing lymphatic endothelial cell shape and alignment by promoting sprouting, development, maturation of the lymphatic network, and coordinating lymphatic valve morphogenesis and the stabilization of lymphatic valves. However, the mechanosignaling and mechanotransduction pathways involved in these processes are poorly understood. Here, we provide an overview of the impact of mechanical forces on lymphatics and summarize the current understanding of the molecular mechanisms involved in the mechanosensation and mechanotransduction by lymphatic endothelial cells. We also discuss how these mechanosensitive pathways affect endothelial cell fate and regulate lymphatic development and function. A better understanding of these mechanisms may provide a deeper insight into the pathophysiology of various diseases associated with impaired lymphatic function, such as lymphedema and may eventually lead to the discovery of novel therapeutic targets for these conditions.

Flow with variable pulse frequencies accelerates vascular recellularization and remodeling of a human bioscaffold

Despite significant advances in vascular tissue engineering, the ideal graft has not yet been developed and autologous vessels remain the gold standard substitutes for small diameter bypass procedures. Here, we explore the use of a flow field with variable pulse frequencies over the regeneration of an ex vivo-derived human scaffold as vascular graft. Briefly, human umbilical veins were decellularized and used as scaffold for cellular repopulation with human smooth muscle cells (SMC) and endothelial cells (EC). Over graft development, the variable flow, which mimics the real-time cardiac output of an individual performing daily activities (e.g., resting vs. exercising), was implemented and compared to the commonly used constant pulse frequency. Results show marked differences on SMC and EC function, with changes at the molecular level reflecting on tissue scales. First, variable frequencies significantly increased SMC proliferation rate and glycosaminoglycan production. These results can be tied with the SMC gene expression that indicates a synthetic phenotype, with a significant downregulation of myosin heavy chain. Additionally and quite remarkably, the variable flow frequencies motivated the re-endothelialization of the grafts, with a quiescent-like structure observed after 10 days of conditioning, contrasting with the low surface coverage and unaligned EC observed under constant frequency (CF). Besides, the overall biomechanics of the generated grafts (conditioned with both pulsed and CFs) evidence a significant remodeling after 55 days of culture, depicted by high burst pressure and Young's modulus. These last results demonstrate the positive recellularization and remodeling of a human-derived scaffold toward an arterial vessel.

Mesangioproliferative glomerulonephritis with extracapillary crescents - unexpected fatal complication in a 17-year-old patient with implanted left ventricular assist device

The continuous flow left ventricular assist device (cf-LVAD) is the life-saving solution for patients with end-stage global heart failure. We present the case of a young patient with biventricular dilated cardiomyopathy, who had a cf-LVAD implantation and died as result of progressive renal failure. In the first year after implantation, he suffered repeated strokes and episodes of pneumonia with Klebsiella pneumoniae and Escherichia coli. The patient had hypertension, which was kept under control with angiotensin-converting enzyme (ACE) inhibitors and beta-blockers. After multiple bleeding episodes, the patient died at 21 months after the LVAD implant. At autopsy, parenchymatous brain hemorrhage was found to be associated with pulmonary hemorrhages. The unexpected features related to mesangioproliferative and extracapillary glomerulonephritis, with focal glomerulosclerosis. The proliferated parietal cells of Bowman’s capsule proved to express cluster of differentiation 44 (CD44), whereas remnant podocytes and mesangial cells showed Wilms tumor 1 (WT1) positivity. Since CD44 might be involved in fibrogenesis, but ACE inhibitors can exert a protective role against glomerular deterioration, we performed a synthesis of literature data which enabled us to propose a hypothesis with a potential clinical impact. We conclude that, in patients with LVAD implants, high blood pressure and high serum level of angiotensin II, the association between ACE inhibitors and anti-CD44 agents might exert glomerular protection and increase the survival time. Experimental studies are necessary to support our hypothesis and to explain the mechanism of possible glomerulopathy installed after LVAD implant.

