Adenosine induces dephosphorylation of myosin II regulatory light chain in cultured bovine corneal endothelial cells

Endothelial tight junctions and their regulatory signaling pathways in vascular homeostasis and disease

Endothelial tight junctions (TJs) regulate the transport of water, ions, and molecules through the paracellular pathway, serving as an important barrier in blood vessels and maintaining vascular homeostasis. In endothelial cells (ECs), TJs are highly dynamic structures that respond to multiple external stimuli and pathological conditions. Alterations in the expression, distribution, and structure of endothelial TJs may lead to many related vascular diseases and pathologies. In this review, we provide an overview of the assessment methods used to evaluate endothelial TJ barrier function both in vitro and in vivo and describe the composition of endothelial TJs in diverse vascular systems and ECs. More importantly, the direct phosphorylation and dephosphorylation of TJ proteins by intracellular kinases and phosphatases, as well as the signaling pathways involved in the regulation of TJs, including and the protein kinase C (PKC), PKA, PKG, Ras homolog gene family member A (RhoA), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/Akt, and Wnt/β-catenin pathways, are discussed. With great advances in this area, targeting endothelial TJs may provide novel treatment for TJ-related vascular pathologies.

Targeting Adenosine in Cancer Immunotherapy to Enhance T-Cell Function

T cells play a critical role in cancer control, but a range of potent immunosuppressive mechanisms can be upregulated in the tumor microenvironment (TME) to abrogate their activity. While various immunotherapies (IMTs) aiming at re-invigorating the T-cell-mediated anti-tumor response, such as immune checkpoint blockade (ICB), and the adoptive cell transfer (ACT) of natural or gene-engineered ex vivo expanded tumor-specific T cells, have led to unprecedented clinical responses, only a small proportion of cancer patients benefit from these treatments. Important research efforts are thus underway to identify biomarkers of response, as well as to develop personalized combinatorial approaches that can target other inhibitory mechanisms at play in the TME. In recent years, adenosinergic signaling has emerged as a powerful immuno-metabolic checkpoint in tumors. Like several other barriers in the TME, such as the PD-1/PDL-1 axis, CTLA-4, and indoleamine 2,3-dioxygenase (IDO-1), adenosine plays important physiologic roles, but has been co-opted by tumors to promote their growth and impair immunity. Several agents counteracting the adenosine axis have been developed, and pre-clinical studies have demonstrated important anti-tumor activity, alone and in combination with other IMTs including ICB and ACT. Here we review the regulation of adenosine levels and mechanisms by which it promotes tumor growth and broadly suppresses protective immunity, with extra focus on the attenuation of T cell function. Finally, we present an overview of promising pre-clinical and clinical approaches being explored for blocking the adenosine axis for enhanced control of solid tumors.

Function-Related Protein Expression in Fuchs Endothelial Corneal Dystrophy Cells and Tissue Models

Fuchs endothelial corneal dystrophy (FECD) is a corneal pathology that affects the endothelial cell's ability to maintain deturgescence, resulting in a progressive loss of corneal transparency. In this study, we investigated the expression of function-related proteins in corneal endothelial cells using FECD or healthy corneal endothelial cells, either in a cell culture two-dimensional model or in an engineered corneal endothelium three-dimensional tissue model. No statistically significant difference in gene regulation was observed for the function-related families ATP1, SLC4, SLC16, AQP, TJP, and CDH between the FECD and the healthy cell models. Similarly, no difference in barrier integrity (transendothelial electrical resistance measurements and permeability assays) was observed in vitro between FECD and healthy cultured cells. Protein expression of the key function-related families was decreased for Na⁺/K⁺-ATPase α1 subunit, monocarboxylate transporters 1 and 4 in native ex vivo end-stage FECD specimens, whereas it returned to levels comparable to that of healthy tissues in the engineered FECD model. These results indicate that cell expansion and tissue engineering culture conditions can generate a corneal endothelium from pathologic FECD cells, with levels of function-related proteins similar to that of healthy tissues. Overall, these results explain why it is possible to reform a functional endothelium using corneal endothelial cells isolated from nonfunctional FECD pathologic specimens.

A novel all-trans retinoid acid derivative N-(3-trifluoromethyl- phenyl)- retinamide inhibits lung adenocarcinoma A549 cell migration through down-regulating expression of myosin light chain kinase

AIM

To observe the effects of a novel all-trans retinoid acid (ATRA) derivative, N-(3-trifluoromethyl-phenyl)- retinamide (ATPR), on lung adenocarcinoma A549 cells and to explore the potential mechanism of ATPR inhibiting of A549 cell migration.

MATERIALS AND METHODS

The cytotoxicity of ATRA and ATPR on A549 cells was assessed using MTT assay. Wound healing assays were used to analyze the influences of ATRA, ATPR, ML-7 (a highly selective inhibitor of myosin light chain kinase (MLCK)), PMA (an activator of MAPKs) and PD98059 (a selective inhibitor of ERK1/2) on the migration of A549 cells. Expression of MLCK and phosphorylation of myosin light chain (MLC) were assessed by Western blotting.

RESULTS

ATRA and ATPR inhibited the proliferation of A549 cells in a dose- and time-dependent manner, and the effect of ATPR was much more remarkable compared with ATRA. Relative migration rate and migration distance of A549 cells both decreased significantly after treatment with ATPR or ML-7. The effect on cell migration of PD98059 combining ATPR treatment was more notable than that of ATPR alone. Moreover, compared with control groups, the expression levels of MLCK and phosphorylated MLC in A549 cells were both clearly reduced in ATRA and ATPR groups.

CONCLUSIONS

ATPR could suppress the migration and invasion of A549 cells, and the mechanism might be concerned with down- regulating the expression of MLCK in the ERK-MAPK signaling pathway, pointing to therapeutic prospects in lung cancer.

