Myosin di-phosphorylation and peripheral actin bundle formation as initial events during endothelial barrier disruption

Salmonella manipulates macrophage migration via SteC-mediated myosin light chain activation to penetrate the gut-vascular barrier

The intestinal pathogen Salmonella enterica rapidly enters the bloodstream after the invasion of intestinal epithelial cells, but how Salmonella breaks through the gut-vascular barrier is largely unknown. Here, we report that Salmonella enters the bloodstream through intestinal CX3CR1+ macrophages during early infection. Mechanistically, Salmonella induces the migration/invasion properties of macrophages in a manner dependent on host cell actin and on the pathogen effector SteC. SteC recruits host myosin light chain protein Myl12a and phosphorylates its Ser19 and Thr20 residues. Myl12a phosphorylation results in actin rearrangement, and enhanced migration and invasion of macrophages. SteC is able to utilize a wide range of NTPs other than ATP to phosphorylate Myl12a. We further solved the crystal structure of SteC, which suggests an atypical dimerization-mediated catalytic mechanism. Finally, in vivo data show that SteC-mediated cytoskeleton manipulation is crucial for Salmonella breaching the gut vascular barrier and spreading to target organs.

Hydrogels with tunable mechanical plasticity regulate endothelial cell outgrowth in vasculogenesis and angiogenesis

The endothelial cell (EC) outgrowth in both vasculogenesis and angiogenesis starts with remodeling surrounding matrix and proceeds with the crosstalk between cells for the multicellular vasculature formation. The mechanical plasticity of matrix, defined as the ability to permanently deform by external traction, is pivotal in modulating cell behaviors. Nevertheless, the implications of matrix plasticity on cell-to-cell interactions during EC outgrowth, along with the molecular pathways involved, remain elusive. Here we develop a collagen-hyaluronic acid based hydrogel platform with tunable plasticity by using compositing strategy of dynamic and covalent networks. We show that although the increasing plasticity of the hydrogel facilitates the matrix remodeling by ECs, the largest tubular lumens and the longest invading distance unexpectedly appear in hydrogels with medium plasticity instead of the highest ones. We unravel that the high plasticity of the hydrogels promotes stable integrin cluster of ECs and recruitment of focal adhesion kinase with an overenhanced contractility which downregulates the vascular endothelial cadherin expression and destabilizes the adherens junctions between individual ECs. Our results, further validated with mathematical simulations and in vivo angiogenic tests, demonstrate that a balance of matrix plasticity facilitates both cell-matrix binding and cell-to-cell adherens, for promoting vascular assembly and invasion.

TNF-α evokes blood-brain barrier dysfunction through activation of Rho-kinase and neurokinin 1 receptor

Ischaemic stroke, accompanied by neuroinflammation, impairs blood-brain barrier (BBB) integrity through a complex mechanism involving activation of both RhoA/Rho kinase/myosin light chain-2 and neurokinin 1 receptor (NK1R). Using an in vitro model of human BBB composed of brain microvascular endothelial cells (BMEC), astrocytes and pericytes, this study examined the potential contributions of these elements to BBB damage induced by elevated availability of pro-inflammatory cytokine, TNF-α. Treatment of human BMECs with TNF-α significantly enhanced RhoA activity and the protein expressions of Rho kinase and phosphorylated Ser19MLC-2 while decreasing that of NK1R. Pharmacological inhibition of Rho kinase by Y-27632 and NK1R by CP96345 neutralised the disruptive effects of TNF-α on BBB integrity and function as ascertained by reversal of decreases in transendothelial electrical resistance and increases in paracellular flux of low molecular weight permeability marker, sodium fluorescein, respectively. Suppression of RhoA activation, mitigation of actin stress fibre formation and restoration of plasma membrane localisation of tight junction protein zonula occludens-1 appeared to contribute to the barrier-protective effects of both Y-27632 and CP96345. Attenuation of TNF-α-mediated increases in NK1R protein expression in BMEC by Y-27632 suggests that RhoA/Rho kinase pathway acts upstream to NK1R. In conclusion, specific inhibition of Rho kinase in cerebrovascular conditions, accompanied by excessive release of pro-inflammatory cytokine TNF-α, helps preserve endothelial cell morphology and inter-endothelial cell barrier formation and may serve as an important therapeutic target.

The interplay between T helper cells and brain barriers in the pathogenesis of multiple sclerosis

The blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) represent two complex structures protecting the central nervous system (CNS) against potentially harmful agents and circulating immune cells. The immunosurveillance of the CNS is governed by immune cells that constantly patrol the BCSFB, whereas during neuroinflammatory disorders, both BBB and BCSFB undergo morphological and functional alterations, promoting leukocyte intravascular adhesion and transmigration from the blood circulation into the CNS. Multiple sclerosis (MS) is the prototype of neuroinflammatory disorders in which peripheral T helper (Th) lymphocytes, particularly Th1 and Th17 cells, infiltrate the CNS and contribute to demyelination and neurodegeneration. Th1 and Th17 cells are considered key players in the pathogenesis of MS and its animal model, experimental autoimmune encephalomyelitis. They can actively interact with CNS borders by complex adhesion mechanisms and secretion of a variety of molecules contributing to barrier dysfunction. In this review, we describe the molecular basis involved in the interactions between Th cells and CNS barriers and discuss the emerging roles of dura mater and arachnoid layer as neuroimmune interfaces contributing to the development of CNS inflammatory diseases.

Critical role of Rho proteins in myosin light chain di-phosphorylation during early phase of endothelial barrier disruption

We previously reported the Rho-associated coiled-coil containing protein kinase (ROCK)-mediated di-phosphorylation of myosin light chain (MLC) and actin bundle formation at the cell periphery as early events of the endothelial barrier disruption. We herein examined the role of RhoA during early events of barrier disruption. Treatment of cultured porcine aortic endothelial cells with simvastatin prevented the decrease in trans-endothelial electrical resistance, MLC di-phosphorylation and peripheral actin bundle formation seen 3 min after thrombin stimulation. Co-treatment with geranylgeranyl pyrophosphate rescued the thrombin-induced events. Thrombin increased a GTP-bound form of RhoA and phosphorylation of myosin phosphatase target subunit 1 (MYPT1) at the ROCK site. The intracellular introduction of the inhibitory protein of RhoA inhibited the thrombin-induced di-phosphorylation of MLC. However, knockdown of either one of RhoA, RhoB or RhoC failed to inhibit thrombin-induced MLC di-phosphorylation. The findings suggest that Rho proteins play a critical role during early events of thrombin-induced barrier disruption.

