Regulation of endothelial cell myosin light chain kinase by Rho, cortactin, and p60(src)

Cortactin is in a complex with VE-cadherin and is required for endothelial adherens junction stability through Rap1/Rac1 activation

Vascular permeability is mediated by Cortactin (Cttn) and regulated by several molecules including cyclic-adenosine-monophosphate, small Rho family GTPases and the actin cytoskeleton. However, it is unclear whether Cttn directly interacts with any of the junctional components or if Cttn intervenes with signaling pathways affecting the intercellular contacts and the cytoskeleton. To address these questions, we employed immortalized microvascular myocardial endothelial cells derived from wild-type and Cttn-knock-out mice. We found that lack of Cttn compromised barrier integrity due to fragmented membrane distribution of different junctional proteins. Moreover, immunoprecipitations revealed that Cttn is within the VE-cadherin-based adherens junction complex. In addition, lack of Cttn slowed-down barrier recovery after Ca2+ repletion. The role of Cttn for cAMP-mediated endothelial barrier regulation was analyzed using Forskolin/Rolipram. In contrast to Cttn-KO, WT cells reacted with increased transendothelial electrical resistance. Absence of Cttn disturbed Rap1 and Rac1 activation in Cttn-depleted cells. Surprisingly, despite the absence of Cttn, direct activation of Rac1/Cdc42/RhoA by CN04 increased barrier resistance and induced well-defined cortical actin and intracellular actin bundles. In summary, our data show that Cttn is required for basal barrier integrity by allowing proper membrane distribution of junctional proteins and for cAMP-mediated activation of the Rap1/Rac1 signaling pathway.

Smooth Muscle Myosin Localizes at the Leading Edge and Regulates the Redistribution of Actin-regulatory Proteins during Migration

Airway smooth muscle cell migration plays an essential role in airway development, repair, and remodeling. Smooth muscle myosin II has been traditionally thought to localize in the cytoplasm solely and regulates cell migration by affecting stress fiber formation and focal adhesion assembly. In this study, we unexpectedly found that 20-kDa myosin light chain (MLC20) and myosin-11 (MYH11), important components of smooth muscle myosin, were present at the edge of lamellipodia. The knockdown of MLC20 or MYH11 attenuated the recruitment of c-Abl, cortactinProfilin-1 (Pfn-1), and Abi1 to the cell edge. Moreover, myosin light chain kinase (MLCK) colocalized with integrin β1 at the tip of protrusion. The inhibition of MLCK attenuated the recruitment of c-Abl, cortactin, Pfn-1, and Abi1 to the cell edge. Furthermore, MLCK localization at the leading edge was reduced by integrin β1 knockdown. Taken together, our results demonstrate that smooth muscle myosin localizes at the leading edge and orchestrates the recruitment of actin-regulatory proteins to the tip of lamellipodia. Mechanistically, integrin β1 recruits MLCK to the leading edge, which catalyzes MLC20 phosphorylation. Activated myosin regulates the recruitment of actin-regulatory proteins to the leading edge, and promotes lamellipodial formation and migration.

Hemin-Induced Endothelial Dysfunction and Endothelial to Mesenchymal Transition in the Pathogenesis of Pulmonary Hypertension Due to Chronic Hemolysis

Pulmonary hypertension in sickle cell disease is an independent predictor of mortality, yet the pathogenesis of pulmonary vascular disease in chronic hemolytic disorders remains incompletely understood and treatment options are limited primarily to supportive care. The release of extracellular hemoglobin has been implicated in the development of pulmonary hypertension, and in this study we explored the direct effects of hemin, the oxidized moiety of heme, on the pulmonary artery endothelium. We found that low dose hemin exposure leads to significantly increased endothelial cell proliferation, migration, and cytokine release as markers of endothelial dysfunction. Protein expression changes in our pulmonary artery endothelial cells showed upregulation of mesenchymal markers after hemin treatment in conjunction with a decrease in endothelial markers. Endothelial to mesenchymal transition (EndoMT) resulting from hemin exposure was further confirmed by showing upregulation of the transcription factors SNAI1 and SLUG, known to regulate EndoMT. Lastly, given the endothelial dysfunction and phenotypic transition observed, the endothelial cytoskeleton was considered a potential novel target. Inhibiting myosin light chain kinase, to prevent phosphorylation of myosin light chain and cytoskeletal contraction, attenuated hemin-induced endothelial hyper-proliferation, migration, and cytokine release. The findings in this study implicate hemin as a key inducer of endothelial dysfunction through EndoMT, which may play an important role in pulmonary vascular remodeling during the development of pulmonary hypertension in chronic hemolytic states.

MRSA-induced endothelial permeability and acute lung injury are attenuated by FTY720 S-phosphonate

Disruption of the lung endothelial barrier is a hallmark of acute respiratory distress syndrome (ARDS), for which no effective pharmacologic treatments exist. Prior work has demonstrated that FTY720 S-phosphonate (Tys), an analog of sphingosine-1-phosphate (S1P) and FTY720, exhibits potent endothelial cell (EC) barrier protective properties. In this study, we investigated the in vitro and in vivo efficacy of Tys against methicillin-resistant Staphylococcus aureus (MRSA), a frequent bacterial cause of ARDS. Tys-protected human lung EC from barrier disruption induced by heat-killed MRSA (HK-MRSA) or staphylococcal α-toxin and attenuated MRSA-induced cytoskeletal changes associated with barrier disruption, including actin stress fiber formation and loss of peripheral VE-cadherin and cortactin. Tys-inhibited Rho and myosin light chain (MLC) activation after MRSA and blocked MRSA-induced NF-κB activation and release of the proinflammatory cytokines, IL-6 and IL-8. In vivo, intratracheal administration of live MRSA in mice caused significant vascular leakage and leukocyte infiltration into the alveolar space. Pre- or posttreatment with Tys attenuated MRSA-induced lung permeability and levels of alveolar neutrophils. Posttreatment with Tys significantly reduced levels of bronchoalveolar lavage (BAL) VCAM-1 and plasma IL-6 and KC induced by MRSA. Dynamic intravital imaging of mouse lungs demonstrated Tys attenuation of HK-MRSA-induced interstitial edema and neutrophil infiltration into lung tissue. Tys did not directly inhibit MRSA growth or viability in vitro. In conclusion, Tys inhibits lung EC barrier disruption and proinflammatory signaling induced by MRSA in vitro and attenuates acute lung injury induced by MRSA in vivo. These results support the potential utility of Tys as a novel ARDS therapeutic strategy.

The cAMP effector PKA mediates Moody GPCR signaling in Drosophila blood-brain barrier formation and maturation

The blood-brain barrier (BBB) of Drosophila comprises a thin epithelial layer of subperineural glia (SPG), which ensheath the nerve cord and insulate it against the potassium-rich hemolymph by forming intercellular septate junctions (SJs). Previously, we identified a novel Gi/Go protein-coupled receptor (GPCR), Moody, as a key factor in BBB formation at the embryonic stage. However, the molecular and cellular mechanisms of Moody signaling in BBB formation and maturation remain unclear. Here, we identify cAMP-dependent protein kinase A (PKA) as a crucial antagonistic Moody effector that is required for the formation, as well as for the continued SPG growth and BBB maintenance in the larva and adult stage. We show that PKA is enriched at the basal side of the SPG cell and that this polarized activity of the Moody/PKA pathway finely tunes the enormous cell growth and BBB integrity. Moody/PKA signaling precisely regulates the actomyosin contractility, vesicle trafficking, and the proper SJ organization in a highly coordinated spatiotemporal manner. These effects are mediated in part by PKA's molecular targets MLCK and Rho1. Moreover, 3D reconstruction of SJ ultrastructure demonstrates that the continuity of individual SJ segments, and not their total length, is crucial for generating a proper paracellular seal. Based on these findings, we propose that polarized Moody/PKA signaling plays a central role in controlling the cell growth and maintaining BBB integrity during the continuous morphogenesis of the SPG secondary epithelium, which is critical to maintain tissue size and brain homeostasis during organogenesis.

