A transmural pressure gradient induces mechanical and biological adaptive responses in endothelial cells

Multiphysics Simulation of Local Transport and Absorption Coupled with Pharmacokinetic Modeling of Systemic Exposure of Subcutaneously Injected Drug Solution

INTRODUCTION

Subcutaneous (SC) injectables have become more acceptable and feasible for administration of biologics and small molecules. However, efficient development of these products is limited to costly and time-consuming techniques, partially because absorption mechanisms and kinetics at the local site of injection remain poorly understood.

OBJECTIVE

To bridge formulation critical quality attributes (CQA) of injectables with local physiological conditions to predict systemic exposure of these products.

METHODOLOGY

We have previously developed a multiscale, multiphysics computational model to simulate lymphatic absorption and whole-body pharmacokinetics of monoclonal antibodies. The same simulation framework was applied in this study to compute the capillary absorption of solubilized small molecule drugs that are injected subcutaneously. Sensitivity analyses were conducted to probe the impact by key simulation parameters on the local and systemic exposures.

RESULTS

This framework was capable of determining which parameters had the biggest impact on small molecule absorption in the SC. Particularly, membrane permeability of a drug was found to have the biggest impact on drug absorption kinetics, followed by capillary density and drug diffusivity.

CONCLUSION

Our modelling framework proved feasible in predicting local transport and systemic absorption from the injection site of small molecules. Understanding the effect of these properties and how to model them may help to greatly expedite the development process.

Biomechanical determinants of endothelial permeability assessed in standard and modified hollow-fibre bioreactors

Effects of mechanical stress on the permeability of vascular endothelium are important to normal physiology and in the development of atherosclerosis. Here we elucidate novel effects using commercially available and modified hollow-fibre bioreactors, in which endothelial cells form confluent monolayers lining plastic capillaries with porous walls, contained in a cartridge. The capillaries were perfused with a near-aortic waveform, and permeability was assessed by the movement of rhodamine-labelled albumin from the intracapillary to the extracapillary space. Permeability was increased by acute application of shear stress and decreased by chronic shear stress compared with a static control: this has previously been shown only for multidirectional flows. Increasing viscosity reduced permeability under both acute and chronic shear; since shear rate remained unchanged, these effects resulted from altered shear stress. Reducing pulsatility increased permeability, contrary to the widely held assumption that flow which is highly oscillatory causes endothelial dysfunction. Chronic convection across the monolayer increased effective permeability more than could be explained by the addition of advective transport, contrary to results from previous acute experiments. The off-the-shelf and modified bioreactors provide an excellent tool for investigating the biomechanics of endothelial permeability and have revealed novel effects of flow duration, viscosity, pulsatility and transmural flow.

Mechanical regulation of the early stages of angiogenesis

Favouring or thwarting the development of a vascular network is essential in fields as diverse as oncology, cardiovascular disease or tissue engineering. As a result, understanding and controlling angiogenesis has become a major scientific challenge. Mechanical factors play a fundamental role in angiogenesis and can potentially be exploited for optimizing the architecture of the resulting vascular network. Largely focusing on in vitro systems but also supported by some in vivo evidence, the aim of this Highlight Review is dual. First, we describe the current knowledge with particular focus on the effects of fluid and solid mechanical stimuli on the early stages of the angiogenic process, most notably the destabilization of existing vessels and the initiation and elongation of new vessels. Second, we explore inherent difficulties in the field and propose future perspectives on the use of in vitro and physics-based modelling to overcome these difficulties.

Quantification of In Vitro Blood-Brain Barrier Permeability

In vitro blood-brain barrier (BBB) models have been widely used to simulate in vivo models due to their low cost, feasibility, and repeatability. To serve as a valid substitute, the in vitro BBB should have the similar barrier function as that in vivo. This chapter summarizes the detailed methods for quantifying the barrier function, e.g., the permeability of the BBB to water, ions, and solutes for an in vitro BBB generated on the Transwell filter.

Location-Specific Comparison Between a 3D In-Stent Restenosis Model and Micro-CT and Histology Data from Porcine In Vivo Experiments

BACKGROUND

Coronary artery restenosis is an important side effect of percutaneous coronary intervention. Computational models can be used to better understand this process. We report on an approach for validation of an in silico 3D model of in-stent restenosis in porcine coronary arteries and illustrate this approach by comparing the modelling results to in vivo data for 14 and 28 days post-stenting.

METHODS

This multiscale model includes single-scale models for stent deployment, blood flow and tissue growth in the stented vessel, including smooth muscle cell (SMC) proliferation and extracellular matrix (ECM) production. The validation procedure uses data from porcine in vivo experiments, by simulating stent deployment using stent geometry obtained from micro computed tomography (micro-CT) of the stented vessel and directly comparing the simulation results of neointimal growth to histological sections taken at the same locations.

RESULTS

Metrics for comparison are per-strut neointimal thickness and per-section neointimal area. The neointimal area predicted by the model demonstrates a good agreement with the detailed experimental data. For 14 days post-stenting the relative neointimal area, averaged over all vessel sections considered, was 20 ± 3% in vivo and 22 ± 4% in silico. For 28 days, the area was 42 ± 3% in vivo and 41 ± 3% in silico.

CONCLUSIONS

The approach presented here provides a very detailed, location-specific, validation methodology for in silico restenosis models. The model was able to closely match both histology datasets with a single set of parameters. Good agreement was obtained for both the overall amount of neointima produced and the local distribution. It should be noted that including vessel curvature and ECM production in the model was paramount to obtain a good agreement with the experimental data.