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.

In vivo dissection of Rhoa function in vascular development using zebrafish

The small monomeric GTPase RHOA acts as a master regulator of signal transduction cascades by activating effectors of cellular signaling, including the Rho-associated protein kinases ROCK1/2. Previous in vitro cell culture studies suggest that RHOA can regulate many critical aspects of vascular endothelial cell (EC) biology, including focal adhesion, stress fiber formation, and angiogenesis. However, the specific in vivo roles of RHOA during vascular development and homeostasis are still not well understood. In this study, we examine the in vivo functions of RHOA in regulating vascular development and integrity in zebrafish. We use zebrafish RHOA-ortholog (rhoaa) mutants, transgenic embryos expressing wild type, dominant negative, or constitutively active forms of rhoaa in ECs, pharmacological inhibitors of RHOA and ROCK1/2, and Rock1 and Rock2a/b dgRNP-injected zebrafish embryos to study the in vivo consequences of RHOA gain- and loss-of-function in the vascular endothelium. Our findings document roles for RHOA in vascular integrity, developmental angiogenesis, and vascular morphogenesis in vivo, showing that either too much or too little RHOA activity leads to vascular dysfunction.

Stiffening Matrix Induces Age-Mediated Microvascular Phenotype Through Increased Cell Contractility and Destabilization of Adherens Junctions

Aging is a major risk factor in microvascular dysfunction and disease development, but the underlying mechanism remains largely unknown. As a result, age-mediated changes in the mechanical properties of tissue collagen have gained interest as drivers of endothelial cell (EC) dysfunction. 3D culture models that mimic age-mediated changes in the microvasculature can facilitate mechanistic understanding. A fibrillar hydrogel capable of changing its stiffness after forming microvascular networks is established. This hydrogel model is used to form vascular networks from induced pluripotent stem cells under soft conditions that mimic young tissue mechanics. Then matrix stiffness is gradually increased, thus exposing the vascular networks to the aging-mimicry process in vitro. It is found that upon dynamic matrix stiffening, EC contractility is increased, resulting in the activation of focal adhesion kinase and subsequent dissociation of β-catenin from VE-Cadherin mediated adherens junctions, leading to the abruption of the vascular networks. Inhibiting cell contractility impedes the dissociation of β-catenin, thereby preventing the deconstruction of adherens junctions, thus partially rescuing the age-mediated vascular phenotype. The findings provide the first direct evidence of matrix's dynamic mechano-changes in compromising microvasculature with aging and highlight the importance of hydrogel systems to study tissue-level changes with aging in basic and translational studies.

Lymphoedema conditions disrupt endothelial barrier function in vitro

Lymphatic vessel contractions generate net antegrade pulsatile lymph flow. By contrast, impaired lymphatic vessels are often associated with lymphoedema and altered lymph flow. The effect of lymphoedema on the lymph flow field and endothelium is not completely known. Here, we characterized the lymphatic flow field of a platelet-specific receptor C-type lectin-like receptor 2 (CLEC2) deficient lymphoedema mouse model. In regions of lymphoedema, collecting vessels were significantly distended, vessel contractility was greatly diminished and pulsatile lymph flow was replaced by quasi-steady flow. In vitro exposure of human dermal lymphatic endothelial cells (LECs) to lymphoedema-like quasi-steady flow conditions increased intercellular gap formation and permeability in comparison to normal pulsatile lymph flow. In the absence of flow, LECs exposed to steady pressure (SP) increased intercellular gap formation in contrast with pulsatile pressure (PP). The absence of pulsatility in steady fluid flow and SP conditions without flow-induced upregulation of myosin light chain (MLCs) regulatory subunits 9 and 12B mRNA expression and phosphorylation of MLCs, in contrast with pulsatile flow and PP without flow. These studies reveal that the loss of pulsatility, which can occur with lymphoedema, causes LEC contraction and an increase in intercellular gap formation mediated by MLC phosphorylation.

A Novel High Content Angiogenesis Assay Reveals That Lacidipine, L-Type Calcium Channel Blocker, Induces In Vitro Vascular Lumen Expansion

Angiogenesis is a critical cellular process toward establishing a functional circulatory system capable of delivering oxygen and nutrients to the tissue in demand. In vitro angiogenesis assays represent an important tool for elucidating the biology of blood vessel formation and for drug discovery applications. Herein, we developed a novel, high content 2D angiogenesis assay that captures endothelial morphogenesis’s cellular processes, including lumen formation. In this assay, endothelial cells form luminized vascular-like structures in 48 h. The assay was validated for its specificity and performance. Using the optimized assay, we conducted a phenotypic screen of a library containing 150 FDA-approved cardiovascular drugs to identify modulators of lumen formation. The screening resulted in several L-type calcium channel blockers being able to expand the lumen space compared to controls. Among these blockers, Lacidipine was selected for follow-up studies. We found that the endothelial cells treated with Lacidipine showed enhanced activity of caspase-3 in the luminal space. Pharmacological inhibition of caspase activity abolished the Lacidipine-enhancing effect on lumen formation, suggesting the involvement of apoptosis. Using a Ca²⁺ biosensor, we found that Lacipidine reduces the intracellular Ca²⁺ oscillations amplitude in the endothelial cells at the early stage, whereas Lacidipine blocks these Ca²⁺ oscillations completely at the late stage. The inhibition of MLCK exhibits a phenotype of lumen expansion similar to that of Lacidipine. In conclusion, this study describes a novel high-throughput phenotypic assay to study angiogenesis. Our findings suggest that calcium signalling plays an essential role during lumen morphogenesis. L-type Ca²⁺ channel blockers could be used for more efficient angiogenesis-mediated therapies.

Senegalia macrostachya seed polysaccharides attenuate inflammation-induced intestinal epithelial barrier dysfunction in a Caco-2 and RAW264.7 macrophage co-culture model by inhibiting the NF-κB/MLCK pathway

Senegalia macrostachya seed polysaccharides improved the Caco-2 cell monolayer integrity from the inflammatory insult. SMSP2 treatment lowered the inflammatory cytokine release, increased TJ proteins, and downregulated the NF-κB/MLCK pathway.