Multivariate analysis of hemodynamic parameters on intracranial aneurysm initiation of the internal carotid artery

Although fluctuating hemodynamic wall stressors are known to impact intracranial aneurysms (IA) initiation, specificity of those stressors has not been evaluated. In this study, using human IA data, we investigated: (1) specificity of stressors in regions with and without IA eventual IA formation; and (2) how combinations of multiple stressors could improve IA formation prediction. 3D computational vasculatures were constructed based on angiographic images of 18 subjects having multiple closely-spaced IAs in the internal carotid artery. Two models were created: Model A with all IAs computationally removed, Model B which kept keep one IA. Computational fluid dynamics (CFD) simulated flow within models. Based on simulated flow fields, wall shear stress and its gradient (WSS, WSSG), oscillatory shear index (OSI), gradient oscillatory number (GON), aneurysm formation index (AFI), and mean number of swirling flow vortices (MV) were analysed. Multivariate logistic regression determined the accuracy of different combinations of those above-mentioned stressors. Overall, we found that combining hemodynamic stressors improves IA formation prediction over individual indices. Both Model A and Model B's parsimonious model was MV+WSS+GON: AUROC 0.88 and 0.83, respectively. Future studies are planned to understand biological meanings induced by fluctuating stressors.

Integrated proteomics and metabolomics analysis reveals differential lipid metabolism in human umbilical vein endothelial cells under high and low shear stress

Atherosclerotic plaque development is closely associated with the hemodynamic forces applied to endothelial cells (ECs). Among these, shear stress (SS) plays a key role in disease development since changes in flow intensity and direction could stimulate an atheroprone or atheroprotective phenotype. ECs under low or oscillatory SS (LSS) show upregulation of inflammatory, adhesion, and cellular permeability molecules. On the contrary, cells under high or laminar SS (HSS) increase their expression of protective and anti-inflammatory factors. The mechanism behind SS regulation of an atheroprotective phenotype is not completely elucidated. Here we used proteomics and metabolomics to better understand the changes in endothelial cells (human umbilical vein endothelial cells) under in vitro LSS and HSS that promote an atheroprone or atheroprotective profile and how these modifications can be connected to atherosclerosis development. Our data showed that lipid metabolism, in special cholesterol metabolism, was downregulated in cells under LSS. The low-density lipoprotein receptor (LDLR) showed significant alterations both at the quantitative expression level as well as regarding posttranslational modifications. Under LSS, LDLR was seen at lower concentrations and with a different glycosylation profile. Finally, modulating LDLR with atorvastatin led to the recapitulation of a HSS metabolic phenotype in EC under LSS. Altogether, our data suggest that there is significant modulation of lipid metabolism in endothelial cells under different SS intensities and that this could contribute to the atheroprone phenotype of LSS. Statin treatment was able to partially recover the protective profile of these cells.

Glioblastoma adhesion in a quick-fit hybrid microdevice

Translational research requires reliable biomedical microdevices (BMMD) to mimic physiological conditions and answer biological questions. In this work, we introduce a reversibly sealed quick-fit hybrid BMMD that is operator-friendly and bubble-free, requires low reagent and cell consumption, enables robust and high throughput performance for biomedical experiments. Specifically, we fabricate a quick-fit poly(methyl methacrylate) and poly(dimethyl siloxane) (PMMA/PDMS) prototype to illustrate its utilities by probing the adhesion of glioblastoma cells (T98G and U251MG) to primary endothelial cells. In static condition, we confirm that angiopoietin-Tie2 signaling increases the adhesion of glioblastoma cells to endothelial cells. Next, to mimic the physiological hemodynamic flow and investigate the effect of physiological electric field, the endothelial cells are pre-conditioned with concurrent shear flow (with fixed 1 Pa shear stress) and direct current electric field (dcEF) in the quick-fit PMMA/PDMS BMMD. With shear flow alone, endothelial cells exhibit classical parallel alignment; while under a concurrent dcEF, the cells align perpendicularly to the electric current when the dcEF is greater than 154 V m^− 1. Moreover, with fixed shear stress of 1 Pa, T98G glioblastoma cells demonstrate increased adhesion to endothelial cells conditioned in dcEF of 154 V m^− 1, while U251MG glioblastoma cells show no difference. The quick-fit hybrid BMMD provides a simple and flexible platform to create multiplex systems, making it possible to investigate complicated biological conditions for translational research.

Ventricular Assist Device Patients Have Different Clinical Outcomes and Altered Patterns of Bleeding with Intracranial Hemorrhage