A gut microbial metabolite of linoleic acid, 10-hydroxy-cis-12-octadecenoic acid, ameliorates intestinal epithelial barrier impairment partially via GPR40-MEK-ERK pathway

Gut microbial metabolites of polyunsaturated fatty acids have attracted much attention because of their various physiological properties. Dysfunction of tight junction (TJ) in the intestine contributes to the pathogenesis of many disorders such as inflammatory bowel disease. We evaluated the effects of five novel gut microbial metabolites on tumor necrosis factor (TNF)-α-induced barrier impairment in Caco-2 cells and dextran sulfate sodium-induced colitis in mice. 10-Hydroxy-cis-12-octadecenoic acid (HYA), a gut microbial metabolite of linoleic acid, suppressed TNF-α and dextran sulfate sodium-induced changes in the expression of TJ-related molecules, occludin, zonula occludens-1, and myosin light chain kinase. HYA also suppressed the expression of TNF receptor 2 (TNFR2) mRNA and protein expression in Caco-2 cells and colonic tissue. In addition, HYA suppressed the protein expression of TNFR2 in murine intestinal epithelial cells. Furthermore, HYA significantly up-regulated G protein-coupled receptor (GPR) 40 expression in Caco-2 cells. It also induced [Ca(2+)]i responses in HEK293 cells expressing human GPR40 with higher sensitivity than linoleic acid, its metabolic precursor. The barrier-recovering effects of HYA were abrogated by a GPR40 antagonist and MEK inhibitor in Caco-2 cells. Conversely, 10-hydroxyoctadacanoic acid, which is a gut microbial metabolite of oleic acid and lacks a carbon-carbon double bond at Δ12 position, did not show these TJ-restoring activities and down-regulated GPR40 expression. Therefore, HYA modulates TNFR2 expression, at least partially, via the GPR40-MEK-ERK pathway and may be useful in the treatment of TJ-related disorders such as inflammatory bowel disease.

Cx43-hemichannel function and regulation in physiology and pathophysiology: insights from the bovine corneal endothelial cell system and beyond

Intercellular communication in primary bovine corneal endothelial cells (BCECs) is mainly driven by the release of extracellular ATP through Cx43 hemichannels. Studying the characteristics of Ca²⁺-wave propagation in BCECs, an important form of intercellular communication, in response to physiological signaling events has led to the discovery of important insights in the functional properties and regulation of native Cx43 hemichannels. Together with ectopic expression models for Cx43 hemichannels and truncated/mutated Cx43 versions, it became very clear that loop/tail interactions play a key role in controlling the activity of Cx43 hemichannels. Interestingly, the negative regulation of Cx43 hemichannels by enhanced actin/myosin contractility seems to impinge upon loss of these loop/tail interactions essential for opening Cx43 hemichannels. Finally, these molecular insights have spurred the development of novel peptide tools that can selectively inhibit Cx43 hemichannels, but neither Cx43 gap junctions nor hemichannels formed by other Cx isoforms. These tools now set the stage to hunt for novel physiological functions for Cx43 hemichannels in primary cells and tissues and to tackle disease conditions associated with excessive, pathological Cx43-hemichannel openings.

Rap1 GTPase activation and barrier enhancement in rpe inhibits choroidal neovascularization in vivo

Loss of barrier integrity precedes the development of pathologies such as metastasis, inflammatory disorders, and blood-retinal barrier breakdown present in neovascular age-related macular degeneration. Rap1 GTPase is involved in regulating both endothelial and epithelial cell junctions; the specific role of Rap1A vs. Rap1B isoforms is less clear. Compromise of retinal pigment epithelium barrier function is a contributing factor to the development of AMD. We utilized shRNA of Rap1 isoforms in cultured human retinal pigment epithelial cells, along with knockout mouse models to test the role of Rap1 on promoting RPE barrier properties, with emphasis on the dynamic junctional regulation that is triggered when the adhesion between cells is challenged. In vitro, Rap1A shRNA reduced steady-state barrier integrity, whereas Rap1B shRNA affected dynamic junctional responses. In a laser-induced choroidal neovascularization (CNV) model of macular degeneration, Rap1b^−/− mice exhibited larger CNV volumes compared to wild-type or Rap1a^−/−. In vivo, intravitreal injection of a cAMP analog (8CPT-2′-O-Me-cAMP) that is a known Rap1 activator significantly reduced laser-induced CNV volume, which correlated with the inhibition of CEC transmigration across 8CPT-2′O-Me-cAMP-treated RPE monolayers in vitro. Rap1 activation by 8CPT-2′-O-Me-cAMP treatment increased recruitment of junctional proteins and F-actin to cell-cell contacts, increasing both the linearity of junctions in vitro and in cells surrounding laser-induced lesions in vivo. We conclude that in vitro, Rap1A may be important for steady state barrier integrity, while Rap1B is involved more in dynamic junctional responses such as resistance to junctional disassembly induced by EGTA and reassembly of cell junctions following disruption. Furthermore, activation of Rap1 in vivo inhibited development of choroidal neovascular lesions in a laser-injury model. Our data suggest that targeting Rap1 isoforms in vivo with 8CPT-2′-O-Me-cAMP may be a viable pharmacological means to strengthen the RPE barrier against the pathological choroidal endothelial cell invasion that occurs in macular degeneration.

Adenosine A1 receptors promote vasa vasorum endothelial cell barrier integrity via Gi and Akt-dependent actin cytoskeleton remodeling

BACKGROUND

In a neonatal model of hypoxic pulmonary hypertension, a dramatic pulmonary artery adventitial thickening, accumulation of inflammatory cells in the adventitial compartment, and angiogenic expansion of the vasa vasorum microcirculatory network are observed. These pathophysiological responses suggest that rapidly proliferating vasa vasorum endothelial cells (VVEC) may exhibit increased permeability for circulating blood cells and macromolecules. However, the molecular mechanisms underlying these observations remain unexplored. Some reports implicated extracellular adenosine in the regulation of vascular permeability under hypoxic and inflammatory conditions. Thus, we aimed to determine the role of adenosine in barrier regulation of VVEC isolated from the pulmonary arteries of normoxic (VVEC-Co) or chronically hypoxic (VVEC-Hyp) neonatal calves.

