Microvascular permeability and number of tight junctions are modulated by cAMP

Edema and lymphatic clearance: molecular mechanisms and ongoing challenges

Resolution of edema remains a significant clinical challenge. Conditions such as traumatic shock, sepsis, or diabetes often involve microvascular hyperpermeability, which leads to tissue and organ dysfunction. Lymphatic insufficiency due to genetic causes, surgical removal of lymph nodes, or infections, leads to varying degrees of tissue swelling that impair mobility and immune defenses. Treatment options are limited to management of edema as there are no specific therapeutics that have demonstrated significant success for ameliorating microvascular leakage or impaired lymphatic function. This review examines current knowledge about the physiological, cellular, and molecular mechanisms that control microvascular permeability and lymphatic clearance, the respective processes for interstitial fluid formation and removal. Clinical conditions featuring edema, along with potential future directions are discussed.

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.

Direct Current Stimulation Disrupts Endothelial Glycocalyx and Tight Junctions of the Blood-Brain Barrier in vitro

Transcranial direct current stimulation (tDCS) is a non-invasive physical therapy to treat many psychiatric disorders and to enhance memory and cognition in healthy individuals. Our recent studies showed that tDCS with the proper dosage and duration can transiently enhance the permeability (P) of the blood-brain barrier (BBB) in rat brain to various sized solutes. Based on the in vivo permeability data, a transport model for the paracellular pathway of the BBB also predicted that tDCS can transiently disrupt the endothelial glycocalyx (EG) and the tight junction between endothelial cells. To confirm these predictions and to investigate the structural mechanisms by which tDCS modulates P of the BBB, we directly quantified the EG and tight junctions of in vitro BBB models after DCS treatment. Human cerebral microvascular endothelial cells (hCMECs) and mouse brain microvascular endothelial cells (bEnd3) were cultured on the Transwell filter with 3 μm pores to generate in vitro BBBs. After confluence, 0.1–1 mA/cm² DCS was applied for 5 and 10 min. TEER and P to dextran-70k of the in vitro BBB were measured, HS (heparan sulfate) and hyaluronic acid (HA) of EG was immuno-stained and quantified, as well as the tight junction ZO-1. We found disrupted EG and ZO-1 when P to dextran-70k was increased and TEER was decreased by the DCS. To further investigate the cellular signaling mechanism of DCS on the BBB permeability, we pretreated the in vitro BBB with a nitric oxide synthase (NOS) inhibitor, L-NMMA. L-NMMA diminished the effect of DCS on the BBB permeability by protecting the EG and reinforcing tight junctions. These in vitro results conform to the in vivo observations and confirm the model prediction that DCS can disrupt the EG and tight junction of the BBB. Nevertheless, the in vivo effects of DCS are transient which backup its safety in the clinical application. In conclusion, our current study directly elucidates the structural and signaling mechanisms by which DCS modulates the BBB permeability.

cAMP Compartmentalization in Cerebrovascular Endothelial Cells: New Therapeutic Opportunities in Alzheimer's Disease

The vascular hypothesis used to explain the pathophysiology of Alzheimer’s disease (AD) suggests that a dysfunction of the cerebral microvasculature could be the beginning of alterations that ultimately leads to neuronal damage, and an abnormal increase of the blood–brain barrier (BBB) permeability plays a prominent role in this process. It is generally accepted that, in physiological conditions, cyclic AMP (cAMP) plays a key role in maintaining BBB permeability by regulating the formation of tight junctions between endothelial cells of the brain microvasculature. It is also known that intracellular cAMP signaling is highly compartmentalized into small nanodomains and localized cAMP changes are sufficient at modifying the permeability of the endothelial barrier. This spatial and temporal distribution is maintained by the enzymes involved in cAMP synthesis and degradation, by the location of its effectors, and by the existence of anchor proteins, as well as by buffers or different cytoplasm viscosities and intracellular structures limiting its diffusion. This review compiles current knowledge on the influence of cAMP compartmentalization on the endothelial barrier and, more specifically, on the BBB, laying the foundation for a new therapeutic approach in the treatment of AD.

A Tissue-Engineered 3D Microvessel Model Reveals the Dynamics of Mosaic Vessel Formation in Breast Cancer

In solid tumors, vascular structure and function varies from the core to the periphery. This structural heterogeneity has been proposed to influence the mechanisms by which tumor cells enter the circulation. Blood vessels exhibit regional defects in endothelial coverage, which can result in cancer cells directly exposed to flow and potentially promoting intravasation. Consistent with prior reports, we observed in human breast tumors and in a mouse model of breast cancer that approximately 6% of vessels consisted of both endothelial cells and tumor cells, so-called mosaic vessels. Due, in part, to the challenges associated with observing tumor-vessel interactions deep within tumors in real-time, the mechanisms by which mosaic vessels form remain incompletely understood. We developed a tissue-engineered model containing a physiologically realistic microvessel in coculture with mammary tumor organoids. This approach allows real-time and quantitative assessment of tumor-vessel interactions under conditions that recapitulate many in vivo features. Imaging revealed that tumor organoids integrate into the endothelial cell lining, resulting in mosaic vessels with gaps in the basement membrane. While mosaic vessel formation was the most frequently observed interaction, tumor organoids also actively constricted and displaced vessels. Furthermore, intravasation of cancer cell clusters was observed following the formation of a mosaic vessel. Taken together, our data reveal that cancer cells can rapidly reshape, destroy, or integrate into existing blood vessels, thereby affecting oxygenation, perfusion, and systemic dissemination. Our novel assay also enables future studies to identify targetable mechanisms of vascular recruitment and intravasation. SIGNIFICANCE: A tissue-engineered microdevice that recapitulates the tumor-vascular microenvironment enables real-time imaging of the cellular mechanisms of mosaic vessel formation and vascular defect generation.

Quantitatively relating brain endothelial cell-cell junction phenotype to global and local barrier properties under varied culture conditions via the Junction Analyzer Program

BACKGROUND

The endothelial cell-cell junctions of the blood-brain barrier (BBB) play a pivotal role in the barrier's function. Altered cell-cell junctions can lead to barrier dysfunction and have been implicated in several diseases. Despite this, the driving forces regulating junctional protein presentation remain relatively understudied, largely due to the lack of efficient techniques to quantify their presentation at sites of cell-cell adhesion. Here, we used our novel Junction Analyzer Program (JAnaP) to quantify junction phenotype (i.e., continuous, punctate, or perpendicular) in response to various substrate compositions, cell culture times, and cAMP treatments in human brain microvascular endothelial cells (HBMECs). We then quantitatively correlated junction presentation with barrier permeability on both a "global" and "local" scale.

METHODS

We cultured HBMECs on collagen I, fibronectin, collagen IV, laminin, fibronectin/collagen IV/laminin, or hyaluronic acid/gelatin for 2, 4, and 7 days with varying cAMP treatment schedules. Images of immunostained ZO-1, VE-cadherin, and claudin-5 were analyzed using the JAnaP to calculate the percent of the cell perimeter presenting continuous, punctate, or perpendicular junctions. Transwell permeability assays and resistance measurements were used to measure bulk ("global") barrier properties, and a "local" permeability assay was used to correlate junction presentation proximal to permeable monolayer regions.

RESULTS

Substrate composition was found to play little role in junction presentation, while cAMP supplements significantly increased the continuous junction architecture. Increased culture time required increased cAMP treatment time to reach similar ZO-1 and VE-cadherin coverage observed with shorter culture, though longer cultures were required for claudin-5 presentation. Prolonged cAMP treatment (6 days) disrupted junction integrity for all three junction proteins. Transwell permeability and TEER assays showed no correlation with junction phenotype, but a local permeability assay revealed a correlation between the number of discontinuous and no junction regions with barrier penetration.

CONCLUSIONS

These results suggest that cAMP signaling influences HBMEC junction architecture more than matrix composition. Our studies emphasized the need for local barrier measurement to mechanistically understand the role of junction phenotype and supported previous results that continuous junctions are indicative of a more mature/stable endothelial barrier. Understanding what conditions influence junction presentations, and how they, in turn, affect barrier integrity, could lead to the development of therapeutics for diseases associated with BBB dysfunction.