Intracranial hemorrhage (ICH) is one of the most feared complications of left ventricular assist device (LVAD) support. However, outcomes in this group have not been well described. We therefore sought to examine clinical outcomes in this patient population in comparison to those with heart failure (HF) and no LVAD, as well as those without HF or LVAD. The National Inpatient Sample database, years 2002-2012, was queried to classify patients into 3 groups: any ICH (group 1), any HF with any ICH (group 2), and any LVAD with any ICH (group 3). Clinical outcomes, hemorrhage type, neurosurgical intervention rates, and hospital factors were collected and analyzed. Group 1 consisted of 419,264 patients, group 2 had 41,186, and group 3 had 118 patients. Group 3 patients were more likely to be in large, academic medical centers, with longer length of stay and higher hospital charges. Inpatient mortality was highest in this group at 39%; however, 46.2% were ultimately discharged to home. Patients in group 3 were more likely to have a subarachnoid or intracerebral hemorrhage versus a subdural or epidural hemorrhage. Neurosurgical intervention rates did not differ between the groups. Although LVAD patients with ICH have worse clinical outcomes, the majority survived their event and nearly half were able to be discharged home. Left ventricular assist device patients also have a distinct pattern of bleeding with ICH. Additional study is required to understand risk factors for the development of ICH in this population and ideal management strategies.

Shear stress in the microvasculature: influence of red blood cell morphology and endothelial wall undulation

The effect of red blood cells and the undulation of the endothelium on the shear stress in the microvasculature is studied numerically using the lattice Boltzmann-immersed boundary method. The results demonstrate a significant effect of both the undulation of the endothelium and red blood cells on wall shear stress. Our results also reveal that morphological alterations of red blood cells, as occur in certain pathologies, can significantly affect the values of wall shear stress. The resulting fluctuations in wall shear stress greatly exceed the nominal values, emphasizing the importance of the particulate nature of blood as well as a more realistic description of vessel wall geometry in the study of hemodynamic forces. We find that within the channel widths investigated, which correspond to those found in the microvasculature, the inverse minimum distance normalized to the channel width between the red blood cell and the wall is predictive of the maximum wall shear stress observed in straight channels with a flowing red blood cell. We find that the maximum wall shear stress varies several factors more over a range of capillary numbers (dimensionless number relating strength of flow to membrane elasticity) and reduced areas (measure of deflation of the red blood cell) than the minimum wall shear stress. We see that waviness reduces variation in minimum and maximum shear stresses among different capillary and reduced areas.

Microfluidics: A high-throughput system for the assessment of the endotheliopathy of trauma and the effect of timing of plasma administration on ameliorating shock-associated endothelial dysfunction

BACKGROUND

Early resuscitation after trauma-hemorrhagic shock with plasma rather than crystalloid may ameliorate systemic endothelial cell (EC) injury and dysfunction (endotheliopathy of trauma). We postulated that endothelial-lined microfluidic networks would be a useful platform to study the EC activation/injury under flow conditions to mimic trauma-hemorrhagic shock. We then used the microfluidic system to further characterize the protective effects and optimal timing of plasma infusion on the development of "endotheliopathy of trauma" in our model.

METHODS

Human umbilical vein ECs were added to microfluidic flow channels, and after overnight perfusion, the cells were subsequently treated with epinephrine and exposed to hypoxia reoxygenation. Media alone or 5% human plasma was perfused either immediately following treatment (early plasma) or after a 3-hour delay (late plasma). Glycocalyx injury was indexed by fluorescent microscopy and shedding of syndecan 1 and hyaluronic acid. Endothelial markers of activation/injury were also measured and included soluble thrombomodulin, tissue plasminogen activator, plasminogen activator inhibitor 1, and angiopoietins 1 and 2. Sheddase activity was indexed by ADAM metallopeptidase domain 17.

RESULTS

Endothelial cell and glycocalyx barrier function studies using microfluidic devices are a more realistic model of the glycocalyx endothelial vascular barrier than studies performed on ECs using static (no flow) conditions. Conditions that mimic the internal milieu following hemorrhagic shock result in glycocalyx degradation and an inflammatory prothrombotic response by the endothelium. "Early" use of plasma in the microfluidic channel perfusate mitigated against these effects. Later perfusion with plasma had no protective effect.

CONCLUSIONS

A temporal effect to plasma administration was noted in our biomimetic model of the endothelial vascular barrier following shock. This suggests a protective role to "early" plasma administration in the severely injured patient.

Blood pressure-induced physiological strain variability modulates wall structure and function in aorta rings

Vascular smooth muscle cells respond to mechanical stretch by reorganizing their cytoskeletal and contractile elements. Recently, we showed that contractile forces in rat aorta rings were maintained when the rings were exposed to 4 h of physiological variability in cycle-by-cycle strain, called variable stretch (VS), mimicking beat-to-beat blood pressure variability. Contractility, however, was reduced when the aorta was exposed to monotonous stretch (MS) with an amplitude equal to the mean peak strain of VS.

OBJECTIVE

Here we reanalyzed the data to obtain wall stiffness as well as added new histologic and inhibitor studies to test the effects of VS on the extracellular matrix.

