Endothelial permeability is controlled by spatially defined cytoskeletal mechanics: atomic force microscopy force mapping of pulmonary endothelial monolayer

Indole-3-Acetic Acid Protects Against Lipopolysaccharide-induced Endothelial Cell Dysfunction and Lung Injury through the Activation of USP40

Lung microvascular endothelial cell (EC) dysfunction is the pathological hallmark of acute respiratory distress syndrome. Heat shock protein 90 (HSP90) is a key regulator in control of endothelial barrier disruption and inflammation. Our recent study has demonstrated that ubiquitin-specific peptidase 40 (USP40) preserves endothelial integrity by targeting HSP90β for its deubiquitination and inactivation. Indole-3-acetic acid (IAA), a plant hormone of the auxin class, can also be catabolized from dietary tryptophan by the intestinal microbiota. Accumulating evidence suggests that IAA reduces oxidative stress and inflammation and promotes intestinal barrier function. However, little is known about the role of IAA in endothelial cells and acute lung injury. In this study, we investigated the role of IAA in lung endothelial cell function in the context of acute lung injury. IAA exhibited EC barrier protection against LPS-induced reduction in transendothelial electrical resistance and inflammatory responses. The underlying mechanism of IAA on EC protective effects was investigated by examining the influence of IAA on degrees of HSP90 ubiquitination and USP40 activity. We identified that IAA, acting as a potential activator of USP40, reduces HSP90 ubiquitination, thereby protecting against LPS-induced inflammation in human lung microvascular endothelial cells as well as alleviating experimental lung injury. Furthermore, the EC protective effects of IAA against LPS-induced EC dysfunction and lung injury were abolished in USP40-deficient human lung microvascular endothelial cell and lungs of USP40 EC-specific knockout (USP40cdh5-ECKO) mice. Taken together, this study reveals that IAA protects against LPS-induced EC dysfunction and lung injury through the activation of USP40.

FAK regulates tension transmission to the nucleus and endothelial transcriptome independent of kinase activity

The mechanical environment generated through the adhesive interaction of endothelial cells (ECs) with the matrix controls nuclear tension, preventing aberrant gene synthesis and the transition from restrictive to leaky endothelium, a hallmark of acute lung injury (ALI). However, the mechanisms controlling tension transmission to the nucleus and EC-restrictive fate remain elusive. Here, we demonstrate that, in a kinase-independent manner, focal adhesion kinase (FAK) safeguards tension transmission to the nucleus to maintain EC-restrictive fate. In FAK-depleted ECs, robust activation of the RhoA-Rho-kinase pathway increased EC tension and phosphorylation of the nuclear envelope protein, emerin, activating DNMT3a. Activated DNMT3a methylates the KLF2 promoter, impairing the synthesis of KLF2 and its target S1PR1 to induce the leaky EC transcriptome. Repleting FAK (wild type or kinase dead) or inhibiting RhoA-emerin-DNMT3a activities in damaged lung ECs restored KLF2 transcription of the restrictive EC transcriptome. Thus, FAK sensing and control of tension transmission to the nucleus govern restrictive endothelium to maintain lung homeostasis.

Establishment of the deuterium oxide dilution method as a new possibility for determining the transendothelial water permeability

Increase in transendothelial water permeability is an essential etiological factor in a variety of diseases like edema and shock. Despite the high clinical relevance, there has been no precise method to detect transendothelial water flow until now. The deuterium oxide (D2O) dilution method, already established for measuring transepithelial water transport, was used to precisely determine the transendothelial water permeability. It detected appropriate transendothelial water flow induced by different hydrostatic forces. This was shown in four different endothelial cell types. The general experimental setup was verified by gravimetry and absorbance spectroscopy. Determination of transendothelial electrical resistance (TEER) and immunocytochemical staining for proteins of the cell-cell contacts were performed to ensure that no damage to the endothelium occurred because of the measurements. Furthermore, endothelial barrier function was modulated. Measurement of transendothelial water flux was verified by measuring the TEER, the apparent permeability coefficient and the electrical capacity. The barrier-promoting substances cyclic adenosine monophosphate and iloprost reduced TEER and electrical capacity and increased permeability. This was accompanied by a reduced transendothelial water flux. In contrast, the barrier-damaging substances thrombin, histamine and bradykinin reduced TEER and electrical capacity, but increased permeability. Here, an increased water flow was shown. This newly established in vitro method for direct measurement of transendothelial water permeability was verified as a highly precise technique in various assays. The use of patient-specific endothelial cells enables individualized precision medicine in the context of basic edema research, for example regarding the development of barrier-protective pharmaceuticals.

The deubiquitinase USP40 preserves endothelial integrity by targeting the heat shock protein HSP90β

Endothelial cell (EC) barrier disruption and inflammation are the pathological hallmarks of vascular disorders and acute infectious diseases and related conditions, including the coronavirus disease 2019 (COVID-19) and sepsis. Ubiquitination plays a critical role in regulating the stability, intracellular trafficking, and enzymatic activity of proteins and is reversed by deubiquitinating enzymes (DUBs). The role of DUBs in endothelial biology is largely unknown. In this study, we report that USP40, a poorly characterized DUB, prevents EC barrier disruption through reductions in the activation of RhoA and phosphorylation of myosin light chain (MLC) and cofilin. Furthermore, USP40 reduces EC inflammation through the attenuation of NF-ĸB activation, ICAM1 expression, and leukocyte-EC adhesion. We further show that USP40 activity and expression are reduced in response to endotoxin challenge. Global depletion of USP40 and EC-targeted USP40 depletion in mice exacerbated experimental lung injury, whereas lentiviral gene transfer of USP40 protected against endotoxin-induced lung injury. Using an unbiased approach, we discovered that the protective effect of USP40 occurs through the targeting of heat shock protein 90β (HSP90β) for its deubiquitination and inactivation. Together, these data reveal a critical protective role of USP40 in vascular injury, identifying a unique mechanistic pathway that profoundly impacts endothelial function via DUBs.

Repair effect of Centella asiatica (L.) extract on damaged HaCaT cells studied by atomic force microscopy

People's choice of cosmetics is no longer just 'Follow the trend', but pays more attention to the ingredients of cosmetics, whether the ingredients of cosmetics are beneficial to people's skin health; therefore, more and more skin-healthy ingredients have been discovered and used in cosmetics. In this work, atomic force microscope (AFM) is used to provide physical information about biomolecules and living cells; it brings us a new method of high-precision physical measurement. Centella asiatica (L.) extract has the ability to promote skin wound healing, but its healing effect on damaged HaCaT cells needs to be investigated, which plays a key role in judging the effectiveness of skincare ingredients. The objective of this study was to explore the impact of Centella asiatica (L.) extract on ethanol-damaged human immortalised epidermal HaCaT cells based on AFM. We established a model of cellular damage and evaluated cell viability using the MTT assay. The physical changes of cell height, roughness, adhesion and Young's modulus were measured by AFM. The findings indicated that the Centella asiatica (L.) extract had a good repair effect on injured HaCaT cells, and the optimal concentration was 75 μg/mL.

