Dynamics between actin and the VE-cadherin/catenin complex: novel aspects of the ARP2/3 complex in regulation of endothelial junctions

Low-energy shock waves evoke intracellular Ca2+ increases independently of sonoporation

Low-energy shock waves (LESWs) accelerate the healing of a broad range of tissue injuries, including angiogenesis and bone fractures. In cells, LESW irradiations enhance gene expression and protein synthesis. One probable mechanism underlying the enhancements is mechanosensing. Shock waves also can induce sonoporation. Thus, sonoporation is another probable mechanism underlying the enhancements. It remains elusive whether LESWs require sonoporation to evoke cellular responses. An intracellular Ca²⁺ increase was evoked with LESW irradiations in endothelial cells. The minimum acoustic energy required for sufficient evocation was 1.7 μJ/mm². With the same acoustic energy, sonoporation, by which calcein and propidium iodide would become permeated, was not observed. It was found that intracellular Ca²⁺ increases evoked by LESW irradiations do not require sonoporation. In the intracellular Ca²⁺ increase, actin cytoskeletons and stretch-activated Ca²⁺ channels were involved; however, microtubules were not. In addition, with Ca²⁺ influx through the Ca²⁺ channels, the Ca²⁺ release through the PLC-IP₃-IP₃R cascade contributed to the intracellular Ca²⁺ increase. These results demonstrate that LESW irradiations can evoke cellular responses independently of sonoporation. Rather, LESW irradiations evoke cellular responses through mechanosensing.

Proteomic Analysis of Endothelial Cells Exposed to Ultrasmall Nanoparticles Reveals Disruption in Paracellular and Transcellular Transport

The large interactive surfaces of nanoparticles (NPs) increase the opportunities to develop NPs for vascular targeting. Proteomic analysis of endothelial cells exposed to NPs reveals the cellular response and turns the focus into the impairment of the endothelial permeability. Here, quantitative proteomics and transcriptome sequencing are combined to evaluate the effects of exposure to sub-lethal concentrations of TiO₂ -USNPs and TiO₂ -NPs on human dermal microvascular endothelial cells. Endothelial cells react to preserve the semi-permeable properties that are essential for vascular tissue fluid homeostasis, vascular development, and angiogenesis. The main impact of the exposure was alteration of functional complexes involved in cell adhesion, vesicular transport, and cytoskeletal structure. Those are the core cellular structures that are linked to the permeability and the integrity of the endothelial tissue. Moreover, the extracellular proteins uptake along wih the NPs into the endothelial cells escape the lysosomal degradation pathway. These findings improve the understanding of the interaction of NPs with endothelial cell. The effects of the studied NPs modulating cell-cell adhesion and vesicular transport can help to evaluate the distribution of NPs via intravenous administration.

Putting VE-cadherin into JAIL for junction remodeling

Junction dynamics of endothelial cells are based on the integration of signal transduction, cytoskeletal remodeling and contraction, which are necessary for the formation and maintenance of monolayer integrity, but also enable repair and regeneration. The VE-cadherin–catenin complex forms the molecular basis of the adherence junctions and cooperates closely with actin filaments. Several groups have recently described small actin-driven protrusions at the cell junctions that are controlled by the Arp2/3 complex, contributing to cell junction regulation. We identified these protrusions as the driving force for VE-cadherin dynamics, as they directly induce new VE-cadherin-mediated adhesion sites, and have accordingly referred to these structures as junction-associated intermittent lamellipodia (JAIL). JAIL extend over only a few microns and thus provide the basis for a subcellular regulation of adhesion. The local (subcellular) VE-cadherin concentration and JAIL formation are directly interdependent, which enables autoregulation. Therefore, this mechanism can contribute a subcellularly regulated adaptation of cell contact dynamics, and is therefore of great importance for monolayer integrity and relative cell migration during wound healing and angiogenesis, as well as for inflammatory responses. In this Review, we discuss the mechanisms and functions underlying these actin-driven protrusions and consider their contribution to the dynamic regulation of endothelial cell junctions.

