Role of cyclic adenosine monophosphate in the induction of endothelial barrier properties

A Focus on the Pathophysiology of Adrenomedullin Expression: Endothelitis and Organ Damage in Severe Viral and Bacterial Infections

Adrenomedullin (ADM) is a peptide hormone produced primarily in the adrenal glands, playing a crucial role in various physiological processes. As well as improving vascular integrity and decreasing vascular permeability, ADM acts as a vasodilator, positive inotrope, diuretic, natriuretic and bronchodilator, antagonizing angiotensin II by inhibiting aldosterone secretion. ADM also has antihypertrophic, anti-apoptotic, antifibrotic, antioxidant, angiogenic and immunoregulatory effects and antimicrobial properties. ADM expression is upregulated by hypoxia, inflammation-inducing cytokines, viral or bacterial substances, strength of shear stress, and leakage of blood vessels. These pathological conditions are established during systemic inflammation that can result from infections, surgery, trauma/accidents or burns. The ability to rapidly identify infections and the prognostic, predictive power makes it a valuable tool in severe viral and bacterial infections burdened by high incidence and mortality. This review sheds light on the pathophysiological processes that in severe viral or bacterial infections cause endothelitis up to the development of organ damage, the resulting increase in ADM levels dosed through its more stable peptide mid-regional proadrenomedullin (MR-proADM), the most significant studies that attest to its diagnostic and prognostic accuracy in highlighting the severity of viral or bacterial infections and appropriate therapeutic insights.

cAMP: A master regulator of cadherin-mediated binding in endothelium, epithelium and myocardium

Regulation of cadherin-mediated cell adhesion is crucial not only for maintaining tissue integrity and barrier function in the endothelium and epithelium but also for electromechanical coupling within the myocardium. Therefore, loss of cadherin-mediated adhesion causes various disorders, including vascular inflammation and desmosome-related diseases such as the autoimmune blistering skin dermatosis pemphigus and arrhythmogenic cardiomyopathy. Mechanisms regulating cadherin-mediated binding contribute to the pathogenesis of diseases and may also be used as therapeutic targets. Over the last 30 years, cyclic adenosine 3',5'-monophosphate (cAMP) has emerged as one of the master regulators of cell adhesion in endothelium and, more recently, also in epithelial cells as well as in cardiomyocytes. A broad spectrum of experimental models from vascular physiology and cell biology applied by different generations of researchers provided evidence that not only cadherins of endothelial adherens junctions (AJ) but also desmosomal contacts in keratinocytes and the cardiomyocyte intercalated discs are central targets in this scenario. The molecular mechanisms involve protein kinase A- and exchange protein directly activated by cAMP-mediated regulation of Rho family GTPases and S665 phosphorylation of the AJ and desmosome adaptor protein plakoglobin. In line with this, phosphodiesterase 4 inhibitors such as apremilast have been proposed as a therapeutic strategy to stabilize cadherin-mediated adhesion in pemphigus and may also be effective to treat other disorders where cadherin-mediated binding is compromised.

Cytoskeleton Elements Contribute to Prion Peptide-Induced Endothelial Barrier Breakdown in a Blood–Brain Barrier In Vitro System

The mechanisms involved in the interaction of PrP 106-126, a peptide corresponding to the prion protein amyloidogenic region, with the blood–brain barrier (BBB) were studied. PrP 106-126 treatment that was previously shown to impair BBB function, reduced cAMP levels in cultured brain endothelial cells, increased nitric oxide (NO) levels, and changed the activation mode of the small GTPases Rac1 (inactivation) and RhoA (activation). The latter are well established regulators of endothelial barrier properties that act via cytoskeletal elements. Indeed, liquid chromatography-mass spectrometry (LC-MS)-based proteomic profiling study revealed extensive changes in expression of cytoskeleton-related proteins. These results shed light on the nature of the interaction between the prion peptide PrP 106-126 and the BBB and emphasize the importance of the cytoskeleton in endothelium response to prion- induced stress.

Substrate stiffness modulates migration and local intercellular membrane motion in pulmonary endothelial cell monolayers

The pulmonary artery endothelium forms a semipermeable barrier that limits macromolecular flux through intercellular junctions. This barrier is maintained by an intrinsic forward protrusion of the interacting membranes between adjacent cells. However, the dynamic interactions of these membranes have been incompletely quantified. Here, we present a novel technique to quantify the motion of the peripheral membrane of the cells, called paracellular morphological fluctuations (PMFs), and to assess the impact of substrate stiffness on PMFs. Substrate stiffness impacted large-length scale morphological changes such as cell size and motion. Cell size was larger on stiffer substrates, whereas the speed of cell movement was decreased on hydrogels with stiffness either larger or smaller than 1.25 kPa, consistent with cells approaching a jammed state. Pulmonary artery endothelial cells moved fastest on 1.25 kPa hydrogel, a stiffness consistent with a healthy pulmonary artery. Unlike these large-length scale morphological changes, the baseline of PMFs was largely insensitive to the substrate stiffness on which the cells were cultured. Activation of store-operated calcium channels using thapsigargin treatment triggered a transient increase in PMFs beyond the control treatment. However, in hypocalcemic conditions, such an increase in PMFs was absent on 1.25 kPa hydrogel but was present on 30 kPa hydrogel-a stiffness consistent with that of a hypertensive pulmonary artery. These findings indicate that 1) PMFs occur in cultured endothelial cell clusters, irrespective of the substrate stiffness; 2) PMFs increase in response to calcium influx through store-operated calcium entry channels; and 3) stiffer substrate promotes PMFs through a mechanism that does not require calcium influx.

Regulation and Dysregulation of Endothelial Permeability during Systemic Inflammation

Systemic inflammation can be triggered by infection, surgery, trauma or burns. During systemic inflammation, an overshooting immune response induces tissue damage resulting in organ dysfunction and mortality. Endothelial cells make up the inner lining of all blood vessels and are critically involved in maintaining organ integrity by regulating tissue perfusion. Permeability of the endothelial monolayer is strictly controlled and highly organ-specific, forming continuous, fenestrated and discontinuous capillaries that orchestrate the extravasation of fluids, proteins and solutes to maintain organ homeostasis. In the physiological state, the endothelial barrier is maintained by the glycocalyx, extracellular matrix and intercellular junctions including adherens and tight junctions. As endothelial cells are constantly sensing and responding to the extracellular environment, their activation by inflammatory stimuli promotes a loss of endothelial barrier function, which has been identified as a hallmark of systemic inflammation, leading to tissue edema formation and hypotension and thus, is a key contributor to lethal outcomes. In this review, we provide a comprehensive summary of the major players, such as the angiopoietin-Tie2 signaling axis, adrenomedullin and vascular endothelial (VE-) cadherin, that substantially contribute to the regulation and dysregulation of endothelial permeability during systemic inflammation and elucidate treatment strategies targeting the preservation of vascular integrity.

Molecular Dambusters: What Is Behind Hyperpermeability in Bradykinin-Mediated Angioedema?

In the last few decades, a substantial body of evidence underlined the pivotal role of bradykinin in certain types of angioedema. The formation and breakdown of bradykinin has been studied thoroughly; however, numerous questions remained open regarding the triggering, course, and termination of angioedema attacks. Recently, it became clear that vascular endothelial cells have an integrative role in the regulation of vessel permeability. Apart from bradykinin, a great number of factors of different origin, structure, and mechanism of action are capable of modifying the integrity of vascular endothelium, and thus, may participate in the regulation of angioedema formation. Our aim in this review is to describe the most important permeability factors and the molecular mechanisms how they act on endothelial cells. Based on endothelial cell function, we also attempt to explain some of the challenging findings regarding bradykinin-mediated angioedema, where the function of bradykinin itself cannot account for the pathophysiology. By deciphering the complex scenario of vascular permeability regulation and edema formation, we may gain better scientific tools to be able to predict and treat not only bradykinin-mediated but other types of angioedema as well.

