Experimental tools to monitor the dynamics of endothelial barrier function: a survey of in vitro approaches

Development of a 3D printed perfusable in vitro blood-brain barrier model for use as a scalable screening tool

Despite recent technological advances in drug discovery, the success rate for neurotherapeutics remains alarmingly low compared to treatments for other areas of the body. One of the biggest challenges for delivering therapeutics to the central nervous system (CNS) is the presence of the blood-brain barrier (BBB). In vitro blood-brain barrier models with high predictability are essential to aid in designing parameters for new therapeutics, assess their ability to cross the BBB, and investigate therapeutic strategies that can be employed to enhance transport. Here, we demonstrate the development of a 3D printable hydrogel blood-brain barrier model that mimics the cellular composition and structure of the blood-brain barrier with human brain endothelial cells lining the surface, pericytes in direct contact with the endothelial cells on the abluminal side of the endothelium, and astrocytes in the surrounding printed bulk matrix. We introduce a simple, static printed hemi-cylinder model to determine design parameters such as media selection, co-culture ratios, and cell incorporation timing in a resource-conservative and high-throughput manner. Presence of cellular adhesion junction, VE-Cadherin, efflux transporters, P-glycoprotein (P-gp) and Breast cancer resistance protein (BCRP), and receptor-mediated transporters, Transferrin receptor (TfR) and low-density lipoprotein receptor-related protein 1 (LRP1) were confirmed via immunostaining demonstrating the ability of this model for screening in therapeutic strategies that rely on these transport systems. Design parameters determined in the hemi-cylinder model were translated to a more complex, perfusable vessel model to demonstrate its utility for determining barrier function and assessing permeability to model therapeutic compounds. This 3D-printed blood-brain barrier model represents one of the first uses of projection stereolithography to fabricate a perfusable blood-brain barrier model, enabling the patterning of complex vessel geometries and precise arrangement of cell populations. This model demonstrates potential as a new platform to investigate the delivery of neurotherapeutic compounds and drug delivery strategies through the blood-brain barrier, providing a useful in vitro screening tool in central nervous system drug discovery and development.

Innovative electrode and chip designs for transendothelial electrical resistance measurements in organs-on-chips

Many different epithelial and endothelial barriers in the human body ensure the proper functioning of our organs by controlling which substances can pass from one side to another. In recent years, organs-on-chips (OoC) have become a popular tool to study such barriers in vitro. To assess the proper functioning of these barriers, we can measure the transendothelial electrical resistance (TEER) which indicates how easily ions can cross the cell layer when a current is applied between electrodes on either side. TEER measurements are a convenient method to quantify the barrier properties since it is a non-invasive and label-free technique. Direct integration of electrodes for TEER measurements into OoC allows for continuous monitoring of the barrier, and fixed integration of the electrodes improves the reproducibility of the measurements. In this review, we will give an overview of different electrode and channel designs that have been used to measure the TEER in OoC. After giving some insight into why biological barriers are an important field of study, we will explain the theory and practice behind measuring the TEER in in vitro systems. Next, this review gives an overview of the state of the art in the field of integrated electrodes for TEER measurements in OoC, with a special focus on alternative chip and electrode designs. Finally, we outline some of the remaining challenges and provide some suggestions on how to overcome these challenges.

A review of electrochemical impedance as a tool for examining cell biology and subcellular mechanisms: merits, limits, and future prospects

Herein the development of cellular impedance biosensors, electrochemical impedance spectroscopy, and the general principles and terms associated with the cell-electrode interface is reviewed. This family of techniques provides quantitative and sensitive information into cell responses to stimuli in real-time with high temporal resolution. The applications of cell-based impedance biosensors as a readout in cell biology is illustrated with a diverse range of examples. The current state of the field, its limitations, the possible available solutions, and the potential benefits of developing biosensors are discussed.

Advances in TEER measurements of biological barriers in microphysiological systems

Biological barriers are multicellular structures that precisely regulate the transport of ions, biomolecules, drugs, cells, and other organisms. Transendothelial/epithelial electrical resistance (TEER) is a label-free method for predicting the properties of biological barriers. Understanding the mechanisms that control TEER significantly enhances our knowledge of the physiopathology of different diseases and aids in the development of new drugs. Measuring TEER values within microphysiological systems called organ-on-a-chip devices that simulate the microenvironment, architecture, and physiology of biological barriers in the body provides valuable insight into the behavior of barriers in response to different drugs and pathogens. These integrated systems should increase the accuracy, reproducibility, sensitivity, resolution, high throughput, speed, cost-effectiveness, and reliable predictability of TEER measurements. Implementing advanced micro and nanoscale manufacturing techniques, surface modification methods, biomaterials, biosensors, electronics, and stem cell biology is necessary for integrating TEER measuring systems with organ-on-chip technology. This review focuses on the applications, advantages, and future perspectives of integrating organ-on-a-chip technology with TEER measurement methods for studying biological barriers. After briefly reviewing the role of TEER in the physiology and pathology of barriers, standard techniques for measuring TEER, including Ohm's law and impedance spectroscopy, and commercially available devices are described. Furthermore, advances in TEER measurement are discussed in multiple barrier-on-a-chip system models representing different organs. Finally, we outline future trends in implementing advanced technologies to design and fabricate nanostructured electrodes, complicated microfluidic chips, and membranes for more advanced and accurate TEER measurements.

Emerging strategies of engineering retinal organoids and organoid-on-a-chip in modeling intraocular drug delivery: current progress and future perspectives

Retinal diseases are a rising concern as major causes of blindness in an aging society; therapeutic options are limited, and the precise pathogenesis of these diseases remains largely unknown. Intraocular drug delivery and nanomedicines offering targeted, sustained, and controllable delivery are the most challenging and popular topics in ocular drug development and toxicological evaluation. Retinal organoids (ROs) and organoid-on-a-chip (ROoC) are both emerging as promising in-vitro models to faithfully recapitulate human eyes for retinal research in the replacement of experimental animals and primary cells. In this study, we review the generation and application of ROs resembling the human retina in cell subtypes and laminated structures and introduce the emerging engineered ROoC as a technological opportunity to address critical issues. On-chip vascularization, perfusion, and close inter-tissue interactions recreate physiological environments in vitro, whilst integrating with biosensors facilitates real-time analysis and monitoring during organogenesis of the retina representing engineering efforts in ROoC models. We also emphasize that ROs and ROoCs hold the potential for applications in modeling intraocular drug delivery in vitro and developing next-generation retinal drug delivery strategies.