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Mechanisms of 15-Epi-LXA4-Mediated HO-1 in Cytoprotection Following Inflammatory Injury

INTRODUCTION

Heme oxygenase-1 (HO-1) is a protective protein in oxidative stress response. LXA4 is an "inflammatory braking signal" that is widely studied at present. The purpose of this study was to elucidate that LXA4 can protect cells by inducing HO-1 in human pulmonary microvascular endothelial cells (HPMECs) as in vitro model to explain acute lung injury after severe acute pancreatitis.

METHODS

This study was performed in two parts: (1) To investigate the mechanisms of lipoxin A4-induced HO-1 expression in vitro, the study subjects were divided into four groups: a control group, LXA4 group (50 ng/mL LXA4), inhibitor group (50 ng/mL LXA4 + 20 μM LY294002 or 50 ng/mL LXA4 + 2 nmol/mL Bis II), and agonist group (50 ng/mL insulin-like growth factor 1, PMA). Western blotting was used to detect the expression of p-Akt, Akt, protein kinase C (PKC), p-Nrf2, Nrf2, and Keap1, and the location of Nrf2 was detected using immunofluorescence. The activation of antioxidant responsive element induced by Nrf2 was detected using Electrophoretic Mobility Shift Assay and (2) to investigate the cytoprotection of HO-1 induced by LXA4 in vitro, the subjects were divided into four groups: a control group, tumor necrosis factor α (TNF-α) group (50 ng/mL), LXA4 group (50 ng/mL TNF-α + 50 ng/mL LXA4), and Zinc protoporphyrin IX group (pretreated with 0.5 μM Zinc protoporphyrin IXfor 12 h, followed by 50 ng/mL TNF-α + 50 ng/mL LXA4). BCECF/AM-labeled THP-1 cells were used to analyze the adhesion of HPMECs, and a mitochondrial membrane potential assay kit with JC-1 was used to analyze the apoptosis of HPMECs.

RESULTS

In part one, (1) LXA4 upregulated the expression of HO-1 in a dose-dependent manner and (2) LXA4 activated the PI3K/Akt and PKC pathways and modulated the phosphorylation and subsequent depolymerization of Nrf2 from Keap1, promoting the translocation of Nrf2 to the nucleus. In part two, (1) LXA4 reversed the changes in mitochondrial membrane potential to alleviate apoptosis in HPMECs and (2) LXA4 attenuated the adhesion of HPMECs induced by TNF-α.

CONCLUSIONS

LXA4 can activate the PI3K/Akt and PKC pathways and induce the phosphorylation of Nrf2, resulting in the upregulation of HO-1. In addition, LXA4 alleviates adhesion and protects mitochondrial function by upregulating the expression of HO-1, which exerts cytoprotection in severe acute pancreatitis-induced lung injury.

BRAF increases endothelial cell stiffness through reorganization of the actin cytoskeleton

The dynamics of the actin cytoskeleton and its connection to endothelial cell-cell junctions determine the barrier function of endothelial cells. The proper regulation of barrier opening/closing is necessary for the normal function of vessels, and its dysregulation can result in chronic and acute inflammation leading to edema formation. By using atomic force microscopy, we show here that thrombin-induced permeability of human umbilical vein endothelial cells, associated with actin stress fiber formation, stiffens the cell center. The depletion of the MEK/ERK kinase BRAF reduces thrombin-induced permeability prevents stress fiber formation and cell stiffening. The peripheral actin ring becomes stabilized by phosphorylated myosin light chain, while cofilin is excluded from the cell periphery. All these changes can be reverted by the inhibition of ROCK, but not of the MEK/ERK module. We propose that the balance between the binding of cofilin and myosin to F-actin in the cell periphery, which is regulated by the activity of ROCK, determines the local dynamics of actin reorganization, ultimately driving or preventing stress fiber formation.

Hypothiocyanous Acid Disrupts the Barrier Function of Brain Endothelial Cells

Inflammation is a common feature of neurological diseases. During neuroinflammation, neutrophils are recruited to the brain vasculature, where myeloperoxidase can produce hypochlorous acid and the less well-studied oxidant hypothiocyanous acid (HOSCN). In this study, we exposed primary brain endothelial cells (BECs) to HOSCN and observed a rapid loss of transendothelial electrical resistance (TEER) at sublethal concentrations. Decreased barrier function was associated with a loss of tight junctions at cellular contacts and a concomitant loss of dynamic microtubules. Both tight junction and cytoskeletal disruptions were visible within 30 min of exposure, whereas significant loss of TEER took more than 1 h. The removal of the HOSCN after 30 min prevented subsequent barrier dysfunction. These results indicate that BECs are sensitive to HOSCN, resulting in the eventual loss of barrier function. We hypothesise that this mechanism may be relevant in neutrophil transmigration, with HOSCN facilitating blood–brain barrier opening at the sites of egress. Furthermore, this mechanism may be a way through which neutrophils, residing in the vasculature, can influence neuroinflammation in diseases.

Escin suppresses HMGB1-induced overexpression of aquaporin-1 and increased permeability in endothelial cells

Escin, a natural triterpene saponin mixture obtained from the horse chestnut tree (Aesculus hippocastanum), has been used for the treatment of chronic venous insufficiency (CVI), hemorrhoids, and edema. However, it is unclear how escin protects against endothelial barrier dysfunction induced by pro‐inflammatory high mobility group protein 1 (HMGB1). Here, we report that escin can suppress (a) HMGB1‐induced overexpression of the aquaporin‐1 (AQP1) water channel in endothelial cells and (b) HMGB1‐induced increases in endothelial cell permeability. This is the first report that escin inhibits AQP1 and alleviates barrier dysfunction in HMGB1‐induced inflammatory response.

Multiplexed, high-throughput measurements of cell contraction and endothelial barrier function

Vascular leakage, protein exudation, and edema formation are events commonly triggered by inflammation and facilitated by gaps that form between adjacent endothelial cells (ECs) of the vasculature. In such paracellular gap formation, the role of EC contraction is widely implicated, and even therapeutically targeted. However, related measurement approaches remain slow, tedious, and complex to perform. Here, we have developed a multiplexed, high-throughput screen to simultaneously quantify paracellular gaps, EC contractile forces, and to visualize F-actin stress fibers, and VE-cadherin. As proof-of-principle, we examined barrier-protective mechanisms of the Rho-associated kinase inhibitor, Y-27632, and the canonical agonist of the Tie2 receptor, Angiopoietin-1 (Angpt-1). Y-27632 reduced EC contraction and actin stress fiber formation, whereas Angpt-1 did not. Yet both agents reduced thrombin-, LPS-, and TNFα-induced paracellular gap formation. This unexpected result suggests that Angpt-1 can achieve barrier defense without reducing EC contraction, a mechanism that has not been previously described. This insight was enabled by the multiplex nature of the force-based platform. The high-throughput format we describe should accelerate both mechanistic studies and the screening of pharmacological modulators of endothelial barrier function.