Pharmacological Benefits and Risk of Using Hormones in Organ Perfusion and Preservation Solutions in the Aspect of Minimizing Hepatic Ischemia-Reperfusion Injury during Storage

For several years, research has been carried out on the effectiveness of solutions for perfusion and preservation of organs, including the liver. There is a search for an optimal pharmacological composition of these solutions, allowing to preserve or improve vital functions of the organ for as long as possible until it is transplanted into a recipient. Hormones due to their properties, often resulting from their pleiotropic effects, may be a valuable component for optimizing the composition of liver perfusion and preservation solutions. The paper presents the current state of knowledge on liver perfusion and preservation solutions modified with hormones. It also shows the characteristics of the hormones evaluated, taking into account their physiological functions in the body.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Cyclic nucleotide phosphodiesterases (PDEs) and endothelial function in ischaemic stroke. A review

BACKGROUND

Endothelial dysfunction is a hallmark of cerebrovascular disease, including ischemic stroke. Modulating endothelial signalling by cyclic nucleotides, cAMP and cGMP, is a potential therapeutic target in stroke. Inhibitors of the cyclic nucleotide degrading phosphodiesterase (PDE) enzymes may restore cerebral endothelial function. Current knowledge on PDE distribution and function in cerebral endothelial cells is sparse. This review explores data on PDE distribution and effects of PDEi in cerebral endothelial cells and identifies which PDEs are potential treatment targets in stroke.

METHOD

We performed a systematic search of electronic databases (Medline and Embase). Our search terms were cerebral ischaemia, cerebral endothelial cells, cyclic nucleotide, phosphodiesterase and phosphodiesterase inhibitors.

RESULTS

We found 23 publications which described effects of selective inhibitors of only three PDE families on endothelial function in ischemic stroke. PDE3 inhibitors (PDE3i) (11 publications) and PDE4 inhibitors (PDE4i) (3 publications) showed anti-inflammatory, anti-apoptotic or pro-angiogenic effects. PDE3i also reduced leucocyte infiltration and MMP-9 expression. Both PDE3i and PDE4i increased expression of tight junction proteins and protected the blood-brain barrier. PDE5 inhibitors (PDE5i) (6 publications) reduced inflammation and apoptosis. In preclinical models, PDE5i enhanced cGMP/NO signalling associated with microvascular angiogenesis, increased cerebral blood flow and improved functional recovery. Non-specific PDEi (3 publications) had mainly anti-inflammatory effects.

CONCLUSION

This review demonstrates that non-selective and selective PDEi of PDE3, PDE4 and PDE5 modulated endothelial function in cerebral ischemic stroke by regulating processes involved in vascular repair and neuroprotection and thus reduced cell death and inflammation. Of note, they promoted angiogenesis, microcirculation and improved functional recovery; all are important in stroke prevention and recovery, and effects should be further evaluated in humans.

12(S)-hydroxyeicosatetraenoic acid impairs vascular endothelial permeability by altering adherens junction phosphorylation levels and affecting the binding and dissociation of its components in high glucose-induced vascular injury

AIMS/INTRODUCTION

Diabetes is an important risk factor for atherosclerotic disease. The initiating factor of atherosclerosis is local endothelial cell injury. The arachidonic acid metabolite, 12(S)-hydroxyeicosatetraenoic acid (12[S]-HETE), might be involved in this process. In recent years, some studies have discussed the effect of 12(S)-HETE on vascular endothelial cell function. In the present study, we investigated the effect of 12(S)-HETE on vascular endothelial cell function in high-glucose conditions and the mechanisms involved.

MATERIALS AND METHODS

Human umbilical vein endothelial cells were cultured in conventional M199 medium and high-glucose M199 medium. Human umbilical vein endothelial cells were stimulated with 12(S)-HETE and cinnamyl-3,4-dihydroxy-α-cyanocinnamate (a 12/15-lipoxygenases inhibitor). A type 1 diabetes mellitus model was established in C57BL/6 or 12/15-lipoxygenases knockout mice with streptozotocin. Aortic tissue was harvested for subsequent testing. The transmembrane transport of dextran and human acute monocytic leukaemia cell line (THP-1) cells was measured. The adherens junction protein, IkBα, nuclear factor kappa Bp65 (P65), intercellular adhesion molecule 1 and vascular cell adhesion protein 1 expression and phosphorylation, and the binding/dissociation of endothelial cell components were observed.

RESULTS

Transendothelial migration of dextran and THP-1 cells was significantly increased by stimulation of human umbilical vein endothelial cell monolayers with high glucose and 12(S)-HETE (P < 0.05). High glucose and 12(S)-HETE altered the vascular endothelial cadherin and β-catenin phosphorylation level, and promoted the dissociation of β-catenin and vascular endothelial cadherin. Expression levels of P-Ikbα, P-P65, intercellular adhesion molecule 1 and vascular cell adhesion protein 1 were elevated in high glucose and 12(S)-HETE treated cells and diabetic mice compared with controls (P < 0.05).

CONCLUSIONS

The lipoxygenases metabolite, 12(S)-HETE, can impair vascular endothelial permeability by altering adherens junction phosphorylation levels, and affecting the binding and dissociation of its components in high-glucose conditions.

Extracellular vesicles: another compartment for the second messenger, cyclic adenosine monophosphate

The second messenger, cAMP, is highly compartmentalized to facilitate signaling specificity. Extracellular vesicles (EVs) are submicron, intact vesicles released from many cell types that can act as biomarkers or be involved in cell-to-cell communication. Although it is well recognized that EVs encapsulate functional proteins and RNAs/miRNAs, currently it is unclear whether cyclic nucleotides are encapsulated within EVs to provide an additional second messenger compartment. Using ultracentrifugation, EVs were isolated from the culture medium of unstimulated systemic and pulmonary endothelial cells. EVs were also isolated from pulmonary microvascular endothelial cells (PMVECs) following stimulation of transmembrane adenylyl cyclase (AC) in the presence or absence of the phosphodiesterase 4 inhibitor rolipram over time. Whereas cAMP was detected in EVs isolated from endothelial cells derived from different vascular beds, it was highest in EVs isolated from PMVECs. Treatment of PMVECs with agents that increase near-membrane cAMP led to an increase in cAMP within corresponding EVs, yet there was no increase in EV number. Elevated cell cAMP, measured by whole cell measurements, peaked 15 min after treatment, yet in EVs the peak increase in cAMP was delayed until 60 min after cell stimulation. Cyclic AMP was also increased in EVs collected from the perfusate of isolated rat lungs stimulated with isoproterenol and rolipram, thus corroborating cell culture findings. When added to unperturbed confluent PMVECs, EVs containing elevated cAMP were not barrier disruptive like cytosolic cAMP but maintained monolayer resistance. In conclusion, PMVECs release EVs containing cAMP, providing an additional compartment to cAMP signaling.

Dynamic Regulation of Vascular Permeability by Vascular Endothelial Cadherin-Mediated Endothelial Cell-Cell Junctions

Endothelial cells lining blood vessels regulate vascular barrier function, which controls the passage of plasma proteins and circulating cells across the endothelium. In most normal adult tissues, endothelial cells preserve basal vascular permeability at a low level, while they increase permeability in response to inflammation. Therefore, vascular permeability is tightly controlled by a number of extracellular stimuli and mediators to maintain tissue homeostasis. Accordingly, impaired regulation of endothelial permeability causes various diseases, including chronic inflammation, asthma, edema, sepsis, acute respiratory distress syndrome, anaphylaxis, tumor angiogenesis, and diabetic retinopathy. Vascular endothelial (VE)-cadherin, a member of the classical cadherin superfamily, is a component of cell-to-cell adherens junctions in endothelial cells and plays an important role in regulating vascular permeability. VE-cadherin mediates intercellular adhesion through trans-interactions formed by its extracellular domain, while its cytoplasmic domain is anchored to the actin cytoskeleton via α- and β-catenins, leading to stabilization of VE-cadherin at cell-cell junctions. VE-cadherin-mediated cell adhesions are dynamically, but tightly, controlled by mechanisms that involve protein phosphorylation and reorganization of the actomyosin cytoskeleton. Phosphorylation of VE-cadherin, and its associated-catenins, results in dissociation of the VE-cadherin/catenin complex and internalization of VE-cadherin, leading to increased vascular permeability. Furthermore, reorganization of the actomyosin cytoskeleton by Rap1, a small GTPase that belongs to the Ras subfamily, and Rho family small GTPases, regulates VE-cadherin-mediated cell adhesions to control vascular permeability. In this review, we describe recent progress in understanding the signaling mechanisms that enable dynamic regulation of VE-cadherin adhesions and vascular permeability. In addition, we discuss the possibility of novel therapeutic approaches targeting the signaling pathways controlling VE-cadherin-mediated cell adhesion in diseases associated with vascular hyper-permeability.