A particle-based model for endothelial cell migration under flow conditions

Endothelial cells (ECs) play a major role in the healing process following angioplasty to inhibit excessive neointima. This makes the process of EC healing after injury, in particular EC migration in a stented vessel, important for recovery of normal vessel function. In that context, we present a novel particle-based model of EC migration and validate it against in vitro experimental data. We have developed a particle-based model of EC migration under flow conditions in an in vitro vessel with obstacles. Cell movement in the model is a combination of random walks and directed movement along the local flow velocity vector. For model calibration, a set of experimental data for cell migration in a similarly shaped channel has been used. We have calibrated the model for a baseline case of a channel with no obstacles and then applied it to the case of a channel with ridges on the bottom surface, representative of stent strut geometry. We were able to closely reproduce the cell migration speed and angular distribution of their movement relative to the flow direction reported in vitro. The model also reproduces qualitative aspects of EC migration, such as entrapment of cells downstream from the flow-disturbing ridge. The model has the potential, after more extensive in vitro validation, to study the effect of variation in strut spacing and shape, through modification of the local flow, on EC migration. The results of this study support the hypothesis that EC migration is strongly affected by the direction and magnitude of local wall shear stress.

Regulation of Epithelial Cell Functions by the Osmolality and Hydrostatic Pressure Gradients: A Possible Role of the Tight Junction as a Sensor

Epithelia act as a barrier to the external environment. The extracellular environment constantly changes, and the epithelia are required to regulate their function in accordance with the changes in the environment. It has been reported that a difference of the environment between the apical and basal sides of epithelia such as osmolality and hydrostatic pressure affects various epithelial functions including transepithelial transport, cytoskeleton, and cell proliferation. In this paper, we review the regulation of epithelial functions by the gradients of osmolality and hydrostatic pressure. We also examine the significance of this regulation in pathological conditions especially focusing on the role of the hydrostatic pressure gradient in the pathogenesis of carcinomas. Furthermore, we discuss the mechanism by which epithelia sense the osmotic and hydrostatic pressure gradients and the possible role of the tight junction as a sensor of the extracellular environment to regulate epithelial functions.

Direct current stimulation of endothelial monolayers induces a transient and reversible increase in transport due to the electroosmotic effect

We investigated the effects of direct current stimulation (DCS) on fluid and solute transport across endothelial cell (EC) monolayers in vitro. Our motivation was transcranial direct current stimulation (tDCS) that has been investigated for treatment of neuropsychiatric disorders, to enhance neurorehabilitation, and to change cognition in healthy subjects. The mechanisms underlying this diversity of applications remain under investigation. To address the possible role of blood-brain barrier (BBB) changes during tDCS, we applied direct current to cultured EC monolayers in a specially designed chamber that generated spatially uniform direct current. DCS induced fluid and solute movement across EC layers that persisted only for the duration of the stimulation suggesting an electroosmosis mechanism. The direction of induced transport reversed with DCS polarity - a hallmark of the electroosmotic effect. The magnitude of DCS-induced flow was linearly correlated to the magnitude of the applied current. A mathematical model based on a two-pore description of the endothelial transport barrier and a Helmholtz model of the electrical double layer describes the experimental data accurately and predicts enhanced significance of this mechanism in less permeable monolayers. This study demonstrates that DCS transiently alters the transport function of the BBB suggesting a new adjunct mechanism of tDCS.

Hypercapnic acidosis attenuates pressure-dependent increase in whole-lung filtration coefficient (Kf)

Hypercapnic acidosis (HCA) has beneficial effects in experimental models of lung injury by attenuating inflammation and decreasing pulmonary edema. However, HCA increases pulmonary vascular pressure that will increase fluid filtration and worsen edema development. To reconcile these disparate effects, we tested the hypothesis that HCA inhibits endothelial mechanotransduction and protects against pressure-dependent increases in the whole lung filtration coefficient (K_f). Isolated perfused rat lung preparation was used to measure whole lung filtration coefficient (K_f) at two levels of left atrial pressure (P_LA = 7.5 versus 15 cm H₂O) and at low tidal volume (LV_t) versus standard tidal volume (STV_t) ventilation. The ratio of K_f2/K_f1 was used as the index of whole lung permeability. Double occlusion pressure, pulmonary artery pressure, pulmonary capillary pressures, and zonal characteristics (ZC) were measured to assess effects of HCA on hemodynamics and their relationship to K_f2/K_f1. An increase in P_LA2 from 7.5 to 15 cm H₂O resulted in a 4.9-fold increase in K_f2/K_f1 during LV_t and a 4.8-fold increase during STV_t. During LV_t, HCA reduced K_f2/K_f1 by 2.7-fold and reduced STV_t K_f2/K_f1 by 5.2-fold. Analysis of pulmonary hemodynamics revealed no significant differences in filtration forces in response to HCA. HCA interferes with lung vascular mechanotransduction and prevents pressure-dependent increases in whole lung filtration coefficient. These results contribute to a further understanding of the lung protective effects of HCA.

A Comparison of Fully-Coupled 3D In-Stent Restenosis Simulations to In-vivo Data

We describe our fully-coupled 3D multiscale model of in-stent restenosis, with blood flow simulations coupled to smooth muscle cell proliferation, and report results of numerical simulations performed with this model. This novel model is based on several previously reported 2D models. We study the effects of various parameters on the process of restenosis and compare with in vivo porcine data where we observe good qualitative agreement. We study the effects of stent deployment depth (and related injury score), reendothelization speed, and simulate the effect of stent width. Also we demonstrate that we are now capable to simulate restenosis in real-sized (18 mm long, 2.8 mm wide) vessel geometries.

Forces and mechanotransduction in 3D vascular biology

The effects of hemodynamic and interstitial mechanical forces on endothelial biology in vivo have been appreciated for over half a century, regulating vessel network development, homeostatic function, and progression of vascular disease. Investigations using cultures of endothelial cells on two-dimensional (2D) substrates have elucidated important mechanisms by which microenvironmental stresses are sensed and transduced into chemical signaling responses. However recent studies in vivo and in three-dimensional (3D) in vitro models of vascular beds have enabled the investigation of forces and cellular behaviors previously not possible in traditional 2D culture systems. These studies support a developing paradigm that the 3D chemo-mechanical architecture of the vascular niche impacts how endothelial cells both sense and respond to microenvironmental forces. We present evolving concepts in endothelial force sensing and mechanical signaling and highlight recent insights gained from in vivo and 3D in vitro vascular models.