Cortical Actin Dynamics in Endothelial Permeability

The pulmonary endothelial cell forms a critical semi-permeable barrier between the vascular and interstitial space. As part of the blood-gas barrier in the lung, the endothelium plays a key role in normal physiologic function and pathologic disease. Changes in endothelial cell shape, defined by its plasma membrane, determine barrier integrity. A number of key cytoskeletal regulatory and effector proteins including non-muscle myosin light chain kinase, cortactin, and Arp 2/3 mediate actin rearrangements to form cortical and membrane associated structures in response to barrier enhancing stimuli. These actin formations support and interact with junctional complexes and exert forces to protrude the lipid membrane to and close gaps between individual cells. The current knowledge of these cytoskeletal processes and regulatory proteins are the subject of this review. In addition, we explore novel advancements in cellular imaging that are poised to shed light on the complex nature of pulmonary endothelial permeability.

Endothelial barrier dysfunction induced by nanoparticle exposure through actin remodeling via caveolae/raft-regulated calcium signalling

The rapid development of modern nanotechnology has resulted in nanomaterial being use in nearly all applications of life, raising the potential risk of nanomaterial exposure alongside the need to design safe and effective materials. Previous work has demonstrated a specific effect of gold nanoparticles (GNPs) of approximately 20 nm on endothelial barrier function in vitro. To expand our understanding of this size-specific effect, titanium dioxide, silicon dioxide, and polystyrene nanoparticles (NPs) in this similar size range were studied. All tested nanoparticles were found to have minimal effects on cell viability, but exhibited a significant detrimental effect on endothelial barrier function. Nanoparticles in the size range of 20 to 30 nm were internalized by endothelial cells through caveolae/raft-mediated endocytosis, causing intracellular calcium elevation by approximately 30% at 2 hours after administration, and triggering myosin light chain kinase (MLCK)-regulated actomyosin contraction. These effects culminated in an increase in endothelial monolayer permeability across all particle types within the 20-30 nm range. This nanoparticle exposure-induced endothelial barrier dysfunction may provide valuable information for designing safer nanomaterials or potential applications of this nanoparticle exposure-induced permeability effect in biomedicine.

Rho-Kinase Inhibition Ameliorates Dasatinib-Induced Endothelial Dysfunction and Pulmonary Hypertension

The multi-kinase inhibitor dasatinib is used for treatment of imatinib-resistant chronic myeloid leukemia, but is prone to induce microvascular dysfunction. In lung this can manifest as capillary leakage with pleural effusion, pulmonary edema or even pulmonary arterial hypertension. To understand how dasatinib causes endothelial dysfunction we examined the effects of clinically relevant concentrations of dasatinib on both human pulmonary arterial macro- and microvascular endothelial cells (ECs). The effects of dasatinib was compared to imatinib and nilotinib, two other clinically used BCR/Abl kinase inhibitors that do not inhibit Src. Real three-dimensional morphology and high resolution stiffness mapping revealed softening of both macro- and microvascular ECs upon dasatinib treatment, which was not observed in response to imatinib. In a dose-dependent manner, dasatinib decreased transendothelial electrical resistance/impedance and caused a permeability increase as well as disruption of tight adherens junctions in both cell types. In isolated perfused and ventilated rat lungs, dasatinib increased mean pulmonary arterial pressure, which was accompanied by a gain in lung weight. The Rho-kinase inhibitor Y27632 partly reversed the dasatinib-induced changes in vitro and ex vivo, presumably by acting downstream of Src. Co-administration of the Rho-kinase inhibitor Y27632 completely blunted the increased pulmonary pressure in response to dasatinib. In conclusion, a dasatinib-induced permeability increase in human pulmonary arterial macro- and microvascular ECs might explain many of the adverse effects of dasatinib in patients. Rho-kinase inhibition might be suitable to ameliorate these effects.

Substrate stiffness-dependent exacerbation of endothelial permeability and inflammation: mechanisms and potential implications in ALI and PH (2017 Grover Conference Series)

The maintenance of endothelial barrier integrity is absolutely essential to prevent the vascular leak associated with pneumonia, pulmonary edema resulting from inhalation of toxins, acute elevation to high altitude, traumatic and septic lung injury, acute lung injury (ALI), and its life-threatening complication, acute respiratory distress syndrome (ARDS). In addition to the long-known edemagenic and inflammatory agonists, emerging evidences suggest that factors of endothelial cell (EC) mechanical microenvironment such as blood flow, mechanical strain of the vessel, or extracellular matrix stiffness also play an essential role in the control of endothelial permeability and inflammation. Recent studies from our group and others have demonstrated that substrate stiffening causes endothelial barrier disruption and renders EC more susceptible to agonist-induced cytoskeletal rearrangement and inflammation. Further in vivo studies have provided direct evidence that proinflammatory stimuli increase lung microvascular stiffness which in turn exacerbates endothelial permeability and inflammation and perpetuates a vicious circle of lung inflammation. Accumulating evidence suggests a key role for RhoA GTPases signaling in stiffness-dependent mechanotransduction mechanisms defining EC permeability and inflammatory responses. Vascular stiffening is also known to be a key contributor to other cardiovascular diseases such as arterial pulmonary hypertension (PH), although the precise role of stiffness in the development and progression of PH remains to be elucidated. This review summarizes the current understanding of stiffness-dependent regulation of pulmonary EC permeability and inflammation, and discusses potential implication of pulmonary vascular stiffness alterations at macro- and microscale in development and modulation of ALI and PH.

Myosin light chain kinase ( MYLK) coding polymorphisms modulate human lung endothelial cell barrier responses via altered tyrosine phosphorylation, spatial localization, and lamellipodial protrusions

Sphingosine 1-phosphate (S1P) is a potent bioactive endogenous lipid that signals a rearrangement of the actin cytoskeleton via the regulation of non-muscle myosin light chain kinase isoform (nmMLCK). S1P induces critical nmMLCK Y⁴⁶⁴ and Y⁴⁷¹ phosphorylation resulting in translocation of nmMLCK to the periphery where spatially-directed increases in myosin light chain (MLC) phosphorylation and tension result in lamellipodia protrusion, increased cell-cell adhesion, and enhanced vascular barrier integrity. MYLK, the gene encoding nmMLCK, is a known candidate gene in lung inflammatory diseases, with coding genetic variants (Pro21His, Ser147Pro, Val261Ala) that confer risk for inflammatory lung injury and influence disease severity. The functional mechanisms by which these MYLK coding single nucleotide polymorphisms (SNPs) affect biologic processes to increase disease risk and severity remain elusive. In the current study, we utilized quantifiable cell immunofluorescence assays to determine the influence of MYLK coding SNPs on S1P-mediated nmMLCK phosphorylation and translocation to the human lung endothelial cell (EC) periphery . These disease-associated MYLK variants result in reduced levels of S1P-induced Y⁴⁶⁴ phosphorylation, a key site for nmMLCK enzymatic regulation and activation. Reduced Y⁴⁶⁴ phosphorylation resulted in attenuated nmMLCK protein translocation to the cell periphery. We further conducted EC kymographic assays which confirmed that lamellipodial protrusion in response to S1P challenge was retarded by expression of a MYLK transgene harboring the three MYLK coding SNPs. These data suggest that ARDS/severe asthma-associated MYLK SNPs functionally influence vascular barrier-regulatory cytoskeletal responses via direct alterations in the levels of nmMLCK tyrosine phosphorylation, spatial localization, and lamellipodial protrusions.