Kinase Inhibition by PKC412 Prevents Epithelial Sheet Damage in Autosomal Dominant Epidermolysis Bullosa Simplex through Keratin and Cell Contact Stabilization

Epidermolysis bullosa simplex (EBS) is a severe and potentially life-threatening disorder for which no adequate therapy exists. Most cases are caused by dominant sequence variations in keratin genes K5 or K14, leading to the formation of cytoplasmic keratin aggregates, profound keratinocyte fragility, and cytolysis. We hypothesized that pharmacological reduction of keratin aggregates, which compromise keratinocyte integrity, represents a viable strategy for the treatment of EBS. In this study, we show that the multikinase inhibitor PKC412, which is currently in clinical use for acute myeloid leukemia and advanced systemic mastocytosis, reduced keratin aggregation by 40% in patient-derived K14.R125C EBS-associated keratinocytes. Using a combination of epithelial shear stress assay and real-time impedance spectroscopy, we show that PKC412 restored intercellular adhesion. Molecularly, global phosphoproteomic analysis together with immunoblots using phosphoepitope-specific antibodies revealed that PKC412 treatment altered phosphorylated sites on keratins and desmoplakin. Thus, our data provide a proof of concept to repurpose existing drugs for the targeted treatment of EBS and showcase how one broad-range kinase inhibitor reduced keratin filament aggregation in patient-derived EBS keratinocytes and the fragility of EBS cell monolayers. Our study paves the way for a clinical trial using PKC412 for systemic or local application in patients with EBS.

In Vitro Methods for Measuring the Permeability of Cell Monolayers

Cell monolayers, including endothelial and epithelial cells, play crucial roles in regulating the transport of biomolecules to underlying tissues and structures via intercellular junctions. Moreover, the monolayers form a semipermeable barrier across which leukocyte transmigration is tightly regulated. The inflammatory cytokines can disrupt the epithelial and endothelial permeability, thus the reduced barrier integrity is a hallmark of epithelial and endothelial dysfunction related with numerous pathological conditions, including cancer-related inflammation. Therefore, the assessment of barrier function is critical in in vitro models of barrier-forming tissues. This review summarizes the commercially available in vitro systems used to measure the permeability of cellular monolayers. The presented techniques are separated in two large groups: macromolecular tracer flux assays, and electrical impedance measurement-based permeability assays. The presented techniques are briefly described and compared.

A 3D printable perfused hydrogel vascular model to assay ultrasound-induced permeability

The development of an in vitro model to study vascular permeability is vital for clinical applications such as the targeted delivery of therapeutics. This work demonstrates the use of a...

Development of an Image-Based HCS-Compatible Method for Endothelial Barrier Function Assessment

The recent renascence of phenotypic drug discovery (PDD) is catalyzed by its ability to identify first-in-class drugs and deliver results when the exact molecular mechanism is partially obscure. Acute respiratory distress syndrome (ARDS) is a severe, life-threatening condition with a high mortality rate that has increased in frequency due to the COVID-19 pandemic. Despite decades of laboratory and clinical study, no efficient pharmacological therapy for ARDS has been found. An increase in endothelial permeability is the primary event in ARDS onset, causing the development of pulmonary edema that leads to respiratory failure. Currently, the detailed molecular mechanisms regulating endothelial permeability are poorly understood. Therefore, the use of the PDD approach in the search for efficient ARDS treatment can be more productive than classic target-based drug discovery (TDD), but its use requires a new cell-based assay compatible with high-throughput (HTS) and high-content (HCS) screening. Here we report the development of a new plate-based image cytometry method to measure endothelial barrier function. The incorporation of image cytometry in combination with digital image analysis substantially decreases assay variability and increases the signal window. This new method simultaneously allows for rapid measurement of cell monolayer permeability and cytological analysis. The time-course of permeability increase in human pulmonary artery endothelial cells (HPAECs) in response to the thrombin and tumor necrosis factor α treatment correlates with previously published data obtained by transendothelial resistance (TER) measurements. Furthermore, the proposed image cytometry method can be easily adapted for HTS/HCS applications.

In-Line Analysis of Organ-on-Chip Systems with Sensors: Integration, Fabrication, Challenges, and Potential

Organ-on-chip systems are promising new in vitro research tools in medical, pharmaceutical, and biological research. Their main benefit, compared to standard cell culture platforms, lies in the improved in vivo resemblance of the cell culture environment. A critical aspect of these systems is the ability to monitor both the cell culture conditions and biological responses of the cultured cells, such as proliferation and differentiation rates, release of signaling molecules, and metabolic activity. Today, this is mostly done using microscopy techniques and off-chip analytical techniques and assays. Integrating in situ analysis methods on-chip enables improved time resolution, continuous measurements, and a faster read-out; hence, more information can be obtained from the developed organ and disease models. Integrated electrical, electrochemical, and optical sensors have been developed and used for chemical analysis in lab-on-a-chip systems for many years, and recently some of these sensing principles have started to find use in organ-on-chip systems as well. This perspective review describes the basic sensing principles, sensor fabrication, and sensor integration in organ-on-chip systems. The review also presents the current state of the art of integrated sensors and discusses future potential. We bring a technological perspective, with the aim of introducing in-line sensing and its promise to advance organ-on-chip systems and the challenges that lie in the integration to researchers without expertise in sensor technology.

In Vitro Investigation of Vascular Permeability in Endothelial Cells from Human Artery, Vein and Lung Microvessels at Steady-State and Anaphylactic Conditions

Human anaphylactic reactions largely involve an increase in vascular permeability, which is mainly controlled by endothelial cells (ECs). Due to the acute and serious nature of human anaphylaxis, in vivo studies of blood vessels must be replaced or supplemented with in vitro models. Therefore, we used a macromolecular tracer assay (MMTA) to investigate the EC permeability of three phenotypes of human ECs: artery (HAECs), vein (HSVECs) and microvessels from lung (HMLECs). ECs were stimulated with two fast-acting anaphylactic mediators (histamine and platelet-activating factor (PAF)) and one longer-lasting mediator (thrombin). At steady-state conditions, HSVEC monolayers were the most permeable and HMLEC the least (15.8% and 8.3% after 60 min, respectively). No response was found in ECs from artery or vein to any stimuli. ECs from microvessels reacted to stimulation with thrombin and also demonstrated a tendency of increased permeability for PAF. There was no reaction for histamine. This was not caused by missing receptor expression, as all three EC phenotypes expressed receptors for both PAF and histamine. The scarce response to fast-acting mediators illustrates that the MMTA is not suitable for investigating EC permeability to anaphylactic mediators.