Enhanced paracellular delivery of vaccine by hydrogel microparticles-mediated reversible tight junction opening for effective oral immunization

The current conventional injectable vaccines face several drawbacks such as inconvenience and ineffectiveness in mucosal immunization. Therefore, the current development of effective oral vaccines is vital to enable the generation of dual systemic and mucosal immunity. In the present study, we examine the potential of pH-responsive bacterial nanocellulose/polyacrylic acid (BNC/PAA) hydrogel microparticles (MPs) as an oral vaccine carrier. In-vitro entrapment efficiency and release study of Ovalbumin (Ova) demonstrated that as high as 72% of Ova were entrapped in the hydrogel, and the release of loaded Ova was pH-dependent. The released Ova remained structurally conserved as evident by Western blot and circular dichroism. Hydrogel MPs reduced the TEER measurement of HT29MTX/Caco2/Raji B triple co-culture monolayer by reversibly opening the tight junctions (TJs) as shown in the TEM images. The ligated ileal loop assay revealed that hydrogel MPs could facilitate the penetration of FITC-Ova into the Peyer's patches in small intestine. Ova and cholera toxin B (CTB) were utilized in in-vivo oral immunization as model antigen and mucosal adjuvant. The in-vivo immunization revealed mice orally administered with Ova and CTB-loaded hydrogel MPs generated significantly higher level of serum anti-Ova IgG and mucosal anti-Ova IgA in the intestinal washes, compared to intramuscular administrated Ova. These results conclude that BNC/PAA hydrogel MPs is a potential oral vaccine carrier for effective oral immunization.

Activation of Notch signaling by soluble Dll4 decreases vascular permeability via a cAMP/PKA-dependent pathway

The Notch ligand delta-like ligand 4 (Dll4), upregulated by VEGF, is a key regulator of vessel morphogenesis and function, controlling tip and stalk cell selection during sprouting angiogenesis. Inhibition of Dll4 results in hypersprouting, nonfunctional, poorly perfused vessels, suggesting a role for Dll4 in the formation of mature, reactive, functional vessels, with low permeability and able to restrict fluid and solute exchange. We tested the hypothesis that Dll4 controls transvascular fluid exchange. A recombinant protein expressing only the extracellular portion of Dll4 [soluble Dll4 (sDll4)] induced Notch signaling in endothelial cells (ECs), resulting in increased expression of vascular-endothelial cadherin, but not the tight junctional protein zonula occludens 1, at intercellular junctions. sDll4 decreased the permeability of FITC-labeled albumin across EC monolayers, and this effect was abrogated by coculture with the γ-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester. One of the known molecular effectors responsible for strengthening EC-EC contacts is PKA, so we tested the effect of modulation of PKA on the sDll4-mediated reduction of permeability. Inhibition of PKA reversed the sDll4-mediated reduction in permeability and reduced expression of the Notch target gene Hey1. Knockdown of PKA reduced sDLL4-mediated vascular-endothelial cadherin junctional expression. sDll4 also caused a significant decrease in the hydraulic conductivity of rat mesenteric microvessels in vivo. This reduction was abolished upon coperfusion with the PKA inhibitor H89 dihydrochloride. These results indicate that Dll4 signaling through Notch activation acts through a cAMP/PKA pathway upon intercellular adherens junctions, but not tight junctions, to regulate endothelial barrier function. NEW & NOTEWORTHY Notch signaling reduces vascular permeability through stimulation of cAMP-dependent protein kinase A.

Neutrophils as sources of dinucleotide polyphosphates and metabolism by epithelial ENPP1 to influence barrier function via adenosine signaling

Extracellular adenosine signaling is established as a protective component in mucosal inflammatory responses. The sources of extracellular adenosine include enzymatic processing from nucleotides, such as ATP and AMP, that can be liberated from a variety of cell types, including infiltrating leukocytes. Here we demonstrate that activated human neutrophils are a source of diadenosine triphosphate (Ap3A), providing an additional source of nucleotides during inflammation. Profiling murine enteroids and intestinal epithelial cell lines revealed that intestinal epithelia prominently express apical and lateral ectonucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1), a member of the ENPP family of enzymes that metabolize diadenosine phosphates, especially Ap3A. Extensions of these studies demonstrated that intestinal epithelia metabolize Ap3A to ADP and AMP, which are further metabolized to adenosine and made available to activate surface adenosine receptors. Using loss and gain of ENPP1 approaches, we revealed that ENPP1 coordinates epithelial barrier formation and promotes epithelial wound healing responses. These studies demonstrate the cooperative metabolism between Ap3A and ENPP1 function to provide a significant source of adenosine, subserving its role in inflammatory resolution.

Right Heart-Pulmonary Circulation at High Altitude and the Development of Subclinical Pulmonary Interstitial Edema

Most healthy subjects can develop a subclinical interstitial pulmonary edema that is a complex and multifactor phenomenon, still with unanswered questions, and might be one line of defense against the development of severe symptomatic lung edema. Whether the acute, reversible increase in lung fluid content is really an innocent and benign part of the adaptation to extreme physiologic condition or rather the clinically relevant marker of an individual vulnerability to life-threatening high altitude pulmonary edema remains to be established in future studies. Thus the question if encouraging more conservative habits to climb is right or not remains open.

Effects of roflumilast, a phosphodiesterase-4 inhibitor, on the lung functions in a saline lavage-induced model of acute lung injury

Acute lung injury (ALI) is associated with deterioration of alveolar-capillary lining and transmigration and activation of inflammatory cells. Whereas a selective phosphodiesterase-4 (PDE4) inhibitor roflumilast has exerted potent anti-inflammatory properties, this study evaluated if its intravenous delivery can influence inflammation, edema formation, and respiratory parameters in rabbits with a lavage-induced model of ALI. ALI was induced by repetitive saline lung lavage (30 ml/kg). Animals were divided into 3 groups: ALI without therapy (ALI), ALI treated with roflumilast i.v. (1 mg/kg; ALI+Rofl), and healthy ventilated controls (Control), and were ventilated for following 4 h. Respiratory parameters (blood gases, ventilatory pressures, lung compliance, oxygenation indexes etc.) were measured and calculated regularly. At the end of experiment, animals were overdosed by anesthetics. Total and differential counts of cells in bronchoalveolar lavage fluid (BAL) were estimated microscopically. Lung edema was expressed as wet/dry lung weight ratio. Treatment with roflumilast reduced leak of cells (P<0.01), particularly of neutrophils (P<0.001), into the lung, decreased lung edema formation (P<0.01), and improved respiratory parameters. Concluding, the results indicate a future potential of PDE4 inhibitors also in the therapy of ALI.

Prolonged activation of cAMP signaling leads to endothelial barrier disruption via transcriptional repression of RRAS

The increase in cAMP levels in endothelial cells triggers cellular signaling to alter vascular permeability. It is generally considered that cAMP signaling stabilizes the endothelial barrier function and reduces permeability. However, previous studies have only examined the permeability shortly after cAMP elevation and thus have only investigated acute responses. Because cAMP is a key regulator of gene expression, elevated cAMP may have a delayed but profound impact on the endothelial permeability by altering the expression of the genes that are vital for the vessel wall stability. The small guanosine triphosphate hydrolase Ras-related protein (R-Ras) stabilizes VE-cadherin clustering and enhances endothelial barrier function, thereby stabilizing the integrity of blood vessel wall. Here we show that cAMP controls endothelial permeability through RRAS gene regulation. The prolonged cAMP elevation transcriptionally repressed RRAS in endothelial cells via a cAMP response element-binding protein (CREB) 3-dependent mechanism and significantly disrupted the adherens junction. These effects resulted in a marked increase of endothelial permeability that was reversed by R-Ras transduction. Furthermore, cAMP elevation in the endothelium by prostaglandin E2 or phosphodiesterase type 4 inhibition caused plasma leakage from intact microvessels in mouse skin. Our study demonstrated that, contrary to the widely accepted notion, cAMP elevation in endothelial cells ultimately increases vascular permeability, and the cAMP-dependent RRAS repression critically contributes to this effect.-Perrot, C. Y., Sawada, J., Komatsu, M. Prolonged activation of cyclic AMP signaling leads to endothelial barrier disruption via transcriptional repression of RRAS.