The Significant Role of c-Abl Kinase in Barrier Altering Agonists-mediated Cytoskeletal Biomechanics

Exploration of human pulmonary artery endothelial cell (EC) as a prototypical biomechanical system has important pathophysiologic relevance because this cell type plays a key role in the development of a wide variety of clinical conditions. The complex hierarchical organization ranging from the molecular scale up to the cellular level has an intimate and intricate relationship to the barrier function between lung tissue and blood. To understand the innate molecule-cell-tissue relationship across varied length-scales, the functional role of c-Abl kinase in the cytoskeletal nano-biomechanics of ECs in response to barrier-altering agonists was investigated using atomic force microscopy. Concurrently, the spatially specific arrangement of cytoskeleton structure and dynamic distribution of critical proteins were examined using scanning electron microscopy and immunofluorescence. Reduction in c-Abl expression by siRNA attenuates both thrombin- and sphingosine 1-phosphate (S1P)-mediated structural changes in ECs, specifically spatially-defined changes in elastic modulus and distribution of critical proteins. These results indicate that c-Abl kinase is an important determinant of cortical actin-based cytoskeletal rearrangement. Our findings directly bridge the gap between kinase activity, structural complexity, and functional connectivity across varied length-scales, and suggest that manipulation of c-Abl kinase activity may be a potential target for the treatment of pulmonary barrier disorders.

Nanoparticle size-specific actin rearrangement and barrier dysfunction of endothelial cells

In this work, we evaluated the impact of gold nanoparticles on endothelial cell behavior and function beyond the influence on cell viability. Five types of gold nanoparticles were studied: 5 nm and 20 nm bare gold nanoparticles, 5 nm and 20 nm gold nanoparticles with biocompatible polyethylene glycol (PEG) coating and 60 nm bare gold nanoparticles. We found that all tested gold nanoparticles did not affect cell viability significantly and reduced the reactive oxygen species (ROS) level in endothelial cells. Only 20 nm bare gold nanoparticles caused an over 50% increase in endothelial barrier permeability and slow recovery of barrier function was observed after the gold nanoparticles were removed. This impairment in endothelial barrier function was caused by unbalanced forces between intracellular tensions and paracellular forces, actin microfilament rearrangement, which occurred through a Rho/ROCK kinase-dependent pathway and broke the force balance between intracellular tensions and paracellular forces. The size-specific effect of gold nanoparticles on endothelial cells may have important implications regarding the behavior of nanoparticles in the biological system and provide valuable guidance in nanomaterial design and biomedical applications.

The ARP 2/3 complex mediates endothelial barrier function and recovery

Pulmonary endothelial cell (EC) barrier dysfunction and recovery is critical to the pathophysiology of acute respiratory distress syndrome. Cytoskeletal and subsequent cell membrane dynamics play a key mechanistic role in determination of EC barrier integrity. Here, we characterizAQe the actin related protein 2/3 (Arp 2/3) complex, a regulator of peripheral branched actin polymerization, in human pulmonary EC barrier function through studies of transendothelial electrical resistance (TER), intercellular gap formation, peripheral cytoskeletal structures and lamellipodia. Compared to control, Arp 2/3 inhibition with the small molecule inhibitor CK-666 results in a reduction of baseline barrier function (1,241 ± 53 vs 988 ± 64 ohm; p < 0.01), S1P-induced barrier enhancement and delayed recovery of barrier function after thrombin (143 ± 14 vs 93 ± 6 min; p < 0.01). Functional changes of Arp 2/3 inhibition on barrier integrity are associated temporally with increased intercellular gap area at baseline (0.456 ± 0.02 vs 0.299 ± 0.02; p < 0.05) and thirty minutes after thrombin (0.885 ± 0.03 vs 0.754 ± 0.03; p < 0.05). Immunofluorescent microscopy reveals reduced lamellipodia formation after S1P and during thrombin recovery in Arp 2/3 inhibited cells. Individual lamellipodia demonstrate reduced depth following Arp 2/3 inhibition vs vehicle at baseline (1.83 ± 0.41 vs 2.55 ± 0.46 µm; p < 0.05) and thirty minutes after S1P treatment (1.53 ± 0.37 vs 2.09 ± 0.36 µm; p < 0.05). These results establish a critical role for Arp 2/3 activity in determination of pulmonary endothelial barrier function and recovery through formation of EC lamellipodia and closure of intercellular gaps.

Photopatterned Hydrogels to Investigate the Endothelial Cell Response to Matrix Stiffness Heterogeneity

Age-related intimal stiffening is associated with increased endothelium permeability, an initiating step in atherosclerosis. Notably, in addition to a bulk increase in matrix stiffness, the aged intima also exhibits increased spatial stiffness heterogeneity. We investigate the effect of heterogeneous matrix stiffness on endothelial cells. Methacrylated hyaluronic acid hydrogels are fabricated and photopatterned to create substrates with 50-and 100 μm squares containing soft and stiff matrix regions of 2.7 ± 0.7 and 10.3 ± 3.9 kPa. On the patterned matrices, endothelial cells integrate subcellular matrix stiffness cues at stiffness interfaces, and focal adhesions are increased in the cell body adhered to stiff matrix regions. Increased matrix stiffness heterogeneity disrupts cell-cell junctions in confluent endothelial monolayers. Together, this work indicates that the spatial presentation of matrix mechanical cues, in addition to bulk substrate compliance, play a role in governing endothelial single cell and monolayer behaviors.

Direct mapping of melanoma cell - endothelial cell interactions

The most life-threatening aspect of cancer is metastasis; cancer patient mortality is mainly due to metastasis. Among all metastases, presence of brain metastasis is one with the poorest prognosis; the median survival time can be counted in months. Therefore, prevention or decreasing their incidence would be highly desired both by patients and physicians. Metastatic cells invading the brain must breach the cerebral vasculature, primarily the blood-brain barrier. The key step in this process is the establishment of firm adhesion between the cancer cell and the cerebral endothelial layer. Using the atomic force microscope, a high-resolution force spectrograph, our aim was to explore the connections among the cell morphology, cellular mechanics, and biological function in the process of transendothelial migration of metastatic cancer cells. By immobilization of a melanoma cell to an atomic force microscope's cantilever, intercellular adhesion was directly measured at quasi-physiological conditions. Hereby, we present our latest results by using this melanoma-decorated probe. Binding characteristics to a confluent layer of brain endothelial cells was directly measured by means of single-cell force spectroscopy. Adhesion dynamics and strength were characterized, and we present data about spatial distribution of elasticity and detachment strength. These results highlight the importance of cellular mechanics in brain metastasis formation and emphasize the enormous potential toward exploration of intercellular dynamic-related processes.