PRINCIPAL FINDINGS

We demonstrate via a transendothelial electrical resistance measurement that exogenous adenosine significantly enhanced the barrier function in VVEC-Co and, to a lesser extent, in VVEC-Hyp. Our data from a quantitative reverse transcription polymerase chain reaction show that both VVEC-Co and VVEC-Hyp express all four adenosine receptors (A1, A2A, A2B, and A3), with the highest expression level of A1 receptors (A1Rs). However, A1R expression was significantly lower in VVEC-Hyp compared to VVEC-Co. By using an A1R-specific agonist/antagonist and siRNA, we demonstrate that A1Rs are mostly responsible for adenosine-induced enhancement in barrier function. Adenosine-induced barrier integrity enhancement was attenuated by pretreatment of VVEC with pertussis toxin and GSK690693 or LY294002, suggesting the involvement of Gi proteins and the PI3K-Akt pathway. Moreover, we reveal a critical role of actin cytoskeleton in VVEC barrier regulation by using specific inhibitors of actin and microtubule polymerization. Further, we show that adenosine pretreatment blocked the tumor necrosis factor alpha (TNF-α)-induced permeability in VVEC-Co, validating its anti-inflammatory effects.

CONCLUSIONS

We demonstrate for the first time that stimulation of A1Rs enhances the barrier function in VVEC by activation of the Gi/PI3K/Akt pathway and remodeling of actin microfilament.

Lack of ecto-5'-nucleotidase (CD73) promotes arteriogenesis

AIMS

Adenosine can stimulate angiogenesis, but its role in the distinct process of arteriogenesis is unknown. We have previously reported that mice lacking ecto-5'-nucleotidase (CD73-/-) show enhanced monocyte adhesion to the endothelium after ischaemia, which is considered to be an important trigger for arteriogenesis.

METHODS AND RESULTS

Hindlimb ischaemia was induced in wild-type (WT) and CD73-/- mice to study the role of extracellularly formed adenosine in arteriogenesis. Magnetic resonance angiography (MRA) was performed for serial visualization of newly developed vessels at a spatial resolution of 1 nL, and high-energy phosphates (HEP) were quantified by (31)P MR spectroscopy (MRS). MRA of CD73-/- mice revealed substantially enhanced collateral artery conductance at day 7 [CD73-/-: 0.73 ± 0.11 a.u. (arbitrary units); WT: 0.44 ± 0.13 a.u.; P < 0.01, n = 6], and MRS of the affected hindlimb showed a faster restoration of HEP in correlation with enhanced functional recovery in the mutant. Additionally, histology showed no differences in capillary density between the groups but showed an increased monocyte infiltration in hindlimbs of CD73-/- mice.

CONCLUSION

Serial assessment of dynamic changes of vessel growth and metabolism in the process of arteriogenesis demonstrate that the lack of CD73-derived adenosine importantly promotes arteriogenesis but does not alter angiogenesis in our model of hindlimb ischaemia.

Cell signaling in regulation of the barrier integrity of the corneal endothelium

The barrier integrity of the corneal endothelium, which is conferred by its tight and adherens junctions, is critical for the maintenance of deturgescence of the corneal stroma. Although characteristically leaky, the barrier integrity restricts fluid leakage into the stroma such that the rate of leak does not exceed the rate of the endothelial active fluid transport directed toward the aqueous humor. At a molecular level, the barrier integrity is influenced by the actin cytoskeleton and microtubules, which are coupled to tight and adherens junctions via a variety of linker proteins. Since the cytoskeleton is affected by Rho family small GTPases and p38 MAP kinase, among others, many pathophysiological stimuli induce plasticity to the cytoskeleton and thereby elicit dynamic regulation of the barrier integrity. This review presents an overview of the impact of several bioactive factors on the barrier integrity of the corneal endothelium through altered actin cytoskeleton and/or disassembly of microtubules. The main focus is on the effect of TNF-α (tumor necrosis factor-α) which is a pro-inflammatory molecule found in the intraocular milieu during allograft rejection and anterior uveitis. This cytokine elicits acute activation of p38 MAP kinase, induces disassembly of microtubules, disrupts the peri-junctional actomyosin ring, and concomitantly breaks down the barrier integrity. These effects of TNF-α could be inhibited by stabilizing the microtubules, co-treating with a selective p38 MAP kinase inhibitor, and elevating intracellular cAMP via A2B receptors or direct exposure to forskolin. Overall, the corneal edema following a potential breakdown of the endothelial barrier integrity can be rescued pharmacologically by inhibiting specific cell-signaling mechanisms.

Role of adenosine A(2B) receptors in inflammation

Recent progress in our understanding of the unique role of A(2B) receptors in the regulation of inflammation, immunity, and tissue repair was considerably facilitated with the introduction of new pharmacological and genetic tools. However, it also led to seemingly conflicting conclusions on the role of A(2B) adenosine receptors in inflammation with some publications indicating proinflammatory effects and others suggesting the opposite. This chapter reviews the functions of A(2B) receptors in various cell types related to inflammation and integrated effects of A(2B) receptor modulation in several animal models of inflammation. It is argued that translation of current findings into novel therapies would require a better understanding of A(2B) receptor functions in diverse types of inflammatory responses in various tissues and at different points of their progression.