My Journey in Academia as a Lipid Biochemist

An overview of Prof. Viswanathan Natarajan's journey in academia as a mentor, teacher, and lipid scientist for nearly 50 years is presented. As a graduate student, Dr. Natarajan interrogated biosynthesis and catabolism of phospholipids in the developing brain; however, in the last five decades, he has been investigating the role of sphingolipids and sphingolipid-metabolizing enzymes in pulmonary endothelial cells, epithelial cells, and fibroblasts under normal conditions and during various lung pathologies such as sepsis, asthma, pulmonary hypertension, idiopathic pulmonary fibrosis, bronchopulmonary dysplasia, and lung cancer. His recent work on sphingosine-1-phosphate and lysophosphatidic acid metabolism in pre-clinical animal models has identified small molecule inhibitors in the signaling pathways that could have therapeutic potential in ameliorating pulmonary fibrosis, hypoxia-induced pulmonary hypertension, lung cancer, and bronchopulmonary dysplasia. Future research in bioactive lipids in combination with OMICS should unravel the importance of various lipid mediators as modulators of cell function under normal and pathological conditions.

Construction for Long Non-Coding RNA (lncRNA)-Associated Competing Endogenous RNA (ceRNA) Network in Human Retinal Detachment (RD) with Proliferative Vitreoretinopathy (PVR)

Background

The aim of this study was to analyze the long non-coding RNA (lncRNA)-associated competing endogenous RNA (ceRNA) network in human retinal tissues following detachment with proliferative vitreoretinopathy (PVR).

Material/Methods

Expression data of 19 human detached retinas with PVR and 19 normal retinas from postmortem donors were downloaded from Gene Expression Omnibust (GEO) database (GSE28133). The R package “limma” was utilized to discriminate the dysregulated lncRNA and mRNA profiles. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of differentially expressed mRNAs were performed using R packages “Clusterprofiler.” The ceRNA network of dysregulated genes was constructed by using mircode, miRDB, miRTarBase and TargetScan databases, and was visualized by Cytoscape v3.6.1.

Results

A total of 23 lncRNAs and 994 mRNAs were identified significantly expressed between the human detached retinas with PVR and the normal retina tissues, with thresholds of |log₂FoldChange| >1.0 and adjusted P-value <0.05. The constructed ceRNA network (lncRNA-miRNA-mRNA regulatory axis) included 9 PVR-specific lncRNAs, as well as 27 miRNAs and 73 mRNAs.

Conclusions

We demonstrated the differential lncRNA expression profile and constructed a lncRNA-associated ceRNA network in human detached retinas with PVR. This may ferret out an unknown ceRNA regulatory network in human retinal detachment with PVR.

Tetraspanin-enriched microdomains regulate digitation junctions

Tetraspanins co-emerged with multi-cellular organisms during evolution and are typically localized at the cell–cell interface, [corrected] and form tetraspanin-enriched microdomains (TEMs) by associating with each other and other membrane molecules. Tetraspanins affect various biological functions, but how tetraspanins engage in multi-faceted functions at the cellular level is largely unknown. When cells interact, the membrane microextrusions at the cell-cell interfaces form dynamic, digit-like structures between cells, which we term digitation junctions (DJs). We found that (1) tetraspanins CD9, CD81, and CD82 and (2) TEM-associated molecules integrin α3β1, CD44, EWI2/PGRL, and PI-4P are present in DJs of epithelial, endothelial, and cancer cells. Tetraspanins and their associated molecules also regulate the formation and development of DJs. Moreover, (1) actin cytoskeleton, RhoA, and actomyosin activities and (2) growth factor receptor-Src-MAP kinase signaling, but not PI-3 kinase, regulate DJs. Finally, we showed that DJs consist of various forms in different cells. Thus, DJs are common, interactive structures between cells, and likely affect cell adhesion, migration, and communication. TEMs probably modulate various cell functions through DJs. Our findings highlight that DJ morphogenesis reflects the transition between cell-matrix adhesion and cell-cell adhesion and involves both cell-cell and cell-matrix adhesion molecules.

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

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

Impaired activity of adherens junctions contributes to endothelial dilator dysfunction in ageing rat arteries

KEY POINTS

Ageing-induced endothelial dysfunction contributes to organ dysfunction and progression of cardiovascular disease. VE-cadherin clustering at adherens junctions promotes protective endothelial functions, including endothelium-dependent dilatation. Ageing increased internalization and degradation of VE-cadherin, resulting in impaired activity of adherens junctions. Inhibition of VE-cadherin clustering at adherens junctions (function-blocking antibody; FBA) reduced endothelial dilatation in young arteries but did not affect the already impaired dilatation in old arteries. After junctional disruption with the FBA, dilatation was similar in young and old arteries. Src tyrosine kinase activity and tyrosine phosphorylation of VE-cadherin were increased in old arteries. Src inhibition increased VE-cadherin at adherens junctions and increased endothelial dilatation in old, but not young, arteries. Src inhibition did not increase dilatation in old arteries treated with the VE-cadherin FBA. Ageing impairs the activity of adherens junctions, which contributes to endothelial dilator dysfunction. Restoring the activity of adherens junctions could be of therapeutic benefit in vascular ageing.

ABSTRACT

Endothelial dilator dysfunction contributes to pathological vascular ageing. Experiments assessed whether altered activity of endothelial adherens junctions (AJs) might contribute to this dysfunction. Aortas and tail arteries were isolated from young (3-4 months) and old (22-24 months) F344 rats. VE-cadherin immunofluorescent staining at endothelial AJs and AJ width were reduced in old compared to young arteries. A 140 kDa VE-cadherin species was present on the cell surface and in TTX-insoluble fractions, consistent with junctional localization. Levels of the 140 kDa VE-cadherin were decreased, whereas levels of a TTX-soluble 115 kDa VE-cadherin species were increased in old compared to young arteries. Acetylcholine caused endothelium-dependent dilatation that was decreased in old compared to young arteries. Disruption of VE-cadherin clustering at AJs (function-blocking antibody, FBA) inhibited dilatation to acetylcholine in young, but not old, arteries. After the FBA, there was no longer any difference in dilatation between old and young arteries. Src activity and tyrosine phosphorylation of VE-cadherin were increased in old compared to young arteries. In old arteries, Src inhibition (saracatinib) increased: (i) 140 kDa VE-cadherin in the TTX-insoluble fraction, (ii) VE-cadherin intensity at AJs, (iii) AJ width, and (iv) acetylcholine dilatation. In old arteries treated with the FBA, saracatinib no longer increased acetylcholine dilatation. Saracatinib did not affect dilatation in young arteries. Therefore, ageing impairs AJ activity, which appears to reflect Src-induced phosphorylation, internalization and degradation of VE-cadherin. Moreover, impaired AJ activity can account for the endothelial dilator dysfunction in old arteries. Restoring endothelial AJ activity may be a novel therapeutic approach to vascular ageing.

Mechanical Stress and Single Nucleotide Variants Regulate Alternative Splicing of the MYLK Gene

The nonmuscle (nm) myosin light-chain kinase isoform (MLCK), encoded by the MYLK gene, is a vital participant in regulating vascular barrier responses to mechanical and inflammatory stimuli. We determined that MYLK is alternatively spliced, yielding functionally distinct nmMLCK splice variants including nmMLCK2, a splice variant highly expressed in vascular endothelial cells (EC) and associated with reduced EC barrier integrity. We demonstrated previously that the nmMLCK2 variant lacks exon 11, which encodes a key regulatory region containing two differentially phosphorylated tyrosine residues (Y⁴⁶⁴ and Y⁴⁷¹) that influence vascular barrier function during inflammation. In this study, we used minigene constructs and RT-PCR to interrogate biophysical factors (mechanical stress) and genetic variants (MYLK single-nucleotide polymorphisms [SNPs]) that are potentially involved in regulating MYLK alternative splicing and nmMLCK2 generation. Human lung EC exposed to pathologic mechanical stress (18% cyclic stretch) produced increased nmMLCK2 expression relative to levels of nmMLCK1 with alternative splicing significantly influenced by MYLK SNPs rs77323602 and rs147245669. In silico analyses predicted that these variants would alter exon 11 donor and acceptor sites for alternative splicing, computational predictions that were confirmed by minigene studies. The introduction of rs77323602 favored wild-type nmMLCK expression, whereas rs147245669 favored alternative splicing and deletion of exon 11, yielding increased nmMLCK2 expression. Finally, lymphoblastoid cell lines selectively harboring these MYLK SNPs (rs77323602 and rs147245669) directly validated SNP-specific effects on MYLK alternative splicing and nmMLCK2 generation. Together, these studies demonstrate that mechanical stress and MYLK SNPs regulate MYLK alternative splicing and generation of a splice variant, nmMLCK2, that contributes to the severity of inflammatory injury.