MAIN RESULTS

The results demonstrate that while the stiffness of the aorta did not change during 4 h MS or VS, nonlinearity in mechanical behavior was slightly stronger following MS. The inhibitor studies also showed that mitochondrial energy production and cytoskeletal organization were involved in this fluctuation-driven mechanotransduction. Reorganization of β-actin in the smooth muscle layer quantified from immunohistochemically labeled images correlated with contractile forces during contraction. Histologic analysis of wall structure provided evidence of reorganization of elastin and collagen fibers following MS but less so following VS. The results suggested that the loss of muscle contraction in MS was compensated by reorganization of fiber structure leading to similar wall stiffness as in VS.

SIGNIFICANCE

We conclude that muscle tone modulated by variability in stretch plays a role in maintaining aortic wall structural and mechanical homeostasis with implications for vascular conditions characterized by a loss or an increase in blood pressure variability.

Fluid shear stress sensing in vascular homeostasis and remodeling: Towards the development of innovative pharmacological approaches to treat vascular dysfunction

Blood circulation, facilitating gas exchange and nutrient transportation, is a quintessential feature of life in vertebrates. Any disruption to blood flow, may it be by blockade or traumatic rupture, irrevocably leads to tissue infarction or death. Therefore, it is not surprising that hemostasis and vascular adaptation measures have been evolutionarily selected to mitigate the adverse consequences of altered circulation. Blood vessels can be broadly categorized as arteries, veins, or capillaries, based on their structure, hemodynamics, and gas exchange. However, all of them share one property: they are lined with an epithelial sheet called the endothelium, which typically lies on a basement membrane. This endothelium is the primary interface between the flowing blood and the rest of the body, and it has highly specialized molecular mechanisms to detect and respond to changes in blood perfusion. The purpose of this commentary will be to highlight some of the recent developments in the research on blood flow sensing, vascular remodeling, and homeostasis and to discuss the development of innovative pharmaceutical approaches targeting mechanosensing mechanisms to prolong patient survival and improve quality of life.

Helical Flow Component of Left Ventricular Assist Devices (LVADs) Outflow Improves Aortic Hemodynamic States

Background

Although LVADs are confirmed to have strong effects on aortic hemodynamics, the precise mechanisms of the helical flow component of LVAD outflow are still unclear.

Material/Methods

To clarify these effects, 3 cases – normal case, flat flow case, and realistic flow case – were designed and studied by using the CFD approach. The normal case denoted the normal aorta without LVAD support, and the flat flow case represented the aorta with the outflow cannula. Similarly, the realistic flow case included the aortic model, the model of outflow cannula, and the model of LVAD. The velocity vector, blood streamline, distribution of wall shear stress (WSS), and the local normalized helicity (LNH) were calculated.

Results

The results showed that the helical component of LVAD outflow significantly improved the aortic hemodynamics. Compared with the flat flow case, the helical flow eliminated the vortex near the outer wall of the aorta and improved the blood flow transport (normal case 0.1 m/s vs. flat flow case 0.14 m/s vs. realistic flow case 0.30 m/s) at the descending aorta. Moreover, the helical flow was confirmed to even the distribution of WSS, reduce the peak value of WSS (normal case 0.92 Pa vs. flat flow case 7.39 Pa vs. realistic flow case 5.2Pa), and maintain a more orderly WSS direction.

Conclusions

The helical flow component of LVAD outflow has significant advantages for improving aortic hemodynamic stability. Our study provides novel insights into LVAD optimization.

VascuTrainer: A Mobile and Disposable Bioreactor System for the Conditioning of Tissue-Engineered Vascular Grafts

In vitro tissue engineering of vascular grafts requires dynamic conditioning in a bioreactor system for in vitro tissue maturation and remodeling to receive a mechanically adequate and hemocompatible implant. The goal of the current work was to develop a bioreactor system for the conditioning of vascular grafts which is (i) able to create a wide range of flow, pressure and frequency conditions, including physiological ones; (ii) compact and easy to assemble; (iii) transportable; (iv) disposable. The system is driven by a small centrifugal pump controlled via a custom-made control unit, which can also be operated on batteries to allow for autonomous transportation. To show the potential of the newly developed bioreactor system small-caliber vascular composite grafts (n = 5, internal diameter = 3 mm, length = 12.5 cm) were fabricated using a fibrin scaffold embedding human umbilical artery smooth muscle cells and a polyvinylidene fluoride warp-knitted macroporous mesh. Subsequently, the vascular grafts were endothelialized and mounted in the bioreactor system for conditioning. The conditioning parameters remained within the predefined range over the complete conditioning period and during operation on batteries as tested for up to 25 h. Fabrication and pre-conditioning under arterial pressure and shear stress conditions resulted in robust and hemocompatible tissue-engineered vascular grafts. Analysis of immunohistochemical stainings against extracellular matrix and cell-specific proteins revealed collagen I and collagen III deposition. The luminal surface was confluently covered with endothelial cells. The developed bioreactor system showed cytocompatibility and pH, pO₂, pCO₂, glucose and lactate stayed constant. Sterility was maintained during the complete fabrication process of the vascular grafts. The potential of a versatile and mobile system and its functionality by conditioning tissue-engineered vascular grafts under physiological pressure and flow conditions could be demonstrated.