Role of ATP synthase alpha subunit in low-dose endothelial monocyte-activating polypeptide-II-induced opening of the blood-tumor barrier

This study was performed to determine whether the α subunit of ATP synthase (α-ATP synthase) on brain microvascular endothelial cells (BMECs) serves as the functional target for endothelial monocyte-activating polypeptide-II (EMAP-II)-induced increase in blood-tumor barrier (BTB) permeability. Using a rat C6 glioma model, we found that low-dose (80 ng/kg) EMAP-II significantly decreased the mRNA and protein expression levels of tight junction (TJ)-related proteins claudin-5, occludin, and ZO-1 on BMECs. Meantime, radioimmunity and Western blot assay showed a significant decrease in the expression levels of cAMP and catalytic subunit of protein kinase A (PKAcs) of tumor tissues. Also, pretreatment with specific α-ATP synthase antibody significantly blocked the effects of EMAP-II on TJ-related proteins, cAMP, and PKAcs. In addition, double immunofluorescence assay identified that EMAP-II was co-localized with α-ATP synthase on BMECs. This in vivo study demonstrated that α subunit of ATP synthase on BMECs serves as the functional target for EMAP-II selective opening of the BTB, and that cAMP/PKA signaling transduction pathway might be involved in the modulating process.

The modulation of protein kinase A and heat shock protein 70 is involved in the reversible increase of blood-brain tumor barrier permeability induced by papaverine

Intra-arterial administration of papaverine has been revealed to cause an increase in the blood-brain tumor barrier (BTB) permeability. The exact mechanism of papaverine opening the BTB in chemotherapy of malignant cerebral tumors, however, has not been well described. We used a rat brain glioma (C6) model for studying how papaverine modulates the permeability of BTB by monitoring the activities of the tight junction (TJ)-associated protein occludin, claudin-5 and cytoskeletal protein filamentous actin (F-actin) and whether protein kinase A (PKA) and heat shock protein 70 (HSP70) were involved in the regulation of this biological process. The levels of occludin, claudin-5 and F-actin protein in the tumor tissues were down-regulated by papaverine via immunohistochemistry, immunofluorescence assays and Western blot, corresponding to the time-dependent change of the BTB permeability. The most obvious attenuation of occludin, claudin-5 and F-actin protein was observed at 1h after papaverine perfusion, companied by a significant decrease in expression levels of PKA protein. The expression level of HSP70 in the tumor tissues was also progressively increased after papaverine perfusion and reached the maximum at 3h. The results demonstrate that the reversible openning of BTB mediated by papaverine may be associated with the functional combination between PKA and HSP70. That is, BTB opening may be attributable to the down-regulation of occludin, claudin-5 and F-actin, and cAMP/PKA signaling pathway might be involved in this process. HSP70 is likely responsible for the BTB closing, which helping the repairment of injured TJ protein and the rebuilding of the BTB.

Lovastatin inhibits the thrombin-induced loss of barrier integrity in bovine corneal endothelium

PURPOSE

Increased actomyosin contraction of the dense band of actin cytoskeleton at the apical junctional complex (perijunctional actomyosin ring, PAMR) breaks down the barrier integrity of corneal endothelium. This study has investigated the efficacy of statins, which inhibit activation of RhoA, in opposing the thrombin-induced loss of barrier integrity of monolayers of cultured bovine corneal endothelium.

METHODS

Myosin light chain (MLC) phosphorylation, a biochemical measure of actomyosin contraction, was assayed by urea-glycerol gel electrophoresis, followed by western blot analysis. The locus of MLC phosphorylation and changes in the organization of the PAMR were visualized by immunostaining. Phosphorylation of MYPT1, a regulatory subunit of myosin light-chain phosphatase (MLCP), was assessed by Western blot analysis to determine down-regulation of RhoA. The barrier integrity was assessed in terms of trans-endothelial electrical resistance (TER), and further confirmed by determining permeability to FITC dextran (10 kDa) and distribution of ZO-1, a marker of tight junctional assembly.

RESULTS

Lovastatin, a prototype of lipophilic statins, induced MLC dephosphorylation under basal conditions. It opposed increase in phosphorylation of MLC and MYPT1 in response to thrombin and nocodazole, agents known to activate RhoA in the endothelium. Pretreatment with the statin opposed the thrombin- and nocodazole-induced disruption of the PAMR and the thrombin-induced decline in TER. Lovastatin also opposed the thrombin- and nocodazole-induced increase in permeability to FITC dextran and redistribution of ZO-1. However, upon supplementation with GGPP (geranylgeranyl pyrophosphate), lovastatin failed to oppose the effects of thrombin and nocodazole on the PAMR, ppMLC, and ZO-1 distribution.

CONCLUSIONS

Lovastatin attenuates RhoA activation in the corneal endothelium presumably by reducing its isoprenylation. This underlies the suppression of the thrombin-induced loss in barrier integrity of the corneal endothelium.

Dynamic regulation of barrier integrity of the corneal endothelium

The corneal endothelium maintains stromal deturgescence, which is a prerequisite for corneal transparency. The principal challenge to stromal deturgescence is the swelling pressure associated with the hydrophilic glycosaminoglycans in the stroma. This negative pressure induces fluid leak into the stroma from the anterior chamber, but the rate of leak is restrained by the tight junctions of the endothelium. This role of the endothelium represents its barrier function. In healthy cornea, the fluid leak is counterbalanced by an active fluid pump mechanism associated with the endothelium itself. Although this pump-leak hypothesis was postulated several decades ago, the mechanisms underlying regulation of the balance between the pump and leak functions remain largely unknown. In the last couple of decades, the ion transport systems that support the fluid pump activity have been discovered. In contrast, despite significant evidence for corneal edema secondary to endothelial barrier dysfunction, the molecular aspects underlying its regulation are relatively unknown. Recent findings in our laboratory, however, indicate that barrier integrity (i.e., structural and functional integrity of the tight junctions) of the endothelium is sensitive to remodeling of its peri-junctional actomyosin ring, which is located at the apical junctional complex. This review provides a focused perspective on dynamic regulation of the barrier integrity of endothelium vis-à-vis plasticity of the peri-junctional actomyosin ring and its association with cell signaling downstream of small GTPases of the Rho family. Based on findings to date, it appears that development of specific pharmacological strategies to treat corneal edema in response to inflammatory stress would be possible in the near future.