Regulation of pulmonary endothelial barrier function by kinases

The pulmonary endothelium is the target of continuous physiological and pathological stimuli that affect its crucial barrier function. The regulation, defense, and repair of endothelial barrier function require complex biochemical processes. This review examines the role of endothelial phosphorylating enzymes, kinases, a class with profound, interdigitating influences on endothelial permeability and lung function.

Myosin light chain kinase steady-state kinetics: comparison of smooth muscle myosin II and nonmuscle myosin IIB as substrates

Myosin light chain kinase (MLCK) phosphorylates S19 of the myosin regulatory light chain (RLC), which is required to activate myosin's ATPase activity and contraction. Smooth muscles are known to display plasticity in response to factors such as inflammation, developmental stage, or stress, which lead to differential expression of nonmuscle and smooth muscle isoforms. Here, we compare steady-state kinetics parameters for phosphorylation of different MLCK substrates: (1) nonmuscle RLC, (2) smooth muscle RLC, and heavy meromyosin subfragments of (3) nonmuscle myosin IIB, and (4) smooth muscle myosin II. We show that MLCK has a ~2-fold higher k_cat for both smooth muscle myosin II substrates compared with nonmuscle myosin IIB substrates, whereas K_m values were very similar. Myosin light chain kinase has a 1.6-fold and 1.5-fold higher specificity (k_cat /K_m ) for smooth versus nonmuscle-free RLC and heavy meromyosin, respectively, suggesting that differences in specificity are dictated by RLC sequences. Of the 10 non-identical RLC residues, we ruled out 7 as possible underlying causes of different MLCK kinetics. The remaining 3 residues were found to be surface exposed in the N-terminal half of the RLC, consistent with their importance in substrate recognition. These data are consistent with prior deletion/chimera studies and significantly add to understanding of MLCK myosin interactions.

SIGNIFICANCE OF THE STUDY

Phosphorylation of nonmuscle and smooth muscle myosin by myosin light chain kinase (MLCK) is required for activation of myosin's ATPase activity. In smooth muscles, nonmuscle myosin coexists with smooth muscle myosin, but the two myosins have very different chemo-mechanical properties relating to their ability to maintain force. Differences in specificity of MLCK for different myosin isoforms had not been previously investigated. We show that the MLCK prefers smooth muscle myosin by a significant factor. These data suggest that nonmuscle myosin is phosphorylated more slowly than smooth muscle myosin during a contraction cycle.

The ZEB1/miR-200c feedback loop regulates invasion via actin interacting proteins MYLK and TKS5

Epithelial to mesenchymal transition (EMT) is a developmental process which is aberrantly activated during cancer invasion and metastasis. Elevated expression of EMT-inducers like ZEB1 enables tumor cells to detach from the primary tumor and invade into the surrounding tissue. The main antagonist of ZEB1 in controlling EMT is the microRNA-200 family that is reciprocally linked to ZEB1 in a double negative feedback loop. Here, we further elucidate how the ZEB1/miR-200 feedback loop controls invasion of tumor cells. The process of EMT is attended by major changes in the actin cytoskeleton. Via in silico screening of genes encoding for actin interacting proteins, we identified two novel targets of miR-200c - TKS5 and MYLK (MLCK). Co-expression of both genes with ZEB1 was observed in several cancer cell lines as well as in breast cancer patients and correlated with low miR-200c levels. Depletion of TKS5 or MYLK in breast cancer cells reduced their invasive potential and their ability to form invadopodia. Whereas TKS5 is known to be a major component, we could identify MYLK as a novel player in invadopodia formation. In summary, TKS5 and MYLK represent two mediators of invasive behavior of cancer cells that are regulated by the ZEB1/miR-200 feedback loop.

The Microtubule Plus End Tracking Protein TIP150 Interacts with Cortactin to Steer Directional Cell Migration

Cell migration is orchestrated by dynamic interactions of microtubules with the plasma membrane cortex. How these interactions facilitate these dynamic processes is still being actively investigated. TIP150 is a newly characterized microtubule plus end tracking protein essential for mitosis and entosis (Ward, T., Wang, M., Liu, X., Wang, Z., Xia, P., Chu, Y., Wang, X., Liu, L., Jiang, K., Yu, H., Yan, M., Wang, J., Hill, D. L., Huang, Y., Zhu, T., and Yao, X. (2013) Regulation of a dynamic interaction between two microtubule-binding proteins, EB1 and TIP150, by the mitotic p300/CBP-associated factor (PCAF) orchestrates kinetochore microtubule plasticity and chromosome stability during mitosis. J. Biol. Chem. 288, 15771-15785; Xia, P., Zhou, J., Song, X., Wu, B., Liu, X., Li, D., Zhang, S., Wang, Z., Yu, H., Ward, T., Zhang, J., Li, Y., Wang, X., Chen, Y., Guo, Z., and Yao, X. (2014) Aurora A orchestrates entosis by regulating a dynamic MCAK-TIP150 interaction. J. Mol. Cell Biol. 6, 240-254). Here we show that TIP150 links dynamic microtubules to steer cell migration by interacting with cortactin. Mechanistically, TIP150 binds to cortactin via its C-terminal tail. Interestingly, the C-terminal TIP150 proline-rich region (CT150) binds to the Src homology 3 domain of cortactin specifically, and such an interaction is negatively regulated by EGF-elicited tyrosine phosphorylation of cortactin. Importantly, suppression of TIP150 or overexpression of phospho-mimicking cortactin inhibits polarized cell migration. In addition, CT150 disrupts the biochemical interaction between TIP150 and cortactin in vitro, and perturbation of the TIP150-cortactin interaction in vivo using a membrane-permeable TAT-CT150 peptide results in an inhibition of directional cell migration. We reason that a dynamic TIP150-cortactin interaction orchestrates directional cell migration via coupling dynamic microtubule plus ends to the cortical cytoskeleton.

Loss of cortactin causes endothelial barrier dysfunction via disturbed adrenomedullin secretion and actomyosin contractility

Changes in vascular permeability occur during inflammation and the actin cytoskeleton plays a crucial role in regulating endothelial cell contacts and permeability. We demonstrated recently that the actin-binding protein cortactin regulates vascular permeability via Rap1. However, it is unknown if the actin cytoskeleton contributes to increased vascular permeability without cortactin. As we consistently observed more actin fibres in cortactin-depleted endothelial cells, we hypothesised that cortactin depletion results in increased stress fibre contractility and endothelial barrier destabilisation. Analysing the contractile machinery, we found increased ROCK1 protein levels in cortactin-depleted endothelium. Concomitantly, myosin light chain phosphorylation was increased while cofilin, mDia and ERM were unaffected. Secretion of the barrier-stabilising hormone adrenomedullin, which activates Rap1 and counteracts actomyosin contractility, was reduced in plasma from cortactin-deficient mice and in supernatants of cortactin-depleted endothelium. Importantly, adrenomedullin administration and ROCK1 inhibition reduced actomyosin contractility and rescued the effect on permeability provoked by cortactin deficiency in vitro and in vivo. Our data suggest a new role for cortactin in controlling actomyosin contractility with consequences for endothelial barrier integrity.