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.

Hydraulic Conductivity of Smooth Muscle Cell-Initiated Arterial Cocultures

The purpose of the study was to examine the effects of arterial coculture conditions on the transport properties of several in vitro endothelial cell (EC)-smooth muscle cell (SMC)-porous filter constructs in which SMC were grown to confluence first and then EC were inoculated. This order of culturing simulates the environment of a blood vessel wall after endothelial layer damage due to stenting, vascular grafting or other vascular wall insult. For all coculture configurations examined, we observed that hydraulic conductivity (L(p)) values were significantly higher than predicted by a resistances-in-series (RIS) model accounting for the L(p) of EC and SMC measured separately. The greatest increases were observed when EC were plated directly on top of a confluent SMC layer without an intervening filter, presumably mediated by direct EC-SMC contacts that were observed under confocal microscopy. The results are the opposite of a previous study that showed L(p) was significantly reduced compared to an RIS model when EC were grown to confluency first. The physiological, pathophysiological and tissue engineering implications of these results are discussed.

Aquaporin-1 facilitates pressure-driven water flow across the aortic endothelium

Aquaporin-1, a ubiquitous water channel membrane protein, is a major contributor to cell membrane osmotic water permeability. Arteries are the physiological system where hydrostatic dominates osmotic pressure differences. In the present study, we show that the walls of large conduit arteries constitute the first example where hydrostatic pressure drives aquaporin-1-mediated transcellular/transendothelial flow. We studied cultured aortic endothelial cell monolayers and excised whole aortas of male Sprague-Dawley rats with intact and inhibited aquaporin-1 activity and with normal and knocked down aquaporin-1 expression. We subjected these systems to transmural hydrostatic pressure differences at zero osmotic pressure differences. Impaired aquaporin-1 endothelia consistently showed reduced engineering flow metrics (transendothelial water flux and hydraulic conductivity). In vitro experiments with tracers that only cross the endothelium paracellularly showed that changes in junctional transport cannot explain these reductions. Percent reductions in whole aortic wall hydraulic conductivity with either chemical blocking or knockdown of aquaporin-1 differed at low and high transmural pressures. This observation highlights how aquaporin-1 expression likely directly influences aortic wall mechanics by changing the critical transmural pressure at which its sparse subendothelial intima compresses. Such compression increases transwall flow resistance. Our endothelial and historic erythrocyte membrane aquaporin density estimates were consistent. In conclusion, aquaporin-1 significantly contributes to hydrostatic pressure-driven water transport across aortic endothelial monolayers, both in culture and in whole rat aortas. This transport, and parallel junctional flow, can dilute solutes that entered the wall paracellularly or through endothelial monolayer disruptions. Lower atherogenic precursor solute concentrations may slow their intimal entrainment kinetics.

Vascular changes in the developing rat retina in response to hypoxia

This study was carried out to investigate the roles of tight junction (TJ) proteins and other factors in the increased permeability of the blood retinal barrier (BRB) affecting the immature neonatal retina following a hypoxic insult. The expression of endothelial TJ proteins such as claudin-5, occludin and zonula occludens-1 (ZO-1) and endothelial cell specific molecule-1 (ESM-1), and associated structural changes in the blood vessels were analyzed in the retinas of 1-day-old Wistar rats subjected to hypoxia for 2 h and subsequently sacrificed at different time points ranging from 3 h to 14 d. The mRNA and protein expression of claudin-5, occludin & ZO-1 was found to be reduced in the hypoxic retina, although, at the ultrastructural level, the TJ between the endothelial cells and retinal pigment epithelial cells appeared to be intact. Following the hypoxic insult vascular endothelial cells frequently showed presence of cytoplasmic vacuoles, vacuolated mitochondria and multivesicular aggregations projecting into the lumen of the capillaries. The expression of ESM-1 in the immature retinas was found to be increased following hypoxic exposure. The structural and molecular changes in the hypoxic neonatal retinas were consistent with a hypoxia induced impairment of the BRB. Hypoxia reduced the expression of TJ proteins in the neonatal retina, but the role of increased ESM-1 expression in this process warrants further investigation.

Effect of shear stress on water and LDL transport through cultured endothelial cell monolayers

Previous animal experiments have shown that the transport of LDL into arterial walls is shear stress dependent. However, little work has probed shear effects on LDL transport in vitro where conditions are well defined and mechanisms are more easily explored. Therefore, we measured shear induced water and LDL fluxes across cultured bovine aortic endothelial (BAEC) monolayers in vitro and developed a three-pore model to describe the transport dynamics. Cell apoptosis was quantified by TdT-mediated dUTP nick end labeling (TUNEL) assay. We also examined the role of nitric oxide (NO) in shear induced water and LDL fluxes by incubating BAEC monolayers with an NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA). Our results show that direct exposure of endothelial monolayers to 12 dyn/cm2 shear stress for 3 h elicited a 2.37-fold increase in water flux (Jv), a 3.00-fold increase in LDL permeability (Pe), a 1.32-fold increase in LDL uptake, and a 1.68-fold increase in apoptotic rate. L-NMMA treatment of BAEC monolayers blocked shear induced Jv response, but had no significant effect on shear responses of Pe and cell apoptosis. A long time shear exposure (12 h) of endothelial monolayers reduced Pe and apoptotic rate close to the baseline. These results suggest that an acute change in shear stress from a static baseline state induces increases in water flux that are mediated by an NO dependent mechanism. On the other hand, the permeability of endothelial monolayers to LDL is enhanced by a short term-shear application and reduced nearly to the baseline level by a longer time shear exposure, positively correlated to the leaky junctions forming around apoptotic cells.