Regulation of Mitochondrial Structure and Dynamics by the Cytoskeleton and Mechanical Factors

Mitochondria supply cells with energy in the form of ATP, guide apoptosis, and contribute to calcium buffering and reactive oxygen species production. To support these diverse functions, mitochondria form an extensive network with smaller clusters that are able to move along microtubules aided by motor proteins. Mitochondria are also associated with the actin network, which is involved in cellular responses to various mechanical factors. In this review, we discuss mitochondrial structure and function in relation to the cytoskeleton and various mechanical factors influencing cell functions. We first summarize the morphological features of mitochondria with an emphasis on fission and fusion as well as how network properties govern function. We then review the relationship between the mitochondria and the cytoskeletal structures, including mechanical interactions. We also discuss how stretch and its dynamic pattern affect mitochondrial structure and function. Finally, we present preliminary data on how extracellular matrix stiffness influences mitochondrial morphology and ATP generation. We conclude by discussing the more general role that mitochondria may play in mechanobiology and how the mechanosensitivity of mitochondria may contribute to the development of several diseases and aging.

Measuring Leukocyte Adhesion to (Primary) Endothelial Cells after Photon and Charged Particle Exposure with a Dedicated Laminar Flow Chamber

The vascular endothelium interacts with all types of blood cells and is a key modulator of local and systemic inflammatory processes, for example, in the adhesion of blood leukocytes to endothelial cells (EC) and the following extravasation into the injured tissue. The endothelium is constantly exposed to mechanical forces caused by blood flow, and the resulting shear stress is essential for the maintenance of endothelial function. Changes in local hemodynamics are sensed by EC, leading to acute or persistent changes. Therefore, in vitro assessment of EC functionality should include shear stress as an essential parameter. Parallel-plate flow chambers with adjustable shear stress can be used to study EC properties. However, commercially available systems are not suitable for radiation experiments, especially with charged particles, which are increasingly used in radiotherapy of tumors. Therefore, research on charged-particle-induced vascular side effects is needed. In addition, α-particle emitters (e.g., radon) are used to treat inflammatory diseases at low doses. In the present study, we established a flow chamber system, applicable for the investigation of radiation induced changes in the adhesion of lymphocytes to EC as readout for the onset of an inflammatory reaction or the modification of a pre-existing inflammatory state. In this system, primary human EC are cultured under physiological laminar shear stress, subjected to a proinflammatory treatment and/or irradiation with X-rays or charged particles, followed by a coincubation with primary human lymphocytes (peripheral blood lymphocytes (PBL)). Analysis is performed by semiautomated quantification of fluorescent staining in microscopic pictures. First results obtained after irradiation with X-rays or helium ions indicate decreased adhesion of PBL to EC under laminar conditions for both radiation qualities, whereas adhesion of PBL under static conditions is not clearly affected by irradiation. Under static conditions, no radiation-induced changes in surface expression of adhesion molecules and activation of nuclear factor kappa B (NF-κB) signaling were observed after single cell-based high-throughput analysis. In subsequent studies, these investigations will be extended to laminar conditions.

Stenosis Hemodynamics Disrupt the Endothelial Cell Glycocalyx by MMP Activity Creating a Proinflammatory Environment

Hemodynamic forces are known to be able to induce an inflammatory phenotype in endothelial cells (ECs). The EC glycocalyx (GCX) is a dynamic structure which is regulated in response to different stimuli and hypothesized as an important contributor to the mechanotransduction of wall shear stresses (WSS). In this work, we used a three dimensional in vitro EC culture model with a 50% asymmetric stenosis to investigate degradation of the GCX by increased matrix metalloproteinase (MMP) activity in regions of WSS gradients and how this degradation might create a proinflammatory environment. Experiments showed GCX degradation was observed in regions of WSSGs created by a 50% asymmetric stenosis. Furthermore, inhibition of MMP activity abolished this regional degradation. The integrity of the GCX altered EC morphological elongation to flow and leukocyte adhesion patterns. These results help strengthen the hypothesis that the EC GCX is involved in the mechanotransduction of hemodynamic forces and that the GCX is regulated by MMP activity in regions of WSSGs.