Myosin light chain kinase facilitates endocytosis of synaptic vesicles at hippocampal boutons

At nerve terminals, endocytosis efficiently recycles vesicle membrane to maintain synaptic transmission under different levels of neuronal activity. Ca(2+) and its downstream signal pathways are critical for the activity-dependent regulation of endocytosis. An activity- and Ca(2+) -dependent kinase, myosin light chain kinase (MLCK) has been reported to regulate vesicle mobilization, vesicle cycling, and motility in different synapses, but whether it has a general contribution to regulation of endocytosis at nerve terminals remains unknown. We investigated this issue at rat hippocampal boutons by imaging vesicle endocytosis as the real-time retrieval of vesicular synaptophysin tagged with a pH-sensitive green fluorescence protein. We found that endocytosis induced by 200 action potentials (5-40 Hz) was slowed by acute inhibition of MLCK and down-regulation of MLCK with RNA interference, while the total amount of vesicle exocytosis and somatic Ca(2+) channel current did not change with MLCK down-regulation. Acute inhibition of myosin II similarly impaired endocytosis. Furthermore, down-regulation of MLCK prevented depolarization-induced phosphorylation of myosin light chain, an effect shared by blockers of Ca(2+) channels and calmodulin. These results suggest that MLCK facilitates vesicle endocytosis through activity-dependent phosphorylation of myosin downstream of Ca(2+) /calmodulin, probably as a widely existing mechanism among synapses. Our study suggests that MLCK is an important activity-dependent regulator of vesicle recycling in hippocampal neurons, which are critical for learning and memory. The kinetics of vesicle membrane endocytosis at nerve terminals has long been known to depend on activity and Ca(2+) . This study provides evidence suggesting that myosin light chain kinase increases endocytosis efficiency at hippocampal neurons by mediating Ca(2+) /calmodulin-dependent phosphorylation of myosin. The authors propose that this signal cascade may serve as a common pathway contributing to the activity-dependent regulation of vesicle endocytosis at synapses.

Loss of MLCK leads to disruption of cell-cell adhesion and invasive behavior of breast epithelial cells via increased expression of EGFR and ERK/JNK signaling

Myosin light chain kinase (MLCK) expression is downregulated in breast cancer, including invasive ductal carcinoma compared with ductal breast carcinoma in situ and metastatic breast tumors. However, little is known about how loss of MLCK expression contributes to tumor progression. MLCK is a component of the actin cytoskeleton and its known role is the phosphorylation of the regulatory light chain of myosin II. To gain insights into the role of MLCK in breast cancer, we perturbed its function using small interfering RNA (siRNA) or pharmacological inhibition in untransformed breast epithelial cells (MCF10A). Loss of MLCK by siRNAs led to increased cell migration and invasion, disruption of cell-cell adhesions and enhanced formation of focal adhesions at the leading edge of migratory cells. In addition, downregulation of MLCK cooperated with HER2 in MCF10A cells to promote cell migration and invasion and low levels of MLCK is associated with a poor prognosis in HER2-positive breast cancer patients. Associated with these altered migratory behaviors were increased expression of epidermal growth factor receptor and activation of extracellular signal-regulated kinase and c-Jun N-terminal kinase signaling pathways in MLCK downregulated MCF10A cells. By contrast, inhibition of the kinase function of MLCK using pharmacological agents inhibited cell migration and invasion, and did not affect cellular adhesions. Our results show that loss of MLCK contributes to the migratory properties of epithelial cells resulting from changes in cell-cell and cell-matrix adhesions, and increased epidermal growth factor receptor signaling. These findings suggest that decreased expression of MLCK may have a critical role during tumor progression by facilitating the metastatic potential of tumor cells.

Structure-Function Analysis of the Non-Muscle Myosin Light Chain Kinase (nmMLCK) Isoform by NMR Spectroscopy and Molecular Modeling: Influence of MYLK Variants

The MYLK gene encodes the multifunctional enzyme, myosin light chain kinase (MLCK), involved in isoform-specific non-muscle and smooth muscle contraction and regulation of vascular permeability during inflammation. Three MYLK SNPs (P21H, S147P, V261A) alter the N-terminal amino acid sequence of the non-muscle isoform of MLCK (nmMLCK) and are highly associated with susceptibility to acute lung injury (ALI) and asthma, especially in individuals of African descent. To understand the functional effects of SNP associations, we examined the N-terminal segments of nmMLCK by ¹H-¹⁵N heteronuclear single quantum correlation (HSQC) spectroscopy, a 2-D NMR technique, and by in silico molecular modeling. Both NMR analysis and molecular modeling indicated SNP localization to loops that connect the immunoglobulin-like domains of nmMLCK, consistent with minimal structural changes evoked by these SNPs. Molecular modeling analysis identified protein-protein interaction motifs adversely affected by these MYLK SNPs including binding by the scaffold protein 14-3-3, results confirmed by immunoprecipitation and western blot studies. These structure-function studies suggest novel mechanisms for nmMLCK regulation, which may confirm MYLK as a candidate gene in inflammatory lung disease and advance knowledge of the genetic underpinning of lung-related health disparities.

Myosin light chain kinase accelerates vesicle endocytosis at the calyx of Held synapse

Neuronal activity triggers endocytosis at synaptic terminals to retrieve efficiently the exocytosed vesicle membrane, ensuring the membrane homeostasis of active zones and the continuous supply of releasable vesicles. The kinetics of endocytosis depends on Ca(2+) and calmodulin which, as a versatile signal pathway, can activate a broad spectrum of downstream targets, including myosin light chain kinase (MLCK). MLCK is known to regulate vesicle trafficking and synaptic transmission, but whether this kinase regulates vesicle endocytosis at synapses remains elusive. We investigated this issue at the rat calyx of Held synapse, where previous studies using whole-cell membrane capacitance measurement have characterized two common forms of Ca(2+)/calmodulin-dependent endocytosis, i.e., slow clathrin-dependent endocytosis and rapid endocytosis. Acute inhibition of MLCK with pharmacological agents was found to slow down the kinetics of both slow and rapid forms of endocytosis at calyces. Similar impairment of endocytosis occurred when blocking myosin II, a motor protein that can be phosphorylated upon MLCK activation. The inhibition of endocytosis was not accompanied by a change in Ca(2+) channel current. Combined inhibition of MLCK and calmodulin did not induce synergistic inhibition of endocytosis. Together, our results suggest that activation of MLCK accelerates both slow and rapid forms of vesicle endocytosis at nerve terminals, likely by functioning downstream of Ca(2+)/calmodulin.

In the absence of effector proteins, the Pseudomonas aeruginosa type three secretion system needle tip complex contributes to lung injury and systemic inflammatory responses

Herein we describe a pathogenic role for the Pseudomonas aeruginosa type three secretion system (T3SS) needle tip complex protein, PcrV, in causing lung endothelial injury. We first established a model in which P. aeruginosa wild type strain PA103 caused pneumonia-induced sepsis and distal organ dysfunction. Interestingly, a PA103 derivative strain lacking its two known secreted effectors, ExoU and ExoT [denoted PA103 (ΔU/ΔT)], also caused sepsis and modest distal organ injury whereas an isogenic PA103 strain lacking the T3SS needle tip complex assembly protein [denoted PA103 (ΔPcrV)] did not. PA103 (ΔU/ΔT) infection caused neutrophil influx into the lung parenchyma, lung endothelial injury, and distal organ injury (reminiscent of sepsis). In contrast, PA103 (ΔPcrV) infection caused nominal neutrophil infiltration and lung endothelial injury, but no distal organ injury. We further examined pathogenic mechanisms of the T3SS needle tip complex using cultured rat pulmonary microvascular endothelial cells (PMVECs) and revealed a two-phase, temporal nature of infection. At 5-hours post-inoculation (early phase infection), PA103 (ΔU/ΔT) elicited PMVEC barrier disruption via perturbation of the actin cytoskeleton and did so in a cell death-independent manner. Conversely, PA103 (ΔPcrV) infection did not elicit early phase PMVEC barrier disruption. At 24-hours post-inoculation (late phase infection), PA103 (ΔU/ΔT) induced PMVEC damage and death that displayed an apoptotic component. Although PA103 (ΔPcrV) infection induced late phase PMVEC damage and death, it did so to an attenuated extent. The PA103 (ΔU/ΔT) and PA103 (ΔPcrV) mutants grew at similar rates and were able to adhere equally to PMVECs post-inoculation indicating that the observed differences in damage and barrier disruption are likely attributable to T3SS needle tip complex-mediated pathogenic differences post host cell attachment. Together, these infection data suggest that the T3SS needle tip complex and/or another undefined secreted effector(s) are important determinants of P. aeruginosa pneumonia-induced lung endothelial barrier disruption.