Hydraulic conductivity of endothelial cell-initiated arterial cocultures

This study describes cocultures of arterial smooth muscle cells (SMCs) and endothelial cells (ECs) and the influences of their heterotypic interactions on hydraulic conductivity (L p ), an important transport property. A unique feature of these cocultures is that ECs were first grown to confluence and then SMCs were inoculated. Bovine aortic smooth muscle cells and bovine aortic endothelial cells (BAECs) were cocultured on Transwell Permeable Supports, and then exposed to a pressure-driven transmural flow. L p across each culture was measured using a bubble tracking apparatus that determined water flux (J v ). Our results indicate that arterial L p is significantly modulated by EC-SMC proximity, and serum content in culture. The L p of cocultures was also compared to the predictions of a resistances-in-series model to distinguish the contributions of heterotypic interactions between SMCs and ECs. Conditions that lead to significantly reduced coculture L p , compared to BAEC monoculture controls, have been uncovered and the lowest L p in the literature for an in vitro system are reported. In addition, VE-cadherin immunostaining of intact BAEC monolayers in each culture configuration reveals that EC-SMC proximity on a porous membrane has a dramatic influence on EC morphology patterns. The cocultures with the lowest L p have ECs with significantly elongated morphology. Confocal imaging indicates that there are no direct EC-SMC contacts in coculture.

High glucose attenuates shear-induced changes in endothelial hydraulic conductivity by degrading the glycocalyx

Diabetes mellitus is a risk factor for cardiovascular disease; however, the mechanisms through which diabetes impairs homeostasis of the vasculature have not been completely elucidated. The endothelium interacts with circulating blood through the surface glycocalyx layer, which serves as a mechanosensor/transducer of fluid shear forces leading to biomolecular responses. Atherosclerosis localizes typically in regions of low or disturbed shear stress, but in diabetics, the distribution is more diffuse, suggesting that there is a fundamental difference in the way cells sense shear forces. In the present study, we examined the effect of hyperglycemia on mechanotranduction in bovine aortic endothelial cells (BAEC). After six days in high glucose media, we observed a decrease in heparan sulfate content coincident with a significant attenuation of the shear-induced hydraulic conductivity response, lower activation of eNOS after exposure to shear, and reduced cell alignment with shear stress. These studies are consistent with a diabetes-induced change to the glycocalyx altering endothelial response to shear stress that could affect the distribution of atherosclerotic plaques.

Acute lung injury and pulmonary vascular permeability: use of transgenic models

Acute lung injury is a general term that describes injurious conditions that can range from mild interstitial edema to massive inflammatory tissue destruction. This review will cover theoretical considerations and quantitative and semi-quantitative methods for assessing edema formation and increased vascular permeability during lung injury. Pulmonary edema can be quantitated directly using gravimetric methods, or indirectly by descriptive microscopy, quantitative morphometric microscopy, altered lung mechanics, high-resolution computed tomography, magnetic resonance imaging, positron emission tomography, or x-ray films. Lung vascular permeability to fluid can be evaluated by measuring the filtration coefficient (Kf) and permeability to solutes evaluated from their blood to lung clearances. Albumin clearances can then be used to calculate specific permeability-surface area products (PS) and reflection coefficients (σ). These methods as applied to a wide variety of transgenic mice subjected to acute lung injury by hyperoxic exposure, sepsis, ischemia-reperfusion, acid aspiration, oleic acid infusion, repeated lung lavage, and bleomycin are reviewed. These commonly used animal models simulate features of the acute respiratory distress syndrome, and the preparation of genetically modified mice and their use for defining specific pathways in these disease models are outlined. Although the initiating events differ widely, many of the subsequent inflammatory processes causing lung injury and increased vascular permeability are surprisingly similar for many etiologies.

Co-regulation of transcellular and paracellular leak across microvascular endothelium by dynamin and Rac

Increased permeability of the microvascular endothelium to fluids and proteins is the hallmark of inflammatory conditions such as sepsis. Leakage can occur between (paracellular) or through (transcytosis) endothelial cells, yet little is known about whether these pathways are linked. Understanding the regulation of microvascular permeability is essential for the identification of novel therapies to combat inflammation. We investigated whether transcytosis and paracellular leakage are co-regulated. Using molecular and pharmacologic approaches, we inhibited transcytosis of albumin in primary human microvascular endothelium and measured paracellular permeability. Blockade of transcytosis induced a rapid increase in paracellular leakage that was not explained by decreases in caveolin-1 or increases in activity of nitric oxide synthase. The effect required caveolin-1 but was observed in cells depleted of clathrin, indicating that it was not due to the general inhibition of endocytosis. Inhibiting transcytosis by dynamin blockade increased paracellular leakage concomitantly with the loss of cortical actin from the plasma membrane and the displacement of active Rac from the plasmalemma. Importantly, inhibition of paracellular leakage by sphingosine-1-phosphate, which activates Rac and induces cortical actin, caused a significant increase in transcytosis of albumin in vitro and in an ex vivo whole-lung model. In addition, dominant-negative Rac significantly diminished albumin uptake by endothelia. Our findings indicate that transcytosis and paracellular permeability are co-regulated through a signaling pathway linking dynamin, Rac, and actin.

Adaptive response of vascular endothelial cells to an acute increase in shear stress magnitude

The adaptation of vascular endothelial cells to shear stress alteration induced by global hemodynamic changes, such as those accompanying exercise or digestion, is an essential component of normal endothelial physiology in vivo. An understanding of the transient regulation of endothelial phenotype during adaptation to changes in mural shear will advance our understanding of endothelial biology and may yield new insights into the mechanism of atherogenesis. In this study, we characterized the adaptive response of arterial endothelial cells to an acute increase in shear stress magnitude in well-defined in vitro settings. Porcine endothelial cells were preconditioned by a basal level shear stress of 15 ± 15 dyn/cm(2) at 1 Hz for 24 h, after which an acute increase in shear stress to 30 ± 15 dyn/cm(2) was applied. Endothelial permeability nearly doubled after 40-min exposure to the elevated shear stress and then decreased gradually. Transcriptomics studies using microarray techniques identified 86 genes that were sensitive to the elevated shear. The acute increase in shear stress promoted the expression of a group of anti-inflammatory and antioxidative genes. The adaptive response of the global gene expression profile is triphasic, consisting of an induction period, an early adaptive response (ca. 45 min) and a late remodeling response. Our results suggest that endothelial cells exhibit a specific phenotype during the adaptive response to changes in shear stress; this phenotype is different than that of fully adapted endothelial cells.