The EPAC-Rap1 pathway prevents and reverses cytokine-induced retinal vascular permeability

Increased retinal vascular permeability contributes to macular edema, a leading cause of vision loss in eye pathologies such as diabetic retinopathy, age-related macular degeneration, and central retinal vein occlusions. Pathological changes in vascular permeability are driven by growth factors such as VEGF and pro-inflammatory cytokines such as TNF-α. Identifying the pro-barrier mechanisms that block vascular permeability and restore the blood-retinal barrier (BRB) may lead to new therapies. The cAMP-dependent guanine nucleotide exchange factor (EPAC) exchange-protein directly activated by cAMP promotes exchange of GTP in the small GTPase Rap1. Rap1 enhances barrier properties in human umbilical endothelial cells by promoting adherens junction assembly. We hypothesized that the EPAC-Rap1 signaling pathway may regulate the tight junction complex of the BRB and may restore barrier properties after cytokine-induced permeability. Here, we show that stimulating EPAC or Rap1 activation can prevent or reverse VEGF- or TNF-α-induced permeability in cell culture and in vivo Moreover, EPAC activation inhibited VEGF receptor (VEGFR) signaling through the Ras/MEK/ERK pathway. We also found that Rap1B knockdown or an EPAC antagonist increases endothelial permeability and that VEGF has no additive effect, suggesting a common pathway. Furthermore, GTP-bound Rap1 promoted tight junction assembly, and loss of Rap1B led to loss of junctional border organization. Collectively, our results indicate that the EPAC-Rap1 pathway helps maintain basal barrier properties in the retinal vascular endothelium and activation of the EPAC-Rap1 pathway may therefore represent a potential therapeutic strategy to restore the BRB.

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.

Inhibition of endothelial nitric oxide synthase decreases breast cancer cell MDA-MB-231 adhesion to intact microvessels under physiological flows

Nitric oxide (NO) at different concentrations may promote or inhibit tumor growth and metastasis under various conditions. To test the hypothesis that tumor cells prefer to adhere to the locations with a higher endothelial NO production in intact microvessels under physiological flows and to further test that inhibiting NO production decreases tumor cell adhesion, we used intravital fluorescence microscopy to measure NO production and tumor cell adhesion in postcapillary venules of rat mesentery under normal and reduced flow conditions, and in the presence of an endothelial nitric oxide synthase (eNOS) inhibitor, N(G)-monomethyl-l-arginine (l-NMMA). Rats (SD, 250-300 g) were anesthetized. A midline incision (∼2 inch) was made in the abdominal wall, and the mesentery was taken out from the abdominal cavity and spread over a coverslip for the measurement. An individual postcapillary venule (35-50 μm) was first loaded with 4,5-diaminofluorescein diacetate (DAF-2 DA), a fluorescent indictor for NO. Then the DAF-2 intensity was measured for 30 min under a normal or reduced flow velocity, with and without perfusion with MDA-MB-231 breast cancer cells, and in the presence of l-NMMA. We found that tumor cells prefer to adhere to the microvessel locations with a higher NO production such as curved portions. Inhibition of eNOS by l-NMMA attenuated the flow-induced NO production and reduced tumor cell adhesion. We also found that l-NMMA treatment for ∼40 min reduced microvessel permeability to albumin. Our results suggest that inhibition of eNOS is a good approach to preventing tumor cell adhesion to intact microvessels under physiological flows.

Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

A novel tension-sensitive junctional protein, synaptopodin, can relay biophysical input from cellular actomyosin contractility to induce biochemical changes at cell–cell contacts, resulting in structural reorganization of the junctional complex and epithelial barrier maturation.

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell–cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces α-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces α-actinin-4–dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of α-actinin-4 as synaptopodin. Synaptopodin forms a complex containing α-actinin-4 and β-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of α-actinin-4, vinculin, and actin. Knockdown of synaptopodin and α-actinin-4 decreases the strength of cell–cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.

Heterogeneity of pulmonary endothelial cyclic nucleotide response to Pseudomonas aeruginosa ExoY infection

Here, we tested the hypothesis that a promiscuous bacterial cyclase synthesizes purine and pyrimidine cyclic nucleotides in the pulmonary endothelium. To test this hypothesis, pulmonary endothelial cells were infected with a strain of the Gram-negative bacterium Pseudomonas aeruginosa that introduces only exoenzyme Y (PA103 ΔexoUexoT::Tc pUCPexoY; ExoY(+)) via a type III secretion system. Purine and pyrimidine cyclic nucleotides were simultaneously detected using mass spectrometry. Pulmonary artery (PAECs) and pulmonary microvascular (PMVECs) endothelial cells both possess basal levels of four different cyclic nucleotides in the following rank order: cAMP > cUMP ≈ cGMP ≈ cCMP. Endothelial gap formation was induced in a time-dependent manner following ExoY(+) intoxication. In PAECs, intercellular gaps formed within 2 h and progressively increased in size up to 6 h, when the experiment was terminated. cGMP concentrations increased within 1 h postinfection, whereas cAMP and cUMP concentrations increased within 3 h, and cCMP concentrations increased within 4 h postinfection. In PMVECs, intercellular gaps did not form until 4 h postinfection. Only cGMP and cUMP concentrations increased at 3 and 6 h postinfection, respectively. PAECs generated higher cyclic nucleotide levels than PMVECs, and the cyclic nucleotide levels increased earlier in response to ExoY(+) intoxication. Heterogeneity of the cyclic nucleotide signature in response to P. aeruginosa infection exists between PAECs and PMVECs, suggesting the intracellular milieu in PAECs is more conducive to cNMP generation.

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.

Isoproternenol increases vascular volume expansion and urinary output after a large crystalloid bolus in healthy volunteers

BACKGROUND

The primary goal of fluid therapy is to maintain fluid homeostasis. Commonly used isotonic crystalloids are only marginally effective and contribute to fluid excess syndrome. In patients with decreased cardiovascular reserve, fluid therapy alone is not sufficient to maintain end-organ perfusion. Therefore, inotropes or vasoactive drugs are used to supplement fluid infusion. Recent animal data suggest that coinfusion of adrenergic agents modulate the distribution of fluid between the vascular and extravascular/interstitial compartments after a fluid bolus. We sought to determine if this effect would translate in humans by coadministering a β-adrenergic agonist with fluid.

METHODS

Nine healthy volunteers (aged 21-50 years) were randomly paired and received either a continuous isoproterenol infusion (ISO: 0.05 μg/kg per minute) or 0.9% saline (control [CON]) 30min prior to a 25 mL/kg 0.9% NaCl fluid bolus. Hemodynamics, ventricular volume and function, and microcirculatory determinants (capillary filtration coefficient and oncotic pressure) were measured. Vascular and extravascular volume and fluid balance were determined.

RESULTS

Compared with CON, ISO significantly increased heart rate (CON: 64.2 ± 4.1 beats/min vs. ISO: 97.4 ± 5.7 beats/min) and cardiac output (CON: 4.4 ± 0.7 L/min vs. ISO: 10.2 ± 0.9) before fluid bolus. Isoproterenol significantly increased urinary output (ISO: 10.86 ± 1.95 vs. control: 6.53 ± 1.45 mL/kg) and reduced extravascular volume (7.98 ± 2.0 vs. 14.15 ± 1.1mL/kg). Isoproterenol prevented an increase in capillary filtration coefficient (1.74 ± 0.4 vs. 3.21 ± 0.4 mL/min per mmHg · 10).

CONCLUSIONS

Isoproterenol, a nonselective β-adrenergic agonist, augments vascular volume expansion and eliminates extravascular volume via enhanced diuresis, which may in part be due to enhanced endothelial barrier function.