CD73 represses pro-inflammatory responses in human endothelial cells

BACKGROUND

CD73 is a 5'-ectonucleotidase that produces extracellular adenosine, which then acts on G protein-coupled purigenic receptors to induce cellular responses. CD73 has been reported to regulate expression of pro-inflammatory molecules in mouse endothelium. Our aim is to determine the function of CD73 in human endothelial cells.

METHODS

We used RNAi to deplete CD73 levels in human umbilical cord endothelial cells (HUVECs).

RESULTS

CD73 depletion resulted in a strong reduction in adenosine production, indicating that CD73 is the major source of extracellular adenosine in HUVECs. We find that CD73 depletion induces a similar response to pro-inflammatory stimuli such as the cytokine TNF-alpha. In CD73-depleted cells, surface levels of the leukocyte adhesion molecules ICAM-1, VCAM-1 and E-selectin increase. This correlates with increased translocation of the transcription factor NF-kB to the nucleus, which is known to regulate ICAM-1, VCAM-1 and E-selectin expression in response to TNF-alpha. Adhesion of monocytic cells to endothelial cells is enhanced. In addition, CD73-depleted cells become elongated, have higher levels of stress fibres and increased endothelial permeability, resembling known responses to TNF-alpha.

CONCLUSIONS

These results indicate that CD73 normally suppresses pro-inflammatory responses in human endothelial cells.

Formation and disassembly of adherens and tight junctions in the corneal endothelium: regulation by actomyosin contraction

Purpose. To determine the role of actin cytoskeleton in the disassembly and reformation of adherens junctions (AJs) and tight junctions (TJs) in bovine corneal endothelial monolayers. Methods. Disassembly and reformation of AJs and TJs were induced by extracellular Ca(2+) depletion and subsequent add-back of Ca(2+), respectively. Resultant changes in the transendothelial electrical resistance (TER), an indicator of integrity of TJs, were measured based on electrical cell-substrate impedance. Phosphorylated myosin light chain (ppMLC), a biochemical measure of actomyosin contraction, and activation of its upstream regulatory molecule RhoA-GTP were assessed by Western blot analysis. Results. Extracellular Ca(2+) depletion led to activation of RhoA, increase in ppMLC, decrease in TER, contraction of the perijunctional actomyosin ring (PAMR), and redistribution of zonula occludens-1 (ZO-1) and cadherins. These effects were reversed on Ca(2+) add-back. Pretreatment with Y-27632 and blebbistatin (as inhibitors of actomyosin contraction) reduced the rate of decline in TER, opposed the contraction of the PAMR, and blocked the redistribution of ZO-1 and cadherins. Both drugs reduced the recovery in TER and opposed the normal redistribution of ZO-1 and cadherins on Ca(2+) add-back. Cytochalasin D, which led to dissolution of the PAMR, also reduced the recovery of TER on Ca(2+) add-back. Conclusions. The (Ca(2+) depletion)-induced disassembly of AJs accelerates the breakdown of TJs through a concomitant increase in the actomyosin contraction of the PAMR. However, these data on reassembly show that a contractile tone of the PAMR is essential for assembly of the apical junctional complex.

Reduced intercellular communication and altered morphology of bovine corneal endothelial cells with prolonged time in cell culture

PURPOSE

Mechanical stimulation induces intercellular Ca(2 +) waves in the corneal endothelium. The extent of the wave propagation is dependent on the activity of gap junctions, hemichannels, and ectonucleotidases. To further establish the use of a cell culture model to investigate intercellular communication, in this study, we have characterized the changes in the Ca(2 +) wave propagation in bovine corneal endothelial cells with prolonged time in culture.

MATERIALS AND METHODS

Freshly isolated BCEC were cultured for a short term (8 to 14 days; referred to as "short term") and a long term (21 to 30 days; referred to as "long term"). Cell surface area and size were measured by confocal microscopy and flow cytometry, respectively. Calcium wave propagation was assayed by imaging spread of the Ca(2 +) waves elicited by mechanical stimulation. ATP release was assayed using Luciferin-Luciferase bioluminescence technique.

RESULTS

Cells cultured for a long term showed larger surface area and size compared to those cultured for a short term, but a reduced spread of the Ca(2 +) wave. Exposure to exogenous apyrases, which can rapidly hydrolyze extracellular ATP, reduced the spread of the Ca(2 +) wave in both groups. The fractional decrease, however, was smaller in cells cultured for a long term. Exposure to ARL-67156 to inhibit the ectonucleotidases led to a larger enhancement of the active area in cells cultured for a long term. However, the active areas of the two groups were not significantly different in the presence of the drug. Furthermore, ATP release in response to mechanical stimulation was lower in cells cultured for a long term in the absence of ARL-67156 but not in its presence.

CONCLUSIONS

BCEC cultured for a long term show an increase in cell surface area and cell size similar to the effect of aging in human corneas. Moreover, the cells cultured for a long term showed a reduced ATP-dependent paracrine intercellular communication, largely due to an increase in the activity of the ectonucleotidases.

Barrier dysfunction of the corneal endothelium in response to TNF-alpha: role of p38 MAP kinase

PURPOSE

TNF-alpha is elevated in the cornea and aqueous humor during allograft rejection and anterior uveitis. The authors investigated the involvement of p38 MAP kinase in the TNF-alpha-induced loss of barrier integrity in monolayers of cultured bovine corneal endothelial cells.

METHODS

Transendothelial electrical resistance (TER), a measure of barrier integrity, was determined by electrical cell-substrate impedance sensing. Barrier integrity was further assessed in terms of permeability to FITC dextran. Reorganization of the apical junctional complex (AJC) in response to TNF-alpha was visualized by immunofluorescence. The expression of TNF-alpha receptors was confirmed by RT-PCR. Activation of p38 MAP kinase in response to TNF-alpha was determined by Western blot analysis.

