Isolation of murine aortic endothelial cells in culture and the effects of sex steroids on their growth

An effective method of isolating endothelial cells from intact rat aorta

Rat aorta has been widely utilized in studies on cardiovascular diseases. Due to its small size, the endothelial cells are difficult to isolate. Here, we developed a modified method to obtain the endothelial cells from the rat aorta. The thoracic aorta of Wistar rat was harvested with its inside was turned out to expose the tunica intima. With its both ends ligated and cultured in ECM in T25 flask, a small amount of cells grew out from the aorta on day 4 and manifested the characteristic cobble stone-like monolayer on day 9-10. The cells were identified to be endothelial cells using reverse transcription (RT)-polymerase chain reaction (PCR), immunocytochemical staining for Von Willebrand Factor (vWF), Dil-Ac-LDL uptake assay, and capillary tube formation assay. This method serves as an easy way to get a large amount of endothelial cells from small-sized vessels, which, therefore, will greatly benefit the researches of vascular disease.

Elevated level of fibrinogen increases caveolae formation; role of matrix metalloproteinase-9

The role of the inflammatory agent fibrinogen (Fg) in increased pial venular permeability has been shown previously. It was suggested that an activation of matrix metalloproteinase-9 (MMP-9) is involved in Fg-induced enhanced transcytosis through endothelial cells (ECs). However, direct link between Fg, caveolae formation, and MMP-9 activity has never been shown. We hypothesized that at an elevated level, Fg enhances formation of functional caveolae through activation of MMP-9. Male wild-type (WT, C57BL/6J) or MMP-9 gene knockout (MMP9-/-) mice were infused with Fg (4 mg/ml, final blood concentration) or equal volume of phosphate buffered saline (PBS). After 2 h, mice were sacrificed and brains were collected for immunohistochemical analyses. Mouse brain ECs were treated with 4 mg/ml of Fg or PBS in the presence or absence of MMP-9 activity inhibitor, tissue inhibitor of metalloproteinases-4 (TIMP-4, 12 ng/ml). Formation of functional caveolae was assessed by confocal microscopy. Fg-induced increased formation of caveolae, which was defined by an increased co-localization of caveolin-1 (Cav-1) and plasmalemmal vesicle-associated protein-1 and was associated with an increased phosphorylation of Cav-1, was ameliorated in the presence of TIMP-4. These results suggest that at high levels, Fg enhances formation of functional caveolae that may involve Cav-1 signaling and MMP-9 activation.

Development of immortalized mouse aortic endothelial cell lines

Background

The understanding of endothelial cell biology has been facilitated by the availability of primary endothelial cell cultures from a variety of sites and species; however, the isolation and maintenance of primary mouse aortic endothelial cells (MAECs) remain a formidable challenge. Culturing MAECs is difficult as they are prone to phenotypic drift during culture. Therefore, there is a need to have a dependable in vitro culture system, wherein the primary endothelial cells retain their properties and phenotypes.

Methods

Here, we developed an effective method to prepare immortalized MAEC (iMAEC) lines. Primary MAECs, initially isolated from aortic explants, were immortalized using a retrovirus expressing polyoma middle T-antigen. Immortalized cells were then incubated with DiI-acetylated-low density lipoprotein and sorted via flow cytometry to isolate iMAECs.

Results

iMAECs expressed common markers of endothelial cells, including PECAM1, eNOS, VE-cadherin, and von Willebrand Factor. iMAECs aligned in the direction of imposed laminar shear and retained the ability to form tubes. Using this method, we have generated iMAEC lines from wild-type and various genetically modified mice such as p47^phox-/-, eNOS^-/-, and caveolin-1^-/-.

Conclusion

In summary, generation of iMAEC lines from various genetically modified mouse lines provides an invaluable tool to study vascular biology and pathophysiology.

Fibrinogen alters mouse brain endothelial cell layer integrity affecting vascular endothelial cadherin

Many inflammatory diseases are associated with elevated blood concentration of fibrinogen (Fg) leading to vascular dysfunction. We showed that pathologically high (4 mg/ml) content of Fg disrupts integrity of endothelial cell (EC) layer and causes macromolecular leakage affecting tight junction proteins. However, role of adherence junction proteins, particularly vascular endothelial cadherin (VE-cadherin) and matrix metalloproteinase-9 (MMP-9) in this process is not clear. We tested the hypothesis that at high levels Fg affects integrity of mouse brain endothelial cell (MBEC) monolayer through activation of MMP-9 and downregulation of VE-cadherin expression and in part its translocation to the cytosol. The effect of Fg on cultured MBEC layer integrity was assessed by measuring transendothelial electrical resistance. Cellular expression and translocation of VE-cadherin were assessed by Western blot and immunohistochemical analyses, (respectively). Our results suggest that high content of Fg decreased VE-cadherin expression at protein and mRNA levels. Fg induced translocation of VE-cadherin to cytosol, which led to disruption of cell-to-cell interaction and cell to subendothelial matrix attachment. Fg-induced alterations in cell layer integrity and their attachment were diminished during inhibition of MMP-9 activity. Thus Fg compromises EC layer integrity causing downregulation and translocation of VE-cadherin and through MMP-9 activation. These results suggest that increased level of Fg could play a significant role in vascular dysfunction and remodeling.