Nitric oxide decreases the permselectivity of the paracellular pathway in thick ascending limbs

Thick ascending limbs reabsorb 25% to 30% of the filtered NaCl. About 50% to 70% is reabsorbed via the transcellular pathway and 30% to 50% is reabsorbed through the Na-selective paracellular pathway. Nitric oxide (NO) inhibits transepithelial Na reabsorption, but its effects on the paracellular pathway are unknown. We hypothesized that NO decreases the selectivity of the paracellular pathway in thick ascending limbs via cGMP-dependent protein kinase. To assess relative Na/Cl permeability ratios (PNa/PCl), we perfused rat thick ascending limbs and measured the effect of reducing bath NaCl on transepithelial voltage, creating dilution potentials, with vehicle, NO donors, and endogenous NO. PNa/PCl was calculated using the Goldman-Hodgkin-Katz equation. Reducing bath Na/Cl to 16/8, 32/24, and 64/56 mmol/L created dilution potentials of -13.6±2.2, -10.8±3.0, and -6.1±0.9 mV, respectively. Calculated PNa/PCls were 2.0±0.2, 2.2±0.5, and 1.9±0.2. The NO donor spermine NONOate (200 µmol/L) blunted the dilution potential caused by 32/24 mmol/L Na/Cl from -11.1±2.1 to -6.5±1.6 mV (P<0.004) and PNa/PCl from 2.2±0.4 to 1.5±0.2. Nitroglycerin (200 µmol/L), another NO donor, also reduced PNa/PCl. Controls showed no significant changes. Dibutyryl-cGMP decreased dilution potentials from -13.4±2.9 to -7.5±1.8 mV (n=6; P<0.01). cGMP-dependent protein kinase inhibition with KT5823 (4 µmol/L) blocked the effect of spermine NONOate, whereas phosphodiesterase 2 inhibition did not. Endogenously produced NO mimicked the effect of the NO donors. In conclusion, NO reduces the selectivity of the paracellular pathway in thick ascending limbs via cGMP and cGMP-dependent protein kinase.

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.

Blood cells and endothelial barrier function

The barrier properties of endothelial cells are critical for the maintenance of water and protein balance between the intravascular and extravascular compartments. An impairment of endothelial barrier function has been implicated in the genesis and/or progression of a variety of pathological conditions, including pulmonary edema, ischemic stroke, neurodegenerative disorders, angioedema, sepsis and cancer. The altered barrier function in these conditions is often linked to the release of soluble mediators from resident cells (e.g., mast cells, macrophages) and/or recruited blood cells. The interaction of the mediators with receptors expressed on the surface of endothelial cells diminishes barrier function either by altering the expression of adhesive proteins in the inter-endothelial junctions, by altering the organization of the cytoskeleton, or both. Reactive oxygen species (ROS), proteolytic enzymes (e.g., matrix metalloproteinase, elastase), oncostatin M, and VEGF are part of a long list of mediators that have been implicated in endothelial barrier failure. In this review, we address the role of blood borne cells, including, neutrophils, lymphocytes, monocytes, and platelets, in the regulation of endothelial barrier function in health and disease. Attention is also devoted to new targets for therapeutic intervention in disease states with morbidity and mortality related to endothelial barrier dysfunction.

Neutrophils and inflammatory resolution in the mucosa

Inflammatory diseases in mucosal organs as diverse as the lung, liver and intestine inevitably require the intimate interactions between neutrophils and epithelia. The physiologic consequences of such interactions often determine endpoint organ function, and for this reason, much recent interest has developed in identifying mechanisms and novel targets to promote the resolution of mucosal inflammation. Physiologically-relevant in vitro and in vivo model systems have aided in discovery of novel pathways to define basic inflammatory mechanisms and approaches to defining the concepts of inflammatory resolution. Here, we will review the recent literature regarding the contribution of neutrophils to inflammatory resolution, with an emphasis on the role of the tissue microenvironment, endogenous pathways for promoting resolution and the molecular determinants of neutrophil-epithelial cell interactions during ongoing inflammation. These recent studies highlight the dynamic nature of pro-resolving pathways and lend insight into the complexity of treating mucosal inflammation.

Diadenosine tetraphosphate induces tight junction disassembly thus increasing corneal epithelial permeability

BACKGROUND AND PURPOSE

Here, we have studied the effects of the dinucleotide P(1), P(4)-Di (adenosine-5') tetraphosphate (Ap4 A) on corneal barrier function conferred by the tight junction (TJ) proteins and its possible involvement in ocular drug delivery and therapeutic efficiency.

EXPERIMENTAL APPROACH

Experiments in vitro were performed using human corneal epithelial cells (HCLEs) treated with Ap4 A (100 μM) for 5 min. Western blot analysis and transepithelial electrical resistance (TEER) were performed to study the TJ protein levels and barrier function respectively. Intracellular pathways involved were determined using an ERK inhibitor and P2Y(2) receptor siRNAs. In in vivo assays with New Zealand rabbits, TJ integrity was examined by zonula occludens-1 (ZO-1) staining. The hypotensive compound 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT) was used to assess improved delivery, measuring its levels by HPLC and measuring intraocular pressure using 5-MCA-NAT, P2Y receptor antagonists and P2Y2 siRNAs.

KEY RESULTS

Two hours after Ap4 A pretreatment, TJ protein levels in HCLE cells were reduced around 40% compared with control. TEER values were significantly reduced at 2 and 4 h (68 and 52% respectively). TJ reduction and ERK activation were blocked by the ERK inhibitor U012 and P2Y(2) siRNAs. In vivo, topical application of Ap4 A disrupted ZO-1 membrane distribution. 5-MCA-NAT levels in the aqueous humour were higher when Ap4 A was previously instilled and its hypotensive effect was also increased. This action was reversed by P2Y receptor antagonists and P2Y(2) siRNA.

CONCLUSIONS AND IMPLICATIONS

Ap4 A increased corneal epithelial barrier permeability. Its application could improve ocular drug delivery and consequently therapeutic efficiency.

Temporal effects of vascular endothelial growth factor and 3,5-cyclic monophosphate on blood-brain barrier solute permeability in vivo

To test the hypothesis that vascular endothelial growth factor (VEGF) can transiently increase the blood-brain barrier permeability, P, as for peripheral microvessels and that the elevation of 3,5-cyclic monophosphate (cAMP) levels can inhibit the VEGF-induced acute hyperpermeability, we employed multiphoton microscopy to quantify the cerebral microvessel permeability P to various-sized solutes under VEGF and cAMP treatments. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a microgrinder. Fluorescein (MW 376Da), fluorescein isothioyanate-dextran-20k (FITC-Dex-20k), FITC-Dex-70k, or Alexa Fluor 488-IgG in 1% bovine serum albumin mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery with a syringe pump. Simultaneously, temporal images were collected from the brain parenchyma ∼100-200 μm below the pia mater. P was determined from the rate of tissue solute accumulation around individual microvessels. Exposure to 1 nM VEGF transiently increased P to 2.2, 10.5, 9.8, and 12.8 times control values, for fluorescein, Dex-20k, Dex-70k, and IgG, respectively, within 30 sec, and all returned to control levels within 2 min. After 20 min of pretreatment with 2 mM of the cAMP analog 8-bromo-cAMP, the initial increase by 1 nM VEGF was completely abolished in P of all solutes. The response pattern of P to VEGF and cAMP and the ratios of the peak to control values for rat cerebral microvessels are similar to those for rat mesenteric (peripheral) microvessels, except that the ratios are higher in P of cerebral microvessels for the intermediate and large solutes. These results imply a new approach for delivering large therapeutic agents to the brain.