Baicalin attenuates angiotensin II-induced blood pressure elevation and modulates MLCK/p-MLC signaling pathway

Scutellaria baicalensis Georgi is an extensively used medicinal herb for the treatment of hypertension in traditional Chinese medicine. Baicalin, is an important flavonoid in Scutellaria baicalensis Georgi extracts, which exhibits therapeutic effects on anti-hypertension, but its underlying mechanisms remain to be further explored. Therefore, we investigated the effects and molecular mechanisms of Baicalin on anti-hypertension. In vivo studies revealed that Baicalin treatment significantly attenuated the elevation in blood pressure, the pulse propagation and thickening of the abdominal aortic wall in C57BL/6 mice infused with Angiotensin II (Ang II). Moreover, RNA-sequencing and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses identified 537 differentially expressed transcripts and multiple enriched signaling pathways (including vascular smooth muscle contraction and calcium signaling pathway). Consistently, we found that Baicalin pretreatment significantly alleviated the Ang II induced constriction of abdominal aortic ring, while promoted NE pre-contracted vasodilation of abdominal aortic ring at least partly dependent on L-type calcium channel. In addition, Ang II stimulation significantly increased cell viability and PCNA expression, while were attenuated after Baicalin treatment. Moreover, Baicalin pretreatment attenuated Ang II-induced intracellular Ca2+ release, Angiotensin II type 1 receptor (AT1R) expression and activation of MLCK/p-MLC pathway in vascular smooth muscle cells (VSMCs). The present work further addressed the pharmacological and mechanistic insights on anti-hypertension of Baicalin, which may help better understand the therapeutic effect of Scutellaria baicalensis Georgi on anti-hypertension.

Immunolocalization of protease-activated receptors in endothelial cells of splenic sinuses

The immunolocalization of protease-activated receptors (PARs) and related proteins in splenic sinus endothelial cells was examined using immunofluorescence and electron microscopy. Immunofluorescence microscopy showed that PAR1 colocalized with PAR2, PAR3, and PAR4. PAR4 colocalized with PAR3 and P2Y12. Myosin heavy chain IIA localized to the outer shape and at the base of cells, but did not colocalize with α-catenin. The localization of di-phosphorylated myosin regulatory light chains (ppMLC) was partially detected on the outer circumference and conspicuously at the base of cells. Macrophage migration inhibitory factor (MIF) also localized in cells. Immunogold electron microscopy revealed the localization of PAR1 on the caveolar membrane, plasma membrane, and junctional membrane of cells. PAR2 and PAR3 localized to the plasma membrane of cells. PAR4 localized to the plasma membrane, depressions in the plasma membrane, and cytoplasmic vesicles. PpMLC was detected in stress fibers, but rarely near the adherens junctions of neighboring cells. MIF localized in vesicles on the apical and basal sides of the Golgi apparatus. Electron microscopy of endothelial cells with saponin extraction showed the depression of many coated pits formed by clathrin from the plasma membrane. Stress fibers developed at the base of cells; however, few actin filaments were observed near adherens junctions. These results indicate that PARs play important roles in splenic sinus endothelial cells, such as in endothelial barrier protection and the maintenance of firm adhesion to ring fibers.

Effect of initial seeding density on cell behavior-driven epigenetic memory and preferential lineage differentiation of human iPSCs

Understanding the cellular behavioral mechanisms underlying memory formation and maintenance in human induced pluripotent stem cell (hiPSC) culture provides key strategies for achieving stability and robustness of cell differentiation. Here, we show that changes in cell behavior-driven epigenetic memory of hiPSC cultures alter their pluripotent state and subsequent differentiation. Interestingly, pluripotency-associated genes were activated during the entire cell growth phases along with increased active modifications and decreased repressive modifications. This memory effect can last several days in the long-term stationary phase and was sustained in the aspect of cell behavioral changes after subculture. Further, changes in growth-related cell behavior were found to induce nucleoskeletal reorganization and active versus repressive modifications, thereby enabling hiPSCs to change their differentiation potential. Overall, we discuss the cell behavior-driven epigenetic memory induced by the culture environment, and the effect of previous memory on cell lineage specification in the process of hiPSC differentiation.

RSV attenuates epithelial cell restitution by inhibiting actin cytoskeleton-dependent cell migration

The airway epithelium's ability to repair itself after injury, known as epithelial restitution, is an essential mechanism enabling the respiratory tract's normal functions. Respiratory Syncytial Virus (RSV) is the leading cause of lower respiratory tract infections worldwide. We sought to determine whether RSV delays the airway epithelium wound repair process both in vitro and in vivo. We found that RSV infection attenuated epithelial cell migration, a step in wound repair, promoted stress fiber formation, and mediated assembly of large focal adhesions (FA). Inhibition of Rho kinase (ROCK), a master regulator of actin function, reversed these effects. There was increased RhoA and phospho-myosin light chain (pMLC2) following RSV infection. In vivo, mice were intraperitoneally inoculated with naphthalene to induce lung injury, followed by RSV infection. RSV infection delayed re-epithelialization. There were increased concentrations of pMLC2 in day 7 naphthalene plus RSV animals which normalized by day 14. This study suggests a key mechanism by which RSV infection delays wound healing.

Moesin Is a Novel Biomarker of Endothelial Injury in Sepsis

Objective

Increased vascular permeability and inflammation are principal hallmark of sepsis. Moesin (MSN) is a membrane-associated cytoskeleton protein and crucial for the vascular endothelial function. This study is aimed at evaluating the role of MSN in endothelial injury during the process of sepsis.

Methods

Serum MSN in septic patients was measured by ELISA. BALB/c mice were injected with different doses of lipopolysaccharide (LPS) or underwent cecal ligation and single or double puncture (CLP) to mimic sublethal and lethal sepsis. After treatment, their serum MSN and PCT levels, wet to dry lung weights (W/D ratio), bronchoalveolar lavage fluid (BALF) protein concentrations, and lung injury scores were measured. The impact of MSN silencing on LPS-altered Rock1/myosin light chain (MLC), NF-κB, and inflammatory factors in human microvascular endothelial cells (HMECs), as well as monolayer HMEC permeability, was tested in vitro.

Results

Compared with healthy controls, serum MSN increased in septic patients and was positively correlated with SOFA scores and serum PCT levels in septic patients. LPS injection significantly increased serum the MSN and PCT expression, BALF protein levels, and W/D ratio, and the serum MSN levels were positively correlated with serum PCT, lung W/D ratio, and lung injury scores in mice. Similar results were obtained in the way of CLP modelling. LPS enhanced MSN, MLC, NF-κB phosphorylation, increased Rock1 expression, and inflammatory factors release in the cultured HMECs, while MSN silencing significantly mitigated the LPS-induced Rock1 and inflammatory factor expression, NF-κB, and MLC phosphorylation as well as the monolayer hyperpermeability in HMECs.