Autoregulatory "Multitasking" at Endothelial Cell Junctions by Junction-Associated Intermittent Lamellipodia Controls Barrier Properties

Vascular endothelial cell (EC) junctions are key structures controlling tissue homeostasis in physiology. In the last three decades, excellent studies have addressed many aspects of this complex and highly dynamic regulation, including cell signaling, remodeling processes of the proteins of tight junctions, adherens junctions, and gap junctions, the cytoskeleton, and post-transcriptional modifications, transcriptional activation, and gene silencing. In this dynamic process, vascular endothelial cadherin (VE-cadherin) provides the core structure of EC junctions mediating the physical adhesion of cells as well as the control of barrier function and monolayer integrity via remodeling processes, regulation of protein expression and post-translational modifications. In recent years, research teams have documented locally restricted dynamics of EC junctions in which actin-driven protrusions in plasma membranes play a central role. In this regard, our research group showed that the dynamics of VE-cadherin is driven by small (1-5 μm) actin-mediated protrusions in plasma membranes that, due to this specific function, were named "junction-associated intermittent lamellipodia" (JAIL). JAIL form at overlapping, adjacent cells, and exactly at this site new VE-cadherin interactions occur, leading to new VE-cadherin adhesion sites, a process that restores weak or lost VE-cadherin adhesion. Mechanistically, JAIL formation occurs locally restricted (1-5 μm) and underlies autoregulation in which the local VE-cadherin concentration is an important parameter. A decrease in the local concentration of VE-cadherin stimulates JAIL formation, whereas an increase in the concentration of VE-cadherin blocks it. JAIL mediated VE-cadherin remodeling at the subjunctional level have been shown to be of crucial importance in angiogenesis, wound healing, and changes in permeability during inflammation. The concept of subjunctional regulation of EC junctions is strongly supported by permeability assays, which can be employed to quantify actin-driven subjunctional changes. In this brief review, we summarize and discuss the current knowledge and concepts of subjunctional regulation in the endothelium.

A label-free impedance assay in endothelial cells differentiates the activation and desensitization properties of clinical S1P1 agonists

Sphingosine-1 phosphate receptor-1 (S1P1 ) activation maintains endothelial barrier integrity, whereas S1P1 desensitization induces peripheral blood lymphopenia. The latter is exploited in the approval and/or late-stage development of receptor-desensitizing agents targeting the S1P1 receptor in multiple sclerosis, such as siponimod, ozanimod, and ponesimod. SAR247799 is a recently described G protein-biased S1P1 agonist that activates S1P1 without desensitization and thus has endothelial-protective properties in patients without reducing lymphocytes. As SAR247799 demonstrated endothelial-protective effects at sub-lymphocyte-reducing doses, the possibility exists that other S1P1 modulators could also exhibit endothelial-protective properties at lower doses. To explore this possibility, we sought to quantitatively compare the biased properties of SAR247799 with the most advanced clinical molecules targeting S1P1 . In this study, we define the β-arrestin pathway component of the impedance profile following S1P1 activation in a human umbilical vein endothelial cell line (HUVEC) and report quantitative indices of the S1P1 activation-to-desensitization ratio of various clinical molecules. In a label-free impedance assay assessing endothelial barrier integrity and disruption, the mean estimates (95% confidence interval) of the activation-to-desensitization ratios of SAR247799, ponesimod, ozanimod, and siponimod were 114 (91.1-143), 7.66 (3.41-17.2), 6.35 (3.21-12.5), and 0.170 (0.0523-0.555), respectively. Thus, we show that SAR247799 is the most G protein-biased S1P1 agonist currently characterized. This rank order of bias among the most clinically advanced S1P1 modulators provides a new perspective on the relative potential of these clinical molecules for improving endothelial function in patients in relation to their lymphocyte-reducing (desensitization) properties.

EPLIN-α and -β Isoforms Modulate Endothelial Cell Dynamics through a Spatiotemporally Differentiated Interaction with Actin

Actin-binding proteins are essential for linear and branched actin filament dynamics that control shape change, cell migration, and cell junction remodeling in vascular endothelium (endothelial cells [ECs]). The epithelial protein lost in neoplasm (EPLIN) is an actin-binding protein, expressed as EPLIN-α and EPLIN-β by alternative promoters; however, the isoform-specific functions are not yet understood. Aortic compared to cava vein ECs and shear stress-exposed cultured ECs express increased EPLIN-β levels that stabilize stress fibers. In contrast, EPLIN-α expression is increased in growing and migrating ECs, is targeted to membrane protrusions, and terminates their growth via interaction with the Arp2/3 complex. The data indicate that EPLIN-α controls protrusion dynamics while EPLIN-β has an actin filament stabilizing role, which is consistent with FRAP analyses demonstrating a lower EPLIN-β turnover rate compared to EPLIN-α. Together, EPLIN isoforms differentially control actin dynamics in ECs, essential in shear stress responses, cell migration, and barrier function.

Integrating Biosensors in Organs-on-Chip Devices: A Perspective on Current Strategies to Monitor Microphysiological Systems

Organs-on-chip (OoC), often referred to as microphysiological systems (MPS), are advanced in vitro tools able to replicate essential functions of human organs. Owing to their unprecedented ability to recapitulate key features of the native cellular environments, they represent promising tools for tissue engineering and drug screening applications. The achievement of proper functionalities within OoC is crucial; to this purpose, several parameters (e.g., chemical, physical) need to be assessed. Currently, most approaches rely on off-chip analysis and imaging techniques. However, the urgent demand for continuous, noninvasive, and real-time monitoring of tissue constructs requires the direct integration of biosensors. In this review, we focus on recent strategies to miniaturize and embed biosensing systems into organs-on-chip platforms. Biosensors for monitoring biological models with metabolic activities, models with tissue barrier functions, as well as models with electromechanical properties will be described and critically evaluated. In addition, multisensor integration within multiorgan platforms will be further reviewed and discussed.