Mechanisms of modulation of brain microvascular endothelial cells function by thrombin

Brain microvascular endothelial cells are a critical component of the blood-brain barrier. They form a tight monolayer which is essential for maintaining the brain homeostasis. Blood-derived proteases such as thrombin may enter the brain during pathological conditions like trauma, stroke, and inflammation and further disrupts the permeability of the blood-brain barrier, via incompletely characterized mechanisms. We examined the underlying mechanisms evoked by thrombin in rat brain microvascular endothelial cells (RBMVEC). Our results indicate that thrombin, acting on protease-activated receptor 1 (PAR1) increases cytosolic Ca²⁺ concentration in RBMVEC via Ca²⁺ release from endoplasmic reticulum through inositol 1,4,5-trisphosphate receptors and Ca²⁺ influx from extracellular space. Thrombin increases nitric oxide production; the effect is abolished by inhibition of the nitric oxide synthase or by antagonism of PAR1 receptors. In addition, thrombin increases mitochondrial and cytosolic reactive oxygen species production via PAR1-dependent mechanisms. Immunocytochemistry studies indicate that thrombin increases F-actin stress fibers, and disrupts the tight junctions. Thrombin increased the RBMVEC permeability assessed by a fluorescent flux assay. Taken together, our results indicate multiple mechanisms by which thrombin modulates the function of RBMVEC and may contribute to the blood-brain barrier dysfunction.

Cell-cell junctional mechanotransduction in endothelial remodeling

The vasculature is one of the most dynamic tissues that encounter numerous mechanical cues derived from pulsatile blood flow, blood pressure, activity of smooth muscle cells in the vessel wall, and transmigration of immune cells. The inner layer of blood and lymphatic vessels is covered by the endothelium, a monolayer of cells which separates blood from tissue, an important function that it fulfills even under the dynamic circumstances of the vascular microenvironment. In addition, remodeling of the endothelial barrier during angiogenesis and trafficking of immune cells is achieved by specific modulation of cell-cell adhesion structures between the endothelial cells. In recent years, there have been many new discoveries in the field of cellular mechanotransduction which controls the formation and destabilization of the vascular barrier. Force-induced adaptation at endothelial cell-cell adhesion structures is a crucial node in these processes that challenge the vascular barrier. One of the key examples of a force-induced molecular event is the recruitment of vinculin to the VE-cadherin complex upon pulling forces at cell-cell junctions. Here, we highlight recent advances in the current understanding of mechanotransduction responses at, and derived from, endothelial cell-cell junctions. We further discuss their importance for vascular barrier function and remodeling in development, inflammation, and vascular disease.

Differential elastic responses to barrier-altering agonists in two types of human lung endothelium

Vascular integrity is primarily determined by endothelial cell (EC) cytoskeletal structure that is differentially regulated by various stimuli. In this study, atomic force microscopy (AFM) was used to characterize structural and mechanical properties in the cytoskeleton of cultured human pulmonary artery EC (HPAEC) and human lung microvascular EC (HLMVEC) by determining elastic properties (Young's modulus) in response to endogenous barrier protective agents sphingosine 1-phosphate (S1P) and hepatocyte growth factor (HGF), or the barrier disruptive molecule thrombin. Initial studies in unstimulated cells indicate higher baseline peripheral elastic modulus values in HPAEC (mean 2.9 KPa) than in HLMVEC (1.8 KPa). After 30 min of stimulation, S1P induced the highest Young's modulus increase (6.1 KPa) compared to the other barrier enhancing stimuli, HGF (5.8 KPa) and the pharmaceutical agent and S1P analog FTY720 (4.1 KPa). In contrast, the barrier disruptive agent thrombin decreased values from 2.5 KPa to 0.7 KPa depending on the cell type and treatment time. AFM topographical imaging supports these quantitative biophysical data regarding differential peripheral elastic properties in EC. Overall, these AFM studies provide novel insights into the biomechanical properties of human lung EC that regulate vascular barrier function and have potential applicability to pathophysiologic vascular leak syndromes such as acute lung injury.

Feeling for Filaments: Quantification of the Cortical Actin Web in Live Vascular Endothelium

Contact-mode atomic force microscopy (AFM) has been shown to reveal cortical actin structures. Using live endothelial cells, we visualized cortical actin dynamics simultaneously by AFM and confocal fluorescence microscopy. We present a method that quantifies dynamic changes in the mechanical ultrastructure of the cortical actin web. We argue that the commonly used, so-called error signal imaging in AFM allows a qualitative, but not quantitative, analysis of cortical actin dynamics. The approach we used comprises fast force-curve-based topography imaging and subsequent image processing that enhances local height differences. Dynamic changes in the organization of the cytoskeleton network can be observed and quantified by surface roughness calculations and automated morphometrics. Upon treatment with low concentrations of the actin-destabilizing agent cytochalasin D, the cortical cytoskeleton network is thinned out and the average mesh size increases. In contrast, jasplakinolide, a drug that enhances actin polymerization, consolidates the cytoskeleton network and reduces the average mesh area. In conclusion, cortical actin dynamics can be quantified in live cells. To our knowledge, this opens a new pathway for conducting quantitative structure-function analyses of the endothelial actin web just beneath the apical plasma membrane.

Dual role of vinculin in barrier-disruptive and barrier-enhancing endothelial cell responses

Endothelial cell (EC) barrier disruption induced by edemagenic agonists such as thrombin is a result of increased actomyosin contraction and enforcement of focal adhesions (FA) anchoring contracting stress fibers, which leads to cell retraction and force-induced disruption of cell junctions. In turn, EC barrier enhancement by oxidized phospholipids (OxPAPC) and other agonists is a result of increased tethering forces due to enforcement of the peripheral actin rim and enhancement of cell-cell adherens junction (AJ) complexes promoting EC barrier integrity. This study tested participation of the mechanosensitive adaptor, vinculin, which couples FA and AJ to actin cytoskeleton, in control of the EC permeability response to barrier disruptive (thrombin) and barrier enhancing (OxPAPC) stimulation. OxPAPC and thrombin induced different patterns of FA remodeling. Knockdown of vinculin attenuated both, OxPAPC-induced decrease and thrombin-induced increase in EC permeability. Thrombin stimulated the vinculin association with FA protein talin and suppressed the interaction with AJ protein, VE-cadherin. In contrast, OxPAPC stimulated the vinculin association with VE-cadherin. Thrombin and OxPAPC induced different levels of myosin light chain (MLC) phosphorylation and caused different patterns of intracellular phospho-MLC distribution. Thrombin-induced talin-vinculin and OxPAPC-induced VE-cadherin-vinculin association were abolished by myosin inhibitor blebbistatin. Expression of the vinculin mutant unable to interact with actin attenuated EC permeability changes and MLC phosphorylation caused by both, thrombin and OxPAPC. These data suggest that the specific vinculin interaction with FA or AJ in different contexts of agonist stimulation is defined by development of regional actyomyosin-based tension and participates in both, the barrier-disruptive and barrier-enhancing endothelial responses.