Tonic regulation of vascular permeability

Our major theme is that the layered structure of the endothelial barrier requires continuous activation of signalling pathways regulated by sphingosine-1-phosphate (S1P) and intracellular cAMP. These pathways modulate the adherens junction, continuity of tight junction strands, and the balance of synthesis and degradation of glycocalyx components. We evaluate recent evidence that baseline permeability is maintained by constant activity of mechanisms involving the small GTPases Rap1 and Rac1. In the basal state, the barrier is compromised when activities of the small GTPases are reduced by low S1P supply or delivery. With inflammatory stimulus, increased permeability can be understood in part as the action of signalling to reduce Rap1 and Rac1 activation. With the hypothesis that microvessel permeability and selectivity under both normal and inflammatory conditions are regulated by mechanisms that are continuously active, it follows that when S1P or intracellular cAMP are elevated at the time of inflammatory stimulus, they can buffer changes induced by inflammatory agents and maintain normal barrier stability. When endothelium is exposed to inflammatory conditions and subsequently exposed to elevated S1P or intracellular cAMP, the same processes restore the functional barrier by first re-establishing the adherens junction, then modulating tight junctions and glycocalyx. In more extreme inflammatory conditions, loss of the inhibitory actions of Rac1-dependent mechanisms may promote expression of more inflammatory endothelial phenotypes by contributing to the up-regulation of RhoA-dependent contractile mechanisms and the sustained loss of surface glycocalyx allowing access of inflammatory cells to the endothelium.

Use of inotropes and vasopressor agents in critically ill patients

Inotropes and vasopressors are biologically and clinically important compounds that originate from different pharmacological groups and act at some of the most fundamental receptor and signal transduction systems in the body. More than 20 such agents are in common clinical use, yet few reviews of their pharmacology exist outside of physiology and pharmacology textbooks. Despite widespread use in critically ill patients, understanding of the clinical effects of these drugs in pathological states is poor. The purpose of this article is to describe the pharmacology and clinical applications of inotropic and vasopressor agents in critically ill patients.

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.

Opening of epithelial tight junctions and enhancement of paracellular permeation by chitosan: microscopic, ultrastructural, and computed-tomographic observations

This study investigates the effects of chitosan (CS) on the opening of epithelial tight junctions (TJs) and paracellular transport at microscopic, ultrastructural, and computed-tomographic levels in Caco-2 cell monolayers and animal models. Using immunofluorescence staining, CS treatment was observed to be associated with the translocation of JAM-1 (a trans-membrane TJ protein), resulting in the disruption of TJs; the removal of CS was accompanied by the recovery of JAM-1. Ultrastructural observations by TEM reveal that CS treatment slightly opened the apical intercellular space, allowing lanthanum (an electron-dense tracer) to stain the intercellular surface immediately beneath the TJs, suggesting the opening of TJs. Following the removal of CS, the TJs were completely recovered. Similar microscopic and ultrastructural findings were obtained in animal studies. CS nanoparticles were prepared as an insulin carrier. The in vivo fluorescence-microscopic results demonstrate that insulin could be absorbed into the systemic circulation, while most CS was retained in the microvilli scaffolds. These observations were verified in a biodistribution study following the oral administration of isotope-labeled nanoparticles by single-photon emission computed tomography. Above results reveal that CS is a safe permeation enhancer and is an effective carrier for oral protein delivery.

Phosphodiesterase-4 inhibition as a therapeutic approach to treat capillary leakage in systemic inflammation

In sepsis and systemic inflammation, increased microvascular permeability and consecutive breakdown of microcirculatory flow significantly contribute to organ failure and death. Evidence points to a critical role of cAMP levels in endothelial cells to maintain capillary endothelial barrier properties in acute inflammation. However, approaches to verify this observation in systemic models are rare. Therefore we tested here whether systemic application of the phosphodiesterase-4-inhibitors (PD-4-Is) rolipram or roflumilast to increase endothelial cAMP was effective to attenuate capillary leakage and breakdown of microcirculatory flow in severe lipopolysaccharide (LPS)-induced systemic inflammation in rats. Measurements of cAMP in mesenteric microvessels demonstrated significant LPS-induced loss of cAMP levels which was blocked by application of rolipram. Increased endothelial cAMP by application of either PD-4-I rolipram or roflumilast led to stabilization of endothelial barrier properties as revealed by measurements of extravasated FITC-albumin in postcapillary mesenteric venules. Accordingly, microcirculatory flow in mesenteric venules was significantly increased following PD-4-I treatment and blood gas analyses indicated improved metabolism. Furthermore application of PD-4-I after manifestation of LPS-induced systemic inflammation and capillary leakage therapeutically stabilized endothelial barrier properties as revealed by significantly reduced volume resuscitation for haemodynamic stabilization. Accordingly microcirculation was significantly improved following treatment with PD-4-Is. Our results demonstrate that inflammation-derived loss of endothelial cAMP contributes to capillary leakage which was blocked by systemic PD-4-I treatment. Therefore these data suggest a highly clinically relevant and applicable approach to stabilize capillary leakage in sepsis and systemic inflammation.

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.

Bacillus anthracis-derived edema toxin (ET) counter-regulates movement of neutrophils and macromolecules through the endothelial paracellular pathway

BACKGROUND

A common finding amongst patients with inhalational anthrax is a paucity of polymorphonuclear leukocytes (PMNs) in infected tissues in the face of abundant circulating PMNs. A major virulence determinant of anthrax is edema toxin (ET), which is formed by the combination of two proteins produced by the organism, edema factor (EF), which is an adenyl cyclase, and protective antigen (PA). Since cAMP, a product of adenyl cyclase, is known to enhance endothelial barrier integrity, we asked whether ET might decrease extravasation of PMNs into tissues through closure of the paracellular pathway through which PMNs traverse.

RESULTS

Pretreatment of human microvascular endothelial cell(EC)s of the lung (HMVEC-L) with ET decreased interleukin (IL)-8-driven transendothelial migration (TEM) of PMNs with a maximal reduction of nearly 60%. This effect required the presence of both EF and PA. Conversely, ET did not diminish PMN chemotaxis in an EC-free system. Pretreatment of subconfluent HMVEC-Ls decreased transendothelial 14 C-albumin flux by ~ 50% compared to medium controls. Coadministration of ET with either tumor necrosis factor-α or bacterial lipopolysaccharide, each at 100 ng/mL, attenuated the increase of transendothelial 14 C-albumin flux caused by either agent alone. The inhibitory effect of ET on TEM paralleled increases in protein kinase A (PKA) activity, but could not be blocked by inhibition of PKA with either H-89 or KT-5720. Finally, we were unable to replicate the ET effect with either forskolin or 3-isobutyl-1-methylxanthine, two agents known to increase cAMP.

CONCLUSIONS

We conclude that ET decreases IL-8-driven TEM of PMNs across HMVEC-L monolayers independent of cAMP/PKA activity.

Mechanical responsiveness of the endothelial cell of Schlemm's canal: scope, variability and its potential role in controlling aqueous humour outflow

Primary open-angle glaucoma is associated with elevated intraocular pressure, which in turn is believed to result from impaired outflow of aqueous humour. Aqueous humour outflow passes mainly through the trabecular meshwork (TM) and then through pores formed in the endothelium of Schlemm's canal (SC), which experiences a basal-to-apical pressure gradient. This gradient dramatically deforms the SC endothelial cell and potentially contributes to the formation of those pores. However, mechanical properties of the SC cell are poorly defined. Using optical magnetic twisting cytometry and traction force microscopy, here we characterize the mechanical properties of primary cultures of the human SC cell, and for the first time, the scope of their changes in response to pharmacological agents that are known to modulate outflow resistance. Lysophosphatidic acid, sphingosine-1-phosphate (S1P) and thrombin caused an increase in cell stiffness by up to 200 per cent, whereas in most cell strains, exposure to latrunculin A, isoproterenol, dibutryl cyclic-AMP or Y-27632 caused a decrease in cell stiffness by up to 80 per cent, highlighting that SC cells possess a remarkably wide contractile scope. Drug responses were variable across donors. S1P, for example, caused 200 per cent stiffening in one donor strain but only 20 per cent stiffening in another. Isoproterenol caused dose-dependent softening in three donor strains but little or no response in two others, a finding mirrored by changes in traction forces and consistent with the level of expression of β(2)-adrenergic receptors. Despite donor variability, those drugs that typically increase outflow resistance systematically caused cell stiffness to increase, while in most cases, those drugs that typically decrease outflow resistance caused cell stiffness to decrease. These findings establish the endothelial cell of SC as a reactive but variable mechanical component of the aqueous humour outflow pathway. Although the mechanism and locus of increased outflow resistance remain unclear, these data suggest the SC endothelial cell to be a modulator of outflow resistance.