PETER-assay: Combined Impedimetric Detection of Permeability (PE) and Resistance (TER) of Barrier-Forming Cell Layers

Epithelial and endothelial barrier function is typically studied in vitro by growing the cells of interest on permeable supports that are sandwiched between two fluid compartments. This setup mimics the physiological situation with the cell layer as the diffusion barrier at the interface between two chemically distinct fluids. Routinely, the barrier function is quantitatively described by two key parameters: (i) the transepithelial or transendothelial electrical resistance (TER) as a measure of the permeability for small inorganic ions and (ii) the permeability coefficient (P_E) as a descriptor of the permeability for molecular tracers. So far the two parameters have been determined in separate experiments. This study introduces a device that allows for simultaneous detection of P_E and TER of the very same cell monolayer in one single experiment (P_ETER-assay). The novel approach is entirely based on AC impedance measurements in two different modes, so that TER and P_E become available in real time. The new approach is demonstrated for three epithelial cell lines derived from the kidney (MDCK-I, MDCK-II, NRK) with very different barrier properties under stationary conditions and when challenged by barrier-breaking fungal toxin cytochalasin D. P_ETER provides an excellent time-resolution and completely automated data collection.

Arg mediates LPS-induced disruption of the pulmonary endothelial barrier

Acute Respiratory Distress Syndrome (ARDS) is a devastating disease process that involves dysregulated inflammation and decreased alveolar-capillary barrier function. Despite increased understanding of the pathophysiology, no effective targeted therapies exist to treat ARDS. Recent preclinical studies suggest that the multi-tyrosine kinase inhibitor, imatinib, which targets the Abl kinases c-Abl and Arg, has the potential to restore endothelial dysfunction caused by inflammatory agonists. Prior work demonstrates that imatinib attenuates LPS (lipopolysaccharide)-induced vascular leak and inflammation; however, the mechanisms underlying these effects remain incompletely understood. In the current study, we demonstrate that imatinib inhibits LPS-induced increase in the phosphorylation of CrkL, a specific substrate of Abl kinases, in human pulmonary endothelial cells. Specific silencing of Arg, and not c-Abl, attenuated LPS-induced pulmonary vascular permeability as measured by electrical cellular impedance sensing (ECIS) and gap formation assays. In addition, direct activation of Abl family kinases with the small molecule activator DPH resulted in endothelial barrier disruption that was attenuated by Arg siRNA. In complementary studies to characterize the mechanisms by which Arg mediates endothelial barrier function, Arg silencing was found to inhibit LPS-induced disruption of adherens junctions and phosphorylation of myosin light chains (MLC). Overall, these results characterize the mechanisms by which imatinib protects against LPS-induced endothelial barrier disruption and suggest that Arg inhibition may represent a novel strategy to enhance endothelial barrier function.

Quantitatively relating brain endothelial cell-cell junction phenotype to global and local barrier properties under varied culture conditions via the Junction Analyzer Program

BACKGROUND

The endothelial cell-cell junctions of the blood-brain barrier (BBB) play a pivotal role in the barrier's function. Altered cell-cell junctions can lead to barrier dysfunction and have been implicated in several diseases. Despite this, the driving forces regulating junctional protein presentation remain relatively understudied, largely due to the lack of efficient techniques to quantify their presentation at sites of cell-cell adhesion. Here, we used our novel Junction Analyzer Program (JAnaP) to quantify junction phenotype (i.e., continuous, punctate, or perpendicular) in response to various substrate compositions, cell culture times, and cAMP treatments in human brain microvascular endothelial cells (HBMECs). We then quantitatively correlated junction presentation with barrier permeability on both a "global" and "local" scale.

METHODS

We cultured HBMECs on collagen I, fibronectin, collagen IV, laminin, fibronectin/collagen IV/laminin, or hyaluronic acid/gelatin for 2, 4, and 7 days with varying cAMP treatment schedules. Images of immunostained ZO-1, VE-cadherin, and claudin-5 were analyzed using the JAnaP to calculate the percent of the cell perimeter presenting continuous, punctate, or perpendicular junctions. Transwell permeability assays and resistance measurements were used to measure bulk ("global") barrier properties, and a "local" permeability assay was used to correlate junction presentation proximal to permeable monolayer regions.

RESULTS

Substrate composition was found to play little role in junction presentation, while cAMP supplements significantly increased the continuous junction architecture. Increased culture time required increased cAMP treatment time to reach similar ZO-1 and VE-cadherin coverage observed with shorter culture, though longer cultures were required for claudin-5 presentation. Prolonged cAMP treatment (6 days) disrupted junction integrity for all three junction proteins. Transwell permeability and TEER assays showed no correlation with junction phenotype, but a local permeability assay revealed a correlation between the number of discontinuous and no junction regions with barrier penetration.

CONCLUSIONS

These results suggest that cAMP signaling influences HBMEC junction architecture more than matrix composition. Our studies emphasized the need for local barrier measurement to mechanistically understand the role of junction phenotype and supported previous results that continuous junctions are indicative of a more mature/stable endothelial barrier. Understanding what conditions influence junction presentations, and how they, in turn, affect barrier integrity, could lead to the development of therapeutics for diseases associated with BBB dysfunction.

Transport Studies Using Blood-Brain Barrier In Vitro Models: A Critical Review and Guidelines

Permeation is one of the most evaluated parameters using preclinical in vitro blood-brain barrier models, as it has long been considered to be one of the major factors influencing central nervous system drug delivery. Blood-brain barrier permeability can be defined as the speed at which a compound crosses the brain endothelial cell barrier and is employed to assess barrier tightness, which is a crucial feature of brain capillaries in vivo. In addition, it is used to assess brain drug penetration. We review traditionally used methods to assess blood-brain barrier permeability in vitro and summarize often neglected in vivo (e.g., plasma protein and brain tissue binding) or in vitro (e.g., culture insert materials or methodology) factors that influence this property. These factors are crucial to consider when performing BBB permeability assessments, and especially when comparing permeability data obtained from different models, since model diversification significantly complicates inter-study comparisons. Finally, measuring transendothelial electrical resistance can be used to describe blood-brain barrier tightness; however, several parameters should be considered while comparing these measurements to the blood-brain barrier permeability to paracellular markers.