Cytocompatibility of electrospun nanofiber tubular scaffolds for small diameter tissue engineering blood vessels

A tubular scaffold was fabricated by using electrospun polymer solution blends of pNSR32 (recombinant spider silk protein), PCL (polycaprolactone) and Gt (gelatin). The physicochemical properties and cytocompatibility of these scaffolds were investigated. Afterwards, the pNSR32/PCL/Gt tubular scaffold (inner diameter=3mm) showed high porosity of 86.2 ± 2.9%, pore size of 2423 ± 979nm and average fibre diameter of 166 ± 85nm. Water uptake and contact angle of the scaffolds reached 112.0 ± 4.4% and 45.7 ± 13.7°, respectively. SDRAECs (Sprague Dawley Rat Aortic Endothelial Cells) grew and proliferated well and phenotype could be maintained on the composite scaffolds after they had been cultured on the composite scaffolds for 7 days. Compared with pure PCL scaffolds a greater density of viable cells was seen on the composites, especially the pNSR32/PCL/Gt scaffolds.

Endonuclease G mediates endothelial cell death induced by carbamylated LDL

End-stage kidney disease is a terminal stage of chronic kidney disease, which is associated with a high incidence of cardiovascular disease. Cardiovascular disease frequently results from endothelial injury caused by carbamylated LDL (cLDL), the product of LDL modification by urea-derived cyanate. Our previous data suggested that cLDL induces mitogen-activated protein kinase-dependent mitotic DNA fragmentation and cell death. However, the mechanism of this pathway is unknown. The current study demonstrated that cLDL-induced endothelial mitotic cell death is independent of caspase-3. The expression of endonuclease G (EndoG), the nuclease implicated in caspase-independent DNA fragmentation, was significantly increased in response to cLDL exposure to the cells. The inhibition of EndoG by RNAi protected cLDL-induced DNA fragmentation, whereas the overexpression of EndoG induced more DNA fragmentation in endothelial cells. Ex vivo experiments with primary endothelial cells isolated from wild-type (WT) and EndoG knockout (KO) mice demonstrated that EndoG KO cells are partially protected against cLDL toxicity compared with WT cells. To determine cLDL toxicity in vivo, we administered cLDL or native LDL (nLDL) intravenously to the WT and EndoG KO mice and then measured floating endothelial cells in blood using flow cytometry. The results showed an increased number of floating endothelial cells after cLDL versus nLDL injection in WT mice but not in EndoG KO mice. Finally, the inhibitors of MEK-ERK1/2 and JNK-c-jun pathways decreased cLDL-induced EndoG overexpression and DNA fragmentation. In summary, our data suggest that cLDL-induced endothelial toxicity is caspase independent and results from EndoG-dependent DNA fragmentation.

Fibrinogen induces alterations of endothelial cell tight junction proteins

We previously showed that an elevated content of fibrinogen (Fg) increased formation of filamentous actin and enhanced endothelial layer permeability. In the present work we tested the hypothesis that Fg binding to endothelial cells (ECs) alters expression of actin-associated endothelial tight junction proteins (TJP). Rat cardiac microvascular ECs were grown in gold plated chambers of an electrical cell-substrate impedance system, 8-well chambered, or in 12-well plates. Confluent ECs were treated with Fg (2 or 4 mg/ml), Fg (4 mg/ml) with mitogen-activated protein kinase (MEK) kinase inhibitors (PD98059 or U0126), Fg (4 mg/ml) with anti-ICAM-1 antibody or BQ788 (endothelin type B receptor blocker), endothelin-1, endothelin-1 with BQ788, or medium alone for 24 h. Fg induced a dose-dependent decrease in EC junction integrity as determined by transendothelial electrical resistance (TEER). Western blot analysis and RT-PCR data showed that the higher dose of Fg decreased the contents of TJPs, occludin, zona occluden-1 (ZO-1), and zona occluden-2 (ZO-2) in ECs. Fg-induced decreases in contents of the TJPs were blocked by PD98059, U0126, or anti-ICAM-1 antibody. While BQ788 inhibited endothelin-1-induced decrease in TEER, it did not affect Fg-induced decrease in TEER. These data suggest that Fg increases EC layer permeability via the MEK kinase signaling pathway by affecting occludin, ZO-1, and ZO-2, TJPs, which are bound to actin filaments. Therefore, increased binding of Fg to its major EC receptor, ICAM-1, during cardiovascular diseases may increase microvascular permeability by altering the content and possibly subcellular localization of endothelial TJPs.