A three-pore model describes transport properties of bovine retinal endothelial cells in normal and elevated glucose

PURPOSE

Changes in blood vessel barrier properties contribute to retinal edema in diabetic retinopathy (DR). However, limited data are available to describe the routes of transport for fluids and solutes across the inner blood-retinal barrier (iBRB). In this study, a three-pore model was developed to characterize such routes in normal and elevated glucose levels.

METHODS

Diffusive and apparent permeabilities to TAMRA (467 Da), dextran (70 kDa), and LDL (2000 kDa), as well as hydraulic conductivity, were measured across bovine retinal endothelial cell (BREC) monolayers after exposure to normal- and high-glucose media for 6 days. The data were used to develop a model of transport dynamics. Claudin 5 and eNOS Western blot analysis were used to measure changes in expression and phosphorylation. Immunolocalization of ZO-1 and VE-cadherin demonstrated organization of the junctional complex. Apoptosis was measured by TUNEL assay.

RESULTS

A three-pore model describes the fractional transport of water and molecular tracers across the retinal endothelial barrier. No change in permeability or hydraulic conductivity was observed after exposure to high glucose, whereas VEGF increased permeability in both normal- and high-glucose environments. The transport results were consistent with ZO-1 and VE-cadherin immunocytochemistry and expression of claudin-5, which were all unaltered by high glucose.

CONCLUSIONS

The data describe, for the first time, a model for transport of various size solutes and fluids across endothelial cells of the iBRB. Further, the results support the existence of an indirect pathway by which iBRB permeability is increased through the upregulation of retinal VEGF in response to hyperglycemia.

Shear stress and the endothelial transport barrier

The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solutes and the hydraulic conductivity. This review begins with a discussion of transport pathways across the endothelium and then considers the experimental evidence from both in vivo and in vitro studies that shows an influence of shear stress on endothelial transport properties after both acute (minutes to hours) and chronic (hours to days) changes in shear stress. Next, the effects of shear stress on individual transport pathways (tight junctions, adherens junctions, vesicles and leaky junctions) are described, and this information is integrated with the transport experiments to suggest mechanisms controlling both acute and chronic responses of transport properties to shear stress. The review ends with a summary of future research challenges.

Effect of mechanical factors on the function of engineered human blood microvessels in microfluidic collagen gels

This work examines how mechanical signals affect the barrier function and stability of engineered human microvessels in microfluidic type I collagen gels. Constructs that were exposed to chronic low flow displayed high permeabilities to bovine serum albumin and 10 kDa dextran, numerous focal leaks, low size selectivity, and short lifespan of less than one week. Higher flows promoted barrier function and increased longevity; at the highest flows, the barrier function rivaled that observed in vivo, and all vessels survived to day 14. By studying the physiology of microvessels of different geometries, we established that shear stress and transmural pressure were the dominant mechanical signals that regulated barrier function and vascular stability, respectively. In microvessels that were exposed to high flow, elevation of intracellular cyclic AMP further increased the selectivity of the barrier and strongly suppressed cell proliferation. Computational models that incorporated stress dependence successfully predicted vascular phenotype. Our results indicate that the mechanical microenvironment plays a major role in the functionality and stability of engineered human microvessels in microfluidic collagen gels.

DBS-relevant electric fields increase hydraulic conductivity of in vitro endothelial monolayers

Deep brain stimulation (DBS) achieves therapeutic outcome through generation of electric fields (EF) in the vicinity of energized electrodes. Targeted brain regions are highly vascularized, and it remains unknown if DBS electric fields modulate blood-brain barrier (BBB) function, either through electroporation of individual endothelial cells or electro-permeation of barrier tight junctions. In our study, we calculated the intensities of EF generated around energized Medtronic 3387 and 3389 DBS leads by using a finite element model. Then we designed a novel stimulation system to study the effects of such fields with DBS-relevant waveforms and intensities on bovine aortic endothelial cell (BAEC) monolayers, which were used as a basic analog for the blood-brain barrier endothelium. Following 5 min of stimulation, we observed a transient increase in endothelial hydraulic conductivity (Lp) that could be related to the disruption of the tight junctions (TJ) between cells, as suggested by zonula occludens-1 (ZO-1) protein staining. This 'electro-permeation' occurred in the absence of cell death or single cell electroporation, as indicated by propidium iodide staining and cytosolic calcein uptake. Our in vitro results, using uniform fields and BAEC monolayers, thus suggest that electro-permeation of the BBB may occur at electric field intensities below those inducing electroporation and within intensities generated near DBS electrodes. Further studies are necessary to address potential BBB disruption during clinical studies, with safety and efficacy implications.

A protein diffusion model of the sealing effect

Water transport across the arterial endothelium is believed primarily to occur through breaks in the tight junction strands at the cell periphery between neighboring cells. Additional proteins arriving at the tight junction can close these breaks, thereby attenuating this water flux. Motivated by evidence that the diffusion of presynthesized protein from the interior of the cell to and incorporation into the cell border is the mechanism of endothelial tight junctional sealing, we develop a diffusion-limited mathematical model of intercellular gap sealing. A single endothelial cell is represented as a thin, axisymmetric disk, initially containing a uniform distribution of junctional protein that does not interact with the apical or basal cell surfaces. Upon application of a transmural pressure gradient, water flows through the junctional cleft, and tight junction remodeling begins. We assume that proteins at the junction are instantaneously incorporated into its strand, dropping the free protein concentration at the cell periphery to zero. This sets the diffusion of intracellular proteins toward the junction in motion. The solution of this one-dimensional initial value problem provides excellent fits to current and previously published experimental data over a wide variety of conditions. It yields three physically meaningful parameters for each fit, including a protein diffusivity in the cytoplasm that varies little within experimental treatments. Statistical variation of these parameters allows rational comparison of experimental runs and identification of outlier runs.