Conclusions

Increased serum MSN contributes to the sepsis-related endothelium damages by activating the Rock1/MLC and NF-κB signaling and may be a potential biomarker for evaluating the severity of sepsis.

The transient receptor potential vanilloid 4 (TRPV4) ion channel mediates protease activated receptor 1 (PAR1)-induced vascular hyperpermeability

Endothelial barrier disruption is a hallmark of tissue injury, edema, and inflammation. Vascular endothelial cells express the G protein-coupled receptor (GPCR) protease acctivated receptor 1 (PAR1) and the ion channel transient receptor potential vanilloid 4 (TRPV4), and these signaling proteins are known to respond to inflammatory conditions and promote edema through remodeling of cell-cell junctions and modulation of endothelial barriers. It has previously been established that signaling initiated by the related protease activated receptor 2 (PAR2) is enhanced by TRPV4 in sensory neurons and that this functional interaction plays a critical role in the development of neurogenic inflammation and nociception. Here, we investigated the PAR1-TRPV4 axis, to determine if TRPV4 plays a similar role in the control of edema mediated by thrombin-induced signaling. Using Evans Blue permeation and retention as an indication of increased vascular permeability in vivo, we showed that TRPV4 contributes to PAR1-induced vascular hyperpermeability in the airways and upper gastrointestinal tract of mice. TRPV4 contributes to sustained PAR1-induced Ca2+ signaling in recombinant cell systems and to PAR1-dependent endothelial junction remodeling in vitro. This study supports the role of GPCR-TRP channel functional interactions in inflammatory-associated changes to vascular function and indicates that TRPV4 is a signaling effector for multiple PAR family members.

Control of adhesive ligand density for modulation of nucleus pulposus cell phenotype

Cells of the nucleus pulposus have been observed to undergo a shift from their notochordal-like juvenile phenotype to a more fibroblast-like state with age and maturation. It has been demonstrated that culture of degenerative adult human nucleus pulposus cells upon soft (<1 kPa) full length laminin-containing hydrogel substrates promotes increased levels of a panel of markers associated with the juvenile nucleus pulposus cell phenotype. In the current work, we observed an ability to use soft polymeric substrates functionalized with short laminin-mimetic peptide sequences to recapitulate the behaviors elicited by soft, full-length laminin containing materials. Furthermore, our work suggests an ability to mimic features of soft systems through control of peptide density upon stiffer substrates. Specifically, results suggest that stiffer polymer-peptide hydrogel substrates can be used to promote the expression of a more juvenile-like phenotype for cells of the nucleus pulposus by reducing adhesive ligand presentation. Here we show how polymer stiffness combined with adhesive ligand presentation can be controlled to be supportive of nucleus pulposus cell phenotype and biosynthesis.

Rho-associated kinase and zipper-interacting protein kinase, but not myosin light chain kinase, are involved in the regulation of myosin phosphorylation in serum-stimulated human arterial smooth muscle cells

Myosin regulatory light chain (LC₂₀) phosphorylation plays an important role in vascular smooth muscle contraction and cell migration. Ca²⁺/calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates LC₂₀ (its only known substrate) exclusively at S19. Rho-associated kinase (ROCK) and zipper-interacting protein kinase (ZIPK) have been implicated in the regulation of LC₂₀ phosphorylation via direct phosphorylation of LC₂₀ at T18 and S19 and indirectly via phosphorylation of MYPT1 (the myosin targeting subunit of myosin light chain phosphatase, MLCP) and Par-4 (prostate-apoptosis response-4). Phosphorylation of MYPT1 at T696 and T853 inhibits MLCP activity whereas phosphorylation of Par-4 at T163 disrupts its interaction with MYPT1, exposing the sites of phosphorylation in MYPT1 and leading to MLCP inhibition. To evaluate the roles of MLCK, ROCK and ZIPK in these phosphorylation events, we investigated the time courses of phosphorylation of LC₂₀, MYPT1 and Par-4 in serum-stimulated human vascular smooth muscle cells (from coronary and umbilical arteries), and examined the effects of siRNA-mediated MLCK, ROCK and ZIPK knockdown and pharmacological inhibition on these phosphorylation events. Serum stimulation induced rapid phosphorylation of LC₂₀ at T18 and S19, MYPT1 at T696 and T853, and Par-4 at T163, peaking within 30–120 s. MLCK knockdown or inhibition, or Ca²⁺ chelation with EGTA, had no effect on serum-induced LC₂₀ phosphorylation. ROCK knockdown decreased the levels of phosphorylation of LC₂₀ at T18 and S19, of MYPT1 at T696 and T853, and of Par-4 at T163, whereas ZIPK knockdown decreased LC₂₀ diphosphorylation, but increased phosphorylation of MYPT1 at T696 and T853 and of Par-4 at T163. ROCK inhibition with GSK429286A reduced serum-induced phosphorylation of LC₂₀ at T18 and S19, MYPT1 at T853 and Par-4 at T163, while ZIPK inhibition by HS38 reduced only LC₂₀ diphosphorylation. We also demonstrated that serum stimulation induced phosphorylation (activation) of ZIPK, which was inhibited by ROCK and ZIPK down-regulation and inhibition. Finally, basal phosphorylation of LC₂₀ in the absence of serum stimulation was unaffected by MLCK, ROCK or ZIPK knockdown or inhibition. We conclude that: (i) serum stimulation of cultured human arterial smooth muscle cells results in rapid phosphorylation of LC₂₀, MYPT1, Par-4 and ZIPK, in contrast to the slower phosphorylation of kinases and other proteins involved in other signaling pathways (Akt, ERK1/2, p38 MAPK and HSP27), (ii) ROCK and ZIPK, but not MLCK, are involved in serum-induced phosphorylation of LC₂₀, (iii) ROCK, but not ZIPK, directly phosphorylates MYPT1 at T853 and Par-4 at T163 in response to serum stimulation, (iv) ZIPK phosphorylation is enhanced by serum stimulation and involves phosphorylation by ROCK and autophosphorylation, and (v) basal phosphorylation of LC₂₀ under serum-free conditions is not attributable to MLCK, ROCK or ZIPK.

The Altered Mucosal Barrier Function in the Duodenum Plays a Role in the Pathogenesis of Functional Dyspepsia

BACKGROUND

An altered gastrointestinal barrier function is reportedly associated with the pathogenesis of functional dyspepsia (FD); however, the pathogenesis of FD has not yet been fully elucidated.