Lipoic Acid Stimulates cAMP Production in Healthy Control and Secondary Progressive MS Subjects

Lipoic acid (LA) exhibits antioxidant and anti-inflammatory properties; supplementation reduces disease severity and T lymphocyte migration into the central nervous system in a murine model of multiple sclerosis (MS), and administration in secondary progressive MS (SPMS) subjects reduces brain atrophy compared to placebo. The mechanism of action (MOA) of LA's efficacy in suppression of MS pathology is incompletely understood. LA stimulates production of the immunomodulator cyclic AMP (cAMP) in vitro. To determine whether cAMP could be involved in the MOA of LA in vivo, we performed a clinical trial to examine whether LA stimulates cAMP production in healthy control and MS subjects, and whether there are differences in the bioavailability of LA between groups. We administered 1200 mg of oral LA to healthy control, relapsing remitting MS (RRMS) and SPMS subjects, and measured plasma LA and cAMP levels in peripheral blood mononuclear cells (PBMCs). There were no significant differences between the groups in pharmacokinetic (PK) parameters. Healthy and SPMS subjects had increased cAMP at 2 and 4 h post-LA treatment compared to baseline, while RRMS subjects showed decreases in cAMP. Additionally, plasma concentrations of prostaglandin E2 (PGE₂, a known cAMP stimulator) were significantly lower in female RRMS subjects compared to female HC and SPMS subjects 4 h after LA ingestion. These data indicate that cAMP could be part of the MOA of LA in SPMS, and that there is a divergent response to LA in RRMS subjects that may have implications in the efficacy of immunomodulatory drugs. This clinical trial, "Defining the Anti-inflammatory Role of Lipoic Acid in Multiple Sclerosis," NCT00997438, is registered at https://clinicaltrials.gov/ct2/show/record/NCT00997438 .

CCM2 and PAK4 act downstream of atrial natriuretic peptide signaling to promote cell spreading

Atrial natriuretic peptide (ANP) is a cardiac hormone released by the atrium in response to stretching forces. Via its receptor, guanylyl cyclase-A (GC-A), ANP maintains cardiovascular homeostasis by exerting diuretic, natriuretic, and hypotensive effects mediated, in part, by endothelial cells. Both in vivo and in vitro, ANP enhances endothelial barrier function by reducing RhoA activity and reorganizing the actin cytoskeleton. We established mouse endothelial cells that stably express GC-A and used them to analyze the molecular mechanisms responsible for actin reorganization. Stimulation by ANP resulted in phosphorylation of myosin light chain (MLC) and promotion of cell spreading. p21-activated kinase 4 (PAK4) and cerebral cavernous malformations 2 (CCM2), a scaffold protein involved in a cerebrovascular disease, were required for the phosphorylation of MLC and promotion of cell spreading by ANP. Finally, in addition to the GC domain, the kinase homology domain of GC-A was also required for ANP/GC-A signaling. Our results indicate that CCM2 and PAK4 are important downstream mediators of ANP/GC-A signaling involved in cell spreading, an important initial step in the enhancement of endothelial barrier function.

Real-time quantification of endothelial response to shear stress and vascular modulators

Quiescence is commonly used to describe the inactive state of endothelial cells (ECs) in monolayers that have reached homeostasis. Experimentally quiescence is usually described in terms of the relative change in cell activity (e.g. turnover, speed, etc.) in response to a perturbation (e.g. solute, shear stress, etc.). The objective of this study is to provide new insight into EC quiescence by quantitatively defining the morphology and activity of confluent cell monolayers in response to shear stress and vascular modulators. Confluent monolayers of human umbilical vein ECs (HUVECs) were subjected to a range of shear stresses (4-16 dyne cm-2) under steady flow. Using phase contrast, time-lapse microscopy and image analysis, we quantified EC morphology, speed, proliferation, and apoptosis rates over time and detected differences in monolayer responses under various media conditions: basal media supplemented with growth factors, interleukin-8, or cyclic AMP. In all conditions, we observed a transition from cobblestone to spindle-like morphology in a dose-dependent manner due to shear stress. Cyclic AMP enhanced the elongation and alignment of HUVECs due to shear stress and reduced steady state cell speed. We observed the lowest proliferation rates below 8 dyne cm-2 and found that growth factors and cyclic AMP reduced proliferation and apoptosis; interleukin-8 similarly decreased proliferation, but increased apoptosis. We have quantified the response of ECs in confluent monolayers to shear stress and vascular modulators in terms of morphology, speed, proliferation and apoptosis and have established quantifiable metrics of cell activity to define vascular quiescence under shear stress.

A Facile Method to Probe the Vascular Permeability of Nanoparticles in Nanomedicine Applications

The effectiveness of nanoparticles (NP) in nanomedicine depends on their ability to extravasate from vasculature towards the target tissue. This is determined by their permeability across the endothelial barrier. Unfortunately, a quantitative study of the diffusion permeability coefficients (P_d) of NPs is difficult with in vivo models. Here, we utilize a relevant model of vascular-tissue interface with tunable endothelial permeability in vitro based on microfluidics. Human umbilical vein endothelial cells (HUVECs) grown in microfluidic devices were treated with Angiopoietin 1 and cyclic adenosine monophosphate (cAMP) to vary the P_d of the HUVECs monolayer towards fluorescent polystyrene NPs (pNPs) of different sizes, which was determined from image analysis of their fluorescence intensity when diffusing across the monolayer. Using 70 kDa dextran as a probe, untreated HUVECs yielded a P_d that approximated tumor vasculature while HUVECs treated with 25 μg/mL cAMP had P_d that approximated healthy vasculature in vivo. As the size of pNPs increased, its P_d decreased in tumor vasculature, but remained largely unchanged in healthy vasculature, demonstrating a trend similar to tumor selectivity for smaller NPs. This microfluidic model of vascular-tissue interface can be used in any laboratory to perform quantitative assessment of the tumor selectivity of nanomedicine-based systems.

Ascorbic acid prevents VEGF-induced increases in endothelial barrier permeability

Vascular endothelial growth factor (VEGF) increases endothelial barrier permeability, an effect that may contribute to macular edema in diabetic retinopathy. Since vitamin C, or ascorbic acid, can tighten the endothelial permeability barrier, we examined whether it could prevent the increase in permeability due to VEGF in human umbilical vein endothelial cells (HUVECs). As previously observed, VEGF increased HUVEC permeability to radiolabeled inulin within 60 min in a concentration-dependent manner. Loading the cells with increasing concentrations of ascorbate progressively prevented the leakage caused by 100 ng/ml VEGF, with a significant inhibition at 13 µM and complete inhibition at 50 µM. Loading cells with 100 µM ascorbate also decreased the basal generation of reactive oxygen species and prevented the increase caused by both 100 ng/ml VEGF. VEGF treatment decreased intracellular ascorbate by 25%, thus linking ascorbate oxidation to its prevention of VEGF-induced barrier leakage. The latter was blocked by treating the cells with 60 µM L-NAME (but not D-NAME) as well as by 30 µM sepiapterin, a precursor of tetrahydrobiopterin that is required for proper function of endothelial nitric oxide synthase (eNOS). These findings suggest that VEGF-induced barrier leakage uncouples eNOS. Ascorbate inhibition of the VEGF effect could thus be due either to scavenging superoxide or to peroxynitrite generated by the uncoupled eNOS, or more likely to its ability to recycle tetrahydrobiopterin, thus avoiding enzyme uncoupling in the first place. Ascorbate prevention of VEGF-induced increases in endothelial permeability opens the possibility that its repletion could benefit diabetic macular edema.