RESULTS

Exposure to TNF-alpha induced a continuous decline in TER that persisted for more than 20 hours. It also led to a significant increase in permeability to FITC dextran. At the AJC, the cytokine caused disassembly of microtubules, disruption of perijunctional actomyosin ring (PAMR), and dislocation of ZO-1 and cadherins. Western blot analysis showed that TNF-alpha also led to the activation of p38 MAP kinase. All these responses to the cytokine were opposed by treatment with SB-203580, a selective p38 MAP kinase inhibitor. TNFR1, but not TNFR2, was expressed in untreated cells with no change in the expression pattern on treatment with the cytokine.

CONCLUSIONS

TNF-alpha breaks down the barrier integrity of corneal endothelium, concomitant with the disruption of PAMR, remodeling of AJC, and disassembly of microtubules. These effects are mediated by transient activation of p38 MAP kinase. Thus, the TNF-alpha-induced barrier dysfunction in the corneal endothelium can be suppressed by inhibitors of p38 MAP kinase and agents downstream of the kinase that affect the cytoskeleton.

Microtubule stabilization opposes the (TNF-alpha)-induced loss in the barrier integrity of corneal endothelium

Microtubule disassembly breaks down the barrier integrity in a number of epithelial and endothelial monolayers. This study has investigated effects of TNF-alpha, which is implicated in corneal allograft rejection, on microtubules and barrier integrity in cultured bovine corneal endothelial cells. Exposure to TNF-alpha led to disassembly of the microtubules, and also caused disruption of the perijunctional actomyosin ring (PAMR). As a measure of barrier integrity, trans-endothelial electrical resistance (TER) was determined based on electrical cell-substrate impedance sensing in realtime. Exposure to TNF-alpha caused a slow decline in TER for > 20 h, and a similar exposure to cells grown on porous culture inserts led to a significant increase in permeability to FITC dextran. These changes, indicating a loss of barrier integrity, were also reflected by dislocation of ZO-1 at the cell border and disassembly of cadherins. These effects of TNF-alpha were inhibited upon stabilization of microtubules by pre-treatment with paclitaxel or epothilone B. Microtubule stabilization may be a useful strategy to overcome (TNF-alpha)-induced loss of the barrier integrity of corneal endothelium during inflammation associated with transplant rejection and uveitis.

Microtubule disassembly breaks down the barrier integrity of corneal endothelium

Increased contractility of the peri-junctional actomyosin ring (PAMR) breaks down the barrier integrity of corneal endothelium. This study has examined the effects of microtubule disassembly on Myosin Light Chain (MLC) phosphorylation, a biochemical marker of actomyosin contraction, and barrier integrity in monolayers of cultured bovine corneal endothelial cells (BCEC). Exposure to nocodazole, which readily induced microtubule disassembly, led to disruption of the characteristically dense assembly of cortical actin cytoskeleton at the apical junctional complex (i.e., PAMR) and dispersion of ZO-1 from its normal locus. Nocodazole also led to an increase in phosphorylation of MLC. Concomitant with these changes, nocodazole caused an increase in permeability to HRP and FITC dextran (10 kDa) and a decrease in trans-endothelial electrical resistance (TER). Y-27632 (a Rho kinase inhibitor) and forskolin (known to inhibit activation of RhoA through direct elevation of cAMP) opposed the nocodazole-induced MLC phosphorylation, decrease in TER, and dispersion of ZO-1. Thrombin, which breaks down the barrier integrity of BCEC monolayers, also induced microtubule disassembly and MLC phosphorylation. Pre-treatment with paclitaxel to stabilize microtubules opposed the thrombin effects. These results suggest that microtubule disassembly breaks down the barrier integrity of BCEC through activation of RhoA and subsequent disruption of the PAMR. The thrombin effect also highlights that signaling downstream of GPCRs can also influence the organization of microtubules.

Forskolin induces myosin light chain dephosphorylation in bovine trabecular meshwork cells

PURPOSE

Enhanced contractility of the actin cytoskeleton in trabecular meshwork (TM) cells is implicated in increased resistance to aqueous humor outflow. In this study, we have investigated effects of forskolin, which is known to elevate cAMP and also enhance aqueous humor outflow, on myosin light chain (MLC) phosphorylation, a biochemical marker of actin contractility.

METHODS

Experiments were performed using cultured bovine TM cells. Phosphorylated MLC (pMLC), expressed as the % of untreated cells, was assessed by urea-glycerol gel electrophoresis and Western blotting. RhoA activity was determined by affinity precipitation of RhoA-GTP to RhoA binding domain of an effector of RhoA. Intracellular cAMP levels were measured by ELISA.

RESULTS

Exposure to LPA (lysophosphatidic acid) led to increased MLC phosphorylation (LPA: pMLC=133%) and activation of RhoA. These responses of LPA were suppressed by co-treatment with forskolin (LPA+forskolin: pMLC=88%). Similarly, ET-1 and nocodazole-induced MLC phosphorylation (ET-1: pMLC=145%; nocodazole: pMLC=145%) as well as RhoA activation were suppressed by co-treatment with forskolin (ET-1+forskolin: pMLC=99%; nocodazole+forskolin: pMLC=107%). Exposure to forskolin alone led to MLC dephosphorylation (pMLC=68%). Forskolin alone led to a 4-fold increase in cAMP levels. This increase was not affected when co-treated with LPA or ET-1.

CONCLUSIONS

Forskolin prevents MLC phosphorylation induced by LPA, ET-1, and nocodazole through inhibition of RhoA-Rho kinase axis. MLC dephosphorylation and consequent relaxation of actin cytoskeleton in TM cells presumably underlies the increased outflow facility reported in response to forskolin.