FBXW7 regulates endothelial barrier function by suppression of the cholesterol synthesis pathway and prenylation of RhoB

Rho GTPases control both the actin cytoskeleton and adherens junction stability and are recognized as essential regulators of endothelial barrier function. They act as molecular switches and are primarily regulated by the exchange of GDP and GTP. However, posttranslational modifications such as phosphorylation, prenylation, and ubiquitination can additionally alter their localization, stability, and activity. F-box proteins are involved in the recognition of substrate proteins predestined for ubiquitination and subsequent degradation. Given the importance of ubiquitination, we studied the effect of the loss of 62 members of the F-box protein family on endothelial barrier function in human umbilical vein endothelial cells. Endothelial barrier function was quantified by electrical cell impedance sensing and macromolecule passage assay. Our RNA interference–based screen identified FBXW7 as a key regulator of endothelial barrier function. Mechanistically, loss of FBXW7 induced the accumulation of the RhoB GTPase in endothelial cells, resulting in their increased contractility and permeability. FBXW7 knockdown induced activation of the cholesterol biosynthesis pathway and changed the prenylation of RhoB. This effect was reversed by farnesyl transferase inhibitors and by the addition of geranylgeranyl pyrophosphate. In summary, this study identifies FBXW7 as a novel regulator of endothelial barrier function in vitro. Loss of FBXW7 indirectly modulates RhoB activity via alteration of the cholesterol biosynthesis pathway and, consequently, of the prenylation status and activity of RhoB, resulting in increased contractility and disruption of the endothelial barrier.

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.

Vascular Endothelial (VE)-Cadherin, Endothelial Adherens Junctions, and Vascular Disease

Endothelial cell-cell adherens junctions (AJs) supervise fundamental vascular functions, such as the control of permeability and transmigration of circulating leukocytes, and the maintenance of existing vessels and formation of new ones. These processes are often dysregulated in pathologies. However, the evidence that links dysfunction of endothelial AJs to human pathologies is mostly correlative. In this review, we present an update of the molecular organization of AJ complexes in endothelial cells (ECs) that is mainly based on observations from experimental models. Furthermore, we report in detail on a human pathology, cerebral cavernous malformation (CCM), which is initiated by loss-of-function mutations in the genes that encode the three cytoplasmic components of AJs (CCM1, CCM2, and CCM3). At present, these represent a unique example of mutations in components of endothelial AJs that cause human disease. We describe also how studies into the defects of AJs in CCM are shedding light on the crucial regulatory mechanisms and signaling activities of these endothelial structures. Although these observations are specific for CCM, they support the concept that dysfunction of endothelial AJs can directly contribute to human pathologies.

CMTM4 regulates angiogenesis by promoting cell surface recycling of VE-cadherin to endothelial adherens junctions

Vascular endothelial (VE) cadherin is a key component of endothelial adherens junctions (AJs) and plays an important role in maintaining vascular integrity. Endocytosis of VE-cadherin regulates junctional strength and a decrease of surface VE-cadherin reduces vascular stability. However, disruption of AJs is also a requirement for vascular sprouting. Identifying novel regulators of endothelial endocytosis could enhance our understanding of angiogenesis. Here, we evaluated the angiogenic potential of (CKLF-like MARVEL transmembrane domain 4) CMTM4 and assessed in which molecular pathway CMTM4 is involved during angiogenesis. Using a 3D vascular assay composed of GFP-labeled HUVECs and dsRED-labeled pericytes, we demonstrated in vitro that siRNA-mediated CMTM4 silencing impairs vascular sprouting. In vivo, CMTM4 silencing by morpholino injection in zebrafish larvae inhibits intersomitic vessel growth. Intracellular staining revealed that CMTM4 colocalizes with Rab4⁺ and Rab7⁺ vesicles, both markers of the endocytic trafficking pathway. CMTM4 colocalizes with both membrane-bound and internalized VE-cadherin. Adenovirus-mediated CMTM4 overexpression enhances the endothelial endocytic pathway, in particular the rapid recycling pathway, shown by an increase in early endosomal antigen-1 positive (EEA1⁺), Rab4⁺, Rab11⁺ , and Rab7⁺ vesicles. CMTM4 overexpression enhances membrane-bound VE-cadherin internalization, whereas CMTM4 knockdown decreases internalization of VE-cadherin. CMTM4 overexpression promotes endothelial barrier function, shown by an increase in recovery of transendothelial electrical resistance (TEER) after thrombin stimulation. We have identified in this study a novel regulatory function for CMTM4 in angiogenesis. CMTM4 plays an important role in the turnover of membrane-bound VE-cadherin at AJs, mediating endothelial barrier function and controlling vascular sprouting.