Regulatory Roles of Fluctuation-Driven Mechanotransduction in Cell Function

Cells in the body are exposed to irregular mechanical stimuli. Here, we review the so-called fluctuation-driven mechanotransduction in which stresses stretching cells vary on a cycle-by-cycle basis. We argue that such mechanotransduction is an emergent network phenomenon and offer several potential mechanisms of how it regulates cell function. Several examples from the vasculature, the lung, and tissue engineering are discussed. We conclude with a list of important open questions.

News in Pathogenesis of Chronic Venous Insufficiency

This review article is concentrating on the news in the pathophysiology of chronic venous insufficiency (CVD). Despite ongoing progress in understanding the molecular aspects of CVD the exact mechanism of its development remains unclear. Many different factors may play role in the pathogenesis of CVD, including changes in hydrostatic pressure, valvular incompetence, increased capillary permeability, endothelial dysfunction, activation of leukocytes, deep venous obstruction, capillary microthrombosis, ineffective function of calf muscle pump, biochemical and structural changes in the vessel wall, extracellular matrix alteration, and several other mechanisms. A better understanding of the pathophysiology is an important step in the finding of new potential treatment.

Hypertension and Stroke in Patients with Left Ventricular Assist Devices (LVADs)

Stroke is one of the most dreaded complications of left ventricular assist device therapy in patients with end-stage congestive heart failure. There is strong evidence linking anticoagulation and infection with ischemic and hemorrhagic strokes, though recent data has emerged regarding the importance of elevated blood pressure. In the recently completed Heartware Ventricular Assist Device studies, a mean arterial pressure greater than 90 mmHg was associated with greater stroke risk, particularly the hemorrhagic subtype. In this review, we discuss recent evidence regarding deleterious effects of uncontrolled hypertension in patients with left ventricular devices, and propose measurement and management strategies.

Flow of Red Blood Cells in Stenosed Microvessels

A computational study is presented on the flow of deformable red blood cells in stenosed microvessels. It is observed that the Fahraeus-Lindqvist effect is significantly enhanced due to the presence of a stenosis. The apparent viscosity of blood is observed to increase by several folds when compared to non-stenosed vessels. An asymmetric distribution of the red blood cells, caused by geometric focusing in stenosed vessels, is observed to play a major role in the enhancement. The asymmetry in cell distribution also results in an asymmetry in average velocity and wall shear stress along the length of the stenosis. The discrete motion of the cells causes large time-dependent fluctuations in flow properties. The root-mean-square of flow rate fluctuations could be an order of magnitude higher than that in non-stenosed vessels. Several folds increase in Eulerian velocity fluctuation is also observed in the vicinity of the stenosis. Surprisingly, a transient flow reversal is observed upstream a stenosis but not downstream. The asymmetry and fluctuations in flow quantities and the flow reversal would not occur in absence of the cells. It is concluded that the flow physics and its physiological consequences are significantly different in micro- versus macrovascular stenosis.

The expression dynamics of mechanosensitive genes in extra-embryonic vasculature after heart starts to beat in chick embryo

BACKGROUND

Fluid flow plays an important role in vascular development. However, the detailed mechanisms, particularly the link between flow and modulation of gene expression during vascular development, remain unexplored. In chick embryo, the key events of vascular development from initiation of heart beat to establishment of effective blood flow occur between the stages HH10 and HH13. Therefore, we propose a novel in vivo model to study the flow experienced by developing endothelium.

OBJECTIVE

Using this model, we aimed to capture the transcriptome dynamics of the pre- and post-flow conditions.

METHODS

RNA was isolated from extra embryonic area vasculosa (EE-AV) pooled from three chick embryos between HH10-HH13 and RNA sequencing was performed.

RESULTS

The whole transcriptome sequencing of chick identified up-regulation of some of the previously well-known mechanosensitive genes including NFR2, HAND1, CTGF and KDR. GO analyses of the up-regulated genes revealed enrichment of several biological processes including heart development, extracellular matrix organization, cell-matrix adhesion, cell migration, blood vessel development, patterning of blood vessels, collagen fibril organization. Genes encoding for gap junctions proteins which are involved in vascular remodeling and arterial-venous differentiation, and genes involved in cell-cell adhesion, and ECM interactions were significantly up-regulated. Validation of selected genes through semi quantitative PCR was performed.