Reinforcing endothelial junctions prevents microvessel permeability increase and tumor cell adhesion in microvessels in vivo

Tumor cell adhesion to the microvessel wall is a critical step during tumor metastasis. Vascular endothelial growth factor (VEGF), a secretion of tumor cells, can increase microvessel permeability and tumor cell adhesion in the microvessel. To test the hypothesis that inhibiting permeability increase can reduce tumor cell adhesion, we used in vivo fluorescence microscopy to measure both microvessel permeability and adhesion rates of human mammary carcinoma MDA-MB-231 cells in post-capillary venules of rat mesentery under the treatment of VEGF and a cAMP analog, 8-bromo-cAMP, which can decrease microvessel permeability. By immunostaining adherens junction proteins between endothelial cells forming the microvessel wall, we further investigated the structural mechanism by which cAMP abolishes VEGF-induced increase in microvessel permeability and tumor cell adhesion. Our results demonstrate that 1) Pretreatment of microvessels with cAMP can abolish VEGF-enhanced microvessel permeability and tumor cell adhesion; 2) Tumor cells prefer to adhere to the endothelial cell junctions instead of cell bodies; 3) VEGF increases microvessel permeability and tumor cell adhesion by compromising endothelial junctions while cAMP abolishes these effects of VEGF by reinforcing the junctions. These results suggest that strengthening the microvessel wall integrity can be a potential approach to inhibiting hematogenous tumor metastasis.

LPP3 localizes LPA6 signalling to non-contact sites in endothelial cells

Lysophosphatidic acid (LPA) is emerging as an angiogenic factor, because knockdown of the enzyme that produces it (autotaxin, also known as ENPP2) and its receptors cause severe developmental vascular defects in both mice and fish. In addition, overexpression of autotaxin in mice causes similar vascular defects, indicating that the extracellular amount of LPA must be tightly regulated. Here, we focused on an LPA-degrading enzyme, lipid phosphate phosphatase 3 (LPP3, also known as PPAP2B), and showed that LPP3 was localized in specific cell–cell contact sites of endothelial cells and suppresses LPA signalling through the LPA₆ receptor (also known as LPAR6). In HEK293 cells, overexpression of LPP3 dramatically suppressed activation of LPA₆. In human umbilical vein endothelial cells (HUVECs), LPA induced actin stress fibre formation through LPA₆, which was substantially upregulated by LPP3 knockdown. LPP3 was localized to cell–cell contact sites and was missing in non-contact sites to which LPA-induced actin stress fibre formation mediated by LPA₆ was restricted. Interestingly, the expression of LPP3 in HUVECs was dramatically increased after forskolin treatment in a process involving Notch signalling. These results indicate that LPP3 regulates and localizes LPA signalling in endothelial cells, thereby stabilizing vessels through Notch signalling for proper vasculature.

Summary: In endothelial cells, the membrane-bound LPA-degrading enzyme LPP3 is specifically expressed at cell–cell contact sites, thereby localizing the signal evoked by extracellularly produced LPA.

Interactions between Epac1 and ezrin in the control of endothelial barrier function

Loss of barrier function in the vasculature promotes inflammatory signalling which in turn contributes to the progression of cardiovascular disease. cAMP can protect against endothelial dysfunction through the effectors PKA (protein kinase A) and Epac (exchange protein directly activated by cAMP). The present review outlines the role of Epac1 signalling within the endothelium and, in particular, the role of Epac1 in cytoskeletal dynamics and the control of cell morphology. The actin/cytoskeleton linker ezrin will be described in terms of the growing body of evidence placing it downstream of cAMP signalling as a mediator of altered cellular morphology.

Metabolic regulation of intestinal epithelial barrier during inflammation

The gastrointestinal mucosa has proven to be an interesting tissue for which to investigate disease-related metabolism. In this review, we outline some evidence that implicates metabolic signaling as important features of barrier in the healthy and disease. Studies from cultured cell systems, animal models and human patients have revealed that metabolites generated within the inflammatory microenvironment are central to barrier regulation. These studies have revealed a prominent role for hypoxia and hypoxia-inducible factor (HIF) at key steps in adenine nucleotide metabolism and within the creatine kinase pathway. Results from animal models of intestinal inflammation have demonstrated an almost uniformly beneficial influence of HIF stabilization on disease outcomes and barrier function. Studies underway to elucidate the contribution of immune responses will provide additional insight into how metabolic changes contribute to the complexity of the gastrointestinal tract and how such information might be harnessed for therapeutic benefit.

PACAP enhances barrier properties of cerebral microvessels

Cerebral microvascular endothelial cells-coming in contact with pericytes and astrocytes-constitute the structural basis of the blood-brain barrier (BBB). The continuous belt of interendothelial tight junctions (TJs) and the presence of specific transport systems, enzymes, and receptors in the brain endothelium regulate the molecular and cellular traffic into the central nervous system. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide having several cellular protective effects. However, little is known about the effects of PACAP on the cerebral endothelium and BBB functions. Here, we show that PACAP has no significant pro-survival role in cerebral microvascular endothelial cells; however, it improves the barrier properties of the brain endothelium. PACAP induces an increase in the transendothelial electrical resistance, which is the most important marker of the tightness of the TJs. Moreover, PACAP has a protective role against glucose deprivation- and oxidative stress-induced junctional damage in microvascular brain endothelial cells.

Reactive oxygen species in inflammation and tissue injury

Abstract Reactive oxygen species (ROS) are key signaling molecules that play an important role in the progression of inflammatory disorders. An enhanced ROS generation by polymorphonuclear neutrophils (PMNs) at the site of inflammation causes endothelial dysfunction and tissue injury. The vascular endothelium plays an important role in passage of macromolecules and inflammatory cells from the blood to tissue. Under the inflammatory conditions, oxidative stress produced by PMNs leads to the opening of inter-endothelial junctions and promotes the migration of inflammatory cells across the endothelial barrier. The migrated inflammatory cells not only help in the clearance of pathogens and foreign particles but also lead to tissue injury. The current review compiles the past and current research in the area of inflammation with particular emphasis on oxidative stress-mediated signaling mechanisms that are involved in inflammation and tissue injury.

Cilostazol prevents retinal ischemic damage partly via inhibition of tumor necrosis factor-α-induced nuclear factor-kappa B/activator protein-1 signaling pathway

Cilostazol is a specific inhibitor of phosphodiesterase III and is widely used to treat ischemic symptoms of peripheral vascular disease. We evaluated the protective effects of cilostazol in a murine model of ocular ischemic syndrome in which retinal ischemia was induced by 5-h unilateral ligation of both the pterygopalatine artery (PPA) and the external carotid artery (ECA) in anesthetized mice. The effects of cilostazol (30 mg/kg, p.o.) on ischemia/reperfusion (I/R)-induced retinal damage were examined by histological, retinal vascular permeability, and electrophysiological analyses. Using immunoblotting, the protective mechanism for cilostazol was evaluated by examining antiinflammatory effects of cilostazol on the expression of tumor necrosis factors-α (TNF-α) and tight junction proteins (ZO-1 and claudin-5), and the phosphorylations of nuclear factor-kappa B (NF-κB) and c-Jun. The histological analysis revealed that I/R decreased the cell number in the ganglion cell layer (GCL) and the thicknesses of the inner plexiform layer (IPL) and inner nuclear layer (INL), and that cilostazol attenuated these decreases. Additionally, cilostazol prevented the hyperpermeability of blood vessels. Electroretinogram (ERG) measurements revealed that cilostazol prevented the I/R-induced reductions in a-, b-, and oscillatory potential (OP) wave amplitudes seen at 5 days after I/R. Cilostazol inhibited the increased expression of TNF-α and the phosphorylation levels of NF-κB and c-Jun in the retina after I/R. In addition, cilostazol prevented TNF-α-induced reduction of ZO-1 and claudin-5 expression in human retinal microvascular endothelial cells (HRMECs). These findings indicate that cilostazol may prevent I/R-induced retinal damage partly through inhibition of TNF-α-induced NF-κB/AP-1 signaling pathway.