Computational design of drainage systems for vascularized scaffolds

This computational study analyzes how to design a drainage system for porous scaffolds so that the scaffolds can be vascularized and perfused without collapse of the vessel lumens. We postulate that vascular transmural pressure--the difference between lumenal and interstitial pressures--must exceed a threshold value to avoid collapse. Model geometries consisted of hexagonal arrays of open channels in an isotropic scaffold, in which a small subset of channels was selected for drainage. Fluid flow through the vessels and drainage channel, across the vascular wall, and through the scaffold were governed by Navier-Stokes equations, Starling's Law of Filtration, and Darcy's Law, respectively. We found that each drainage channel could maintain a threshold transmural pressure only in nearby vessels, with a radius-of-action dependent on vascular geometry and the hydraulic properties of the vascular wall and scaffold. We illustrate how these results can be applied to microvascular tissue engineering, and suggest that scaffolds be designed with both perfusion and drainage in mind.

The endothelial glycocalyx mediates shear-induced changes in hydraulic conductivity

Recent in vitro and in vivo studies have reported fluid shear stress-induced increases in endothelial layer hydraulic conductivity (L(p)) that are mediated by an increased production of nitric oxide (NO). Other recent studies have shown that NO induction by shear stress is mediated by the glycocalyx that decorates the surface of endothelial cells. Here we find that a selective depletion of the major components of the glycocalyx with enzymes can block the shear stress-induced response of L(p). Heparinase and hyaluronidase block shear-induced increases in L(p), which is consistent with their effects on NO production. But chondroitinase, which does not suppress shear-induced NO production, also inhibits shear-induced L(p). A further surprise is that treatment with the general proteolytic enzyme pronase does not suppress the shear L(p) response. We also find that heparinase does not alter baseline L(p) significantly, whereas chondroitinase, hyaluronidase, and pronase increase it significantly.

Occludin independently regulates permeability under hydrostatic pressure and cell division in retinal pigment epithelial cells

PURPOSE

The aim of this study was to determine the function of the tight junction protein occludin in the control of permeability, under diffusive and hydrostatic pressures, and its contribution to the control of cell division in retinal pigment epithelium.

METHODS

Occludin expression was inhibited in the human retinal pigment epithelial cell line ARPE-19 by siRNA. Depletion of occludin was confirmed by Western blot, confocal microscopy, and RT-PCR. Paracellular permeability of cell monolayers to fluorescently labeled 70 kDa dextran, 10 kDa dextran, and 467 Da tetramethylrhodamine (TAMRA) was examined under diffusive conditions or after the application of 10 cm H2O transmural pressure. Cell division rates were determined by tritiated thymidine incorporation and Ki67 immunoreactivity. Cell cycle inhibitors were used to determine whether changes in cell division affected permeability.

RESULTS

Occludin depletion increased diffusive paracellular permeability to 467 Da TAMRA by 15%, and permeability under hydrostatic pressure was increased 50% compared with control. Conversely, depletion of occludin protein with siRNA did not alter diffusive permeability to 70 kDa and 10 kDa RITC-dextran, and permeability to 70 kDa dextran was twofold lower in occludin-depleted cells under hydrostatic pressure conditions. Occludin depletion also increased thymidine incorporation by 90% and Ki67-positive cells by 50%. Finally, cell cycle inhibitors did not alter the effect of occludin siRNA on paracellular permeability.

CONCLUSIONS

The data suggest that occludin regulates tight junction permeability in response to changes in hydrostatic pressure. Furthermore, these data suggest that occludin also contributes to the control of cell division, demonstrating a novel function for this tight junction protein.

Vascular permeability in ocular disease and the role of tight junctions

Vascular permeability is closely linked with angiogenesis in a number of pathologies. In the retina, the normally well-developed blood-retinal barrier is altered in a host of eye diseases preceding or commensurate with angiogenesis. This review examines the literature regarding the tight junction complex that establishes the blood-retinal barrier focusing on the transmembrane proteins occludin and the claudin family and the membrane associated protein zonula occludens. The changes observed in these proteins associated with vascular and epithelial permeability is discussed. Finally, novel literature addressing the link between the tight junction complex and angiogenesis is considered.

In vitro study of LDL transport under pressurized (convective) conditions

It is difficult to assess the transport pathways that carry low-density lipoprotein (LDL) into the artery wall in vivo, and there has been no previous in vitro study that has examined transendothelial transport under physiologically relevant pressurized (convective) conditions. Therefore, we measured water, albumin, and LDL fluxes across bovine aortic endothelial cell (BAEC) monolayers in vitro and determined the relative contributions of vesicles, paracellular transport through "breaks" in the tight junction, and "leaky" junctions associated with dying or dividing cells. Our results show that leaky junctions are the dominant pathway for LDL transport (>90%) under convective conditions and that albumin also has a significant component of transport through leaky junctions (44%). Transcellular transport of LDL by receptor-mediated processes makes a minor contribution (<10%) to overall transport under convective conditions.