Defective endothelial cell migration in the absence of Cdc42 leads to capillary-venous malformations

Formation and homeostasis of the vascular system requires several coordinated cellular functions, but their precise interplay during development and their relative importance for vascular pathologies remain poorly understood. Here, we investigated the endothelial functions regulated by Cdc42 and their in vivo relevance during angiogenic sprouting and vascular morphogenesis in the postnatal mouse retina. We found that Cdc42 is required for endothelial tip cell selection, directed cell migration and filopodia formation, but dispensable for cell proliferation or apoptosis. Although the loss of Cdc42 seems generally compatible with apical-basal polarization and lumen formation in retinal blood vessels, it leads to defective endothelial axial polarization and to the formation of severe vascular malformations in capillaries and veins. Tracking of Cdc42-depleted endothelial cells in mosaic retinas suggests that these capillary-venous malformations arise as a consequence of defective cell migration, when endothelial cells that proliferate at normal rates are unable to re-distribute within the vascular network.

Atorvastatin enhances endothelial adherens junctions through promoting VE-PTP gene transcription and reducing VE-cadherin-Y731 phosphorylation

Vascular endothelial protein tyrosine phosphatase (VE-PTP) is essential for endothelial cells (ECs) adherens junction and vascular homeostasis; however, the regulatory mechanism of VE-PTP transcription is unknown, and a drug able to promote VE-PTP expression in ECs has not yet been reported in the literature. In this study, we used human ECs as a model to explore small molecule compounds able to promote VE-PTP expression, and found that atorvastatin, a HMG-CoA reductase inhibitor widely used in the clinic to treat hypercholesterolemia-related cardiovascular diseases, strongly promoted VE-PTP transcription in ECs through activating the VE-PTP promoter and upregulating the expression of the transcription factor, specificity protein 1 (SP1). Additionally, atorvastatin markedly reduced VE-cadherin-Y731 phosphorylation induced by cigarette smoke extract and significantly enhanced stability of endothelial adherens junctions. Together, our findings reveal that atorvastatin up-regulates VE-PTP expression, increases VE-cadherin protein levels, and decreases VE-cadherin-Y731 phosphorylation to strengthen EC adherens junctions and maintain vascular cell monolayer integrity, offering a new mechanism of atorvastatin against CSE-induced disruption of vascular integrity and relevant cardio-cerebrovascular disease.

YAP and TAZ regulate adherens junction dynamics and endothelial cell distribution during vascular development

Formation of blood vessel networks by sprouting angiogenesis is critical for tissue growth, homeostasis and regeneration. How endothelial cells arise in adequate numbers and arrange suitably to shape functional vascular networks is poorly understood. Here we show that YAP/TAZ promote stretch-induced proliferation and rearrangements of endothelial cells whilst preventing bleeding in developing vessels. Mechanistically, YAP/TAZ increase the turnover of VE-Cadherin and the formation of junction associated intermediate lamellipodia, promoting both cell migration and barrier function maintenance. This is achieved in part by lowering BMP signalling. Consequently, the loss of YAP/TAZ in the mouse leads to stunted sprouting with local aggregation as well as scarcity of endothelial cells, branching irregularities and junction defects. Forced nuclear activity of TAZ instead drives hypersprouting and vascular hyperplasia. We propose a new model in which YAP/TAZ integrate mechanical signals with BMP signaling to maintain junctional compliance and integrity whilst balancing endothelial cell rearrangements in angiogenic vessels.