Pseudomonas aeruginosa exotoxin Y-mediated tau hyperphosphorylation impairs microtubule assembly in pulmonary microvascular endothelial cells

Pseudomonas aeruginosa uses a type III secretion system to introduce the adenylyl and guanylyl cyclase exotoxin Y (ExoY) into the cytoplasm of endothelial cells. ExoY induces Tau hyperphosphorylation and insolubility, microtubule breakdown, barrier disruption and edema, although the mechanism(s) responsible for microtubule breakdown remain poorly understood. Here we investigated both microtubule behavior and centrosome activity to test the hypothesis that ExoY disrupts microtubule dynamics. Fluorescence microscopy determined that infected pulmonary microvascular endothelial cells contained fewer microtubules than control cells, and further studies demonstrated that the microtubule-associated protein Tau was hyperphosphorylated following infection and dissociated from microtubules. Disassembly/reassembly studies determined that microtubule assembly was disrupted in infected cells, with no detectable effects on either microtubule disassembly or microtubule nucleation by centrosomes. This effect of ExoY on microtubules was abolished when the cAMP-dependent kinase phosphorylation site (Ser-214) on Tau was mutated to a non-phosphorylatable form. These studies identify Tau in microvascular endothelial cells as the target of ExoY in control of microtubule architecture following pulmonary infection by Pseudomonas aeruginosa and demonstrate that phosphorylation of tau following infection decreases microtubule assembly.

Bicarbonate disruption of the pulmonary endothelial barrier via activation of endogenous soluble adenylyl cyclase, isoform 10

It is becoming increasingly apparent that cAMP signals within the pulmonary endothelium are highly compartmentalized, and this compartmentalization is critical to maintaining endothelial barrier integrity. Studies demonstrate that the exogenous soluble bacterial toxin, ExoY, and heterologous expression of the forskolin-stimulated soluble mammalian adenylyl cyclase (AC) chimera, sACI/II, elevate cytosolic cAMP and disrupt the pulmonary microvascular endothelial barrier. The barrier-disruptive effects of cytosolic cAMP generated by exogenous soluble ACs are in contrast to the barrier-protective effects of subplasma membrane cAMP generated by transmembrane AC, which strengthens endothelial barrier integrity. Endogenous soluble AC isoform 10 (AC10 or commonly known as sAC) lacks transmembrane domains and localizes within the cytosolic compartment. AC10 is uniquely activated by bicarbonate to generate cytosolic cAMP, yet its role in regulation of endothelial barrier integrity has not been addressed. Here we demonstrate that, within the pulmonary circulation, AC10 is expressed in pulmonary microvascular endothelial cells (PMVECs) and pulmonary artery endothelial cells (PAECs), yet expression in PAECs is lower. Furthermore, pulmonary endothelial cells selectively express bicarbonate cotransporters. While extracellular bicarbonate generates a phosphodiesterase 4-sensitive cAMP pool in PMVECs, no such cAMP response is detected in PAECs. Finally, addition of extracellular bicarbonate decreases resistance across the PMVEC monolayer and increases the filtration coefficient in the isolated perfused lung above osmolality controls. Collectively, these findings suggest that PMVECs have a bicarbonate-sensitive cytosolic cAMP pool that disrupts endothelial barrier integrity. These studies could provide an alternative mechanism for the controversial effects of bicarbonate correction of acidosis of acute respiratory distress syndrome patients.

Neural Wiskott-Aldrich syndrome protein (N-WASP)-mediated p120-catenin interaction with Arp2-Actin complex stabilizes endothelial adherens junctions

Stable adherens junctions (AJs) are required for formation of restrictive endothelial barrier. Vascular endothelial cadherin from contiguous endothelial cells forms AJs, which are stabilized intracellularly by binding of p120-catenin and cortical actin. Mechanisms inducing cortical actin formation and enabling its linkage with p120-catenin remain enigmatic. We altered the function of neural Wiskott-Aldrich syndrome protein (N-WASP), which induces actin polymerization through actin-related protein 2/3 complex (Arp2/3), to address the role of N-WASP in regulating AJ stability and thereby endothelial permeability. We show that depletion of N-WASP in endothelial cells impaired AJ adhesion and favored the organization of actin from cortical actin to stress fibers, resulting thereby in formation of leaky endothelial barrier. Exposure of the N-WASP-depleted endothelial cell monolayer to the permeability-increasing mediator, thrombin, exaggerated AJ disruption and stress fiber formation, leading to an irreversible increase in endothelial permeability. We show that N-WASP binds p120-catenin through its verprolin cofilin acid (VCA) domain, induces cortical actin formation through Arp2, and links p120-catenin with cortical actin. The interaction of N-WASP with p120-catenin, actin, and Arp2 requires phosphorylation of N-WASP at the Tyr-256 residue by focal adhesion kinase. Expression of the VCA domain of N-WASP or phosphomimicking (Y256D)-N-WASP mutant in endothelial cells stabilizes AJs and facilitates barrier recovery after thrombin stimulation. Our study demonstrates that N-WASP, by mediating p120-catenin interaction with actin-polymerizing machinery, maintains AJs and mitigates disruption of endothelial barrier function by edemagenic agents, therefore representing a novel target for preventing leaky endothelial barrier syndrome.

Pseudomonas aeruginosa exotoxin Y is a promiscuous cyclase that increases endothelial tau phosphorylation and permeability

Exotoxin Y (ExoY) is a type III secretion system effector found in ~ 90% of the Pseudomonas aeruginosa isolates. Although it is known that ExoY causes inter-endothelial gaps and vascular leak, the mechanisms by which this occurs are poorly understood. Using both a bacteria-delivered and a codon-optimized conditionally expressed ExoY, we report that this toxin is a dual soluble adenylyl and guanylyl cyclase that results in intracellular cAMP and cGMP accumulation. The enzymatic activity of ExoY caused phosphorylation of endothelial Tau serine 214, accumulation of insoluble Tau, inter-endothelial cell gap formation, and increased macromolecular permeability. To discern whether the cAMP or cGMP signal was responsible for Tau phosphorylation and barrier disruption, pulmonary microvascular endothelial cells were engineered for the conditional expression of either wild-type guanylyl cyclase, which synthesizes cGMP, or a mutated guanylyl cyclase, which synthesizes cAMP. Sodium nitroprusside stimulation of the cGMP-generating cyclase resulted in transient Tau serine 214 phosphorylation and gap formation, whereas stimulation of the cAMP-generating cyclase induced a robust increase in Tau serine 214 phosphorylation, gap formation, and macromolecular permeability. These results indicate that the cAMP signal is the dominant stimulus for Tau phosphorylation. Hence, ExoY is a promiscuous cyclase and edema factor that uses cAMP and, to some extent, cGMP to induce the hyperphosphorylation and insolubility of endothelial Tau. Because hyperphosphorylated and insoluble Tau are hallmarks in neurodegenerative tauopathies such as Alzheimer disease, acute Pseudomonas infections cause a pathophysiological sequela in endothelium previously recognized only in chronic neurodegenerative diseases.

The effect of β-2 adrenoreceptor agonist inhalation on lungs donated after cardiac death in a canine lung transplantation model

BACKGROUND

It is a matter of great importance in a donation after cardiac death to attenuate ischemia-reperfusion injury (IRI) related to the inevitable warm ischemic time.