Cellular Biomechanics in Drug Screening and Evaluation: Mechanopharmacology

The study of mechanobiology is now widespread. The impact of cell and tissue mechanics on cellular responses is well appreciated. However, knowledge of the impact of cell and tissue mechanics on pharmacological responsiveness, and its application to drug screening and mechanistic investigations, have been very limited in scope. We emphasize the need for a heightened awareness of the important bidirectional influence of drugs and biomechanics in all living systems. We propose that the term 'mechanopharmacology' be applied to approaches that employ in vitro systems, biomechanically appropriate to the relevant (patho)physiology, to identify new drugs and drug targets. This article describes the models and techniques that are being developed to transform drug screening and evaluation, ranging from a 2D environment to the dynamic 3D environment of the target expressed in the disease of interest.

Cell-stiffness-induced mechanosignaling - a key driver of leukocyte transendothelial migration

The breaching of cellular and structural barriers by migrating cells is a driving factor in development, inflammation and tumor cell metastasis. One of the most extensively studied examples is the extravasation of activated leukocytes across the vascular endothelium, the inner lining of blood vessels. Each step of this leukocyte transendothelial migration (TEM) process is regulated by distinct endothelial adhesion receptors such as the intercellular adhesion molecule 1 (ICAM1). Adherent leukocytes exert force on these receptors, which sense mechanical cues and transform them into localized mechanosignaling in endothelial cells. In turn, the function of the mechanoreceptors is controlled by the stiffness of the endothelial cells and of the underlying substrate representing a positive-feedback loop. In this Commentary, we focus on the mechanotransduction in leukocytes and endothelial cells, which is induced in response to variations in substrate stiffness. Recent studies have described the first key proteins involved in these mechanosensitive events, allowing us to identify common regulatory mechanisms in both cell types. Finally, we discuss how endothelial cell stiffness controls the individual steps in the leukocyte TEM process. We identify endothelial cell stiffness as an important component, in addition to locally presented chemokines and adhesion receptors, which guides leukocytes to sites that permit TEM.

Nano-Biomechanical Study of Spatio-Temporal Cytoskeleton Rearrangements that Determine Subcellular Mechanical Properties and Endothelial Permeability

The endothelial cell (EC) lining of the pulmonary vascular system forms a semipermeable barrier between blood and the interstitium and regulates various critical biochemical functions. Collectively, it represents a prototypical biomechanical system, where the complex hierarchical architecture, from the molecular scale to the cellular and tissue level, has an intimate and intricate relationship with its biological functions. We investigated the mechanical properties of human pulmonary artery endothelial cells (ECs) using atomic force microscopy (AFM). Concurrently, the wider distribution and finer details of the cytoskeletal nano-structure were examined using fluorescence microscopy (FM) and scanning transmission electron microscopy (STEM), respectively. These correlative measurements were conducted in response to the EC barrier-disrupting agent, thrombin, and barrier-enhancing agent, sphingosine 1-phosphate (S1P). Our new findings and analysis directly link the spatio-temporal complexities of cell re-modeling and cytoskeletal mechanical properties alteration. This work provides novel insights into the biomechanical function of the endothelial barrier and suggests similar opportunities for understanding the form-function relationship in other biomechanical subsystems.

Control of vascular permeability by adhesion molecules

Vascular permeability is a vital function of the circulatory system that is regulated in large part by the limited flux of solutes, water, and cells through the endothelial cell layer. One major pathway through this barrier is via the inter-endothelial junction, which is driven by the regulation of cadherin-based adhesions. The endothelium also forms attachments with surrounding proteins and cells via 2 classes of adhesion molecules, the integrins and IgCAMs. Integrins and IgCAMs propagate activation of multiple downstream signals that potentially impact cadherin adhesion. Here we discuss the known contributions of integrin and IgCAM signaling to the regulation of cadherin adhesion stability, endothelial barrier function, and vascular permeability. Emphasis is placed on known and prospective crosstalk signaling mechanisms between integrins, the IgCAMs- ICAM-1 and PECAM-1, and inter-endothelial cadherin adhesions, as potential strategic signaling nodes for multipartite regulation of cadherin adhesion.

Actin-binding proteins differentially regulate endothelial cell stiffness, ICAM-1 function and neutrophil transmigration

Chronic vascular inflammation is driven by interactions between activated leukocytes and the endothelium. Leukocyte β2-integrins bind to endothelial intercellular adhesion molecule 1 (ICAM-1), which allows leukocyte spreading, crawling and transendothelial migration. Leukocytes scan the vascular endothelium for permissive sites to transmigrate, which suggests that there is apical membrane heterogeneity within the endothelium. However, the molecular basis for this heterogeneity is unknown. Leukocyte adhesion induces ICAM-1 clustering, which promotes its association to the actin-binding proteins filamin B, α-actinin-4 and cortactin. We show that these endothelial proteins differentially control adhesion, spreading and transmigration of neutrophils. Loss of filamin B, α-actinin-4 and cortactin revealed adaptor-specific effects on a nuclear-to-peripheral gradient of endothelial cell stiffness. By contrast, increasing endothelial cell stiffness stimulates ICAM-1 function. We identify endothelial α-actinin-4 as a key regulator of endothelial cell stiffness and of ICAM-1-mediated neutrophil transmigration. Finally, we found that the endothelial lining of human and murine atherosclerotic plaques shows elevated levels of α-actinin-4. These results identify endothelial cell stiffness as an important regulator of endothelial surface heterogeneity and of ICAM-1 function, which in turn controls the adhesion and transmigration of neutrophils.