Short interfering RNA-mediated silencing of actin-related protein 2/3 complex subunit 4 inhibits the migration of SW620 human colorectal cancer cells

Actin-related protein 2/3 complex subunit 4 (ARPC4) acts as an actin nucleator in actin cytoskeleton branching and contributes to cell migration. ARPC4 has previously been demonstrated to be abnormally expressed in various colorectal carcinoma cell lines, particularly SW620 cells. The present study explored the biological action and the possible mechanisms underlying the function of ARPC4 in the progression of carcinoma. The proliferation and migration of SW620 cells transfected with ARPC4-specific short interfering (si)RNAs were assessed using western blot, cell counting, flow cytometry and transwell assays. SW620 cells exhibited the highest ARPC4 expression of the cell lines investigated, and siRNA538 was the most effective of the siRNAs considered. The results of the present study demonstrated that ARPC4-silencing exhibited a significant effect on the capacity of cells for migration, but did not affect their proliferative ability. ARPC4-silencing inhibited human SW620 cell migration, but not proliferation, in vitro, suggesting that ARPC4 may be a putative therapeutic target for colorectal carcinoma.

Expression analysis of E-cad and vascular endothelial growth factor in triple-negative breast cancer patients of different ethnic groups in western China

The aim of this article is to investigate the expression of E-cadherin (E-cad) and vascular endothelial growth factor (VEGF) in triple-negative breast cancer (TNBC) of Han and Uygur women patients in western China, and their relationship with clinical features of TNBC.Totally, 172 cases of Han TNBC patients and 79 cases of Uighur TNBC patients were enrolled. The expressions of E-cad and VEGF were detected with immunohistochemistry. The correlation of E-cad and VEGF expression with lymph node metastasis, TNM stage, and histological grade were analyzed. The 5-year disease-free survival rate of the 2 groups was also evaluated.There was no significant difference in the 5-year disease-free survival rate (P > .05) and the expression of E-cad between the 2 groups. The positive rate of VEGF in Han was significantly lower than that in Uygur (P < .05). The expression of E-cad was negatively correlated with lymph node metastasis, TNM stage, and histological grade (-1≤r < 1, P < .05). However, the expression of VEGF was positively correlated with lymph node metastasis and TNM staging (0 < r < 1, P < 0.05), but not with histological grading.The expression of E-cad and VEGF and their relationship with clinical features of TNBC suggest that Uygur TNBC patients might have different prognostic factors as compared with Han patients.

RhoA, RhoB and RhoC differentially regulate endothelial barrier function

RhoGTPases are known regulators of intracellular actin dynamics that are important for maintaining endothelial barrier function. RhoA is most extensively studied as a key regulator of endothelial barrier function, however the function of the 2 highly homologous family-members (> 88%) RhoB and RhoC in endothelial barrier function is still poorly understood.

This study aimed to determine whether RhoA, RhoB and RhoC have overlapping or distinct roles in barrier function and permeability in resting and activated endothelium. By using primary endothelial cells in combination with siRNA transfection to establish individual, double or triple knockdown of the RhoA/B/C RhoGTPases, we found that RhoB, but not RhoA or RhoC, is in resting endothelium a negative regulator of permeability. Loss of RhoB accounted for an accumulation of VE-cadherin at cell-cell contacts. Thrombin-induced loss of endothelial integrity is mediated primarily by RhoA and RhoB. Combined loss of RhoA/B showed decreased phosphorylation of Myosin Light Chain and increased expression of VE-cadherin at cell-cell contacts after thrombin stimulation. RhoC contributes to the Rac1-dependent restoration of endothelial barrier function.

In summary, this study shows that these highly homologous RhoGTPases differentially control the dynamics of endothelial barrier function.