AIMS

The objective of the present study was to determine whether the mucosal barrier function is impaired in patients with FD and to investigate the mechanisms underlying FD.

METHODS

The present study included patients with FD (FD group, n = 24), non-FD patients with abdominal symptoms (symptomatic control group, n = 14), and patients with no abdominal symptoms (asymptomatic control group, n = 20). The groups were compared regarding the mucosal electrical impedance (MI) values of the stomach and duodenum, which were measured using a tissue conductance meter during esophagogastroduodenoscopy.

RESULTS

There were no significant differences between the three groups in the MI of the stomach. In contrast, the duodenal MI of the FD group (17.8 ± 4.3 Ω) was significantly lower than those of the symptomatic control group (27.2 ± 6.4 Ω, p < 0.0001) and asymptomatic control group (23.0 ± 7.4 Ω, p = 0.016). The expression of zonula occludens-1 (ZO-1) was significantly lower in the FD group than in the symptomatic control group (p = 0.011), where ZO-1 was positively correlated with the duodenal MI (β = 0.513, p = 0.017). The interleukin (IL)-1β expression was significantly higher in the FD group than in the symptomatic control group (p = 0.041), where IL-1β was inversely correlated with the duodenal MI (β = - 0.600, p = 0.004).

CONCLUSIONS

The mucosal barrier function of the duodenum was altered in patients with FD. Both a decreased ZO-1 and increased IL-1β may play a role in the pathogenesis of FD.

Signaling pathways in cytoskeletal responses to plasma membrane depolarization in corneal endothelial cells

In previous work, we reported that plasma membrane potential depolarization (PMPD) provokes cortical F-actin remodeling in bovine corneal endothelial (BCE) cells in culture, which eventually leads to the appearance of intercellular gaps. In kidney epithelial cells it has been shown that PMPD determines an extracellular-signal-regulated kinase (ERK)/Rho-dependent increase in diphosphorylated myosin light chain (ppMLC). The present study investigated the signaling pathways involved in the response of BCE cells to PMPD. Differently to renal epithelial cells, we observed that PMPD leads to a decrease in monophosphorylated MLC (pMLC) without affecting diphosphorylated MLC. Also, that the pMLC reduction is a consequence of cyclic adenosine 3',5'-monophosphate (cAMP)/protein kinase A (PKA) activation. In addition, we found evidence that the cAMP increase mostly depends on soluble adenylyl cyclase activity. Inhibition of this enzyme reduces the effect of PMPD on the cAMP rise, F-actin remodeling, and pMLC decrease. No changes in phosho-ERK were observed, although we could determine that RhoA undergoes activation. Our results suggested that active RhoA is not involved in the intercellular gap formation. Overall, the findings of this study support the view that, differently to renal epithelial cells, in BCE cells PMPD determines cytoskeletal reorganization via activation of the cAMP/PKA pathway.

Gamma Interferon Alters Junctional Integrity via Rho Kinase, Resulting in Blood-Brain Barrier Leakage in Experimental Viral Encephalitis

Blood-brain barrier (BBB) breakdown is a hallmark of many diseases of the central nervous system (CNS). Loss of BBB integrity in CNS diseases such as viral encephalitis results in the loss of nutrient/oxygen delivery, rapid infiltration of immune cells, and brain swelling that can exacerbate neuronal injury. Despite this, the cellular and molecular mechanisms that underlie BBB breakdown in viral encephalitis are incompletely understood. We undertook a comprehensive analysis of the cellular and molecular signaling events that induce BBB breakdown in an experimental model of virus-induced encephalitis in which neonatal mice are infected with reovirus (serotype 3 strain Abney). We show that BBB leakage during reovirus infection correlates with morphological changes in the vasculature, reductions in pericytes (BBB supporting cells), and disorganization of vascular junctions. Pathway analysis on RNA sequencing from brain endothelial cells identified the activation of interferon (IFN) signaling within the brain vasculature following reovirus infection. Our in vitro and in vivo studies show that type II IFN mediated by IFN-γ, a well known antiviral signal, is a major contributor to BBB leakage during reovirus infection. We show that IFN-γ reduces barrier properties in cultured brain endothelial cells through Rho kinase (ROCK)-mediated cytoskeletal contractions, resulting in junctional disorganization and cell-cell separations. In vivo neutralization of IFN-γ during reovirus infection significantly improved BBB integrity, pericyte coverage, attenuated vascular ROCK activity, and junctional disorganization. Our work supports a model in which IFN-γ acts directly on the brain endothelium to induce BBB breakdown through a mechanism involving ROCK-induced junctional disorganization.IMPORTANCE In an experimental viral encephalitis mouse model in which mice are infected with reovirus, we show that IFN-γ induces blood-brain barrier leakage. We show that IFN-γ promotes Rho kinase activity, resulting in actin cytoskeletal contractions in the brain endothelium that lead to vascular junctional disorganization and cell-cell separations. These studies now provide insight into a previously unknown mechanism for how blood-brain barrier breakdown occurs in viral encephalitis and implicates IFN-γ-Rho kinase activity as major contributor to this phenomenon. By identifying this mechanism of blood-brain barrier breakdown, we now provide potential therapeutic targets in treating patients with viral causes of encephalitis with the hope of limiting damage to the central nervous system.

ZIPK mediates endothelial cell contraction through myosin light chain phosphorylation and is required for ischemic-reperfusion injury

The paracellular gap formed by endothelial cell (EC) contraction is fundamental for endothelium permeability, but the mechanism underlying EC contraction has yet to be determined. Here, we identified the zipper-interacting protein kinase (ZIPK) as the kinase for EC contraction and myosin light chain (MLC) phosphorylation. Inhibition of ZIPK activity by pharmacological inhibitors and small interfering RNAs led to a significant decrease in the mono- and diphosphorylation of MLCs along with a contractile response to thrombin, suggesting an essential role of ZIPK in EC paracellular permeability. To assess the role of ZIPK in vivo, we established mouse lines with conditional deletion of Zipk gene. The endothelium-specific deletion of Zipk led to embryonic lethality, whereas the UBC-Cre^ERT2-mediated deletion of Zipk by tamoxifen induction at adulthood caused no apparent phenotype. The induced deletion of Zipk significantly inhibited ischemia-reperfusion-induced blood-brain barrier dysfunction and neuronal injuries from middle cerebral artery occlusion and reperfusion, as evidenced by reduced infarct and edema volume, attenuated Evans blue dye leakage, and improved neuronal behavior. We thus concluded that ZIPK and its phosphorylation of MLC were required for EC contraction and ischemic neuronal injuries. ZIPK may be a prospective therapeutic target for stroke.-Zhang, Y., Zhang, C., Zhang, H., Zeng, W., Li, S., Chen, C., Song, X., Sun, J., Sun, Z., Cui, C., Cao, X., Zheng, L., Wang, P., Zhao, W., Zhang, Z., Xu, Y., Zhu, M., Chen, H. ZIPK mediates endothelial cell contraction through myosin light chain phosphorylation and is required for ischemic-reperfusion injury.