Bradykinin increases blood-tumor barrier permeability by down-regulating the expression levels of ZO-1, occludin, and claudin-5 and rearranging actin cytoskeleton

Bradykinin (BK) has been shown to open blood-tumor barrier (BTB) selectively and to increase permeability of the BTB transiently, but the mechanism is unclear. This study was performed to determine whether BK opens the BTB by affecting the tight junction (TJ)-associated proteins zonula occluden-1 (ZO-1), occludin, and caludin-5 and cytoskeleton protein filamentous actin (F-actin). In rat brain glioma model and BTB model in vitro, we find that the protein expression levels of ZO-1, occludin, and claudin-5 are attenuated by BK induction. Immunohistochemistry and immunofluorescence assays show that the attenuated expression of ZO-1, occludin, and claudin-5 and F-actin is most obvious in the smaller tumor capillaries (<20 microm) after BK infusion, and there is no change in the larger tumor capillaries (>20 microm). The redistribution of ZO-1, occludin, and claudin-5 and rearrangement of F-actin in brain microvascular endothelial cells are observed at the same time. Meanwhile, Evans blue assay shows that the permeability of BTB increases after BK infusion. Transmission electron microscopy indicates that TJ is opened and that pinocytotic vesicular density is increased. Transendothelial electrical resistance (TEER) and horseradish peroxidase flux assays also reveal that TJ is opened by BK induction. In addition, radioimmunity and Western blot assay reveal a significant decrease in expression levels of cAMP and catalytic subunit of protien kinase A (PKAcs) of tumor tissue. This study demonstrates that the increase of BK-mediated BTB permeability is associated with the down-regulation of ZO-1, occludin, and claudin-5 and the rearrangement of F-actin and that cAMP/PKA signal transduction system might be involved in the modulating process.

Dependence of cAMP meditated increases in Cl- and HCO(3)- permeability on CFTR in bovine corneal endothelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is present on the apical membrane of corneal endothelial cells. Increasing intracellular [cAMP] with forskolin stimulates an NPPB and glibenclamide-inhibitable apical Cl(-) and HCO(3)(-) permeability [Sun, X.C., Bonanno, J.A., 2002. Expression, localization, and functional evaluation of CFTR in bovine corneal endothelial cells. Am. J. Physiol. Cell Physiol. 282, C673-C683]. To definitively determine that the increased permeability is dependent on CFTR, we used an siRNA knockdown approach. Apical Cl(-) and HCO(3)(-) permeability and steady-state HCO(3)(-) flux were measured in the presence or absence of forskolin using cultured bovine corneal endothelial cells that were transfected with CFTR siRNA or a scrambled sequence control. CFTR protein expression was reduced by approximately 80% in CFTR siRNA treated cultures. Forskolin (10 microM) increased apical chloride permeability by 7-fold, which was reduced to control level in siRNA treated cells. CFTR siRNA treatment had no effect on baseline apical chloride permeability. Apical HCO(3)(-) permeability was increased 2-fold by 10 microM forskolin, which was reduced to control level in siRNA treated cultures. Similarly, there was no effect on baseline apical HCO(3)(-) permeability by knocking down CFTR expression. The steady-state apical-basolateral pH gradient (DeltapH) at 4h in control cultures was increased approximately 2.5-fold by forskolin. In CFTR siRNA treated cells, the baseline DeltapH was similar to control, however forskolin did not have a significant effect. We conclude that forskolin induced increases in apical HCO(3)(-) permeability in bovine corneal endothelium requires CFTR. However, CFTR does not have a major role in determining baseline apical chloride or HCO(3)(-) permeability.

Histamine-induced myosin light chain phosphorylation breaks down the barrier integrity of cultured corneal epithelial cells

PURPOSE

To investigate changes in the phosphorylation of myosin light chain (MLC) in response to histamine and its effect on the barrier integrity of corneal epithelial cells.

MATERIALS AND METHODS

Experiments were performed in bovine corneal epithelial cells (BCEC). RT-PCR and Western blotting were employed to characterize expression of H1 receptors and MLC kinase (MLCK). Phosphorylation of MLC was assessed by urea-glycerol gel electrophoresis and Western blotting. Barrier integrity was determined as permeability to horseradish peroxidase (HRP; 44 kDa) across monolayers grown on porous filters.

RESULTS

Expression of both H1 receptors and MLCK was found in BCEC. Exposure to histamine induced significant MLC phosphorylation concomitant with an increase in HRP permeability. In addition, organization of the cortical actin found in resting cells was disrupted. In contrast to histamine, ATP (a P2Y receptor agonist) induced dephosphorylation of MLC. Pre-exposure to ATP reduced the effect of histamine on HRP permeability and disruption of cortical actin.

CONCLUSION

MLC phosphorylation, a biochemical pre-requisite for increased contractility of the actin cytoskeleton, led to histamine-induced breakdown of the barrier integrity in the corneal epithelial cells. This is attributed to weakening of the tethering forces at the tight junctions by the centripetal forces produced by increased actin contractility.

Phosphorylation of myosin phosphatase targeting subunit 3 (MYPT3) and regulation of protein phosphatase 1 by protein kinase A

Myosin phosphatase targeting subunit 3 (MYPT3) and transforming growth factor-beta-inhibited membrane-associated protein (TIMAP) are two closely related myosin-binding targeting subunits of protein phosphatase 1 (PP1c) with a characteristic CAAX (where AA indicates aliphatic amino acid) box at the C termini. Here we show that MYPT3 can be a substrate for protein kinase A (PKA). We first mapped the multiple phosphorylation sites within a central conserved motif. Deletion or mutations of this motif resulted in enhancement of the associated PP1c activity, suggesting that phosphorylation of MYPT3 may play an important role in regulating PP1c catalytic activity. However, unlike the other known MYPTs, which upon phosphorylation inhibit PP1c, PKA phosphorylation of MYPT3 resulted in PP1c activation, indicating a different mode of action. There is a direct interaction between the central conserved phosphorylated site motif with the N-terminal ankyrin repeat region; this interaction was significantly reduced with MYPT3 phosphorylation or acidic phosphorylation site mutations, with concomitant alterations in biochemical and morphological consequences. We therefore propose a novel mechanism for the phosphorylation of MYPT3 by PKA and activation of the catalytic activity through direct interaction of a central region of MYPT3 with its N-terminal region.