CONCLUSION

The study indicates that shear stress plays a major role in development. Through appropriate validation, this platform can serve as an in vivo model to study conditions of disturbed flow in pathology as well as normal flow during development.

Physiologically Modeled Pulse Dynamics to Improve Function in In Vitro-Endothelialized Small-Diameter Vascular Grafts

The lack of a functional endothelium on small-diameter vascular grafts leads to intimal hyperplasia and thrombotic occlusion. Shear stress conditioning through controlled hydrodynamics within in vitro perfusion bioreactors has shown promise as a mechanism to drive endothelial cell (EC) phenotype from an activated, pro-inflammatory wound state toward a quiescent functional state that inhibits responses that lead to occlusive failure. As part of an overall design strategy to engineer functional vascular grafts, we present a novel two-phase shear conditioning approach to improve graft endothelialization. Axial rotation was first used to seed uniform EC monolayers onto the lumenal surface of decellularized scaffolds derived from the human umbilical vein. Using computer-controlled perfusion circuits, a flow-ramping paradigm was applied to adapt endothelia to arterial levels of fluid shear stress and pressure without graft denudation. The effects of constant pulse frequencies (CF) on EC quiescence were then compared with pulse frequencies modeled from temporal fluctuations in blood flow observed in vivo, termed physiologically modeled pulse dynamics (PMPD). Constructs exposed to PMPD for 72 h expressed a more functional transcriptional profile, lower metabolic activity (39.8% ± 8.4% vs. 62.5% ± 11.5% reduction, p = 0.012), and higher nitric oxide production (80.42 ± 23.93 vs. 48.75 ± 6.93 nmol/10(5) cells, p = 0.028) than those exposed to CF. By manipulating in vitro flow conditions to mimic natural physiology, endothelialized vascular grafts can be stimulated to express a nonactivated phenotype that would better inhibit peripheral cell adhesion and smooth muscle cell hyperplasia, conditions that typically lead to occlusive failure. Development of robust, functional endothelia on vascular grafts by modulation of environmental conditions within perfusion bioreactors may ultimately improve clinical outcomes in vascular bypass grafting.

Temporal and spatial variations of wall shear stress in the entrance region of microvessels

Using a simplified two-dimensional divider-channel setup, we simulate the development process of red blood cell (RBC) flows in the entrance region of microvessels to study the wall shear stress (WSS) behaviors. Significant temporal and spatial variation in WSS is noticed. The maximum WSS magnitude and the strongest variation are observed at the channel inlet due to the close cell-wall contact. From the channel inlet, both the mean WSS and variation magnitude decrease, with a abrupt drop in the close vicinity near the inlet and then a slow relaxation over a relatively long distance; and a relative stable state with approximately constant mean and variation is established when the flow is well developed. The correlations between the WSS variation features and the cell free layer (CFL) structure are explored, and the effects of several hemodynamic parameters on the WSS variation are examined. In spite of the model limitations, the qualitative information revealed in this study could be useful for better understanding relevant processes and phenomena in the microcirculation.

Decreased platelet aggregation by shear stress-stimulated endothelial cells in vitro: description of a method and first results in diabetes

The interaction between platelets and endothelium in vivo is a complex phenomenon. Our aim was to develop an in vitro system that mimics the in vivo environment and investigate platelet function in a common pathological condition. Human umbilical vein endothelial cells were used and platelets from 28 type 2 diabetes patients were studied under shear stress conditions. Mean coefficient of variation of platelet aggregation was 10% in dynamic conditions in the presence of endothelium. Endothelial cells increased the concentration of inductor needed to achieve 50% platelet aggregation to adenosine diphosphate from 2.6 ± 1.3 in static conditions to 3.7 ± 1.3 µM in dynamic conditions. A similar pattern was observed when collagen was used for platelet activation. Incubation of endothelium with a nitric oxide inhibitor abolished this effect, indicating platelet inhibitory effect of endothelial cells is nitric oxide mediated. Platelet reactivity of healthy controls was less influenced by the presence of endothelial cells and displayed reduced basal platelet reactivity compared with platelets from diabetes patients. We show that platelet aggregation in diabetes as commonly reported in vitro may not fully reflect the in vivo pathophysiological process. Future studies are warranted to investigate other pathological conditions and analyse the effects of antiplatelet agents using this system.