Proline-rich region of non-muscle myosin light chain kinase modulates kinase activity and endothelial cytoskeletal dynamics

Disruption of the pulmonary endothelial barrier and subsequent vascular leak is a hallmark of acute lung injury. Dynamic rearrangements in the endothelial cell (EC) peripheral membrane and underlying cytoskeleton are critical determinants of barrier function. The cytoskeletal effector protein non-muscle myosin light chain kinase (nmMLCK) and the actin-binding regulatory protein cortactin are important regulators of the endothelial barrier. In the present study we functionally characterize a proline-rich region of nmMLCK previously identified as the possible site of interaction between nmMLCK and cortactin. A mutant nmMLCK construct deficient in proline residues at the putative sites of cortactin binding (amino acids 973, 976, 1019, 1022) was generated. Co-immunoprecipitation studies in human lung EC transfected with wild-type or mutant nmMLCK demonstrated similar levels of cortactin interaction at baseline and after stimulation with the barrier-enhancing agonist, sphingosine 1-phosphate (S1P). In contrast, binding studies utilizing recombinant nmMLCK fragments containing the wild-type or proline-deficient sequence demonstrated a two-fold increase in cortactin binding (p<0.01) to the mutant construct. Immunofluorescent microscopy revealed an increased stress fiber density in ECs expressing GFP-labeled mutant nmMLCK at baseline (p=0.02) and after thrombin (p=0.01) or S1P (p=0.02) when compared to wild-type. Mutant nmMLCK demonstrated an increase in kinase activity in response to thrombin (p<0.01). Kymographic analysis demonstrated an increased EC membrane retraction distance and velocity (p<0.01) in response to the barrier disrupting agent thrombin in cells expressing the mutant vs. the wild-type nmMLCK construct. These results provide evidence that critical prolines within nmMLCK (amino acids 973, 976, 1019, 1022) regulate cytoskeletal and membrane events associated with pulmonary endothelial barrier function.

Probing the biomechanical contribution of the endothelium to lymphocyte migration: diapedesis by the path of least resistance

Immune cell trafficking requires the frequent breaching of the endothelial barrier either directly through individual cells ('transcellular' route) or through the inter-endothelial junctions ('paracellular' route). What determines the loci or route of breaching events is an open question with important implications for overall barrier regulation. We hypothesized that basic biomechanical properties of the endothelium might serve as crucial determinants of this process. By altering junctional integrity, cytoskeletal morphology and, consequently, local endothelial cell stiffness of different vascular beds, we could modify the preferred route of diapedesis. In particular, high barrier function was associated with predominantly transcellular migration, whereas negative modulation of junctional integrity resulted in a switch to paracellular diapedesis. Furthermore, we showed that lymphocytes dynamically probe the underlying endothelium by extending invadosome-like protrusions (ILPs) into its surface that deform the nuclear lamina, distort actin filaments and ultimately breach the barrier. Fluorescence imaging and pharmacologic depletion of F-actin demonstrated that lymphocyte barrier breaching efficiency was inversely correlated with local endothelial F-actin density and stiffness. Taken together, these data support the hypothesis that lymphocytes are guided by the mechanical 'path of least resistance' as they transverse the endothelium, a process we term 'tenertaxis'.

Metabolic theory of septic shock

Septic shock is a life threatening condition that can develop subsequent to infection. Mortality can reach as high as 80% with over 150000 deaths yearly in the United States alone. Septic shock causes progressive failure of vital homeostatic mechanisms culminating in immunosuppression, coagulopathy and microvascular dysfunction which can lead to refractory hypotension, organ failure and death. The hypermetabolic response that accompanies a systemic inflammatory reaction places high demands upon stored nutritional resources. A crucial element that can become depleted early during the progression to septic shock is glutathione. Glutathione is chiefly responsible for supplying reducing equivalents to neutralize hydrogen peroxide, a toxic oxidizing agent that is produced during normal metabolism. Without glutathione, hydrogen peroxide can rise to toxic levels in tissues and blood where it can cause severe oxidative injury to organs and to the microvasculature. Continued exposure can result in microvascular dysfunction, capillary leakage and septic shock. It is the aim of this paper to present evidence that elevated systemic levels of hydrogen peroxide are present in septic shock victims and that it significantly contributes to the development and progression of this frequently lethal condition.

Mechanical control of the endothelial barrier

The integrity of the endothelial barrier is controlled by the combined action of chemical and mechanical signaling systems. Permeability-regulating factors signal through small GTPases to regulate the architecture of the cytoskeleton and this has a strong impact on the morphology and stability of VE-cadherin-based cell-cell junctions. The details of how structural and mechanical properties of the actin cytoskeleton influence cell-cell adhesion and how this impacts the dynamic regulation of the endothelial barrier, are beginning to be elucidated. In this review, we discuss the physical and regulatory interactions between the VE-cadherin complex and the actomysoin cytoskeleton, as they are the main determinants of cell-cell adhesion and the mechanical architecture of the cytoskeleton. We discuss, based on recent in vitro data, how a balance between Linear Adherens Junctions, paralleled by cortical actin bundles and Focal Adherens Junctions, connected to radial action bundles, determines endothelial barrier function. We discuss how small GTPases control this balance by regulating the spatial organization and mechanics of actomyosin. We propose a hypothetical model of how biochemical and mechanical signals cooperate locally, at the actomyosin-adhesion interface to open and re-seal the barrier in a rapid and controlled manner.

New approach to investigate the cytotoxicity of nanomaterials using single cell mechanics

Current in vitro methods to assess nanomaterial cytotoxicity involve various assays to monitor specific cellular dysfunction, such as metabolic imbalance or inflammation. Although high throughput, fast, and animal-free, these in vitro methods suffer from unreliability and lack of relevance to in vivo situations. New approaches, especially with the potential to reliably relate to in vivo studies directly, are in critical need. This work introduces a new approach, single cell mechanics, derived from atomic force microscopy-based single cell compression. The single cell based approach is intrinsically advantageous in terms of being able to directly correlate to in vivo investigations. Its reliability and potential to measure cytotoxicity is evaluated using known systems: zinc oxide (ZnO) and silicon dioxide (SiO2) nanoparticles (NP) on human aortic endothelial cells (HAECs). This investigation clearly indicates the reliability of single cell compression. For example, ZnO NPs cause significant changes in force vs relative deformation profiles, whereas SiO2 NPs do not. New insights into NPs-cell interactions pertaining to cytotoxicity are also revealed from this single cell mechanics approach, in addition to a qualitative cytotoxicity conclusion. The advantages and disadvantages of this approach are also compared with conventional cytotoxicity assays.