Ultrastructural analysis reveals cAMP-dependent enhancement of microvascular endothelial barrier functions via Rac1-mediated reorganization of intercellular junctions

Evidence exists that cAMP stabilizes the endothelial barrier, in part via activation of the small GTPase Rac1. However, despite the high medical relevance of this signaling pathway, the mechanistic effects on intercellular contacts on the ultrastructural level are largely unknown. In microvascular endothelial cell monolayers, in which increased cAMP strengthened barrier properties, similar to intact microvessels in vivo, both forskolin and rolipram (F/R) to increase cAMP and 8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphorothioate (O-Me-cAMP) to stimulate exchange protein directly activated by cAMP/Ras proximate-1 (EPac/Rap 1) signaling enhanced transendothelial electrical resistance and induced activation of Rac1. Concurrently, augmented immunofluorescence intensity and linearization of signals at cell borders were observed for intercellular junction proteins VE-cadherin and claudin 5. Ultrastructural analysis of the intercellular contact zone architecture documented that exposure to F/R or O-Me-cAMP led to a significant increase in the proportion of contact sites displaying complex interdigitations of cell borders, in which membranes of neighboring cells were closely apposed over comparatively long distances; in addition, they were stabilized by numerous intercellular junctions. Interference with Rac1 activation by NSC-23766 completely abolished both barrier stabilization and contact zone reorganization in response to O-Me-cAMP, whereas F/R-mediated Rac1 activation and barrier enhancement were not affected by NSC-23766. In parallel experiments using macrovascular endothelium, increased cAMP failed to induce Rac1 activation, barrier enhancement, and contact zone reorganization. These results indicate that, in microvascular endothelium, Rac1-mediated alterations in contact zone architecture contribute to cAMP-induced barrier stabilization.

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.

Intrinsic sex-specific differences in microvascular endothelial cell phosphodiesterases

The importance of gonadal hormones in the regulation of vascular function has been documented. An alternate and essential contribution of the sex chromosomes to sex differences in vascular function is poorly understood. We reported previously sex differences in microvessel permeability (P(s)) responses to adenosine that were mediated by the cAMP signaling pathway (Wang J, PhD thesis, 2005; Wang J and Huxley V, Proceedings of the VIII World Congress of Microcirculation, 2007; Wang J and Huxley VH, Am J Physiol Heart Circ Physiol 291: H3094-H3105, 2006). The two cyclic nucleotides, cAMP and cGMP, central to the regulation of vascular barrier integrity, are hydrolyzed by phosphodiesterases (PDE). We hypothesized that microvascular endothelial cells (EC) would retain intrinsic and inheritable sexually dimorphic genes with respect to the PDEs modulating EC barrier function. Primary cultured microvascular EC from skeletal muscles isolated from male and female rats, respectively, were used. SRY (a sex-determining region Y gene) mRNA expression was observed exclusively in male, not female, cells. The predominant isoform among PDE1-5, present in both XY and XX EC, was PDE4. Expression mRNA levels of PDE1A (male > female) and PDE3B (male < female) were sex dependent; PDE2A, PDE4D, and PDE5A were sex independent. Barrier function, P(s), was determined from measures of albumin flux across confluent primary cultured microvessel XY and XX EC monolayers. Consistent with intact in situ microvessels, basal monolayer P(s) did not differ between XY (1.7 +/- 0.2 x 10(-6) cm/s; n = 8) and XX (1.8 +/- 0.1 x 10(-6) cm/s; n = 10) EC. Cilostazol, a PDE3 inhibitor, reduced (11%, P < 0.05) P(s) in XX, not XY, cells. These findings demonstrate the presence and maintenance of intrinsic sex-related differences in gene expression and cellular phenotype by microvascular EC in a gonadal-hormone-free environment. Furthermore, intrinsic cell-sex likely contributes significantly to sexual dimorphism in cardiovascular function.

Glucagon-like peptide-1 protects mesenteric endothelium from injury during inflammation

Glucagon-like peptide-1 (GLP-1) is a proglucagon-derived hormone with cellular protective actions. We hypothesized that GLP-1 would protect the endothelium from injury during inflammation. Our aims were to determine the: (1) effect of GLP-1 on basal microvascular permeability, (2) effect of GLP-1 on increased microvascular permeability induced by lipopolysaccaride (LPS), (3) involvement of the GLP-1 receptor in GLP-1 activity, and (4) involvement of the cAMP/PKA pathway in GLP-1 activity. Microvascular permeability (L(p)) of rat mesenteric post-capillary venules was measured in vivo. First, the effect of GLP-1 on basal L(p) was measured. Second, after systemic LPS injection, L(p) was measured after subsequent perfusion with GLP-1. Thirdly, L(p) was measured after LPS injection and perfusion with GLP-1+GLP-1 receptor antagonist. Lastly, L(p) was measured after LPS injection and perfusion with GLP-1+inhibitors of the cAMP/PKA pathway. Results are presented as mean area under the curve (AUC)+/-SEM. GLP-1 had no effect on L(p) (AUC: baseline=27+/-1.4, GLP-1=1+/-0.4, p=0.08). LPS increased L(p) two-fold (AUC: LPS=54+/-1.7, p<0.0001). GLP-1 reduced the LPS increase in L(p) by 75% (AUC: LPS+GLP-1=34+/-1.5, p<0.0001). GLP-1 antagonism reduced the effects of GLP-1 by 60% (AUC: LPS+GLP-1+antagonist=46+/-2.0, p<0.001). The cAMP synthesis inhibitor reduced the effects of GLP-1 by 60% (AUC: LPS+GLP-1+cAMP inhibitor=46+/-1.5, p<0.0001). The PKA inhibitor reduced the effects of GLP-1 by 100% (AUC: LPS+GLP-1+PKA inhibitor=56+/-1.5, p<0.0001). GLP-1 attenuates the increase in microvascular permeability induced by LPS. GLP-1 may protect the endothelium during inflammation, thus decreasing third-space fluid loss.

Angiopoietin-1 alters microvascular permeability coefficients in vivo via modification of endothelial glycocalyx

AIMS

In this study, we wished to determine whether angiopoietin-1 (Ang1) modified the permeability coefficients of non-inflamed, intact continuous, and fenestrated microvessels in vivo and to elucidate the underlying cellular mechanisms.

METHODS AND RESULTS

Permeability coefficients were measured using the Landis-Michel technique (in frog and rat mesenteric microvessels) and an oncopressive permeability technique (in glomeruli). Ang1 decreased water permeability (L(P): hydraulic conductivity) in continuous and fenestrated microvessels and increased the retention of albumin (sigma: reflection coefficient) in continuous microvessels. Endothelial glycocalyx is common to these anatomically distinct microvascular beds, and contributes to the magnitude of both L(P) and sigma. Ang1 treatment increased the depth of endothelial glycocalyx in intact microvessels and increased the content of glycosaminoglycan of cultured microvascular endothelial cell supernatant. Ang1 also prevented the pronase-induced increase in L(P) (attributable to selective removal of endothelial glycocalyx by pronase) by restoration of glycocalyx at the endothelial cell surface. The reduction in permeability was inhibited by a cell transport inhibitor, Brefeldin.

CONCLUSION

Ang1 modifies basal microvessel permeability coefficients, in keeping with previous reports demonstrating reduced solute flux in inflamed vessels. Anatomical, biochemical, and physiological evidence indicates that modification of endothelial glycocalyx is a novel mechanism of action of Ang1 that contributes to these effects.