Variation in Cardiac Pulse Frequencies Modulates vSMC Phenotype Switching During Vascular Remodeling

In vitro perfusion systems have exposed vascular constructs to mechanical conditions that emulate physiological pulse pressure and found significant improvements in graft development. However, current models maintain constant, or set pulse/shear mechanics that do not account for the natural temporal variation in frequency. With an aim to develop clinically relevant small diameter vascular grafts, these investigations detail a perfusion culture model that incorporates temporal pulse pressure variation. Our objective was to test the hypothesis that short-term variation in heart rate, such as changes in respiratory activity, plays a significant role in vascular remodeling and graft development. The pulse rate of a healthy volunteer was logged to model the effect of daily activities on heart rate. Vascular bioreactors were used to deliver perfusion conditions based on modeled frequencies of temporal pulse variability, termed Physiologically Modeled Pulse Dynamics (PMPD). Acellular scaffolds derived from the human umbilical vein were seeded with human vascular smooth muscle cells and perfused under defined pulsatile conditions. vSMC exposed to constant pulse frequencies expressed a contractile phenotype, while exposure to PMPD drove cells to a synthetic state with continued cell proliferation, increased tensile strength and stiffness as well as diminished vasoactivity. Results show the temporal variation associated with normal heart physiology to have a profound effect on vascular remodeling and vasoactive function. While these models are representative of vascular regeneration further investigation is required to understanding these and other key regulators in vSMC phenotype switching in non-pathological or wound healing states. This understanding has important clinical implications that may lead to improved treatments that enhance vessel regeneration.

The wall-stress footprint of blood cells flowing in microvessels

It is well known that mechanotransduction of hemodynamic forces mediates cellular processes, particularly those that lead to vascular development and maintenance. Both the strength and space-time character of these forces have been shown to affect remodeling and morphogenesis. However, the role of blood cells in the process remains unclear. We investigate the possibility that in the smallest vessels blood's cellular character of itself will lead to forces fundamentally different than the time-averaged forces usually considered, with fluctuations that may significantly exceed their mean values. This is quantitated through the use of a detailed simulation model of microvessel flow in two principal configurations: a diameter D=6.5 μm tube-a model for small capillaries through which red blood cells flow in single-file-and a D=12 μm tube-a model for a nascent vein or artery through which the cells flow in a confined yet chaotic fashion. Results in both cases show strong sensitivity to the mean flow speed U. Peak stresses exceed their means by greater than a factor of 10 when U/D≲10 s(-1), which corresponds to the inverse relaxation time of a healthy red blood cell. This effect is more significant for smaller D cases. At faster flow rates, including those more commonly observed under normal, nominally static physiological conditions, the peak fluctuations are more comparable with the mean shear stress. Implications for mechanotransduction of hemodynamic forces are discussed.

Novel high-throughput in vitro model for identifying hemodynamic-induced inflammatory mediators of cerebral aneurysm formation

Cerebral aneurysms are thought to develop at locations of hemodynamic shear stress, via an inflammatory process. The molecular mechanism that links shear stress to inflammation, however, is not completely understood. Progress in studying this disease is limited by a lack of a suitable in vitro model. To address this, we designed novel in vitro parallel-plate flow chamber models of a straight artery, a bifurcation, and a bifurcation aneurysm. We compared endothelial cell phenotypes across the 3 different models and among microenvironments within each flow model by cytokine array, ELISA, and relative immunofluorescence. Human aneurysms express interleukin-8 and chemokine (C-X-C motif) ligand 1 (CXCL1), whereas normal arteries do not. The bifurcation aneurysm model showed significantly higher interleukin-8 and CXCL1 levels than both the straight artery and bifurcation models. Within the bifurcation and bifurcation aneurysm models, endothelial cells near the bifurcation or within the aneurysm sac microenvironments have significantly higher expression of CXCL1, and interleukin-8 and CXCL1, respectively, than at the straight proximal segment or the limbs of the bifurcation. Murine aneurysms express CXCL1, and it is the primary ELR+ CXC chemokine expressed, whereas normal arteries do not. CXCL1 antibody blockade results in significantly fewer murine aneurysms (13.3 versus 66.7%; P=0.0078), decreased neutrophil infiltration, and vascular cell adhesion molecule 1 expression than an immunoglobulin G control. We successfully designed and validated a novel hemodynamic model of cerebral aneurysms in vitro. We also show that shear stress-induced CXCL1 plays a critical role in cerebral aneurysm formation.