Modification of plasma membrane organization in tobacco cells elicited by cryptogein

Lipid mixtures within artificial membranes undergo a separation into liquid-disordered and liquid-ordered phases. However, the existence of this segregation into microscopic liquid-ordered phases has been difficult to prove in living cells, and the precise organization of the plasma membrane into such phases has not been elucidated in plant cells. We developed a multispectral confocal microscopy approach to generate ratiometric images of the plasma membrane surface of Bright Yellow 2 tobacco (Nicotiana tabacum) suspension cells labeled with an environment sensitive fluorescent probe. This allowed the in vivo characterization of the global level of order of this membrane, by which we could demonstrate that an increase in its proportion of ordered phases transiently occurred in the early steps of the signaling triggered by cryptogein and flagellin, two elicitors of plant defense reactions. The use of fluorescence recovery after photobleaching revealed an increase in plasma membrane fluidity induced by cryptogein, but not by flagellin. Moreover, we characterized the spatial distribution of liquid-ordered phases on the membrane of living plant cells and monitored their variations induced by cryptogein elicitation. We analyze these results in the context of plant defense signaling, discuss their meaning within the framework of the "membrane raft" hypothesis, and propose a new mechanism of signaling platform formation in response to elicitor treatment.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Heterogeneous elastic response of human lung microvascular endothelial cells to barrier modulating stimuli

In this study we employ atomic force microscopy, supported by finite element analysis and fluorescence microscopy, to characterize the elastic properties accompanying cytoskeletal structural rearrangements of lung microvascular endothelial cells in response to barrier altering stimuli. Statistical analysis of elasticity data obtained from multiple cells demonstrates a heterogeneous cellular elastic response to barrier-enhancing and barrier-disrupting agents; sphingosine 1-phosphate (S1P) and thrombin, respectively. A small but detectable (10%) increase in the average elastic modulus of all cells is observed for S1P, which is accompanied by a corresponding significant decrease in cell thickness. Variable effects of thrombin on these parameters were observed. To account for possible substrate effects in our elasticity analysis, we analyzed only the low-force sections of the force-displacement curves and utilized a finite-thickness correction to the Hertzian model. Our finite element analysis results substantiate this approach. The heterogeneous elastic behavior correlates with differential cytoskeletal rearrangements observed with fluorescence microscopy.

FROM THE CLINICAL EDITOR

This team of investigators employed atomic force microscopy coupled with finite element analysis and fluorescence microscopy to characterize the elastic properties accompanying cytoskeletal structural rearrangements of lung microvascular endothelial cells in response to barrier altering stimuli, demonstrating the validity of their approach.

High glucose enhances LPS-stimulated human PMVEC hyperpermeability via the NO pathway

Chronic hyperglycemia is an established risk factor for endothelial damage. It remains unclear, however, whether brief hyperglycemic exposure exacerbates the damage to vascular endothelial cells induced by endotoxin. We hypothesize that brief hyperglycemic exposure enhances the permeability of the endothelium following stimulation with lipopolysaccharide (LPS). Correlations between modulation of nitric oxide synthase (NOS) pathways and altered endothelial homeostasis have been studied and demonstrated in various pathophysiological conditions. NOS activities are regulated by endogenous inhibitors, including asymmetric dimethylarginine (ADMA), which is metabolized by dimethylarginine dimethylaminohydrolase (DDAH). Since previous data demonstrated that endothelial dysfunction may be related to reduced expression and/or activity of DDAH, in this study, we aimed to determine the effect of increased glucose levels on pulmonary microvascular endothelial cell (PMVEC) permeability, including effects on the NOS pathways. Human PMVECs were incubated with normal (5.5 mM) and high (33 mM) concentrations of D-glucose for 5 days to create a monolayer of cells prior to LPS stimulation (10 μg/ml) for 12 h. When stimulated with LPS, cells incubated with a high glucose (HG) concentration had significant microfilament rearrangement compared with cells incubated with a normal glucose concentration, as determined by immunofluorescence. Scanning electron microscopy revealed a larger average diameter and increased number of fenestrae on the hyperglycemic PMVECs when stimulated with LPS, compared with PMVECs cultured with a normal glucose concentration. The results demonstrated that a high concentration of glucose increases the LPS-stimulated horseradish peroxidase (HRP) permeability compared with a normal concentration of glucose. Furthermore, a HG concentration upregulated LPS-stimulated inducible NOS (iNOS) production and down-regulated endothelial NOS (eNOS) and DDAH-2 expression. Hyperglycemia significantly increased LPS-stimulated nitrite/nitrate production (stable NO end-products). Our results, thus, demonstrate that in vitro HG concentrations exacerbate LPS-stimulated cytoskeletal rearrangement and hyperpermeability of an endothelial monolayer, and cause further imbalance of the NO pathway. These results suggest that it is important to manage even short-term increases in blood glucose, particularly following acute infection.

Double-stranded RNA attenuates the barrier function of human pulmonary artery endothelial cells

Circulating RNA may result from excessive cell damage or acute viral infection and can interact with vascular endothelial cells. Despite the obvious clinical implications associated with the presence of circulating RNA, its pathological effects on endothelial cells and the governing molecular mechanisms are still not fully elucidated. We analyzed the effects of double stranded RNA on primary human pulmonary artery endothelial cells (hPAECs). The effect of natural and synthetic double-stranded RNA (dsRNA) on hPAECs was investigated using trans-endothelial electric resistance, molecule trafficking, calcium (Ca²⁺) homeostasis, gene expression and proliferation studies. Furthermore, the morphology and mechanical changes of the cells caused by synthetic dsRNA was followed by in-situ atomic force microscopy, by vascular-endothelial cadherin and F-actin staining. Our results indicated that exposure of hPAECs to synthetic dsRNA led to functional deficits. This was reflected by morphological and mechanical changes and an increase in the permeability of the endothelial monolayer. hPAECs treated with synthetic dsRNA accumulated in the G1 phase of the cell cycle. Additionally, the proliferation rate of the cells in the presence of synthetic dsRNA was significantly decreased. Furthermore, we found that natural and synthetic dsRNA modulated Ca²⁺ signaling in hPAECs by inhibiting the sarco-endoplasmic Ca²⁺-ATPase (SERCA) which is involved in the regulation of the intracellular Ca²⁺ homeostasis and thus cell growth. Even upon synthetic dsRNA stimulation silencing of SERCA3 preserved the endothelial monolayer integrity. Our data identify novel mechanisms by which dsRNA can disrupt endothelial barrier function and these may be relevant in inflammatory processes.

AFM nanoindentation detection of the elastic modulus of tongue squamous carcinoma cells with different metastatic potentials

Although significant advances have been made in understanding the molecular mechanisms that influence tongue squamous cell carcinoma (TSCC) metastasis, less is known about the association between the cellular elastic modulus and TSCC metastasis. Atomic force microscopy (AFM) nanoindentation via the rate-jump method was used to detect the elastic modulus of TSCC cells from patients and cell lines with different metastatic potentials. TSCC cells with higher metastatic potential showed decreases in the elastic modulus compared to TSCC cells with lower metastatic potential. Moreover, the decrease in elastic modulus was accompanied with epithelial-mesenchymal transition (EMT), cytoskeleton (F-actin and β-tubulin) changes, small nucleus size and large nucleus/cytoplasm (N/C) ratio. The present findings demonstrate a close relationship between the cellular elastic modulus and the metastasis of TSCC. The elastic modulus detected by AFM nanoindentation via the rate-jump method can potentially be used to grade the metastatic potential of TSCC.

FROM THE CLINICAL EDITOR

This team of investigators report the use of an atomic force microscopy-based method to determine the elastic modulus of tongue squamous cell carcinoma cells, and demonstrate that such cells with higher metastatic potential show decreased elastic modulus compared to cells with lower metastatic potential.