Transient blood-brain barrier disruption is induced by low pulsed electrical fields in vitro: an analysis of permeability and trans-endothelial electric resistivity

The blood–brain barrier (BBB) is limiting transcellular and paracellular movement of molecules and cells, controls molecular traffic, and keeps out toxins. However, this protective function is the major hurdle for treating brain diseases such as brain tumors, Parkinson’s disease, Alzheimer’s disease, etc. It was previously demonstrated that high pulsed electrical fields (PEFs) can disrupt the BBB by inducing electroporation (EP) which increases the permeability of the transcellular route. Our goal was to study the effects of low PEFs, well below the threshold of EP on the integrity and function of the BBB. Ten low voltage pulses (5–100 V) were applied to a human in vitro BBB model. Changes in permeability to small molecules (NaF) were studied as well as changes in impedance spectrum and trans-endothelial electric resistivity. Viability and EP were evaluated by Presto-Blue and endogenous Lactate dehydrogenase release assays. The effect on tight junction and adherent junction protein was also studied. The results of low voltage experiments were compared to high voltage experiments (200–1400 V). A significant increase in permeability was found at voltages as low as 10 V despite EP only occurring from 100 V. The changes in permeability as a function of applied voltage were fitted to an inverse-exponential function, suggesting a plateau effect. Staining of VE-cadherin showed specific changes in protein expression. The results indicate that low PEFs can transiently disrupt the BBB by affecting the paracellular route, although the mechanism remains unclear.

Impedance-based cellular assays for regenerative medicine

Therapies based on regenerative techniques have the potential to radically improve healthcare in the coming years. As a result, there is an emerging need for non-destructive and label-free technologies to assess the quality of engineered tissues and cell-based products prior to their use in the clinic. In parallel, the emerging regenerative medicine industry that aims to produce stem cells and their progeny on a large scale will benefit from moving away from existing destructive biochemical assays towards data-driven automation and control at the industrial scale. Impedance-based cellular assays (IBCA) have emerged as an alternative approach to study stem-cell properties and cumulative studies, reviewed here, have shown their potential to monitor stem-cell renewal, differentiation and maturation. They offer a novel method to non-destructively assess and quality-control stem-cell cultures. In addition, when combined with in vitro disease models they provide complementary insights as label-free phenotypic assays. IBCA provide quantitative and very sensitive results that can easily be automated and up-scaled in multi-well format. When facing the emerging challenge of real-time monitoring of three-dimensional cell culture dielectric spectroscopy and electrical impedance tomography represent viable alternatives to two-dimensional impedance sensing.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

A Novel Microscopic Assay Reveals Heterogeneous Regulation of Local Endothelial Barrier Function

Blood vessels are covered with endothelial cells on their inner surfaces, forming a selective and semipermeable barrier between the blood and the underlying tissue. Many pathological processes, such as inflammation or cancer metastasis, are accompanied by an increased vascular permeability. Progress in live cell imaging techniques has recently revealed that the structure of endothelial cell contacts is constantly reorganized and that endothelial junctions display high heterogeneities at a subcellular level even within one cell. Although it is assumed that this dynamic remodeling is associated with a local change in endothelial barrier function, a direct proof is missing mainly because of a lack of appropriate experimental techniques. Here, we describe a new assay to dynamically measure local endothelial barrier function with a lateral resolution of ∼15 μm and a temporal resolution of 1 min. In this setup, fluorescence-labeled molecules are added to the apical compartment of an endothelial monolayer, and the penetration of molecules from the apical to the basal compartment is recorded by total internal reflection fluorescence microscopy utilizing the generated evanescent field. With this technique, we found a remarkable heterogeneity in the local permeability for albumin within confluent endothelial cell layers. In regions with low permeability, stimulation with the proinflammatory agent histamine results in a transient increase in paracellular permeability. The effect showed a high variability along the contact of one individual cell, indicating a local regulation of endothelial barrier function. In regions with high basal permeability, histamine had no obvious effect. In contrast, the barrier-enhancing drug forskolin reduces the permeability for albumin and dextran uniformly along the cell junctions. Because this new approach can be readily combined with other live cell imaging techniques, it will contribute to a better understanding of the mechanisms underlying subcellular junctional reorganization during wound healing, inflammation, and angiogenesis.

Microfluidic assay for the on-chip electrochemical measurement of cell monolayer permeability

Cell monolayers, including endothelial cells lining the vasculature and blood-brain barrier, and epithelial cells lining the lung airways and gut, form a semipermeable barrier across which transport of biomolecules is tightly regulated. The assessment of barrier function is therefore critical in in vitro models of barrier-forming tissues, including microfluidic organ-on-a-chip models. Cell monolayer barrier function is commonly assessed using a fluorescent tracer-based permeability assay in both conventional Transwell and organ-on-a-chip models, but this method requires laborious manual sampling, bulky instrumentation and offline sample processing. In this work, we introduce a novel on-chip microfluidic permeability assay that replaces the traditional fluorescent tracer with an electroactive tracer. Similar to methods such as TEER, the electrochemical permeability assay eliminates the need for manual sampling and complex optical instrumentation. We validated the method by demonstrating close agreement between experimental and numerically-simulated diffusive and convective transport in the microfluidic device. Different electroactive tracers were screened for efficient electron transfer, stability and inertness relative to the cell monolayer. The assay was then used to measure the permeability of endothelial cells cultured under both static and flow culture conditions, and after exposure to a permeability mediator. In summary, the electrochemical permeability assay combines the simplicity of tracer-based permeability methods with the benefits of on-chip integration, which will ultimately facilitate the robust multiparametric characterization of barrier function in microfluidic organs-on-chips.