METHODS

Donor dogs were rendered cardiac-dead and left at room temperature. The dogs were allocated into 2 groups: the β-2 group (n = 5) received an aerosolized β-2 adrenoreceptor agonist (procaterol, 350 μg) and ventilation with 100% oxygen for 60 minutes starting at 240 minutes after cardiac arrest, and the control group (n = 6) received an aerosolized control solvent with the ventilation. Lungs were recovered 300 minutes after cardiac arrest. Recipient dogs underwent left single-lung transplantation to evaluate the functions of the left transplanted lung for 240 minutes after the reperfusion.

RESULTS

Oxygenation and dynamic compliance were significantly higher in the β-2 group than in the control group. The β-2 group revealed significantly higher levels of cyclic adenosine monophosphate and high-energy phosphates in the donor lung after the inhalation than before it. Histologic findings revealed that the β-2 group had less edema and fewer inflammatory cells.

CONCLUSION

Our results suggest that β-2 adrenoreceptor agonist inhalation during the pre-procurement period may ameliorate IRI.

Attenuation by sphingosine-1-phosphate of rat microvessel acute permeability response to bradykinin is rapidly reversible

To evaluate the hypothesis that sphingosine-1-phosphate (S1P) and cAMP attenuate increased permeability of individually perfused mesenteric microvessels through a common Rac1-dependent pathway, we measured the attenuation of the peak hydraulic conductivity (L(p)) in response to the inflammatory agent bradykinin (BK) by either S1P or cAMP. We varied the extent of exposure to each agent (test) and measured the ratio L(p)(test)/L(p)(BK alone) for each vessel (anesthetized rats). S1P (1 μM) added at the same time as BK (concurrent, no pretreatment) was as effective to attenuate the response to BK (L(p) ratio: 0.14 ± 0.05; n = 5) as concurrent plus pretreatment with S1P for 30 min (L(p) ratio: 0.26 ± 0.06; n = 11). The same pretreatment with S1P, but with no concurrent S1P, caused no inhibition of the BK response (L(p) ratio 1.07 ± 0.11; n = 8). The rapid on and off action of S1P demonstrated by these results was in contrast to cAMP-dependent changes induced by rolipram and forskolin (RF), which developed more slowly, lasted longer, and resulted in partial inhibition when given either as pretreatment or concurrent with BK. In cultured endothelium, there was no Rac activation or peripheral cortactin localization at 1 min with RF, but cortactin localization and Rac activation were maximal at 1 min with S1P. When S1P was removed, Rac activation returned to control within 2 min. Because of such differing time courses, S1P and cAMP are unlikely to act through fully common effector mechanisms.

Targeting Hypoxia to Augment Mucosal Barrier Function

Sites of inflammation are associated with profound changes in tissue metabolism. Studies in vitro and in vivo have shown that the activation of the hypoxia-inducible factor (HIF) serves as an adaptive pathway for the resolution of inflammation associated with various murine disease models. The resolution of disease occurs, at least in part, through transcriptional regulation of non-classical epithelial barrier genes. There is significant recent interest in harnessing hypoxia-inducible pathways, including targeting the HIF and the proyl-hydroxylase (PHD) enzymes that stabilize HIF, to promote mucosal healing. Here, we review the signaling pathways involved and define how hypoxia-associated signaling provides mechanistic insight into augmenting barrier function in mucosal inflammatory disease.

Epac1 and PDZ-GEF cooperate in Rap1 mediated endothelial junction control

Epac1 and its effector Rap1 are important mediators of cAMP induced tightening of endothelial junctions and consequential increased barrier function. We have investigated the involvement of Rap1 signalling in basal, unstimulated, barrier function of a confluent monolayer of HUVEC using real time Electric Cell-substrate Impedance Sensing. Depletion of Rap1, but not Epac1, results in a strong decrease in barrier function. This decrease is also observed when cells are depleted of the cAMP independent Rap exchange factors PDZ-GEF1 and 2, showing that PDZ-GEFs are responsible for Rap1 activity in control of basal barrier function. Monolayers of cells depleted of PDZ-GEF or Rap1 show an irregular, zipper-like organization of VE-cadherin and live imaging of VE-cadherin-GFP reveals enhanced junction motility upon depletion of PDZ-GEF or Rap1. Importantly, activation of Epac1 increases the formation of cortical actin bundles at the cell-cell junctions, inhibits junction motility and restores barrier function of PDZ-GEFs depleted, but not Rap1 depleted cells. We conclude that PDZ-GEF activates Rap1 under resting conditions to stabilize cell-cell junctions and maintain basal integrity. Activation of Rap1 by cAMP/Epac1 induces junctional actin to further tighten cell-cell contacts.

Intermedin induces loss of coronary microvascular endothelial barrier via derangement of actin cytoskeleton: role of RhoA and Rac1

AIMS

Intermedin (IMD) is a novel member of the calcitonin gene-related peptide family, which acts via calcitonin receptor-like receptors (CLRs), mediating activation of cAMP signalling. The main objective of the present study was to analyse the molecular mechanisms of the differential effects of IMD on the macromolecule permeability of endothelial cells of different vascular beds.

METHODS AND RESULTS

Here we demonstrate that IMD increases permeability of rat coronary microvascular endothelial cells (RCECs) and reduces permeability of human umbilical vein endothelial cells (HUVECs) and rat aortic endothelial cells via CLRs and cAMP. Intermedin causes a derangement of the actin cytoskeleton accompanied by loss of vascular endothelial cadherin (VE-cadherin) in RCECs, while it causes a rearrangement of the actin cytoskeleton and VE-cadherin at cell-cell junctions in HUVECs. Intermedin inactivates the RhoA/Rho-kinase (Rock) pathway in both cell types; however, it inactivates Rac1 in RCECs but not in HUVECs. Inhibition and rescue experiments demonstrate that both RhoA and Rac1 are required for the RCEC barrier stability, while in HUVECs the inhibition of RhoA/Rock signalling does not interfere with basal permeability.

CONCLUSION

The opposite effects of IMD on permeability of RCECs and HUVECs are due to differential regulation of actin cytoskeleton dynamics via RhoA and Rac1. Moreover, Rac1 activity is regulated by the RhoA/Rock pathway in RCECs but not in HUVECs.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

CD73 has distinct roles in nonhematopoietic and hematopoietic cells to promote tumor growth in mice

CD73 is overexpressed in many types of human and mouse cancers and is implicated in the control of tumor progression. However, the specific contribution from tumor or host CD73 expression to tumor growth remains unknown to date. Here, we show that host CD73 promotes tumor growth in a T cell-dependent manner and that the optimal antitumor effect of CD73 blockade requires inhibiting both tumor and host CD73. Notably, enzymatic activity of CD73 on nonhematopoietic cells limited tumor-infiltrating T cells by controlling antitumor T cell homing to tumors in multiple mouse tumor models. In contrast, CD73 on hematopoietic cells (including CD4⁺CD25⁺ Tregs) inhibited systemic antitumor T cell expansion and effector functions. Thus, CD73 on hematopoietic and nonhematopoietic cells has distinct adenosinergic effects in regulating systemic and local antitumor T cell responses. Importantly, pharmacological blockade of CD73 using its selective inhibitor or an anti-CD73 mAb inhibited tumor growth and completely restored efficacy of adoptive T cell therapy in mice. These findings suggest that both tumor and host CD73 cooperatively protect tumors from incoming antitumor T cells and show the potential of targeting CD73 as a cancer immunotherapy strategy.