MASP-1 Increases Endothelial Permeability

Pathologically increased vascular permeability is an important dysfunction in the pathomechanism of life-threatening conditions, such as sepsis, ischemia/reperfusion, or hereditary angioedema (HAE), diseases accompanied by uncontrolled activation of the complement system. HAE for example is caused by the deficiency of C1-inhibitor (the main regulator of early complement activation), which leads to edematous attacks threatening with circulatory collapse. We have previously reported that endothelial cells become activated during HAE attacks. A natural target of C1-inhibitor is mannan-binding lectin-associated serine protease-1 (MASP-1), a multifunctional serine protease, which plays a key role in the activation of complement lectin pathway. We have previously shown that MASP-1 induces the pro-inflammatory activation of endothelial cells and in this study we investigated whether MASP-1 can directly affect endothelial permeability. All experiments were performed on human umbilical vein endothelial cells (HUVECs). Real-time micro electric sensing revealed that MASP-1 decreases the impedance of HUVEC monolayers and in a recently developed permeability test (XperT), MASP-1 dose-dependently increased endothelial paracellular transport. We show that protease activated receptor-1 mediated intracellular Ca2+-mobilization, Rho-kinase activation dependent myosin light chain (MLC) phosphorylation, cytoskeletal actin rearrangement, and disruption of interendothelial junctions are underlying this phenomenon. Furthermore, in a whole-transcriptome microarray analysis MASP-1 significantly changed the expression of 25 permeability-related genes in HUVECs-for example it up-regulated bradykinin B2 receptor expression. According to our results, MASP-1 has potent permeability increasing effects. During infections or injuries MASP-1 may help eliminate the microbes and/or tissue debris by enhancing the extravasation of soluble and cellular components of the immune system, however, it may also play a role in the pathomechanism of diseases, where edema formation and complement lectin pathway activation are simultaneously present. Our findings also raise the possibility that MASP-1 may be a promising target of anti-edema drug development.

Endothelial Nanomechanics in the Context of Endothelial (Dys)function and Inflammation

SIGNIFICANCE

Stiffness of endothelial cells is closely linked to the function of the vasculature as it regulates the release of vasoactive substances such as nitric oxide (NO) and reactive oxygen species. The outer layer of endothelial cells, consisting of the glycocalyx above and the cortical zone beneath the plasma membrane, is a vulnerable compartment able to adapt its nanomechanical properties to any changes of forces exerted by the adjacent blood stream. Sustained stiffening of this layer contributes to the development of endothelial dysfunction and vascular pathologies. Recent Advances: The development of specific techniques to quantify the mechanical properties of cells enables the detailed investigation of the mechanistic link between structure and function of cells.

CRITICAL ISSUES

Challenging the mechanical stiffness of cells, for instance, by inflammatory mediators can lead to the development of endothelial dysfunction. Prevention of sustained stiffening of the outer layer of endothelial cells in turn improves endothelial function.

FUTURE DIRECTIONS

The mechanical properties of cells can be used as critical marker and test system for the proper function of the vascular system. Pharmacological substances, which are able to improve endothelial nanomechanics and function, could take a new importance in the prevention and treatment of vascular diseases. Thus, detailed knowledge acquisition about the structure/function relationship of endothelial cells and the underlying signaling pathways should be promoted.

The cerebral endothelial cell as a key regulator of inflammatory processes in sterile inflammation

Cerebral endothelial cells accomplish numerous tasks connected to the maintenance of homeostasis of the central nervous system. They create a barrier between the central nervous system and peripheral blood and regulate mechanotransduction, vascular permeability, rheology, thrombogenesis, and leukocyte adhesion. In pathophysiological conditions (e.g., stroke or ischemia-reperfusion injury) the endothelial functions are impaired, leading to increased vascular permeability, vascular inflammation, leukocyte-endothelium interactions, and transendothelial migration, driving CNS inflammation and neuronal destruction. This review describes the current knowledge on the regulatory roles of endothelial cells in neuroinflammatory processes.

MicroRNA dysregulation in lung injury: the role of the miR-26a/EphA2 axis in regulation of endothelial permeability

MicroRNAs (miRNAs) are noncoding RNAs that regulate gene expression in many diseases, although the contribution of miRNAs to the pathophysiology of lung injury remains obscure. We hypothesized that dysregulation of miRNA expression drives the changes in key genes implicated in the development of lung injury. To test our hypothesis, we utilized a model of lung injury induced early after administration of intratracheal bleomycin (0.1 U). Wild-type mice were treated with bleomycin or PBS, and lungs were collected at 4 or 7 days. A profile of lung miRNA was determined by miRNA array and confirmed by quantitative PCR and flow cytometry. Lung miR-26a was significantly decreased 7 days after bleomycin injury, and, on the basis of enrichment of predicted gene targets, it was identified as a putative regulator of cell adhesion, including the gene targets EphA2, KDR, and ROCK1, important in altered barrier function. Lung EphA2 mRNA, and protein increased in the bleomycin-injured lung. We further explored the miR-26a/EphA2 axis in vitro using human lung microvascular endothelial cells (HMVEC-L). Cells were transfected with miR-26a mimic and inhibitor, and expression of gene targets and permeability was measured. miR-26a regulated expression of EphA2 but not KDR or ROCK1. Additionally, miR-26a inhibition increased HMVEC-L permeability, and the disrupted barrier integrity due to miR-26a was blocked by EphA2 knockdown, shown by VE-cadherin staining. Our data suggest that miR-26a is an important epigenetic regulator of EphA2 expression in the pulmonary endothelium. As such, miR-26a may represent a novel therapeutic target in lung injury by mitigating EphA2-mediated changes in permeability.