Barrier enhancing signals in pulmonary edema

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

TRPV4 calcium entry and surface expression attenuated by inhibition of myosin light chain kinase in rat pulmonary microvascular endothelial cells

In previous studies, blockade or gene deletion of either myosin light chain kinase (MLCK) or the mechanogated transient receptor potential vanilloid 4 (TRPV4) channel attenuated mechanical lung injury. To determine their effects on calcium entry, rat pulmonary microvascular endothelial cells (RPMVEC) were labeled with fluo-4 and calcium entry initiated with the TRPV4 agonist, 4α-phorbol 12, 13-didecanoate (4αPDD). Mean calcium transients peaked at ∼25 sec and persisted ∼500 sec. The 4αPDD response was essentially abolished in calcium-free media, or after pretreatment with the MLCK inhibitor, ML-7. ML-7 also attenuated the 4αPDD-induced inward calcium current measured directly using whole-cell patch clamp. Pretreatment with dynasore, an inhibitor of dynamin produced an initial calcium transient followed by a 4αPDD transient of unchanged peak intensity. Automated averaging of areas under the curve (AUC) of calcium transients in individual cells indicated total calcium activity with a relationship between treatment groups of ML-7 + 4αPDD < 4αPDD only < dynasore + 4αPDD. Measurement of biotinylated surface TRPV4 protein indicated a significant reduction after ML-7 pretreatment, but no significant change with dynasore treatment. RPMVEC monolayer electrical resistances were decreased by only 3% with 10 μmol/L 4αPDD and the response was dose-related. Dynasore alone produced a 29% decrease in resistance, but neither ML-7 nor dynasore affected the subsequent 4αPDD resistance response. These studies suggest that MLCK may inhibit mechanogated calcium responses through reduced surface expression of stretch activated TRPV4 channels in the plasma membrane.

A critical role for Lyn kinase in strengthening endothelial integrity and barrier function

The Src family kinases (SFKs) c-Src and Yes mediate vascular leakage in response to various stimuli including lipopolysaccharide (LPS) and vascular endothelial growth factor (VEGF). Here, we define an opposing function of another SFK, Lyn, which in contrast to other SFKs, strengthens endothelial junctions and thereby restrains the increase in vascular permeability. Mice lacking Lyn displayed increased mortality in LPS-induced endotoxemia and increased vascular permeability in response to LPS or VEGF challenge compared with wild-type littermates. Lyn knockout mice repopulated with wild-type bone marrow-derived cells have higher vascular permeability than wild-type mice, suggesting a role of endothelial Lyn in the maintenance of the vascular barrier. Small interfering RNA-mediated down-regulation of Lyn disrupted endothelial barrier integrity, whereas expression of a constitutively active mutant of Lyn enhanced the barrier. However, down-regulation of Lyn did not affect LPS-induced endothelial permeability. We demonstrate that Lyn association with focal adhesion kinase (FAK) and phosphorylation of FAK at tyrosine residues 576/577 and 925 were required for Lyn-dependent stabilization of endothelial adherens junctions. Thus, in contrast to c-Src and Yes, which increase vascular permeability in response to stimuli, Lyn stabilizes endothelial junctions through phosphorylation of FAK. Therefore, therapeutics activating Lyn kinase may strengthen the endothelial barrier junction and hence have anti-inflammatory potential.

Phospholipase D signaling mediates reactive oxygen species-induced lung endothelial barrier dysfunction

Reactive oxygen species (ROS) have emerged as critical players in the pathophysiology of pulmonary disorders and diseases. Earlier, we have demonstrated that ROS stimulate lung endothelial cell (EC) phospholipase D (PLD) that generates phosphatidic acid (PA), a second messenger involved in signal transduction. In the current study, we investigated the role of PLD signaling in the ROS-induced lung vascular EC barrier dysfunction. Our results demonstrated that hydrogen peroxide (H₂O₂), a typical physiological ROS, induced PLD activation and altered the barrier function in bovine pulmonary artery ECs (BPAECs). 1-Butanol, the quencher of PLD, generated PA leading to the formation of physiologically inactive phosphatidyl butanol but not its biologically inactive analog, 2-butanol, blocked the H₂O₂-mediated barrier dysfunction. Furthermore, cell permeable C2 ceramide, an inhibitor of PLD but not the C2 dihydroceramide, attenuated the H₂O₂-induced PLD activation and enhancement of paracellular permeability of Evans blue conjugated albumin across the BPAEC monolayers. In addition, transfection of BPAECs with adenoviral constructs of hPLD₁ and mPLD₂ mutants attenuated the H₂O₂-induced barrier dysfunction, cytoskeletal reorganization and distribution of focal adhesion proteins. For the first time, this study demonstrated that the PLD-generated intracellular bioactive lipid signal mediator, PA, played a critical role in the ROS-induced barrier dysfunction in lung vascular ECs. This study also underscores the importance of PLD signaling in vascular leak and associated tissue injury in the etiology of lung diseases among critically ill patients encountering oxygen toxicity and excess ROS production during ventilator-assisted breathing.

Phytoestrogen genistein protects against endothelial barrier dysfunction in vascular endothelial cells through PKA-mediated suppression of RhoA signaling

The soy-derived phytoestrogen genistein has received attention for its potential to improve vascular function, but its mechanism remains unclear. Here, we report that genistein at physiologically relevant concentrations (0.1-10 μM) significantly inhibited thrombin-induced increase in endothelial monolayer permeability. Genistein also reduced the formation of stress fibers by thrombin and suppressed thrombin-induced phosphorylation of myosin light chain (MLC) on Ser(19)/Thr(18) in endothelial cells (ECs). Genistein had no effect on resting intracellular [Ca(2+)] or thrombin-induced increase in Ca(2+) mobilization. Addition of the inhibitors of endothelial nitric oxide synthase or estrogen receptor did not alter the protective effect of genistein. RhoA is a small GTPase that plays an important role in actin-myosin contraction and endothelial barrier dysfunction. RhoA inhibitor blocked the protective effect of genistein on endothelial permeability and also ablated thrombin-induced MLC-phosphorylation in ECs. Inhibition of PKA significantly attenuated the effect of genistein on thrombin-induced EC permeability, MLC phosphorylation, and RhoA membrane translocation in ECs. Furthermore, thrombin diminished cAMP production in ECs, which were prevented by treatment with genistein. These findings demonstrated that genistein improves thrombin-induced endothelial barrier dysfunction in ECs through PKA-mediated suppression of RhoA signaling.

Extracellular signal-regulated kinase (ERK) regulates cortactin ubiquitination and degradation in lung epithelial cells

Cortactin, an actin-binding protein, is essential for cell growth and motility. We have shown that cortactin is regulated by reversible phosphorylation, but little is known regarding cortactin protein stability. Here, we show that lipopolysaccharide (LPS)-induced cortactin degradation is mediated by extracellular regulated signal kinase (ERK). LPS induces cortactin serine phosphorylation, ubiquitination, and degradation in mouse lung epithelia, an effect abrogated by ERK inhibition. Serine phosphorylation sites mutant, cortactin(S405A/S418A), enhances its protein stability. Cortactin is polyubiquitinated and degraded within the proteasome, whereas a cortactin(K79R) mutant exhibited proteolytic stability during cyclohexamide (CHX) or LPS treatment. The E3 ligase subunit β-Trcp interacts with cortactin, and its overexpression reduced cortactin protein levels, an effect attenuated by ERK inhibition. Overexpression of β-Trcp was sufficient to reduce the protective effects of exogenous cortactin on epithelial cell barrier integrity, an effect not observed after expression of a cortactin(K79R) mutant. These results provide evidence that LPS modulation of cortactin stability is coordinately regulated by stress kinases and the ubiquitin-proteasomal network.

Absence of integrin αvβ3 enhances vascular leak in mice by inhibiting endothelial cortical actin formation

RATIONALE

Sepsis and acute lung injury (ALI) have devastatingly high mortality rates. Both are associated with increased vascular leak, a process regulated by complex molecular mechanisms.