Are intramural suction-squeezing effects generated by the variations in radial wall stress during each heart beat the motor of atherosclerosis? A new concept

In the early sixties, the existence of predilection sites for atherosclerotic lesions inside the arterial circulation led to the concept that low wall shear stress (WSS) was responsible, together with systemic factors like high blood pressure, hypercholesterolemia, or diabetes, for the genesis and progression of atherosclerosis. It was found later that oscillating WSS and high WSS gradients could also be incriminated. Yet, this concept, which is broadly accepted today, fails to explains several facts, for instance that some arteries (e.g. epicardial coronary arteries) are more prone to become atherosclerotic than other ones exposed to the same systemic factors (e.g. hepatic and brain arteries). In this paper, we present a quite different concept. It is based on the fact that the increase in intravascular pressure and flow that occur in the arteries during systole generates, at the predilection sites of atherosclerotic lesions (bends, bifurcations, and branchings), an increase of radial wall stress in the outer layers of the arterial wall so that this stress becomes momentarily tensile. These cyclic stress increases have a suction effect that is likely to facilitate the diffusion of atherogenic cells and substances inside the wall. Furthermore, since arteries are not primarily structured to resist inversions of radial stress, they may also create damages (e.g. disruptions of cell membranes and elastic lamellae) followed by inflammations and micro-haemorrhages in the wall. This new concept may provide a complementary (or possibly alternative) explanation of atherosclerosis.

Human cytomegalovirus (HCMV) infection of endothelial cells promotes naive monocyte extravasation and transfer of productive virus to enhance hematogenous dissemination of HCMV

Human cytomegalovirus (HCMV) pathogenesis is dependent on the hematogenous spread of the virus to host tissue. While data suggest that infected monocytes are required for viral dissemination from the blood to the host organs, infected endothelial cells are also thought to contribute to this key step in viral pathogenesis. We show here that HCMV infection of endothelial cells increased the recruitment and transendothelial migration of monocytes. Infection of endothelial cells promoted the increased surface expression of cell adhesion molecules (intercellular cell adhesion molecule 1, vascular cell adhesion molecule 1, E-selectin, and platelet endothelial cell adhesion molecule 1), which were necessary for the recruitment of naïve monocytes to the apical surface of the endothelium and for the migration of these monocytes through the endothelial cell layer. As a mechanism to account for the increased monocyte migration, we showed that HCMV infection of endothelial cells increased the permeability of the endothelium. The cellular changes contributing to the increased permeability and increased naïve monocyte transendothelial migration include the disruption of actin stress fiber formation and the decreased expression of lateral junction proteins (occludin and vascular endothelial cadherin). Finally, we showed that the migrating monocytes were productively infected with the virus, documenting that the virus was transferred to the migrating monocyte during passage through the lateral junctions. Together, our results provide evidence for an active role of the infected endothelium in HCMV dissemination and pathogenesis.

Hydraulic conductance of pulmonary microvascular and macrovascular endothelial cell monolayers

Endothelial cells isolated from pulmonary arteries (RPAEC) and microcirculation (RPMVEC) of rat lungs were grown to confluence on porous filters and mounted on an Ussing-type chamber. Transmembrane pressure (deltaP) was controlled by the reservoir height, and the filtration rate corrected for surface area (J(v)/A) was measured by timing fluid movement in a calibrated micropipette. These parameters were used to calculate hydraulic conductance (Lp) by using linear regression of J(v)/A on deltaP. Mean Lp values for newly confluent RPAEC monolayers were 22 times higher than those for RPMVEC monolayers (28.6 +/- 5.6 vs. 1.30 +/- 0.50 x 10(-7) cm x s(-1) x cmH2O(-1); P < or = 0.01). After confluence was reached, electrical resistance and Lp remained stable in RPAEC but continued to change in RPMVEC with days in culture. Both phenotypes exhibited an initial time-dependent sealing response, but Lp also had an inverse relationship to deltaP in RPMVEC monolayers > or = 4 days postconfluence that was attributed to cell overgrowth rather than junctional length. In a comparison of the cadherin contents, E-cadherin was predominant in RPMVEC, but VE-cadherin was predominant in RPAEC. At a constant deltaP of 40-45 cmH2O for 2 h, J(v)/A increased 225% in RPAEC monolayers but did not change significantly in RPMVEC monolayers. Significant decreases in Lp were obtained after treatment with 5% albumin, GdCl3, or isoproterenol plus rolipram in both phenotypes. Thus lung microvascular endothelial cells exhibited a significantly lower Lp than conduit vessel endothelium, which would limit alveolar flooding relative to perivascular edema cuff formation during increased pulmonary vascular pressures.

Transport in rat vessel walls. I. Hydraulic conductivities of the aorta, pulmonary artery, and inferior vena cava with intact and denuded endothelia

In this study, filtration flows through the walls of the rat aorta, pulmonary artery (PA), and inferior vena cava (IVC), vessels with very different susceptibilities to atherosclerosis, were measured as a function of transmural pressure (DeltaP), with intact and denuded endothelium on the same vessel. Aortic hydraulic conductivity (L(p)) is high at 60 mmHg, drops approximately 40% by 100 mmHg, and is pressure independent to 140 mmHg. The trends are similar in the PA and IVC, dropping 42% from 10 to 40 mmHg and flat to 100 mmHg (PA) and dropping 33% from 10 to 20 mmHg and essentially flat to 60 mmHg (IVC). Removal of the endothelium renders L(p)(DeltaP) flat: it increases L(p) of the aorta by approximately 75%, doubles L(p) of the PA, and quadruples L(p) of the IVC. Specific resistance (1/L(p)) of the aortic endothelium is approximately 47% of total resistance; i.e., the endothelium accounts for approximately 47% of the DeltaP drop at 100 mmHg. The PA value is 55% at >40 mmHg, and the IVC value is 23% at 10 mmHg. L(p) of the intact aorta, PA, and IVC are order 10(-8), 10(-7), and 5 x 10(-7) cm.s(-1).mmHg(-1), and wall thicknesses are 145.8 microm (SD 9.3), 78.9 microm (SD 3.3), and 66.1 microm (SD 4.1), respectively. These data are consistent with the different wall structures of the three vessels. The rat aortic L(p) data are quantitatively consistent with rabbit L(p)(DeltaP) (Tedgui A and Lever MJ. Am J Physiol Heart Circ Physiol 247: H784-H791, 1984; Baldwin AL and Wilson LM. Am J Physiol Heart Circ Physiol 264: H26-H32, 1993), suggesting that intimal compression under pressure loading may also play a role in L(p)(DeltaP) in these other vessels. Despite very different driving DeltaP, nominal transmural water fluxes of these three vessels are very similar and, therefore, cannot alone account for their differences in disease susceptibility. The different fates of macromolecular tracers convected by these water fluxes into the walls of these vessels may account for this difference.