Dabigatran reduces endothelial permeability through inhibition of thrombin-induced cytoskeleton reorganization

Dabigatran etexilate (DE), a new oral anti-coagulant, is a direct thrombin inhibitor. Clinical trials showed the favorable benefit-to-risk profile of DE compared to warfarin for the prevention of ischemic stroke in patients with atrial fibrillation. Remarkably, patients treated with dabigatran showed reduced rates of intracerebral hemorrhage compared to warfarin. As the breakdown of endothelial barrier integrity is associated with hemorrhagic events and as thrombin increases endothelial permeability, we hypothesized that dabigatran preserves the endothelial barrier by inhibiting thrombin-induced permeability. We assessed leakage of fluorescein isothiocyanate (FITC)-dextran through the endothelial monolayer and measured trans-endothelial electrical resistance of the endothelial monolayer after treatment of thrombin or thrombin pre-incubated with dabigatran. Thrombin increased the permeability of endothelial cells. Dabigatran effectively blocked the ability of thrombin to increase permeability. Dabigatran inhibited the formation of actin stress fibers induced by thrombin and inhibited consequent destabilization of junctional protein complexes and intercellular gap formation. The interaction of thrombin with protease activated receptor-1 activates the Rho A guanosine triphosphate (GTP)ase-myosin light chain (MLC) phosphorylation signaling axis, leading to actin cytoskeleton changes. This signaling pathway was effectively inhibited by dabigatran in endothelial cells. Consistently, the number of phosphorylated MLC-positive cells was significantly decreased in ischemic tissue of rat brains. These results indicate dabigatran blocks the ability of thrombin to induce vascular permeability and the resulting underlying signaling cascade in endothelial cells. Our findings provide evidence that dabigatran may confer a lower risk of intracerebral hemorrhage by preserving endothelial barrier integrity.

Aberrant endoplasmic reticulum stress mediates coronary artery spasm through regulating MLCK/MLC2 pathway

Coronary artery spasm (CAS) is a pathophysiological phenomenon that may cause myocardial infarction and lead to circulatory collapse and death. Aberrant endoplasmic reticulum (ER) stress causes accumulation of misfolding proteins and has been reported to be involved in a variety of vascular diseases. The present study investigated the role of ER stress in the development of CAS and explored the possible molecular mechanisms. Initially, it was found that ER stress markers were elevated in response to drug-induced vascular smooth muscle cells (VSMCs) contraction. Pharmacologic activation of ER stress using Tunicamycin (Tm) persistently induced CAS and significantly promoted Pituitrin-induced CAS in mice as well as in a collagen gel contraction assay. On the contrary, pharmacologic inhibition of ER stress using 4-phenylacetic acid (4-PBA) completely blunted Pituitrin-induced CAS development in mice. Moreover, during the drug-induced VSMCs contraction, expression of ER stress markers were increased in parallel to those of myosin light chain kinase (MLCK) and phosphor-MLC2 (p-MLC2, at Ser19). After inhibiting MLCK activity by using its specific inhibitor ML-7, the ER stress activator Tm failed to activate the MLCK/MLC2 pathway and could neither trigger CAS in mice nor induce VSMCs contraction in vitro. Our results suggested that aberrant ER stress mediated CAS via regulating the MLCK/MLC2 pathway. ER stress activators might be more robust than the common drugs (Pituitrin or acetylcholine) as to induce vasocontraction and thus may serve as potential therapeutics against chronic bleeding, while its inhibitor might be useful for treatment of severe CAS caused by other medication.

Localized Myosin II Activity Regulates Assembly and Plasticity of the Axon Initial Segment

The axon initial segment (AIS) is the site of action potential generation and a locus of activity-dependent homeostatic plasticity. A multimeric complex of sodium channels, linked via a cytoskeletal scaffold of ankyrin G and beta IV spectrin to submembranous actin rings, mediates these functions. The mechanisms that specify the AIS complex to the proximal axon and underlie its plasticity remain poorly understood. Here we show phosphorylated myosin light chain (pMLC), an activator of contractile myosin II, is highly enriched in the assembling and mature AIS, where it associates with actin rings. MLC phosphorylation and myosin II contractile activity are required for AIS assembly, and they regulate the distribution of AIS components along the axon. pMLC is rapidly lost during depolarization, destabilizing actin and thereby providing a mechanism for activity-dependent structural plasticity of the AIS. Together, these results identify pMLC/myosin II activity as a common link between AIS assembly and plasticity.

Mind the gap: mechanisms regulating the endothelial barrier

The endothelial barrier consists of intercellular contacts localized in the cleft between endothelial cells, which is covered by the glycocalyx in a sievelike manner. Both types of barrier-forming junctions, i.e. the adherens junction (AJ) serving mechanical anchorage and mechanotransduction and the tight junction (TJ) sealing the intercellular space to limit paracellular permeability, are tethered to the actin cytoskeleton. Under resting conditions, the endothelium thereby builds a selective layer controlling the exchange of fluid and solutes with the surrounding tissue. However, in the situation of an inflammatory response such as in anaphylaxis or sepsis intercellular contacts disintegrate in post-capillary venules leading to intercellular gap formation. The resulting oedema can cause shock and multi-organ failure. Therefore, maintenance as well as coordinated opening and closure of interendothelial junctions is tightly regulated. The two principle underlying mechanisms comprise spatiotemporal activity control of the small GTPases Rac1 and RhoA and the balance of the phosphorylation state of AJ proteins. In the resting state, junctional Rac1 and RhoA activity is enhanced by junctional components, actin-binding proteins, cAMP signalling and extracellular cues such as sphingosine-1-phosphate (S1P) and angiopoietin-1 (Ang-1). In addition, phosphorylation of AJ components is prevented by junction-associated phosphatases including vascular endothelial protein tyrosine phosphatase (VE-PTP). In contrast, inflammatory mediators inhibiting cAMP/Rac1 signalling cause strong activation of RhoA and induce AJ phosphorylation finally leading to endocytosis and cleavage of VE-cadherin. This results in dissolution of TJs the outcome of which is endothelial barrier breakdown.

Activation of ROCK and MLCK tunes regional stress fiber formation and mechanics via preferential myosin light chain phosphorylation

Graded induction of regulatory light chain (RLC) activators MLCK and ROCK were used to explore the relationship between RLC phosphorylation and actin-myosin stress fiber viscoelasticity. MLCK controls peripheral stress fiber mechanics by monophosphorylation of RLC, whereas ROCK acts on central stress fibers via diphosphorylation.