Epithelial protein lost in neoplasm (EPLIN): Beyond a tumor suppressor

The majority of cancer-related deaths are caused by tumor recurrence, metastasis and therapeutic resistance. During the late stages of tumor progression, multiple factors are involved, including the downregulation and/or loss of function of metastasis suppressors. Epithelial protein lost in neoplasm (EPLIN), an actin-binding protein, was initially identified as a putative tumor suppressor that is frequently downregulated in epithelial tumors. Recent evidence indicates that EPLIN may negatively regulate epithelia-to-mesenchymal transition (EMT), a crucial process by which cancer cells acquire invasive capabilities and therapeutic resistance. Importantly, downregulation of EPLIN is associated with clinical metastasis in a variety of solid tumors, suggesting that EPLIN could be a suppressor of metastasis. In this review, I will discuss the regulation and function of EPLIN in human cancer cells and explore the clinical significance of EPLIN in metastatic disease.

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.

From basic mechanisms to clinical applications in heart protection, new players in cardiovascular diseases and cardiac theranostics: meeting report from the third international symposium on "New frontiers in cardiovascular research"

In this meeting report, particularly addressing the topic of protection of the cardiovascular system from ischemia/reperfusion injury, highlights are presented that relate to conditioning strategies of the heart with respect to molecular mechanisms and outcome in patients' cohorts, the influence of co-morbidities and medications, as well as the contribution of innate immune reactions in cardioprotection. Moreover, developmental or systems biology approaches bear great potential in systematically uncovering unexpected components involved in ischemia-reperfusion injury or heart regeneration. Based on the characterization of particular platelet integrins, mitochondrial redox-linked proteins, or lipid-diol compounds in cardiovascular diseases, their targeting by newly developed theranostics and technologies opens new avenues for diagnosis and therapy of myocardial infarction to improve the patients' outcome.

VE-cadherin involved in the pulmonary microvascular endothelial cell barrier injury induced by angiotensin II through modulating the cellular apoptosis and skeletal rearrangement

OBJECTIVE

Angiotensin II (AngII) involved in the pathogenesis of pulmonary injury through impairing the integrity of pulmonary microvascular endothelial barrier, but the mechanism is still not clear. We aim to determine the roles of VE-cadherin, playing crucial roles in the adhesion of the vascular endothelial barrier and the barrier function, in the pulmonary microvascular endothelial cell (PMVEC) barrier injury mediated by AngII.

METHODS

Mice acute lung injury (ALI) model was induced through pumping of AngII. The infiltration of macrophages and neutrophils as well as the PMVEC permeability were determined in order to determine the barrier injury in vivo and in vitro. Knockdown of VE-cadherin was established using siRNA technique, and its roles in the apoptosis and skeletal rearrangement in the PMVECs were evaluated.

RESULTS

After AngII interference, the expression of VE-cadherin in the PMVECs and pulmonary tissues in mice was down-regulated. Upon VE-cadherin knockdown through siRNA technique, AngII induced susceptibility of PMVECs to apoptosis. Knockdown of VE-cadherin contributed to the skeletal rearrangement in the endothelial cells, together with increase of permeability.

CONCLUSIONS

VE-cadherin expression is closely related to the apoptosis and skeletal rearrangement of PMVECs induced by AngII.

Parallels between single cell migration and barrier formation: The case of RhoB and Rac1 trafficking

The appearance of multicellularity implied the adaptation of signaling networks required for unicellular life to new functions arising in this remarkable evolutionary transition. A hallmark of multicellular organisms is the formation of cellular barriers that compartmentalize spaces and functions. Here we discuss recent findings concerning the role of RhoB in the negative control of Rac1 trafficking from endosomes to the cell border, in order to induce membrane extensions to restore endothelial barrier function after acute contraction. This role closely resembles that proposed for RhoB in controlling single cell migration through Rac1, which has also been observed in cancer cell invasion. We highlight these similarities as a signaling paradigm that shows that endothelial barrier integrity is controlled not only by the formation of cell-cell junctions, but also by a balance between ancestral mechanisms of cell spreading and contraction conserved from unicellular organisms and orchestrated by Rho GTPases.