Regulation of hydraulic conductivity in response to sustained changes in pressure

The present study addresses the effect of a sustained change in pressure on microvascular permeability assessed by hydraulic conductivity ( Lp) measurements from microvessels of the rat mesentery. With a microperfusion technique, transvascular filtration (normalized to surface area; Jv/ S) and Lp were measured in small arterioles (baseline Lp = 0.26 × 10−7 cm·s−1·cmH2O−1) and venules (baseline Lp = 2.88 × 10−7 cm·s−1·cmH2O−1). The main finding of this study is that step increases in microvascular pressure led to time-dependent alterations of Lp. Immediately after a twofold step increase in pressure, Jv/ S increased in proportion to the pressure change. This observation is consistent with Starling's law that predicts filtration proportional to the overall pressure gradient when Lp is constant. However, when Jv/ S measurements continued for 60–90 min past the step in pressure, there was an initial decrease in Jv/ S for 30 min (“sealing effect”) followed by a substantial increase in Jv/ S out to 90 min. The sustained increase in Jv/ S suggests an increase in Lp of 36 ± 7% for small arterioles and 42 ± 5% for small venules ( P < 0.05 for both). In addition, the increase in Lp in response to an increase in pressure was attenuated significantly by nitric oxide synthase inhibition. These results indicate that a pressure-induced mechanical stimulus (possibly Jv) activates a NO-dependent biochemical response that leads to an increase in hydraulic conductivity.

Shear Stress Regulates HUVEC Hydraulic Conductivity by Occludin Phosphorylation

Human umbilical vein endothelial cells (HUVECs) display hydraulic conductivity (L(P)) responses to shear stress that differ markedly from the responses of bovine aortic endothelial cells (BAECs). In HUVECs, 5, 10, and 20 dyn cm(-2) steady shear stress transiently increased L(P) with a return to preshear baseline after a 2-h exposure to shear stress. Pure oscillatory shear stress of 0 +/- 20 dyn cm(-2) (mean+/-amplitude) had no effect on L(P), whereas superposition of oscillatory shear stress on steady shear stress suppressed the effect induced by steady shear stress alone. Shear reversal (amplitude greater than mean) was not necessary for the inhibitory influence of oscillatory shear stress. The transient increase of L(P) by steady shear stress was not affected by incubation with BAPTA-AM (10 microM), suggesting calcium independence of the shear response. Decreasing nitric oxide (NO) concentration with L-NMMA (100 microM), a nitric oxide synthase (NOS) inhibitor, did not inhibit the HUVEC L(P) response to shear stress. At the protein level, 10 dyn cm(-2) shear stress did not affect the total content of occludin, but it did elevate the phosphorylation level transiently. The positive correlation between occludin phosphorylation and hydraulic conductivity parallels observations in BAECs and suggests that occludin phosphorylation may be a general mediator of shear-L(P) responses in diverse endothelial cell types.

Regulation of capillary hydraulic conductivity in response to an acute change in shear

The effects of mechanical perturbations (shear stress, pressure) on microvascular permeability primarily have been examined in micropipette-cannulated vessels or in endothelial monolayers in vitro. The objective of this study is to determine whether acute changes in blood flow shear stress might influence measurements of hydraulic conductivity (L(p)) in autoperfused microvessels in vivo. Rat mesenteric microvessels were observed via intravital microscopy. Occlusion of a third-order arteriole with a micropipette was used to divert and increase flow through a nonoccluded capillary or fourth-order arteriolar branch. Transvascular fluid filtration rate in the branching vessel was measured with a Landis technique. Flow (shear)-induced increases in L(p) disappeared within 20-30 s of the removal of the shear and could be eliminated with nitric oxide synthase inhibition. The shear-induced increase in L(p) was greater in capillaries compared with terminal arterioles. An acute change in shear may regulate L(p) by a nitric oxide-dependent mechanism that displays heterogeneity within a microvascular network.

Sexual dimorphism in the permeability response of coronary microvessels to adenosine

Gender influences volume regulation via several mechanisms; whether these include microvascular exchange, especially in the heart, is not known. In response to adenosine (Ado), permeability (P(s)) to protein of coronary arterioles of female pigs decreases acutely. Whether Ado induces similar P(s) changes in arterioles from males or whether equivalent responses occur in coronary venules of either sex has not been determined. Hypotheses that 1) basal P(s) properties and 2) P(s) responses to vasoactive stimuli are sex independent were evaluated from measures of P(s) to two hydrophilic proteins, alpha-lactalbumin and porcine serum albumin (PSA), in arterioles and venules isolated from hearts of adult male and female pigs. Consistent with hypothesis 1, basal P(s) values of both microvessel types were independent of sex. Contrary to hypothesis 2, P(s) responses to Ado varied with sex, protein, and vessel type. Confirming earlier studies, Ado induced a approximately 20% decrease in P(s) to both proteins in coronary arterioles from females. In arterioles from males, Ado did not change P(s) for alpha-lactalbumin (P(s)(alpha-lactalb), 3 +/- 13%), whereas P(s) for PSA (P(s)(PSA)) decreased by 27 +/- 8% (P < 0.005). In venules from females, Ado elevated P(s)(PSA) by 44 +/- 20% (P < 0.05), whereas in those from males, Ado reduced P(s)(PSA) by 24 +/- 5% (P < 0.05). The variety of outcomes is consistent with transvascular protein and protein-carried solute flux being regulated by multiple sex-dependent mechanisms in the heart and provides evidence of differences in exchange homeostasis of males and females in health and, likely, disease.