OBJECTIVES

We hypothesized that integrin αvβ3 could be an important determinant of vascular leak and endothelial permeability in sepsis and ALI.

METHODS

β3 subunit knockout mice were tested for lung vascular leak after endotracheal LPS, and systemic vascular leak and mortality after intraperitoneal LPS and cecal ligation and puncture. Possible contributory effects of β3 deficiency in platelets and other hematopoietic cells were excluded by bone marrow reconstitution experiments. Endothelial cells treated with αvβ3 antibodies were evaluated for sphingosine-1 phosphate (S1P)–mediated alterations in barrier function, cytoskeletal arrangement, and integrin localization.

MEASUREMENTS AND MAIN RESULTS

β3 knockout mice had increased vascular leak and pulmonary edema formation after endotracheal LPS, and increased vascular leak and mortality after intraperitoneal LPS and cecal ligation and puncture. In endothelial cells, αvβ3 antibodies inhibited barrier-enhancing and cortical actin responses to S1P. Furthermore, S1P induced translocation of αvβ3 from discrete focal adhesions to cortically distributed sites through Gi- and Rac1-mediated pathways. Cortical αvβ3 localization after S1P was decreased by αvβ3 antibodies, suggesting that ligation of the αvβ3 with its extracellular matrix ligands is required to stabilize cortical αvβ3 focal adhesions.

CONCLUSIONS

Our studies identify a novel mechanism by which αvβ3 mitigates increased vascular leak, a pathophysiologic function central to sepsis and ALI. These studies suggest that drugs designed to block αvβ3 may have the unexpected side effect of intensifying sepsis- and ALI-associated vascular endothelial leak.

Novel role for non-muscle myosin light chain kinase (MLCK) in hyperoxia-induced recruitment of cytoskeletal proteins, NADPH oxidase activation, and reactive oxygen species generation in lung endothelium

We recently demonstrated that hyperoxia (HO) activates lung endothelial cell NADPH oxidase and generates reactive oxygen species (ROS)/superoxide via Src-dependent tyrosine phosphorylation of p47(phox) and cortactin. Here, we demonstrate that the non-muscle ~214-kDa myosin light chain (MLC) kinase (nmMLCK) modulates the interaction between cortactin and p47(phox) that plays a role in the assembly and activation of endothelial NADPH oxidase. Overexpression of FLAG-tagged wild type MLCK in human pulmonary artery endothelial cells enhanced interaction and co-localization between cortactin and p47(phox) at the cell periphery and ROS production, whereas abrogation of MLCK using specific siRNA significantly inhibited the above. Furthermore, HO stimulated phosphorylation of MLC and recruitment of phosphorylated and non-phosphorylated cortactin, MLC, Src, and p47(phox) to caveolin-enriched microdomains (CEM), whereas silencing nmMLCK with siRNA blocked recruitment of these components to CEM and ROS generation. Exposure of nmMLCK(-/-) null mice to HO (72 h) reduced ROS production, lung inflammation, and pulmonary leak compared with control mice. These results suggest a novel role for nmMLCK in hyperoxia-induced recruitment of cytoskeletal proteins and NADPH oxidase components to CEM, ROS production, and lung injury.

Role of myosin light chain kinase in cardiotrophin-1-induced cardiac myofibroblast cell migration

Chemotactic movement of myofibroblasts is recognized as a common means for their sequestration to the site of tissue injury. Following myocardial infarction (MI), recruitment of cardiac myofibroblasts to the infarct scar is a critical step in wound healing. Contractile myofibroblasts express embryonic smooth muscle myosin, α-smooth muscle actin, as well as collagens I and III. We examined the effects of cardiotrophin-1 (CT-1) in the induction of primary rat ventricular myofibroblast motility. Changes in membrane potential (Em) and Ca2+entry were studied to reveal the mechanisms for induction of myofibroblast migration. CT-1-induced cardiac myofibroblast cell migration, which was attenuated through the inhibition of JAK2 (25 μM AG490), and myosin light chain kinase (20 μM ML-7). Inhibition of K+channels (1 mM tetraethylammonium or 100 μM 4-aminopyridine) and nonselective cation channels by 10 μM gadolinium (Gd3+) significantly reduced migration in the presence of CT-1. CT-1 treatment caused a significant increase in myosin light chain phosphorylation, which could be inhibited by incubation in Ca2+-free conditions or by application of AG490, ML-7, and W7 (100 μM; calmodulin inhibitor). Monitoring myofibroblast membrane potential with potentiometric fluorescent DiBAC4( 3 ) dye revealed a biphasic response to CT-1 consisting of an initial depolarization followed by hyperpolarization. Increased intracellular Ca2+, as assessed by fluo 3, occurred immediately after membrane depolarization and attenuated at the time of maximal hyperpolarization. CT-1 exerts chemotactic effects via multiple parallel signaling modalities in ventricular myofibroblasts, including changes in membrane potential, alterations in intracellular calcium, and activation of a number of intracellular signaling pathways. Further study is warranted to determine the precise role of K+currents in this process.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Nonmuscle myosin light-chain kinase deficiency attenuates atherosclerosis in apolipoprotein E-deficient mice via reduced endothelial barrier dysfunction and monocyte migration

BACKGROUND

Endothelial dysfunction and monocyte migration are key events in the pathogenesis of atherosclerosis. Nonmuscle myosin light-chain kinase (nmMLCK), the predominant MLCK isoform in endothelial cells, has been shown to contribute to vascular inflammation by altering endothelial barrier function. However, its impact on atherogenesis remains unknown.

METHODS AND RESULTS

We investigated the role of nmMLCK in the development of atherosclerotic lesions in apolipoprotein E-deficient (apoE(-/-)) mice fed an atherogenic diet for 12 weeks. Histopathological examination demonstrated that nmMLCK deficiency (apoE(-/-)nmmlck(-/-)) reduced the size of aortic lesions by 53%, lipid contents by 44%, and macrophage deposition by 40%. Western blotting and reverse-transcription polymerase chain reaction revealed the expression of nmMLCK in aortic endothelial cells and peripheral blood monocytes. Measurements of transendothelial electric resistance indicated that nmMLCK deficiency attenuated endothelial barrier dysfunction caused by thrombin, oxidized low-density lipoprotein, and tumor necrosis factor α. In monocytes, nmMLCK deficiency reduced their migration in response to the chemokine monocyte chemoattractant protein-1. Further mechanistic studies showed that nmMLCK acted through both myosin light chain phosphorylation-coupled and -uncoupled pathways; the latter involved Rous sacracoma virus homolog genes-encoded tyrosine kinases (Src) signaling. Moreover, depletion of Src via gene silencing, site-specific mutagenesis, or pharmacological inhibition of Src greatly attenuated nmMLCK-dependent endothelial barrier dysfunction and monocyte migration.

CONCLUSIONS

Nonmuscle myosin light-chain kinase contributes to atherosclerosis by regulating endothelial barrier function and monocyte migration via mechanisms involving not only kinase-mediated MLC phosphorylation but also Src activation.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Myosin light chain kinase and Src control membrane dynamics in volume recovery from cell swelling

MLCK resolves membrane blebbing induced by osmotic cell swelling. Cell swelling also stimulates the formation of a Src–MLCK complex, which with cortactin and dynamin forms actin-based structures at the base of the cell to facilitate membrane retrieval for volume recovery.

 The expansion of the plasma membrane, which occurs during osmotic swelling of epithelia, must be retrieved for volume recovery, but the mechanisms are unknown. Here we have identified myosin light chain kinase (MLCK) as a regulator of membrane internalization in response to osmotic swelling in a model liver cell line. On hypotonic exposure, we found that there was time-dependent phosphorylation of the MLCK substrate myosin II regulatory light chain. At the sides of the cell, MLCK and myosin II localized to swelling-induced membrane blebs with actin just before retraction, and MLCK inhibition led to persistent blebbing and attenuated cell volume recovery. At the base of the cell, MLCK also localized to dynamic actin-coated rings and patches upon swelling, which were associated with uptake of the membrane marker FM4-64X, consistent with sites of membrane internalization. Hypotonic exposure evoked increased biochemical association of the cell volume regulator Src with MLCK and with the endocytosis regulators cortactin and dynamin, which colocalized within these structures. Inhibition of either Src or MLCK led to altered patch and ring lifetimes, consistent with the concept that Src and MLCK form a swelling-induced protein complex that regulates volume recovery through membrane turnover and compensatory endocytosis under osmotic stress.