Contraction of endothelial cells: 40 years of research, but the debate still lives

Force generation in non-muscle cells is vital for many cellular and tissue functions. Force-generating mechanisms include actomyosin-mediated contraction, actin polymerization that drives plasma membrane protrusions and filopodia as well as kinesin- and dynein-controlled transport of vesicles and organelles along the microtubule cytoskeleton. The actomyosin-mediated contractility and actin remodeling in both epithelium and endothelium were shown to have significant impact on cell migration, shape change and formation and control of intercellular junctions. In endothelium, contraction is supposed to control permeability for fluid and solutes. However, recent studies demonstrated the constitutive appearance of junction-associated intermittent lamellipodia (JAIL) that drive vascular endothelial cadherin (VE-cadherin) dynamics and control endothelial permeability. Since thrombin blocks JAIL formation and thus increases endothelial permeability, the concept of a simple Rho GTPase-controlled contraction, which is supposed to open endothelial junctions, becomes challenged. Furthermore, specific tyrosine phosphorylation sites of VE-cadherin and catenins have been shown to be involved in control of VE-cadherin-mediated cell adhesion. How the causal-mechanistic interdependency between contractility, VE-cadherin and catenin phosphorylation and JAIL-mediated dynamic remodeling of VE-cadherin is regulated is still an open question and needs to be further addressed.

Interleukin 2 Activates Brain Microvascular Endothelial Cells Resulting in Destabilization of Adherens Junctions

The pleiotropic cytokine interleukin 2 (IL2) disrupts the blood-brain barrier and alters brain microcirculation, underlying vascular leak syndrome that complicates cancer immunotherapy with IL2. The microvascular effects of IL2 also play a role in the development of multiple sclerosis and other chronic neurological disorders. The mechanism of IL2-induced disruption of brain microcirculation has not been determined previously. We found that both human and murine brain microvascular endothelial cells express constituents of the IL2 receptor complex. Then we established that signaling through this receptor complex leads to activation of the transcription factor, nuclear factor κB, resulting in expression of proinflammatory interleukin 6 and monocyte chemoattractant protein 1. We also discovered that IL2 induces disruption of adherens junctions, concomitant with cytoskeletal reorganization, ultimately leading to increased endothelial cell permeability. IL2-induced phosphorylation of vascular endothelial cadherin (VE-cadherin), a constituent of adherens junctions, leads to dissociation of its stabilizing adaptor partners, p120-catenin and β-catenin. Increased phosphorylation of VE-cadherin was also accompanied by a reduction of Src homology 2 domain-containing protein-tyrosine phosphatase 2, known to maintain vascular barrier function. These results unravel the mechanism of deleterious effects induced by IL2 on brain microvascular endothelial cells and may inform the development of new measures to improve IL2 cancer immunotherapy, as well as treatments for autoimmune diseases affecting the central nervous system.

Traction force dynamics predict gap formation in activated endothelium

In many pathological conditions the endothelium becomes activated and dysfunctional, resulting in hyperpermeability and plasma leakage. No specific therapies are available yet to control endothelial barrier function, which is regulated by inter-endothelial junctions and the generation of acto-myosin-based contractile forces in the context of cell-cell and cell-matrix interactions. However, the spatiotemporal distribution and stimulus-induced reorganization of these integral forces remain largely unknown. Traction force microscopy of human endothelial monolayers was used to visualize contractile forces in resting cells and during thrombin-induced hyperpermeability. Simultaneously, information about endothelial monolayer integrity, adherens junctions and cytoskeletal proteins (F-actin) were captured. This revealed a heterogeneous distribution of traction forces, with nuclear areas showing lower and cell-cell junctions higher traction forces than the whole-monolayer average. Moreover, junctional forces were asymmetrically distributed among neighboring cells. Force vector orientation analysis showed a good correlation with the alignment of F-actin and revealed contractile forces in newly formed filopodia and lamellipodia-like protrusions within the monolayer. Finally, unstable areas, showing high force fluctuations within the monolayer were prone to form inter-endothelial gaps upon stimulation with thrombin. To conclude, contractile traction forces are heterogeneously distributed within endothelial monolayers and force instability, rather than force magnitude, predicts the stimulus-induced formation of intercellular gaps.