Protection of vascular cells from oxidative stress by proteasome inhibition depends on Nrf2

AIMS

Increased levels of reactive oxygen species cause oxidative stress and severely damage lipids, proteins, and DNA. We have previously shown that partial proteasome inhibition induces an antioxidative gene pattern in endothelial cells. Here, we elucidate the mechanisms of proteasome inhibitor-mediated upregulation of antioxidative enzymes and cytoprotection.

METHODS AND RESULTS

Non-toxic proteasome inhibition upregulated mRNA and protein expression of superoxide dismutase 1 (SOD1) and haem oxygenase 1 (HO1) in several human endothelial and vascular smooth muscle cell types. Transcriptional activation of these enzymes was shown by inhibition of RNA polymerase II and nuclear run-on assays. Transfection of endothelial cells with luciferase reporter constructs revealed that upregulation can be largely confined to an antioxidant response element (ARE), which proved to be sufficient for transcriptional activation of SOD1 and HO1. Co-transfection studies and bandshift analyses confirmed binding of the antioxidative transcription factor NF-E2-related factor 2 (Nrf2)-which was stabilized by proteasome inhibition as shown by immunoblots-to the ARE site of HO1. Experiments with aortic endothelial and smooth muscle cells from Nrf2 wild-type and knockout mice revealed an essential role of Nrf2: in wild-type cells, proteasome inhibitor-mediated induction of SOD1 and HO1 was accompanied by protection of vascular cells against oxidative stress as determined by lactate dehydrogenase release assays. In contrast, proteasome inhibitor-mediated induction of antioxidative enzymes and cytoprotection were completely lost in cells from Nrf2 knockout mice.

CONCLUSION

Nrf2-dependent transcriptional activation of antioxidative enzymes is crucial for proteasome inhibitor-mediated protection of vascular cells against oxidative stress.

Fibrinogen-induced endothelin-1 production from endothelial cells

We previously demonstrated that fibrinogen (Fg) binding to the vascular endothelial intercellular adhesion molecule-1 (ICAM-1) leads to microvascular constriction in vivo and in vitro. Although a role of endothelin-1 (ET-1) in this Fg-induced vasoconstriction was suggested, the mechanism of action was not clear. In the current study, we tested the hypothesis that Fg-induced vasoconstriction results from ET-1 production by vascular endothelial cells (EC) and is mediated by activation of extracellular signal-regulated kinase -1/2 (ERK-1/2). Confluent, rat heart microvascular endothelial cells (RHMECs) were treated with one of the following: Fg (2 or 4 mg/ml), Fg (4 mg/ml) with ERK-1/2 kinase inhibitors (PD-98059 or U-0126), Fg (4 mg/ml) with an antibody against ICAM-1, or medium alone for 45 min. The amount of ET-1 formed and the concentration of released von Willebrand factor (vWF) in the cell culture medium were measured by ELISAs. Fg-induced exocytosis of Weibel-Palade bodies (WPBs) was assessed by immunocytochemistry. Phosphorylation of ERK-1/2 was detected by Western blot analysis. Fg caused a dose-dependent increase in ET-1 formation and release of vWF from the RHMECs. This Fg-induced increase in ET-1 production was inhibited by specific ERK-1/2 kinase inhibitors and by anti-ICAM-1 antibody. Immunocytochemical staining showed that an increase in Fg concentration enhanced exocytosis of WPBs in ECs. A specific endothelin type B receptor blocker, BQ-788, attenuated the enhanced phosphorylation of ERK-1/2 in ECs caused by increased Fg content in the culture medium. The presence of an endothelin converting enzyme inhibitor, SM-19712, slightly decreased Fg-induced phosphorylation of ERK-1/2, but inhibited production of Fg-induced ET-1 production. These results suggest that Fg-induced vasoconstriction may be mediated, in part, by activation of ERK-1/2 signaling and increased production of ET-1 that further increases EC ERK-1/2 signaling. Thus, an increased content of Fg may enhance vasoconstriction through increased production of ET-1.

gamma-Aminbuturic acid A receptor mitigates homocysteine-induced endothelial cell permeability