Towards non-invasive characterisation of coronary stent re-endothelialisation - An in-vitro, electrical impedance study

The permanent implantation of a stent has become the most common method for ameliorating coronary artery narrowing arising from atherosclerosis. Following the procedure, optimal arterial wall healing is characterised by the complete regrowth of an Endothelial Cell monolayer over the exposed stent surface and surrounding tissue, thereby reducing the risk of thrombosis. However, excessive proliferation of Smooth Muscle Cells, within the artery wall can lead to unwanted renarrowing of the vessel lumen. Current imaging techniques are unable to adequately identify re-endothelialisation, and it has previously been reported that the stent itself could be used as an electrode in combination with electrical impedance spectroscopic techniques to monitor the post-stenting recovery phase. The utility of such a device will be determined by its ability to characterise between vascular cell types. Here we present in-vitro impedance spectroscopy measurements of pulmonary artery porcine Endothelial Cells, Human Umbilical Vein Endothelial Cells and coronary artery porcine Smooth Muscle Cells grown to confluence over platinum black electrodes in clinically relevant populations. These measurements were obtained, using a bespoke impedance spectroscopy system that autonomously performed impedance sweeps in the 1kHz to 100kHz frequency range. Analysis of the reactance component of impedance revealed distinct frequency dependent profiles for each cell type with post confluence reactance declines in Endothelial Cell populations that have not been previously reported. Such profiles provide a means of non-invasively characterising between the cell types and give an indication that impedance spectroscopic techniques may enable the non-invasive characterisation of the arterial response to stent placement.

The expression of VE-cadherin in breast cancer cells modulates cell dynamics as a function of tumor differentiation and promotes tumor-endothelial cell interactions

The cadherin switch has profound consequences on cancer invasion and metastasis. The endothelial-specific vascular endothelial cadherin (VE-cadherin) has been demonstrated in diverse cancer types including breast cancer and is supposed to modulate tumor progression and metastasis, but underlying mechanisms need to be better understood. First, we evaluated VE-cadherin expression by tissue microarray in 392 cases of breast cancer tumors and found a diverse expression and distribution of VE-cadherin. Experimental expression of fluorescence-tagged VE-cadherin (VE-EGFP) in undifferentiated, fibroblastoid and E-cadherin-negative MDA-231 (MDA-VE-EGFP) as well as in differentiated E-cadherin-positive MCF-7 human breast cancer cell lines (MCF-VE-EGFP), respectively, displayed differentiation-dependent functional differences. VE-EGFP expression reversed the fibroblastoid MDA-231 cells to an epithelial-like phenotype accompanied by increased β-catenin expression, actin and vimentin remodeling, increased cell spreading and barrier function and a reduced migration ability due to formation of VE-cadherin-mediated cell junctions. The effects were largely absent in both MDA-VE-EGFP and in control MCF-EGFP cell lines. However, MCF-7 cells displayed a VE-cadherin-independent planar cell polarity and directed cell migration that both developed in MDA-231 only after VE-EGFP expression. Furthermore, VE-cadherin expression had no effect on tumor cell proliferation in monocultures while co-culturing with endothelial cells enhanced tumor cell proliferation due to integration of the tumor cells into monolayer where they form VE-cadherin-mediated cell contacts with the endothelium. We propose an interactive VE-cadherin-based crosstalk that might activate proliferation-promoting signals. Together, our study shows a VE-cadherin-mediated cell dynamics and an endothelial-dependent proliferation in a differentiation-dependent manner.

Statin regulated ERK5 stimulates tight junction formation and reduces permeability in human cardiac endothelial cells

The MEKK3/MEK5/ERK5 signaling axis is required for cardiovascular development in vivo. We analyzed the physiological role of ERK5 in cardiac endothelial cells and the consequence of activation of this kinase by the statin class of HMG Co‐A reductase inhibitor drugs. We utilized human cardiac microvascular endothelial cells (HCMECs) and altered ERK5 expression using siRNA mediated gene silencing or overexpression of constitutively active MEK5 and ERK5 to reveal a role for ERK5 in regulating endothelial tight junction formation and cell permeability. Statin treatment of HCMECs stimulated activation of ERK5 and translocation to the plasma membrane resulting in co‐localization with the tight junction protein ZO‐1 and a concomitant reduction in endothelial cell permeability. Statin mediated activation of ERK5 was a consequence of reduced isoprenoid synthesis following HMG Co‐A reductase inhibition. Statin pretreatment could overcome the effect of doxorubicin in reducing endothelial tight junction formation and prevent increased permeability. Our data provide the first evidence for the role of ERK5 in regulating endothelial tight junction formation and endothelial cell permeability. Statin mediated ERK5 activation and the resulting decrease in cardiac endothelial cell permeability may contribute to the cardioprotective effects of statins in reducing doxorubicin‐induced cardiotoxicity.

VEGF isoforms have differential effects on permeability of human pulmonary microvascular endothelial cells

BACKGROUND

Alternative splicing of Vascular endothelial growth factor-A mRNA transcripts (commonly referred as VEGF) leads to the generation of functionally differing isoforms, the relative amounts of which have potentially significant physiological outcomes in conditions such as acute respiratory distress syndrome (ARDS). The effect of such isoforms on pulmonary vascular permeability is unknown. We hypothesised that VEGF₁₆₅a and VEGF₁₆₅b isoforms would have differing effects on pulmonary vascular permeability caused by differential activation of intercellular signal transduction pathways.

METHOD

To test this hypothesis we investigated the physiological effect of VEGF₁₆₅a and VEGF₁₆₅b on Human Pulmonary Microvascular Endothelial Cell (HPMEC) permeability using three different methods: trans-endothelial electrical resistance (TEER), Electric cell-substrate impedance sensing (ECIS) and FITC-BSA passage. In addition, potential downstream signalling pathways of the VEGF isoforms were investigated by Western blotting and the use of specific signalling inhibitors.

RESULTS

VEGF₁₆₅a increased HPMEC permeability using all three methods (paracellular and transcellular) and led to associated VE-cadherin and actin stress fibre changes. In contrast, VEGF₁₆₅b decreased paracellular permeability and did not induce changes in VE-cadherin cell distribution. Furthermore, VEGF₁₆₅a and VEGF₁₆₅b had differing effects on both the phosphorylation of VEGF receptors and downstream signalling proteins pMEK, p42/44MAPK, p38 MAPK, pAKT and peNOS. Interestingly specific inhibition of the pMEK, p38 MAPK, PI3 kinase and eNOS pathways blocked the effects of both VEGF₁₆₅a and VEGF₁₆₅b on paracellular permeability and the effect of VEGF₁₆₅a on proliferation/migration, suggesting that this difference in cellular response is mediated by an as yet unidentified signalling pathway(s).