Spatiotemporal analysis of RhoA/B/C activation in primary human endothelial cells

Endothelial cells line the vasculature and are important for the regulation of blood pressure, vascular permeability, clotting and transendothelial migration of leukocytes and tumor cells. A group of proteins that that control the endothelial barrier function are the RhoGTPases. This study focuses on three homologous (>88%) RhoGTPases: RhoA, RhoB, RhoC of which RhoB and RhoC have been poorly characterized. Using a RhoGTPase mRNA expression analysis we identified RhoC as the highest expressed in primary human endothelial cells. Based on an existing RhoA FRET sensor we developed new RhoB/C FRET sensors to characterize their spatiotemporal activation properties. We found all these RhoGTPase sensors to respond to physiologically relevant agonists (e.g. Thrombin), reaching transient, localized FRET ratio changes up to 200%. These RhoA/B/C FRET sensors show localized GEF and GAP activity and reveal spatial activation differences between RhoA/C and RhoB. Finally, we used these sensors to monitor GEF-specific differential activation of RhoA/B/C. In summary, this study adds high-contrast RhoB/C FRET sensors to the currently available FRET sensor toolkit and uncover new insights in endothelial and RhoGTPase cell biology. This allows us to study activation and signaling by these closely related RhoGTPases with high spatiotemporal resolution in primary human cells.

Meeting report from the 2nd International Symposium on New Frontiers in Cardiovascular Research. Protecting the cardiovascular system from ischemia: between bench and bedside

Recent advances in basic cardiovascular research as well as their translation into the clinical situation were the focus at the last "New Frontiers in Cardiovascular Research meeting". Major topics included the characterization of new targets and procedures in cardioprotection, deciphering new players and inflammatory mechanisms in ischemic heart disease as well as uncovering microRNAs and other biomarkers as versatile and possibly causal factors in cardiovascular pathogenesis. Although a number of pathological situations such as ischemia-reperfusion injury or atherosclerosis can be simulated and manipulated in diverse animal models, also to challenge new drugs for intervention, patient studies are the ultimate litmus test to obtain unequivocal information about the validity of biomedical concepts and their application in the clinics. Thus, the open and bidirectional exchange between bench and bedside is crucial to advance the field of ischemic heart disease with a particular emphasis of understanding long-lasting approaches in cardioprotection.

Endothelial monolayers and transendothelial migration depend on mechanical properties of the substrate

Endothelial cells (ECs) line the microvasculature and constitute a barrier between the vessel lumen and surrounding tissues. ECs inform circulating immune cells of the health and integrity of surrounding tissues, recruiting them in response to pathogens and tissue damage. ECs play an active role in the transmigration of immune cells across the vessel wall. We have discovered important differences in the properties of ECs on soft hydrogel substrates of varying stiffness, in comparison to glass. Primary ECs from several human sources were tested; all formed monolayers normally on soft substrates. EC monolayers formed more mature cell-cell junctions on soft substrates, relative to glass, based on increased recruitment of vinculin and F-actin. EC monolayers supported transendothelial migration (TEM) on soft substrates. Immune cells, including peripheral blood lymphocytes (PBLs) and natural killer cells, showed decreasing numbers of paracellular (between-cell) transmigration events with decreasing substrate stiffness, while the number of transcellular (through-cell) events increased for PBLs. Melanoma cancer cells showed increased transmigration with decreased stiffness. Our findings demonstrate that endothelial monolayers respond to the mechanical properties of their surroundings, which can regulate the integrity and function of the monolayer independently from inflammatory signals. Soft hydrogel substrates are a more appropriate and physiological model for tissue environments than hard substrates, with important implications for the experimental analysis of TEM.