The assembly and mechanics of actomyosin stress fibers (SFs) depend on myosin regulatory light chain (RLC) phosphorylation, which is driven by myosin light chain kinase (MLCK) and Rho-associated kinase (ROCK). Although previous work suggests that MLCK and ROCK control distinct pools of cellular SFs, it remains unclear how these kinases differ in their regulation of RLC phosphorylation or how phosphorylation influences individual SF mechanics. Here, we combine genetic approaches with biophysical tools to explore relationships between kinase activity, RLC phosphorylation, SF localization, and SF mechanics. We show that graded MLCK overexpression increases RLC monophosphorylation (p-RLC) in a graded manner and that this p-RLC localizes to peripheral SFs. Conversely, graded ROCK overexpression preferentially increases RLC diphosphorylation (pp-RLC), with pp-RLC localizing to central SFs. Interrogation of single SFs with subcellular laser ablation reveals that MLCK and ROCK quantitatively regulate the viscoelastic properties of peripheral and central SFs, respectively. The effects of MLCK and ROCK on single-SF mechanics may be correspondingly phenocopied by overexpression of mono- and diphosphomimetic RLC mutants. Our results point to a model in which MLCK and ROCK regulate peripheral and central SF viscoelastic properties through mono- and diphosphorylation of RLC, offering new quantitative connections between kinase activity, RLC phosphorylation, and SF viscoelasticity.

Endothelial cell-cell adhesion and signaling

Endothelial cells line blood vessels and provide a dynamic interface between the blood and tissues. They remodel to allow leukocytes, fluid and small molecules to enter tissues during inflammation and infections. Here we compare the signaling networks that contribute to endothelial permeability and leukocyte transendothelial migration, focusing particularly on signals mediated by small GTPases that regulate cell adhesion and the actin cytoskeleton. Rho and Rap GTPase signaling is important for both processes, but they differ in that signals are activated locally under leukocytes, whereas endothelial permeability is a wider event that affects the whole cell. Some molecules play a unique role in one of the two processes, and could therefore be targeted to selectively alter either endothelial permeability or leukocyte transendothelial migration.

VE-Cadherin Disassembly and Cell Contractility in the Endothelium are Necessary for Barrier Disruption Induced by Tumor Cells

During metastasis, breakdown of the endothelial barrier is critical for tumor cell extravasation through blood vessel walls and is mediated by a combination of tumor secreted soluble factors and receptor-ligand interactions. However, a complete mechanism governing tumor cell transendothelial migration remains unclear. Here, we investigate the roles of tumor-associated signals in regulating endothelial cell contractility and adherens junction disassembly leading to endothelial barrier breakdown. We show that Src mediates VE-cadherin disassembly in response to metastatic melanoma cells. Through the use of pharmacological inhibitors of cytoskeletal contractility we find that endothelial cell contractility is responsive to interactions with metastatic cancer cells and that reducing endothelial cell contractility abrogates migration of melanoma cells across endothelial monolayers. Furthermore, we find that a combination of tumor secreted soluble factors and receptor-ligand interactions mediate activation of Src within endothelial cells that is necessary for phosphorylation of VE-cadherin and for breakdown of the endothelial barrier. Together, these results provide insight into how tumor cell signals act in concert to modulate cytoskeletal contractility and adherens junctions disassembly during extravasation and may aid in identification of therapeutic targets to block metastasis.

Interplay of myosin phosphatase and protein phosphatase-2A in the regulation of endothelial nitric-oxide synthase phosphorylation and nitric oxide production

The inhibitory phosphorylation of endothelial nitric oxide (NO) synthase (eNOS) at Thr497 (eNOS^pThr497) by protein kinase C or RhoA-activated kinase is a major regulatory determinant of eNOS activity. The signalling mechanisms involved in the dephosphorylation of eNOS^pThr497 have not yet been clarified. This study identifies myosin phosphatase (MP) holoenzyme consisting of protein phosphatase-1 catalytic subunit (PP1c) and MP target subunit-1 (MYPT1) as an eNOS^pThr497 phosphatase. In support of this finding are: (i) eNOS and MYPT1 interacts in various endothelial cells (ECs) and in in vitro binding assays (ii) MYPT1 targets and stimulates PP1c toward eNOS^pThr497 substrate (iii) phosphorylation of MYPT1 at Thr696 (MYPT1^pThr696) controls the activity of MP on eNOS^pThr497. Phosphatase inhibition suppresses both NO production and transendothelial resistance (TER) of ECs. In contrast, epigallocatechin-3-gallate (EGCG) signals ECs via the 67 kDa laminin-receptor (67LR) resulting in protein kinase A dependent activation of protein phosphatase-2A (PP2A). PP2A dephosphorylates MYPT1^pThr696 and thereby stimulates MP activity inducing dephosphorylation of eNOS^pThr497 and the 20 kDa myosin II light chains. Thus an interplay of MP and PP2A is involved in the physiological regulation of EC functions implying that an EGCG dependent activation of these phosphatases leads to enhanced NO production and EC barrier improvement.

AM966, an Antagonist of Lysophosphatidic Acid Receptor 1, Increases Lung Microvascular Endothelial Permeability through Activation of Rho Signaling Pathway and Phosphorylation of VE-Cadherin

Maintenance of pulmonary endothelial barrier integrity is important for reducing severity of lung injury. Lysophosphatidic acid (LPA) regulates cell motility, cytoskeletal rearrangement, and cell growth. Knockdown of LPA receptor 1 (LPA1) has been shown to mitigate lung injury and pulmonary fibrosis. AM966, an LPA1 antagonist exhibiting an antifibrotic property, has been considered to be a future antifibrotic medicine. Here, we report an unexpected effect of AM966, which increases lung endothelial barrier permeability. An electric cell-substrate sensing (ECIS) system was used to measure permeability in human lung microvascular endothelial cells (HLMVECs). AM966 decreased the transendothelial electrical resistance (TEER) value immediately in a dose-dependent manner. VE-cadherin and f-actin double immunostaining reveals that AM966 increases stress fibers and gap formation between endothelial cells. AM966 induced phosphorylation of myosin light chain (MLC) through activation of RhoA/Rho kinase pathway. Unlike LPA treatment, AM966 had no effect on phosphorylation of extracellular signal-regulated kinases (Erk). Further, in LPA1 silencing cells, we observed that AM966-increased lung endothelial permeability as well as phosphorylation of VE-cadherin and focal adhesion kinase (FAK) were attenuated. This study reveals that AM966 induces lung endothelial barrier dysfunction, which is regulated by LPA1-mediated activation of RhoA/MLC and phosphorylation of VE-cadherin.

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.