VASP is involved in cAMP-mediated Rac 1 activation in microvascular endothelial cells

Accumulating evidence points to a significant role of vasodilator-stimulated phosphoprotein (VASP) in the maintenance of endothelial barrier functions. We have recently shown that impaired barrier functions in VASP-deficient microvascular myocardial endothelial cells (MyEnd VASP(-/-)) correlated with decreased Rac 1 activity. To further test the hypothesis that VASP is involved in regulation of Rac 1 activity, we studied cAMP-dependent Rac 1 activation. Both inhibition of Rac 1 activation by NSC-23766 and inhibition of PKA by PKI completely blunted the efficacy of forskolin/rolipram (F/R)-mediated cAMP increase to stabilize barrier functions as revealed by measurements of transendothelial resistance (TER). Because these results indicate that PKA/Rac 1 activation is important for barrier stabilization, we tested this signaling pathway in VASP(-/-) cells. We found that F/R and isoproterenol reduced permeability measured as FITC-dextran flux across VASP(-/-) monolayers, but not below baseline levels of wild-type cells (WT). Moreover, cAMP-mediated Rac 1 activation was reduced to approximately 50% of WT levels, and both PKA inhibition by PKI and PKA anchoring via A kinase anchoring peptides (AKAPs) by HT31 almost completely abolished Rac 1 activation in VASP(-/-) and WT endothelium. Accordingly, HT31 significantly reduced F/R-mediated TER increase in WT cells and completely blocked the protective effect of cAMP on endothelial barrier properties. Together, our data underline the significant role of cAMP-mediated Rac 1 activation for endothelial barrier stabilization and demonstrate that both AKAP-mediated PKA anchoring and VASP are required for this process.

Three-dimensional localization and quantification of PAF-induced gap formation in intact venular microvessels

Combining single-vessel perfusion technique with confocal microscopy, this study presents a new approach that allows three-dimensional visualization and quantification of endothelial gaps under experimental conditions identical to those used to measure permeability coefficients, endothelial calcium concentration, and nitric oxide production in individually perfused intact microvessels. This approach provides an efficient means for defining the transport pathways and cellular mechanisms of increased microvascular permeability during inflammation. Platelet-activating factor (PAF) was used to increase the permeability of individually perfused rat mesenteric venules. Fluorescent microspheres (FMs, 100 nm) were used as leakage markers, and confocal images were acquired at successive focal planes through the perfused microvessel. Perfusion of FMs under control conditions produced a thin, uniform layer of FMs in the vessel lumen, but in PAF-stimulated microvessels significant amounts of FMs accumulated at endothelial junctions. Reconstructed confocal images three-dimensionally delineated the temporal and spatial development of endothelial gaps in PAF-stimulated microvessels. The FM accumulation, quantified as the total fluorescence intensity per square micrometer of vessel wall, was 8.4 +/- 1.8 times the control value within 10 min of PAF perfusion and declined to 5.0 +/- 0.6 and 1.4 +/- 0.2 times the control value when FMs were applied 30 and 60 min after PAF perfusion. The changes in the magnitude of FM accumulation closely correlated with the time course of PAF-induced increases in hydraulic conductivity (L(p)), indicating that the opening and closing of endothelial gaps contributed to the transient increase in L(p) in PAF-stimulated microvessels. Electron microscopic evaluations confirmed PAF-induced gap formation and FM accumulation at endothelial clefts.

cAMP induced Rac 1-mediated cytoskeletal reorganization in microvascular endothelium

It is well established that cAMP stabilizes endothelial barrier functions, in part by regulation of VE-cadherin via EPAC/Rap 1. The aim of the present study was to investigate whether cAMP activates Rac 1 in microvascular endothelium. In human dermal microvascular endothelial cells (HDMEC), treatment with forskolin/rolipram (F/R) to increase cAMP by as well as the Epac/Rap 1-stimulating cAMP analogue 8-pCPT-2'-O-methyl-cAMP (O-Me-cAMP) stabilized endothelial barrier properties as revealed by raised transendothelial electrical resistance (TER). Under these conditions, immunostaining of VE-cadherin and claudin 5 were increased and linearized. This was paralleled by activation of Rac 1 by 153 +/- 16% (F/R) or 281 +/- 65% (O-Me-cAMP) whereas activity of Rho A was unchanged. F/R and O-Me-cAMP increased the peripheral actin belt and recruited the Rac 1 effector cortactin to cell junctions, similar to direct activation of Rac 1 by CNF-1. Thrombin was used to further test the physiologic relevance of cAMP-mediated Rac 1 activation. Thrombin-induced drop of TER was paralleled by intercellular gap formation, inactivation of Rac 1 and activation of Rho A at 5 and 15 min whereas baseline conditions where re-established following 60 min. Both, F/R and O-Me-cAMP completely blocked the thrombin-induced barrier breakdown. F/R completely abolished thrombin-induced Rac 1 inactivation and Rho A activation whereas O-Me-cAMP only partially blocked Rac 1 inactivation. Taken together, these results indicate that Rac 1 activation likely contributes to the barrier-stabilizing effects of cAMP in microvascular endothelium and that these effects may in part be mediated by Epac/Rap 1.

Effect of cyclic AMP on barrier function of human lymphatic microvascular tubes

This work examines the effect of cyclic AMP (cAMP) on the in vitro barrier function of tubes of human dermal lymphatic microvascular endothelial cells (LECs). Under baseline conditions, the barrier function of LEC tubes was weak, with diffusional permeability coefficients to bovine serum albumin and 10 kDa dextran of 1.4(-0.6)(+0.9)x10(-6) cm/s and 1.7(-0.5)(+0.8)x10(-6) cm/s (geometric mean+/-95% CI), respectively, and 1.2+/-0.5 (mean+/-95% CI) focal leaks per mm. Exposure to low concentrations (3 microM) of a cell-permeant analog of cAMP did not alter the barrier function. Exposure to higher concentrations (80 and 400 microM) and/or the phosphodiesterase inhibitor Ro-20-1724 (20 microM) lowered permeabilities and the number of focal leaks, and increased the selectivity of the barrier. Decreased permeabilities were accompanied by an increase in continuous VE-cadherin staining at cell-cell borders. Exposure to 1 mM 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase, did not increase permeabilities. LECs expressed the lymphatic-specific master transcription factor Prox-1, regardless of whether barrier function was weak or strong. Our results indicate that the permeability of LEC tubes in vitro responds to cAMP in a manner similar to that well-described for the permeability of blood microvessels.

Epac/Rap1 pathway regulates microvascular hyperpermeability induced by PAF in rat mesentery

Experiments in cultured endothelial cell monolayers demonstrate that increased intracellular cAMP strongly inhibits the acute permeability responses by both protein kinase A (PKA)-dependent and -independent pathways. The contribution of the PKA-independent pathways to the anti-inflammatory mechanisms of cAMP in intact mammalian microvessels has not been systematically investigated. We evaluated the role of the cAMP-dependent activation of the exchange protein activated by cAMP (Epac), a guanine nucleotide exchange factor for the small GTPase Rap1, in rat venular microvessels exposed to the platelet-activating factor (PAF). The cAMP analog 8-pCPT-2'-O-methyl-cAMP (O-Me-cAMP), which stimulates the Epac/Rap1 pathway but has no effect on PKA, significantly attenuated the PAF increase in microvessel permeability as measured by hydraulic conductivity (Lp). We also demonstrated that PAF induced a rearrangement of vascular endothelial (VE)-cadherin seen as numerous lateral spikes and frequent short breaks in the otherwise continuous peripheral immunofluorescent label. Pretreatment with O-Me-cAMP completely prevented the PAF-induced rearrangement of VE-cadherin. We conclude that the action of the Epac/Rap1 pathway to stabilize cell-cell adhesion is a significant component of the activity of cAMP to attenuate an acute increase in vascular permeability. Our results indicate that increased permeability in intact microvessels by acute inflammatory agents such as PAF is the result of the decreased effectiveness of the Epac/Rap1 pathway modulation of cell-cell adhesion.