CONCLUSION

This study demonstrates that the novel isoform VEGF₁₆₅a and VEGF₁₆₅b induce differing effects on permeability in pulmonary microvascular endothelial cells.

Independent impedimetric analysis of two cell populations co-cultured on opposite sides of a porous support

The transepithelial or -endothelial electrical resistance (TEER) is a very common and routinely recorded parameter describing the expression of barrier-forming cell-cell contacts (tight junctions) in quantitative terms. To determine TEER, barrier-forming cell monolayers are cultured on porous filter supports that separate two fluid compartments. The frequency-dependent impedance of the cell layer is then recorded and analyzed by means of equivalent circuit modelling providing TEER and the cell layer capacitance. The latter serves as a quantitative indicator for membrane topography. When cells are co-cultured on opposite sides of such a porous support to model more complex biological barriers, TEER readings will integrate over both cell layers and the individual contributions are not assessable. This study describes the modification of commonly used porous filter inserts by coating their backside with a thin gold-film. When this gold-film is used as an additional electrode, both cell layers can be studied separately by impedance analysis. The electrical parameters of either cell layer are assessable independently by switching between different electrode combinations. The performance of this new approach is illustrated and documented by experiments that (i) follow the de novo formation of cell junctions between initially suspended cells and (ii) the manipulation of mature cell-cell junctions by cytoskeleton-active drugs. Both assays confirm that both cell layers are monitored entirely independently.

Low-dose acetaminophen induces early disruption of cell-cell tight junctions in human hepatic cells and mouse liver

Dysfunction of cell-cell tight junction (TJ) adhesions is a major feature in the pathogenesis of various diseases. Liver TJs preserve cellular polarity by delimiting functional bile-canalicular structures, forming the blood-biliary barrier. In acetaminophen-hepatotoxicity, the mechanism by which tissue cohesion and polarity are affected remains unclear. Here, we demonstrate that acetaminophen, even at low-dose, disrupts the integrity of TJ and cell-matrix adhesions, with indicators of cellular stress with liver injury in the human hepatic HepaRG cell line, and primary hepatocytes. In mouse liver, at human-equivalence (therapeutic) doses, dose-dependent loss of intercellular hepatic TJ-associated ZO-1 protein expression was evident with progressive clinical signs of liver injury. Temporal, dose-dependent and specific disruption of the TJ-associated ZO-1 and cytoskeletal-F-actin proteins, correlated with modulation of hepatic ultrastructure. Real-time impedance biosensing verified in vitro early, dose-dependent quantitative decreases in TJ and cell-substrate adhesions. Whereas treatment with NAPQI, the reactive metabolite of acetaminophen, or the PKCα-activator and TJ-disruptor phorbol-12-myristate-13-acetate, similarly reduced TJ integrity, which may implicate oxidative stress and the PKC pathway in TJ destabilization. These findings are relevant to the clinical presentation of acetaminophen-hepatotoxicity and may inform future mechanistic studies to identify specific molecular targets and pathways that may be altered in acetaminophen-induced hepatic depolarization.

Pitfalls in assessing microvascular endothelial barrier function: impedance-based devices versus the classic macromolecular tracer assay

The most frequently used parameters to describe the barrier properties of endothelial cells (ECs) in vitro are (i) the macromolecular permeability, indicating the flux of a macromolecular tracer across the endothelium, and (ii) electrical impedance of ECs grown on gold-film electrodes reporting on the cell layer’s tightness for ion flow. Due to the experimental differences between these approaches, inconsistent observations have been described. Here, we present the first direct comparison of these assays applied to one single cell type (human microvascular ECs) under the same experimental conditions. The impact of different pharmacological tools (histamine, forskolin, Y-27632, blebbistatin, TRAP) on endothelial barrier function was analyzed by Transwell^® tracer assays and two commercial impedance devices (xCELLigence^®, ECIS^®). The two impedance techniques provided very similar results for all compounds, whereas macromolecular permeability readings were found to be partly inconsistent with impedance. Possible reasons for these discrepancies are discussed. We conclude that the complementary combination of both approaches is highly recommended to overcome the restrictions of each assay. Since the nature of the growth support may contribute to the observed differences, structure-function relationships should be based on cells that are consistently grown on either permeable or impermeable growth supports in all experiments.

Intact endothelial autophagy is required to maintain vascular lipid homeostasis

The physiological role of autophagic flux within the vascular endothelial layer remains poorly understood. Here, we show that in primary endothelial cells, oxidized and native LDL stimulates autophagosome formation. Moreover, by both confocal and electron microscopy, excess native or modified LDL appears to be engulfed within autophagic structures. Transient knockdown of the essential autophagy gene ATG7 resulted in higher levels of intracellular (125) I-LDL and oxidized LDL (OxLDL) accumulation, suggesting that in endothelial cells, autophagy may represent an important mechanism to regulate excess, exogenous lipids. The physiological importance of these observations was assessed using mice containing a conditional deletion of ATG7 within the endothelium. Following acute intravenous infusion of fluorescently labeled OxLDL, mice lacking endothelial expression of ATG7 demonstrated prolonged retention of OxLDL within the retinal pigment epithelium (RPE) and choroidal endothelium of the eye. In a chronic model of lipid excess, we analyzed atherosclerotic burden in ApoE(-/-) mice with or without endothelial autophagic flux. The absence of endothelial autophagy markedly increased atherosclerotic burden. Thus, in both an acute and chronic in vivo model, endothelial autophagy appears critically important in limiting lipid accumulation within the vessel wall. As such, strategies that stimulate autophagy, or prevent the age-dependent decline in autophagic flux, might be particularly beneficial in treating atherosclerotic vascular disease.