Many cerebrovascular disorders are accompanied by an increased homocysteine (Hcy) levels. We have previously shown that acute hyperhomocysteinemia (HHcy) leads to an increased microvascular permeability in the mouse brain. Hcy competitively binds to gamma -aminbuturic acid (GABA) receptors and may increase vascular permeability by acting as an excitatory neurotransmitter. However, the role of GABA-A (GABA(A)) receptor in Hcy-induced endothelial cell (EC) permeability remains unclear. In the present study we attempted to determine the role of GABA(A) receptor and the possible mechanisms involved in Hcy-induced EC layer permeability. Mouse aortic and brain ECs were grown in Transwells and treated with 50 mu M Hcy in the presence or absence of GABA(A)-specific agonist muscimol. Role of matrix metalloproteinase-9 (MMP-9) was determined using its activity inhibitor GM-6001. Involvement of extracellular signal-regulated kinase (ERK) signaling was assessed using its kinase activity inhibitors PD98059 or U0126. EC permeability to the known content of bovine serum albumin (BSA)-conjugated with Alexa Flour-488 was assessed by measuring fluorescence intensity of the solutes in the Transwell's lower chambers. It was found that Hcy induced the formation of filamentous actin (F-actin). Hcy-induced EC permeability to BSA was significantly decreased by GABA and muscimol treatments. Presence of MMP-9 or ERK kinase activity inhibitors restored the Hcy-induced EC permeability to its baseline level. The mediation BSA leakage through the ECs was further confirmed in the experiments where Hcy-induced alterations in transendothelial electrical resistance of confluent ECs were assessed. The data suggest that Hcy increases EC layer permeability through inhibition of GABA(A) receptor and F-actin formation, in part, by transducing ERK and MMP-9 activation.

Homocysteine causes cerebrovascular leakage in mice

Elevated plasma homocysteine (Hcy) is associated with cerebrovascular disease and activates matrix metalloproteinases (MMPs), which lead to vascular remodeling that could disrupt the blood-brain barrier. To determine whether Hcy administration can increase brain microvascular leakage secondary to activation of MMPs, we examined pial venules by intravital video microscopy through a craniotomy in anesthetized mice. Bovine serum albumin labeled with fluorescein isothiocyanate (BSA-FITC) was injected into a carotid artery to measure extravenular leakage. Hcy (30 microM/total blood volume) was injected 10 min after FITC-BSA injection. Four groups of mice were examined: 1) wild type (WT) given vehicle; 2) WT given Hcy (WT + Hcy); 3) MMP-9 gene knockout given Hcy (MMP-9-/- + Hcy); and 4) MMP-9-/- with topical application of histamine (10(-4) M) (MMP-9-/- + histamine). In the WT + Hcy mice, leakage of FITC-BSA from pial venules was significantly (P < 0.05) greater than in the other groups. There was no significant leakage of pial microvessels in MMP-9-/- + Hcy mice. Increased cerebrovascular leakage in the MMP-9-/- + histamine group showed that microvascular permeability could still increase by a mechanism independent of MMP-9. Treatment of cultured mouse microvascular endothelial cells with 30 microM Hcy resulted in significantly greater F-actin formation than in control cells without Hcy. Treatment with a broad-range MMP inhibitor (GM-6001; 1 microM) ameliorated Hcy-induced F-actin formation. These data suggest that Hcy increases microvascular permeability, in part, through MMP-9 activation.

A method of isolation and culture of microvascular endothelial cells from mouse skin

OBJECTIVES

The study of isolated microvascular endothelial cells from mice has long been impeded due to the many difficulties encountered in isolating and culturing these cells. We focused on developing a method to isolate microvascular endothelial cells from the skin fragments of newborn mice. We also aimed at establishing optimal culture conditions to sustain the growth of these cells.

METHODS AND RESULTS

Isolation of murine dermal microvascular endothelial cells (mDMEC) from P3 newborn mice was based first on enzymatic separation of the skin epidermal layer from the dermis using dispase and then on disaggregating dermal cellular elements using collagenase. The cells obtained from the dermis were subjected to a continuous density gradient centrifugation. Cells situated between densities 1.033 and 1.047 were then cultured on collagen IV-coated culture flasks using optimized growth culture conditions. Cells were characterized by endothelial appearance and by the presence and genetic expression of endothelial markers like CD31, NOS3, VEGFR-2 and Tie-2. Uptake of acetylated low-density lipoprotein (Ac-LDL) was used as a functional assay.

CONCLUSIONS

The methodology described herein for isolation and culture of murine microvascular endothelium offers a distinctive advantage for those using mouse models to study endothelial cell biology.