Molecular expression and functional involvement of the bovine calcium-activated chloride channel 1 (bCLCA1) in apical HCO3- permeability of bovine corneal endothelium

Corneal endothelium secretes HCO(3)(-) from basolateral (stroma) to apical (anterior chamber) compartments. Apical HCO(3)(-) permeability can be enhanced by increasing [Ca(2+)](i). We hypothesized that the bovine calcium-activated chloride channel 1 (bCLCA1), shown previously by PCR screening to be expressed in corneal endothelium, is involved in Ca(2+) activated apical HCO(3)(-) permeability. bCLCA1 expression in cultured bovine corneal endothelial cells (CBCEC) was examined by in situ hybridization analysis, immunoblotting, immunofluorescence and confocal microscopy. Rabbit polyclonal antibodies were generated using a 14 aa polypeptide (417-430) from the predicted sequence of bCLCA1. The small interference RNA (siRNA) knock down technique was used to evaluate the functional involvement of bCLCA1 in apical HCO(3)(-) permeability. In situ hybridization confirmed prominent bCLCA1-specific mRNA expression in CBCEC. bCLCA1 antiserum detected the heterologously expressed bCLCA1 in HEK293 cells and a 90kDa band in CBCEC, which was absent when using the pre-immune serum or antigen absorption of serum. Immunofluoresence staining with anti-bCLCA1 antibody and confocal microscopy indicates an apical membrane location in CBCEC. In CBCEC transfected with bCLCA1 specific siRNA, bCLCA1 expression was reduced by 80%, while transfection with siControl scrambled sequence had no effect. Increasing [Ca(i)(2+)] by application of ATPgammaS or cyclopiazonic acid (CPA) increased apical HCO(3)(-) permeability in siControl transfected CBCEC, while having no effect on apical HCO(3)(-) permeability in bCLCA1 specific siRNA transfected cells. Baseline HCO(3)(-) permeability, however, was not different between controls and siRNA treated cells. We conclude that the calcium-activated chloride channel (bCLCA1) is expressed in bovine corneal endothelial cells and can contribute to Ca(2+) dependent apical HCO(3)(-) permeability, but not resting permeability, across the corneal endothelium.

Extracellular ATP opposes thrombin-induced myosin light chain phosphorylation and loss of barrier integrity in corneal endothelial cells

Increased contractility of the actin cytoskeleton by phosphorylation of the regulatory myosin light chain (MLC) results in a loss of barrier integrity in corneal endothelial cells. This study has investigated the effect of extracellular ATP, which may influence both Ca2+ and cAMP signalling, on MLC phosphorylation and barrier integrity in cultured bovine corneal endothelial cells (BCEC) known to express A2B and P2Y purinergic receptors, and ecto-nucleotidases. Extracellular ATP (100 microM) promoted MLC dephosphorylation (pMLC=61.8% at 18 min; n=9). Pre-exposure to ARL-67156, an ecto-nucleotidase inhibitor, prevented ATP-induced dephosphorylation. Other P2Y agonists, UTP and ATPgammaS, also induced MLC dephosphorylation but to a lesser degree compared to ATP. Thrombin (2 U/ml), which activate Rho kinase through PAR-1 receptors in the endothelium, induced MLC phosphorylation (pMLC=129.2%; n=14). This phosphorylation was completely abolished by concomitant exposure to ATP. When cells were pretreated with adenosine (100 microM; A2B agonist) or forskolin (10 microM), thrombin-induced phosphorylation was suppressed. ATP also led to a significant increase in cAMP (> 3-fold compared to 10 microM adenosine). Thrombin-induced increase in trans-endothelial flux of horseradish peroxidase (44 kDa) and disruption of the cortical actin were suppressed by ATP. These findings indicate that in BCEC (1) ATP induces elevated cAMP through its metabolite adenosine leading to MLC dephosphorylation, (2) Stimulation of P2Y2 receptors also leads to activation of MLCP since UTP- and ATPgammaS caused MLC dephosphorylation, and (3) ATP is antagonistic to thrombin since the latter inhibits MLCP through increased activity of Rho kinase. These findings further emphasize the role of contractility of the actin cytoskeleton in regulating the barrier integrity of corneal endothelium.

Characterization of adenosine receptors in bovine corneal endothelium

Previous studies indicated that adenosine can increase [cAMP](i) and stimulate fluid transport by corneal endothelium. The purpose of this study was to determine which adenosine receptor subtype(s) are expressed and to examine their functional roles in modulating [cAMP](i), [Ca(2+)](i) and effects on Cl(-) permeability in corneal endothelium. We screened bovine corneal endothelium (BCE) for adenosine receptor subtypes by RT-PCR and immunoblotting, and examined the effects of pharmacological agents on adenosine stimulated Cl(-) transport, [cAMP](i) and [Ca(2+)](i). RT-PCR indicated the presence of A(1) and A(2b) adenosine receptors, while A(2a) and A(3) were negative. Western blot (WB) confirmed the presence of A(2b) ( approximately 50 kDa) and A(1) ( approximately 40 kDa) in fresh and cultured BCE. Ten micromolar adenosine increased [cAMP](i) by 2.7-fold over control and this was inhibited 66% by 10 microm alloxazine, a specific A(2b) blocker. A(1) activation with 1 micromN(6)-CPA (a specific A(1) agonist) or 100 nm adenosine decreased [cAMP](i) by 23 and 6%, respectively. Adenosine had no effect on [Ca(2+)](i) mobilization. Indirect immunofluorescence localized A(2b) receptors to the lateral membrane and A(1) to the apical surface in cultured BCE. Adenosine significantly increased apical Cl(-) permeability by 2.2 times and this effect was nearly abolished by DMPX (10 microm), a general A(2) blocker. Adenosine-induced membrane depolarization was also inhibited by 33% (n=6) in the presence of alloxazine. Bovine corneal endothelium expresses functional A(1) and A(2b) adenosine receptors. A(1), preferentially activated at <1 microm adenosine, acts to decrease [cAMP](i) and A(2b), activated at >1 microm adenosine, increase [cAMP](i).