Pigment Epithelium-Derived Factor Induces Endothelial Barrier Dysfunction via p38/MAPK Phosphorylation

Endothelial barrier dysfunction, which is a serious problem that occurs in various inflammatory conditions, permits extravasation of serum components into the surrounding tissues, leading to edema formation and organ failure. Pigment epithelium-derived factor (PEDF), which is a major endogenous antagonist, has been implicated in diverse biological process, but its role in endothelial barrier dysfunction has not been defined. To assess the role of PEDF in the vasculature, we evaluated the effects of exogenous PEDF using human umbilical vein endothelial cells (HUVECs) in vitro. Our results demonstrated that exogenous PEDF activated p38/MAPK signalling pathway in a dose- and time-dependent manner and induced vascular hyperpermeability as measured by the markedly increased FITC-dextran leakage and the decreased transendothelial electrical resistance (TER) across the monolayer cells, which was accompanied by microtubules (MTs) disassembly and F-actin rearrangement. However, the aforementioned alterations can be arrested by the application of low concentration of p38/MAPK inhibitor SB203580. These results reveal a novel role for PEDF as a potential vasoactive substance in inducing hyperpermeability. Furthermore, our results suggest that PEDF and p38/MAPK may serve as therapeutic targets for maintaining vascular integrity.

Impedance analysis of GPCR-mediated changes in endothelial barrier function: overview and fundamental considerations for stable and reproducible measurements

The past 20 years has seen significant growth in using impedance-based assays to understand the molecular underpinning of endothelial and epithelial barrier function in response to physiological agonists and pharmacological and toxicological compounds. Most studies on barrier function use G protein-coupled receptor (GPCR) agonists which couple to fast and transient changes in barrier properties. The power of impedance-based techniques such as electric cell-substrate impedance sensing (ECIS) resides in its ability to detect minute changes in cell layer integrity label-free and in real-time ranging from seconds to days. We provide a comprehensive overview of the biophysical principles, applications, and recent developments in impedance-based methodologies. Despite extensive application of impedance analysis in endothelial barrier research, little attention has been paid to data analysis and critical experimental variables, which are both essential for signal stability and reproducibility. We describe the rationale behind common ECIS data presentation and interpretation and illustrate practical guidelines to improve signal intensity by adapting technical parameters such as electrode layout, monitoring frequency, or parameter (resistance versus impedance magnitude). Moreover, we discuss the impact of experimental parameters, including cell source, liquid handling, and agonist preparation on signal intensity and kinetics. Our discussions are supported by experimental data obtained from human microvascular endothelial cells challenged with three GPCR agonists, thrombin, histamine, and sphingosine-1-phosphate.

Cell-Based Microarrays for In Vitro Toxicology

DNA/RNA and protein microarrays have proven their outstanding bioanalytical performance throughout the past decades, given the unprecedented level of parallelization by which molecular recognition assays can be performed and analyzed. Cell microarrays (CMAs) make use of similar construction principles. They are applied to profile a given cell population with respect to the expression of specific molecular markers and also to measure functional cell responses to drugs and chemicals. This review focuses on the use of cell-based microarrays for assessing the cytotoxicity of drugs, toxins, or chemicals in general. It also summarizes CMA construction principles with respect to the cell types that are used for such microarrays, the readout parameters to assess toxicity, and the various formats that have been established and applied. The review ends with a critical comparison of CMAs and well-established microtiter plate (MTP) approaches.

Endothelial to Mesenchymal Transition Contributes to Endothelial Dysfunction in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by lung endothelial cell dysfunction and vascular remodeling. Normally, the endothelium forms an integral cellular barrier to regulate vascular homeostasis. During embryogenesis endothelial cells exhibit substantial plasticity that contribute to cardiac development by undergoing endothelial-to-mesenchymal transition (EndoMT). We determined the presence of EndoMT in the pulmonary vasculature in vivo and the functional effects on pulmonary artery endothelial cells (PAECs) undergoing EndoMT in vitro. Histologic assessment of patients with systemic sclerosis-associated PAH and the hypoxia/SU5416 mouse model identified the presence von Willebrand factor/α-smooth muscle actin-positive endothelial cells in up to 5% of pulmonary vessels. Induced EndoMT in PAECs by inflammatory cytokines IL-1β, tumor necrosis factor α, and transforming growth factor β led to actin cytoskeleton reorganization and the development of a mesenchymal morphology. Induced EndoMT cells exhibited up-regulation of mesenchymal markers, including collagen type I and α-smooth muscle actin, and a reduction in endothelial cell and junctional proteins, including von Willebrand factor, CD31, occludin, and vascular endothelial-cadherin. Induced EndoMT monolayers failed to form viable biological barriers and induced enhanced leak in co-culture with PAECs. Induced EndoMT cells secreted significantly elevated proinflammatory cytokines, including IL-6, IL-8, and tumor necrosis factor α, and supported higher immune transendothelial migration compared with PAECs. These findings suggest that EndoMT may contribute to the development of PAH.

Temporal studies into attachment, VE-cadherin perturbation, and paracellular migration of human umbilical mesenchymal stem cells across umbilical vein endothelial monolayers

Mesenchymal stem cells from Wharton's jelly of human umbilical cords (WJ-MSC) are a valuable alternate source of stem cells. Their role in situ and whether they can interact and cross intact endothelial monolayers requires elucidation. The aim of this study was to investigate the dynamic interactions between WJ-MSC and human umbilical vein endothelial cells (HUVEC), including attachment, transit times, extravasation pathway, and the effects of WJ-MSC on junctional vascular endothelial (VE)-cadherin. HUVEC were grown to near confluence in endothelial media and to full confluence in mixed media before the addition of PKH26-labelled WJ-MSC. Time lapse fluorescence microscopy showed stem cells undergoing membrane blebbing followed by amoeboid movement on HUVEC monolayers before rounding up and changing shape toward the spindle-shaped morphology during/after transmigration to subendothelial positions. Cells demonstrated a time lag of 60 min before paracellular extravasation, confirmed by confocal microscopy. Forty-six percent of attached cells crossed in the first 2 h. By 16 h, a majority of cells had transmigrated with >96% of cells crossing by 22 h. There were concomitant changes in endothelial junctional VE-cadherin with statistically significant increases in discontinuous staining at 2 h, return to control values at 16 h, even as from 22 h onward HUVEC displayed increased percentage of junctions with continuous staining and upregulation of protein. Our data suggests that WJ-MSC crosses the endothelial barrier through the paracellular pathway and can influence junctional organization of HUVEC with discreet perturbation of VE-cadherin preceding transmigration followed by upregulation once the adluminal side is reached. The latter may reflect a perivascular support function of WJ-MSC in the umbilical cord.