Emerging themes of cAMP regulation of the pulmonary endothelial barrier

The presence of excess fluid in the interstitium and air spaces of the lung presents severe restrictions to gas exchange. The pulmonary endothelial barrier regulates the flux of fluid and plasma proteins from the vascular space into the underlying tissue. The integrity of this endothelial barrier is dynamically regulated by transitions in cAMP (3',5'-cyclic adenosine monophosphate), which are synthesized in discrete subcellular compartments. Cyclic AMP generated in the subplasma membrane compartment acts through PKA and Epac (exchange protein directly activated by cAMP) to tighten cell adhesions, strengthen cortical actin, reduce actomyosin contraction, and decrease permeability. Confining cAMP within the subplasma membrane space is critical to its barrier-protective properties. When cAMP escapes the near membrane compartment and gains access to the cytosolic compartment, or when soluble adenylyl cyclases generate cAMP within the cytosolic compartment, this second messenger activates established cytosolic cAMP signaling cascades to perturb the endothelial barrier through PKA-mediated disruption of microtubules. Thus the concept of cAMP compartmentalization in endothelial barrier regulation is gaining momentum and new possibilities are being unveiled for cytosolic cAMP signaling with the emergence of the bicarbonate-regulated mammalian soluble adenylyl cyclase (sAC or AC10).

Phosphodiesterase-III inhibitor prevents hemorrhagic transformation induced by focal cerebral ischemia in mice treated with tPA

The purpose of the present study was to investigate whether cilostazol, a phosphodiesterase-III inhibitor and antiplatelet drug, would prevent tPA-associated hemorrhagic transformation. Mice subjected to 6-h middle cerebral artery occlusion were treated with delayed tPA alone at 6 h, with combined tPA plus cilostazol at 6 h, or with vehicle at 6 h. We used multiple imaging (electron microscopy, spectroscopy), histological and neurobehavioral measures to assess the effects of the treatment at 18 h and 7 days after the reperfusion. To further investigate the mechanism of cilostazol to beneficial effect, we also performed an in vitro study with tPA and a phosphodiesterase-III inhibitor in human brain microvascular endothelial cells, pericytes, and astrocytes. Combination therapy with tPA plus cilostazol prevented development of hemorrhagic transformation, reduced brain edema, prevented endothelial injury via reduction MMP-9 activity, and prevented the blood-brain barrier opening by inhibiting decreased claudin-5 expression. These changes significantly reduced the morbidity and mortality at 18 h and 7 days after the reperfusion. Also, the administration of both drugs prevented injury to brain human endothelial cells and human brain pericytes. The present study indicates that a phosphodiesterase-III inhibitor prevents the hemorrhagic transformation induced by focal cerebral ischemia in mice treated with tPA.

Phosphodiesterase 4 inhibition attenuates atrial natriuretic peptide-induced vascular hyperpermeability and loss of plasma volume

Inhibition of phosphodiesterase 4 (PDE4) to increase endothelial cAMP and stabilize the endothelial barrier attenuates acute inflammatory increases in vascular permeability.We extended this approach to attenuate physiological increases in vascular permeability in response to atrial natriuretic peptide (ANP), which acts with the kidney to regulate plasma volume. We measured blood-to-tissue albumin clearance and changes in plasma volume in isoflurane-anaesthetized mice (C57BL/6J) pre-treated with rolipram (8 mg kg(-1) I.P., 30 min). Rolipram significantly reduced albumin permeability, measured using a dual-label fluorescence method, in skin and skeletal muscle compared with ANP alone (500 ng kg(-1) min(-1)). Skin and muscle tissue accounted for 70% of the reduction in whole body albumin clearance taking into account albumin clearance in gastrointestinal (GI) tissue, heart and kidney. The action of ANP and rolipram to modify albumin clearances in duodenum and jejunum could be accounted for by local increases in vascular perfusion to increase surface area for exchange. ANP increased haematocrit from 40.6% to 46.8%, corresponding to an average loss of 22% plasma fluid volume (227 μl), and this was almost completely reversed with rolipram. Renal water excretion accounted for less than 30% of plasma fluid loss indicating that reduced albumin permeability and reduced filtration into vasodilated GI tissue were the predominant actions of PDE4 inhibition. Similar fluid retention was measured in mice with endothelial-restricted deletion of the guanylyl cyclase-A receptor for ANP. Stabilizing the endothelial barrier to offset ANP-induced increases in vascular permeability may be part of a strategy to maintain plasma volume.

Temporally resolved cAMP monitoring in endothelial cells uncovers a thrombin-induced [cAMP] elevation mediated via the Ca²+-dependent production of prostacyclin

The barrier function of the endothelium is controlled by the second messengers Ca2+ and cAMP that differentially regulate the permeability of endothelial cells. The Ca2+-elevating agent thrombin has been demonstrated to increase endothelial permeability and to decrease cAMP levels as detected via enzyme immunoassays. To study the effects of thrombin on cAMP with high temporal resolution, we utilised the FRET-based cAMP sensor Epac1-camps in single intact human umbilical vein endothelial cells (HUVECs). In these cells, thrombin induced a delayed increase in [cAMP], initiating after about 40 s, with maximum cAMP levels after 130 s of thrombin application. This increase of cAMP levels was Ca2+-dependent, but did not require calmodulin (CaM). Pharmacological approaches revealed that phospholipase A2 (PLA2) activity and cyclooxygenase (COX)-mediated synthesis of prostaglandins was required for the thrombin-induced elevation of [cAMP]. Furthermore, preincubation of HUVECs with a prostacyclin-receptor antagonist significantly reduced the thrombin-induced increase in [cAMP]. We conclude that thrombin causes the synthesis of prostacyclin in endothelial cells and that the subsequent stimulation of Gs-coupled prostacyclin receptors then results in an increase in [cAMP].

Caffeine protects against disruptions of the blood-brain barrier in animal models of Alzheimer's and Parkinson's diseases

Sporadic Alzheimer's disease (AD) and Parkinson's disease (PD) are two of the most common neurodegenerative diseases and as such they represent major public health problems. Finding effective treatments for AD and PD represents an unmet and elusive goal largely because these diseases are chronic and progressive, and have a complicated and ill-understood pathogenesis. Although the underlying mechanisms are not fully understood, caffeine, the most commonly ingested psychoactive drug in the world, has been shown in human and animal studies to be protective against AD and PD. One mechanism implicated in the pathogenesis of AD and PD is blood-brain barrier (BBB) dysfunction and we reported recently that caffeine exerts protective effects against AD and PD at least in part by keeping the BBB intact. The present review focuses on the role of BBB dysfunction in the pathogenesis of AD and PD, caffeine's protective effects against AD and PD, and potential mechanisms whereby caffeine protects against BBB leakage.

Protein kinase A phosphorylation of tau-serine 214 reorganizes microtubules and disrupts the endothelial cell barrier

Intracellular cAMP is compartmentalized to near membrane domains in endothelium, where it strengthens endothelial cell barrier function. Phosphodiesterase 4D4 (PDE4D4) interacts with the spectrin membrane skeleton and prevents cAMP from accessing microtubules. Expression of a dominant-negative PDE4D4 peptide enables cAMP to access microtubules, where it results in phosphorylation of the nonneuronal microtubule-associated protein tau at serine 214. Presently, we sought to determine whether PKA is responsible for tau-Ser214 phosphorylation and furthermore whether PKA phosphorylation of tau-Ser214 is sufficient to reorganize microtubules and induce endothelial cell gaps. In cells expressing the dominant-negative PDE4D4 peptide, forskolin activated transmembrane adenylyl cyclases, increased cAMP, and induced tau-Ser214 phosphorylation that was accompanied by microtubule reorganization. PKA catalytic and regulatory I subunits, but not the regulatory II subunit, coassociated with reorganized microtubules. To determine the functional consequence of tau-Ser214 phosphorylation, wild-type human tau40 and tau40 engineered to possess an alanine point mutation (S214A) were stably expressed in endothelium. In cells expressing the dominant-negative PDE4D4 peptide and tau-S214A, PKA-dependent phosphorylation of both the endogenous and heterologously expressed tau were abolished. Expression of tau-S214A prevented forskolin from depolymerizing microtubules, inducing intercellular gaps, and increasing macromolecular permeability. These findings therefore identify nonneuronal tau as a critical cAMP-responsive microtubule-associated protein that controls microtubule architecture and endothelial cell barrier function.