Abl tyrosine kinase phosphorylates nonmuscle Myosin light chain kinase to regulate endothelial barrier function

This study identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy and examined their influence on nmMLCK function and human lung endothelial barrier regulation. The data indicate an essential role for Abl kinase in vascular barrier regulation via phosphorylation of nmMLCK and the actin-binding protein cortactin.

Nonmuscle myosin light chain kinase (nmMLCK), a multi-functional cytoskeletal protein critical to vascular homeostasis, is highly regulated by tyrosine phosphorylation. We identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy analysis (including Y²³¹, Y⁴⁶⁴, Y⁵⁵⁶, Y⁸⁴⁶) and examined their influence on nmMLCK function and human lung endothelial cell (EC) barrier regulation. Tyrosine phosphorylation of nmMLCK increased kinase activity, reversed nmMLCK-mediated inhibition of Arp2/3-mediated actin polymerization, and enhanced binding to the critical actin-binding phosphotyrosine protein, cortactin. EC challenge with sphingosine 1-phosphate (S1P), a potent barrier-enhancing agonist, resulted in c-Abl and phosphorylated nmMLCK recruitment into caveolin-enriched microdomains, rapid increases in Abl kinase activity, and spatial targeting of c-Abl to barrier-promoting cortical actin structures. Conversely, reduced c-Abl expression in EC (siRNA) markedly attenuated S1P-mediated cortical actin formation, reduced the EC modulus of elasticity (assessed by atomic force microscopy), reduced nmMLCK and cortactin tyrosine phosphorylation, and attenuated S1P-mediated barrier enhancement. These studies indicate an essential role for Abl kinase in vascular barrier regulation via posttranslational modification of nmMLCK and strongly support c-Abl-cortactin-nmMLCK interaction as a novel determinant of cortical actin-based cytoskeletal rearrangement critical to S1P-mediated EC barrier enhancement.

Rho kinase: an important mediator of atherosclerosis and vascular disease

Atherosclerosis is a complex inflammatory process characterized by the cross-talk between excessive inflammation and lipid accumulation. In the past few years, compelling evidence suggests that statins can decrease vascular inflammation and attenuate the development of atherosclerosis through their so-called "pleiotropic effects". These cholesterol-independent effects are predominantly due to their ability to inhibit isoprenoid synthesis. In particular, inhibition of geranylgeranylpyrophosphate synthesis leads to inhibition of Rho and its downstream target, Rho-kinase (ROCK). Thus, one of the beneficial effects of statin therapy could be due to inhibitory effects on ROCK. ROCK is involved in mediating diverse cellular functions such as smooth muscle contraction, cell migration and proliferation. While increased ROCK activity is associated with endothelial dysfunction, cerebral ischemia, coronary vasospasms and metabolic syndrome, the inhibition of ROCK by statins or selective ROCK inhibitors leads to up-regulation of endothelial nitric oxide synthase (eNOS), decreased vascular inflammation, and reduced atherosclerotic plaque formation. This review will focus on the impact of ROCK in cardiovascular disease and its contributory role to vascular inflammation and the atherosclerosis.

Inflammation-associated repression of vasodilator-stimulated phosphoprotein (VASP) reduces alveolar-capillary barrier function during acute lung injury

Acute lung injury (ALI) is an inflammatory disorder associated with reduced alveolar-capillary barrier function, increased pulmonary vascular permeability, and infiltration of leukocytes into the alveolar space. Pulmonary function might be compromised, its most severe form being the acute respiratory distress syndrome. A protein central to physiological barrier properties is vasodilator-stimulated phosphoprotein (VASP). Given the fact that VASP expression is reduced during periods of cellular hypoxia, we investigated the role of VASP during ALI. Initial studies revealed reduced VASP expressional levels through cytokines in vitro. Studies in the putative human VASP promoter identified NF-kappaB as a key regulator of VASP transcription. This VASP repression results in increased paracellular permeability and migration of neutrophils in vitro. In a model of LPS-induced ALI, VASP(-/-) mice demonstrated increased pulmonary damage compared with wild-type animals. These findings were confirmed in a second model of ventilator-induced lung injury. Studies employing bone marrow chimeric animals identified tissue-specific repression of VASP as the underlying cause of decreased barrier properties of the alveolar-capillary barrier during ALI. Taken together these studies identify tissue-specific VASP as a central protein in the control of the alveolar-capillary barrier properties during ALI.

Lung inflammation and endothelial cell damage are decreased after treatment with phototherapy (PhT) in a model of acute lung injury induced by Escherichia coli lipopolysaccharide in the rat

Lipopolysaccharide (LPS) mimics the symptoms of acute lung injury (ALI), which is characterized by the accumulation in the lungs of neutrophils producing inflammatory mediators. Because of the lack of information about phototherapy (PhT) effects on ALI, we investigated whether PhT (685nm InGaAlP) attenuates LPS-induced ALI. PhT reduced lung edema, the accumulation of TNF-alpha in the lung, and myeloperoxidase (MPO) activity. However, PhT was not efficient in reducing of TNF-alpha concentration in both serum and neutrophils of blood after LPS. In another series of experiments, in vitro assays of the effects of PhT effect on mouse pulmonary arterial endothelium cells (MPAECs) after TNF-alpha showed that the laser restores the MPAECs damage induced at 6 or 24h after TNF-alpha. These results suggest the PhT effect on ALI is partly due to inhibition of TNF-alpha release from neutrophils and lung cells.

The actin cytoskeleton in endothelial cell phenotypes

Endothelium forms a semi-permeable barrier that separates blood from the underlying tissue. Barrier function is largely determined by cell-cell and cell-matrix adhesions that define the limits of cell borders. Yet, such cell-cell and cell-matrix tethering is critically reliant upon the nature of adherence within the cell itself. Indeed, the actin cytoskeleton fulfills this essential function, to provide a strong, dynamic intracellular scaffold that organizes integral membrane proteins with the cell's interior, and responds to environmental cues to orchestrate appropriate cell shape. The actin cytoskeleton is comprised of three distinct, but inter-related structures, including actin cross-linking of spectrin within the membrane skeleton, the cortical actin rim, and actomyosin-based stress fibers. This review addresses each of these actin-based structures, and discusses cellular signals that control the disposition of actin in different endothelial cell phenotypes.

Regulation of vascular permeability by sphingosine 1-phosphate

A significant and sustained increase in vascular permeability is a hallmark of acute inflammatory diseases such as acute lung injury (ALI) and sepsis and is an essential component of tumor metastasis, angiogenesis, and atherosclerosis. Sphingosine 1-phosphate (S1P), an endogenous bioactive lipid produced in many cell types, regulates endothelial barrier function by activation of its G-protein coupled receptor S1P(1). S1P enhances vascular barrier function through a series of profound events initiated by S1P(1) ligation with subsequent downstream activation of the Rho family of small GTPases, cytoskeletal reorganization, adherens junction and tight junction assembly, and focal adhesion formation. Furthermore, recent studies have identified transactivation of S1P(1) signaling by other barrier-enhancing agents as a common mechanism for promoting endothelial barrier function. This review summarizes the state of our current knowledge about the mechanisms through which the S1P/S1P(1) axis reduces vascular permeability, which remains an area of active investigation that will hopefully produce novel therapeutic agents in the near future.

Src family kinases as mediators of endothelial permeability: effects on inflammation and metastasis

Src family kinases (SFKs) are signaling enzymes that have long been recognized to regulate critical cellular processes such as proliferation, survival, migration, and metastasis. Recently, considerable work has elucidated mechanisms by which SFKs regulate normal and pathologic processes in vascular biology, including endothelial cell proliferation and permeability. Further, when inappropriately activated, SFKs promote pathologic inflammatory processes and tumor metastasis, in part through their effects on the regulation of endothelial monolayer permeability. In this review, we discuss the roles of aberrantly activated SFKs in mediating endothelial permeability in the context of inflammatory states and tumor cell metastasis. We further summarize recent efforts to translate Src-specific inhibitors into therapy for systemic inflammatory conditions and numerous solid organ cancers.