Spectrin-anchored phosphodiesterase 4D4 restricts cAMP from disrupting microtubules and inducing endothelial cell gap formation

Dynamic cAMP fluctuations that are restricted to a sub-plasma-membrane domain strengthen endothelial barrier integrity. Phosphodiesterases (PDEs) localize within this domain where they limit cAMP diffusion into the bulk cytosolic compartment; however, the molecular identity of PDEs responsible for endothelial cell membrane cAMP compartmentation remain poorly understood. Our present findings reveal that the D4 splice variant of the PDE4 phosphodiesterase family - PDE4D4 - is expressed in pulmonary microvascular endothelial cells, and is found in plasma membrane fractions. PDE4D4 interacts with alpha II spectrin within this membrane domain. Although constitutive PDE4D4 activity limits cAMP access to the bulk cytosol, inhibiting its activity permits cAMP to access a cytosolic domain that is rich in microtubules, where it promotes protein kinase A (PKA) phosphorylation of tau at Ser214. Such phosphorylation reorganizes microtubules and induces interendothelial cell gap formation. Thus, spectrin-anchored PDE4D4 shapes the physiological response to cAMP by directing it to barrier-enhancing effectors while limiting PKA-mediated microtubule reorganization.

Adaptation of coronary microvascular exchange in arterioles and venules to exercise training and a role for sex in determining permeability responses

Studies of physical performance and energy metabolism during and following exercise have shown significant sex-specific musculoskeletal adaptations; less is known of vascular adaptations, particularly with respect to exchange capacity. In response to adenosine (ADO), a metabolite produced during exercise, permeability (P(s)) of coronary arterioles from female pigs changed acutely; the magnitude and direction of the change (Delta P(s)) were determined by training status. In the present study P(s) to albumin was assessed in arterioles (n = 138) and venules (n = 24) isolated from hearts of male (N = 27) and female (N = 59) pigs in the exercise training group (EX). We evaluated the hypothesis that coronary microvessel exchange adapts to endurance exercise training not by altering basal P(s), per se, but by elevating P(s) on exposure to ADO. In contrast, training resulted in a reduction of basal P(s) in all arterioles, and in venules from males, with no change in venules from EX females. Exposure to ADO resulted in the predicted increase in P(s) except for venules from EX males where P(s) was reduced. Delta P(s) responses of arterioles to mediators of adenylyl cyclase (isoproterenol)- and guanylyl cyclase (atrial natriuretic peptide)-signaling pathways were attenuated in EX pigs relative to pigs in the sedentary group. The adaptation of EX arterioles involves an upregulation of a nitric oxide-dependent pathway since nitric oxide synthase inhibition blocks Delta P(s) by ADO. Thus adaptation of microvascular exchange capacity to endurance exercise training not only occurs but also involves multiple mechanisms that differ in arterioles and venules with their relative contribution to net flux being a function of sex.

Prostaglandin E2-EP4 receptor signalling promotes tumorigenic behaviour of HT-29 human colorectal cancer cells

The predominant product of cyclooxygenase (COX) activity in the colon, prostaglandin (PG) E2 promotes intestinal tumorigenesis. Expression of the PGE2 receptor EP4 is upregulated during colorectal carcinogenesis. Therefore, we investigated the role of elevated PGE2-EP4 receptor signalling in the protumorigenic activity of PGE2 by increasing EP4 receptor expression in HT-29 human colorectal cancer (CRC) cells (HT-29-EP4) by stable transfection. Elevated PGE2-induced EP4 receptor activity in HT-29 cells increased resistance to spontaneous apoptosis and promoted anchorage-independent growth, but had no effect on proliferation of HT-29-EP4 cells. EP4 receptor activation by PGE2 in HT-29-EP4 cells also led to development of fluid-filled cysts, which was associated with increased tight junction protein (occludin and zonula occludens-1) expression. Overexpression of the EP4 receptor in HT-29 cells led to basal EP4 receptor signalling in the absence of exogenous PGE2, which was explained by autocrine activity of endogenous, COX-2-derived PGE2 and constitutive, ligand-independent EP4 receptor activity. The predominant signalling pathway mediating antiapoptotic activity downstream of PGE2-EP4 receptor activation in HT-29-EP4 cells was elevation of cyclic adenosine monophosphate (cAMP) levels, which was associated with phosphorylation of cAMP-response element binding protein. EP4 receptor activation led to a small increase in phosphorylated extracellular signal-regulated kinase (ERK) 2 protein levels but inhibition of ERK phosphorylation did not abrogate the antiapoptotic activity of PGE2. However, PGE2-EP4 receptor signalling did not lead to trans-activation of the epidermal growth factor receptor in HT-29 cells. Inhibition of protumorigenic PGE2-EP4 receptor signalling represents a potential strategy for anti-CRC therapy that may avoid the toxicity associated with systemic COX inhibition.

Leukocyte-platelet aggregate adhesion and vascular permeability in intact microvessels: role of activated endothelial cells

Leukocyte-platelet aggregation and aggregate adhesion have been indicated as biomarkers of the severity of tissue injury during inflammation or ischemic reperfusion. The objective of this study is to investigate the mechanisms of the aggregate adhesion and quantitatively evaluate its relationship with microvessel permeability. A combined autologous blood perfusion with single microvessel perfusion technique was employed in rat mesenteric venular microvessels. The aggregate adhesion was induced by systemic application of TNF-α plus local application of platelet-activating factor (PAF). Changes in permeability were determined by measurements of hydraulic conductivity ( Lp) before and after aggregate adhesion in the same individually perfused microvessels. The compositions of the adherent aggregates were identified with fluorescent labeling and confocal imaging. In contrast to leukocyte adhesion as single cells resulting in no increase in microvessel permeability, aggregate adhesion induced prolonged increases in microvessel Lp(6.1 ± 0.9 times the control, n = 9) indicated by the initial Lpmeasurements after 3 h of blood perfusion, which is distinct from the transient Lpincrease caused by PAF-induced endothelial activation in the absence of blood. Isoproteronol (Iso) attenuated aggregate adhesion-mediated Lpincreases if applied after autologous blood perfusion and prevented the aggregate adhesion if the initial endothelial activation is inhibited by applying Iso before PAF administration but showed less effect on single leukocyte adhesion. This study demonstrated that leukocyte-platelet aggregate adhesion via a mechanism different from that of single leukocyte adhesion caused a prolonged increase in microvessel permeability. Our results also indicate that the initial activation of endothelial cells by PAF plays a crucial role in the initiation of leukocyte-platelet aggregate adhesion.

Adenosine A2A receptor modulation of juvenile female rat skeletal muscle microvessel permeability

Little is known of the regulation of skeletal muscle microvascular exchange under resting or stimulating conditions. Adenosine (ADO) levels in skeletal muscle increase during physiological (exercise) and pathological (hypoxia, inflammation, and ischemia) conditions. Later stages of these pathologies are characterized by the loss of vascular barrier integrity. This study focused on determining which ADO receptor mediates the robust reduction in microvessel permeability to rat serum albumin (P(s)(RSA)) observed in juvenile female rats. In microvessels isolated from abdominal skeletal muscle, ADO suffusion induced a concentration-dependent reduction in arteriolar [log(IC(50)) = -9.8 +/- 0.2 M] and venular [log(IC(50)) = -8.4 +/- 0.2 M] P(s)(RSA). RT-PCR and immunoblot analysis demonstrated mRNA and protein expression of ADO A(1), A(2A), A(2B), and A(3) receptors in both vessel types, and immunofluorescence assay revealed expression of the four subtype receptors in the microvascular walls (endothelium and smooth muscle). P(s)(RSA) responses of arterioles and venules to ADO were blocked by 8-(p-sulphophenyl)theophylline, a nonselective A(1) and A(2) antagonist. An A(2A) agonist, CGS21680, was more potent than the A(1) agonist, cyclopentyladenosine, or the most-selective A(2B) agonist, 5'-(N-ethylcarboxamido)adenosine. The ability of CGS21680 or ADO to reduce P(s)(RSA) was abolished by the A(2A) antagonist, ZM241385. An adenylyl cyclase inhibitor, SQ22536, blocked the permeability response to ADO. In aggregate, these results demonstrate that, in juvenile females (before the production of the reproductive hormones), ADO enhances skeletal muscle arteriole and venule barrier function predominantly via A(2A) receptors using activation of adenylyl cyclase-signaling mechanisms.