Sphingosine-1-phosphate modulation of basal permeability and acute inflammatory responses in rat venular microvessels

AIMS

Although several cultured endothelial cell studies indicate that sphingosine-1-phosphate (S1P), via GTPase Rac1 activation, enhances endothelial barriers, very few in situ studies have been published. We aimed to further investigate the mechanisms whereby S1P modulates both baseline and increased permeability in intact microvessels.

METHODS AND RESULTS

We measured attenuation by S1P of platelet-activating factor (PAF)- or bradykinin (Bk)-induced hydraulic conductivity (L(p)) increase in mesenteric microvessels of anaesthetized rats. S1P alone (1-5 µM) attenuated by 70% the acute L(p) increase due to PAF or Bk. Immunofluorescence methods in the same vessels under identical experimental conditions showed that Bk or PAF stimulated the loss of peripheral endothelial cortactin and rearrangement of VE-cadherin and occludin. Our results are the first to show in intact vessels that S1P pre-treatment inhibited rearrangement of VE-cadherin and occludin induced by PAF or Bk and preserved peripheral cortactin. S1P (1-5 µM, 30 min) did not increase baseline L(p). However, 10 µM S1P (60 min) increased L(p) two-fold.

CONCLUSION

Our results conform to the hypothesis that S1P inhibits acute permeability increase in association with enhanced stabilization of peripheral endothelial adhesion proteins. These results support the idea that S1P can be useful to attenuate inflammation by enhancing endothelial adhesion through activation of Rac-dependent pathways.

Molecular mechanisms involved in adenosine-induced endothelial cell barrier enhancement

Extracellular adenosine is a physiologically relevant agonist released by various sources, including endothelial cells (EC) and activated platelets, with complex effects mediated via activation of P1 purinergic receptors. Adenosine-induced EC production of glutathione peroxidase1 and nitric oxide is recognized, and an anti-inflammatory mechanism has been described. Effects of extracellular adenosine on the pulmonary EC barrier function and vascular permeability, however, remain poorly characterized. In this study, we demonstrated the adenosine-induced rapid dose-dependent barrier enhancement in human pulmonary artery EC (HPAEC) as measured by an increase in transendothelial electrical resistance (TER). We have shown that HPAEC express only A2A and A2B adenosine receptors. Pharmacological and siRNA depletion studies indicate that A2A, but not A2B receptor activation is required for the adenosine-induced TER increase. Depletion of Galphas with a specific siRNA significantly attenuated the adenosine-induced TER response in HPAEC. In contrast, depletion of either Galphaq or Galphai2 did not affect the adenosine-induced TER increase. This suggests that the adenosine-induced TER increase is cAMP-dependent. The adenosine-induced barrier enhancement effects were associated with a rearrangement of the EC F-actin component of the cytoskeleton, enhanced cell-surface presentation of cell-cell junctional protein VE-cadherin and an involvement of Myosin-light-chain phosphatase (MLCP). Our results suggest, for the first time, that adenosine regulates the EC barrier function via A2A receptors followed by Galphas engagement and is associated with cytoskeletal activation.

The role of cyclic AMP in normalizing the function of engineered human blood microvessels in microfluidic collagen gels

Nearly all engineered tissues must eventually be vascularized to survive. To this end, we and others have recently developed methods to synthesize extracellular matrix-based scaffolds that contain open microfluidic networks. These scaffolds serve as templates for the formation of endothelial tubes that can be perfused; whether such microvascular structures are stable and/or functional is largely unknown. Here, we show that compounds that elevate intracellular concentrations of the second messenger cyclic AMP (cAMP) strongly normalize the phenotype of engineered human microvessels in microfluidic type I collagen gels. Cyclic AMP-elevating agents promoted vascular stability and barrier function, and reduced cellular turnover. Under conditions that induced the highest levels of cAMP, the physiology of engineered microvessels in vitro quantitatively mirrored that of native vessels in vivo. Computational analysis indicated that cAMP stabilized vessels partly via its enhancement of barrier function.

cAMP/PKA antagonizes thrombin-induced inactivation of endothelial myosin light chain phosphatase: role of CPI-17

AIMS

Activation of cAMP signalling abrogates thrombin-induced hyperpermeability. One of the mechanisms underlying this protective effect is the inactivation of endothelial contractile machinery, one of the major determinants of endothelial barrier function, mainly via the activation of myosin light chain phosphatase (MLCP). To date, the mechanisms of cAMP-mediated MLCP activation are only partially understood. Here the contribution of two cAMP effectors, PKA and Epac, in the regulation of endothelial contractile machinery and barrier function was studied.

METHODS AND RESULTS

Endothelial contractile machinery and barrier function were analysed in cultured human umbilical vein endothelial cells (HUVEC). The cAMP analogues 8-CPT-cAMP and 6-Bnz-cAMP were used to activate Epac and PKA, respectively, and forskolin (FSK) was used to activate adenylyl cyclase. The cells were challenged by thrombin to inhibit MLCP via the RhoA/Rock pathway. Activation of either PKA or Epac partially blocked thrombin-induced hyperpermeability. Simultaneous activation of PKA and Epac had additive effects that were comparable to that of FSK. Activation of PKA but not Epac inhibited thrombin-induced phosphorylation of MLC and the MLCP regulatory subunit MYPT1, partly via inhibition of the RhoA/Rock pathway. FSK activated the MLCP catalytic subunit PP1 via dephosphorylation and dissociation of the PP1 inhibitory protein CPI-17. FSK blunted thrombin-induced CPI-17 phosphorylation, CPI-17/PP1 complex formation, and PP1 inactivation. Down-regulation of CPI-17 attenuated thrombin-induced hyperpermeability and abolished the antagonistic effect of the PKA activator, whereas the Epac activator retained its antagonistic effect.

CONCLUSION

cAMP/PKA regulates the endothelial barrier via inhibition of the contractile machinery, mainly by the activation of MLCP via inhibition of CPI-17 and RhoA/Rock. The permeability-lowering effect of the cAMP/Epac pathway is independent of CPI-17.

Endothelial-specific deletion of connexin40 promotes atherosclerosis by increasing CD73-dependent leukocyte adhesion

BACKGROUND

Endothelial dysfunction is the initiating event of atherosclerosis. The expression of connexin40 (Cx40), an endothelial gap junction protein, is decreased during atherogenesis. In the present report, we sought to determine whether Cx40 contributes to the development of the disease.

METHODS AND RESULTS

Mice with ubiquitous deletion of Cx40 are hypertensive, a risk factor for atherosclerosis. Consequently, we generated atherosclerosis-susceptible mice with endothelial-specific deletion of Cx40 (Cx40del mice). Cx40del mice were indeed not hypertensive. The progression of atherosclerosis was increased in Cx40del mice after 5 and 10 weeks of a high-cholesterol diet, and spontaneous lesions were observed in the aortic sinuses of young mice without such a diet. These lesions showed monocyte infiltration into the intima, increased expression of vascular cell adhesion molecule-1, and decreased expression of the ecto-enzyme CD73 in the endothelium. The proinflammatory phenotype of Cx40del mice was confirmed in another model of induced leukocyte recruitment from the lung microcirculation. Endothelial CD73 is known to induce antiadhesion signaling via the production of adenosine. We found that reducing Cx40 expression in vitro with small interfering RNA or antisense decreased CD73 expression and activity and increased leukocyte adhesion to mouse endothelial cells. These effects were reversed by an adenosine receptor agonist.

CONCLUSIONS

Cx40-mediated gap junctional communication contributes to a quiescent nonactivated endothelium by propagating adenosine-evoked antiinflammatory signals between endothelial cells. Alteration in this mechanism by targeting Cx40 promotes leukocyte adhesion to the endothelium, thus accelerating atherosclerosis.