Src-mediated cortactin phosphorylation regulates actin localization and injurious blebbing in acinar cells

Suprastimulation of pancreatic acini is a well-known model for pancreatitis, and it is characterized by actin reorganization and cell blebbing. Currently, however, the mechanisms underlying regulation of these aberrant cytoskeletal and membrane dynamics and how they contribute to cell injury are unclear. We observed that suprastimulation results in a rapid activation of Src and relocalization of the actin-binding protein cortactin from the apical to the basolateral domain at the necks of membrane blebs. Furthermore, Src-mediated cortactin tyrosine phosphorylation was markedly increased after suprastimulation. Pretreatment of acini with Src inhibitors or expression of a cortactin tyrosine phospho-inhibitory mutant reduced actin redistribution and bleb formation induced by suprastimulation in vitro. Importantly, inhibition of Src activity in rat models of suprastimulation-induced pancreatitis substantially reduced disease severity, as indicated by a reduction in serum amylase and pancreatic edema and a striking improvement in tissue histology. These findings indicate a novel, disease-relevant role for Src-mediated cortactin phosphorylation in aberrant reorganization of the actin cytoskeleton, a mechanism that is likely to have implications in other types of cell injury. In addition, they suggest a potential use for Src inhibitors as an approach to reduce cell injury.

Heat shock protein 90 inhibitors attenuate LPS-induced endothelial hyperpermeability

Endothelial hyperperme ability leading to vascular leak is an important consequence of sepsis and sepsis-induced lung injury. We previously reported that heat shock protein (hsp) 90 inhibitor pretreatment improved pulmonary barrier dysfunction in a murine model of sepsis-induced lung injury. We now examine the effects of hsp90 inhibitors on LPS-mediated endothelial hyperpermeability, as reflected in changes in transendothelial electrical resistance (TER) of bovine pulmonary arterial endothelial cells (BPAEC). Vehicle-pretreated cells exposed to endotoxin exhibited a concentration-dependent decrease in TER, activation of pp60(Src), phosphorylation of the focal adhesion protein paxillin, and reduced expression of the adherens junction proteins, vascular endothelial (VE)-cadherin and beta-catenin. Pretreatment with the hsp90 inhibitor, radicicol, prevented the decrease in TER, maintained VE-cadherin and beta-catenin expression, and inhibited activation of pp60(Src) and phosphorylation of paxillin. Similarly, when BPAEC hyperpermeability was induced by endotoxin-activated neutrophils, pretreatment of neutrophils and/or endothelial cells with radicicol protected against the activated neutrophil-induced decrease in TER. Increased paxillin phosphorylation and decreased expression of beta-catenin and VE-cadherin were also observed in mouse lungs 12 h after intraperitoneal endotoxin and attenuated in mice pretreated with radicicol. These results suggest that hsp90 plays an important role in sepsis-associated endothelial barrier dysfunction.

Regulation of endothelial permeability by Src kinase signaling: vascular leakage versus transcellular transport of drugs and macromolecules

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways have been identified controlling endothelial barrier function. The normally restrictive paracellular pathway, which can become "leaky" during inflammation when gaps are induced between endothelial cells at the level of adherens and tight junctional complexes, and the transcellular pathway, which transports plasma proteins the size of albumin via transcytosis in vesicle carriers originating from cell surface caveolae. During non-inflammatory conditions, caveolae-mediated transport may be the primary mechanism of vascular permeability regulation of fluid phase molecules as well as lipids, hormones, and peptides that bind avidly to albumin. Src family protein tyrosine kinases have been implicated in the upstream signaling pathways that lead to endothelial hyperpermeability through both the paracellular and transcellular pathways. Endothelial barrier dysfunction not only affects vascular homeostasis and cell metabolism, but also governs drug delivery to underlying cells and tissues. In this review of the field, we discuss the current understanding of Src signaling in regulating paracellular and transcellular endothelial permeability pathways and effects on endogenous macromolecule and drug delivery.

Regulation of vascular endothelial cell barrier function and cytoskeleton structure by protein phosphatases of the PPP family

Reversible phosphorylation of cytoskeletal and cytoskeleton-associated proteins is a significant element of endothelial barrier function regulation. Therefore, understanding the mechanisms of phosphorylation/dephosphorylation of endothelial cell cytoskeletal proteins is vital to the treatment of severe lung disorders such as high permeability pulmonary edema. In vivo, there is a controlled balance between the activities of protein kinases and phosphatases. Due to various external or internal signals, this balance may be shifted. The actual balances at a given time alter the phosphorylation level of certain proteins with appropriate physiological consequences. The latest information about the structure and regulation of different types of Ser/Thr protein phosphatases participating in the regulation of endothelial cytoskeletal organization and barrier function will be reviewed here.

Src phosphorylation of cortactin enhances actin assembly

Src kinase mediates growth factor signaling and causes oncogenic transformation, which includes dramatic changes in the actin cytoskeleton, cell shape, and motility. Cortactin was discovered as a substrate for Src. How phosphorylation of cortactin can enhance actin assembly is unknown. Here, using an actin assembly system reconstituted from purified components, we demonstrate for the first time a biochemical mechanism by which Src phosphorylation of cortactin affects actin assembly. The adaptor Nck is an important component of the system, linking phosphorylated cortactin with neuronal WASp (N-WASp) and WASp-interacting protein (WIP) to activate Arp2/3 complex.

Integrin alphavbeta5 regulates lung vascular permeability and pulmonary endothelial barrier function

Increased lung vascular permeability is an important contributor to respiratory failure in acute lung injury (ALI). We found that a function-blocking antibody against the integrin alphavbeta5 prevented development of lung vascular permeability in two different models of ALI: ischemia-reperfusion in rats (mediated by vascular endothelial growth factor [VEGF]) and ventilation-induced lung injury (VILI) in mice (mediated, at least in part, by transforming growth factor-beta [TGF-beta]). Knockout mice homozygous for a null mutation of the integrin beta5 subunit were also protected from lung vascular permeability in VILI. In pulmonary endothelial cells, both the genetic absence and blocking of alphavbeta5 prevented increases in monolayer permeability induced by VEGF, TGF-beta, and thrombin. Furthermore, actin stress fiber formation induced by each of these agonists was attenuated by blocking alphavbeta5, suggesting that alphavbeta5 regulates induced pulmonary endothelial permeability by facilitating interactions with the actin cytoskeleton. These results identify integrin alphavbeta5 as a central regulator of increased pulmonary vascular permeability and a potentially attractive therapeutic target in ALI.

Bluetongue virus and double-stranded RNA increase human vascular permeability: role of p38 MAPK

Endothelial cell (EC) involvement in viral hemorrhagic fevers has been clearly established. However, virally activated mechanisms leading to endothelial activation and dysfunction are not well understood. Several different potential mechanisms such as direct viral infection, alterations in procoagulant/anticoagulant balance, and increased cytokine production have been suggested. We utilized a model of EC barrier dysfunction and vascular endothelial leakage to explore the effect of bluetongue virus (BTV), a hemorrhagic fever virus of ruminants, on human lung endothelial cell barrier properties. Infection of human lung EC with BTV induced a significant and dose-dependent decrease in trans-endothelial electrical resistance (TER). Furthermore, decreases in TER occurred in conjunction with cytoskeletal rearrangement, suggesting a direct mechanism for viral infection-mediated endothelial barrier disruption. Interestingly, double-stranded RNA (dsRNA) mimicked the effects of BTV on endothelial barrier properties. Both BTV- and dsRNA-induced endothelial barrier dysfunction was blocked by treatment with a pharmacological inhibitor of p38 MAPK. The induction of vascular permeability by dsRNA treatment or BTV infection was concomitent with induction of inflammatory cytokines. Taken together, our data suggest that the presence of dsRNA during viral infections and subsequent activation of p38 MAPK is a potential molecular pathway for viral induction of hemorrhagic fevers. Collectively, our data suggest that inhibition of p38 MAPK may be a possible therapeutic approach to alter viral-induced acute hemorrhagic diseases.