Vasoactive substances produced by cultured rat brain endothelial cells

Effects of sub-chronic, in vivo administration of sigma-1 receptor ligands on platelet and aortic arachidonate cascade in streptozotocin-induced diabetic rats

Background

Diabetes mellitus is a chronic metabolic disorder which induces endothelial dysfunction and platelet activation. Eicosanoids produced from arachidonic acid regulate cellular and vascular functions. Sigma-1 receptors (S1R) are expressed in platelets and endothelial cells and S1R expression is protective in diabetes.

Objectives

Our aim was to examine the influence of sub-chronic, in vivo administered S1R ligands PRE-084, (S)-L1 (a new compound) and NE-100 on the ex vivo arachidonic acid metabolism of platelets and aorta in streptozotocin-induced diabetic rats.

Methods

The serum level of the S1R ligands was detected by LC-MS/MS before the ex vivo analysis. Sigma-1 receptor and cyclooxygenase gene expression in platelets were determined by RT-qPCR. The eicosanoid synthesis was examined with a radiolabelled arachidonic acid substrate and ELISA.

Results

One month after the onset of STZ-induced diabetes, in vehicle-treated, diabetic rat platelet TxB2 and aortic 6-k-PGF1α production dropped. Sub-chronic in vivo treatment of STZ-induced diabetes in rats for one week with PRE-084 enhanced vasoconstrictor and platelet aggregator and reduced vasodilator and anti-aggregator cyclooxygenase product formation. (S)-L1 reduced the synthesis of vasodilator and anti-aggregator cyclooxygenase metabolites and promoted the recovery of physiological platelet function in diabetic rats. The S1R antagonist NE-100 produced no significant changes in platelet arachidonic acid metabolism. (S)-L1 decreased the synthesis of vasoconstrictor and platelet aggregator cyclooxygenase metabolites, whereas NE-100 increased the quantity of aortic vasodilator and anti-aggregator cyclooxygenase products and promoted the recovery of diabetic endothelial dysfunction in the aorta. The novel S1R ligand, (S)-L1 had similar effects on eicosanoid synthesis in platelets as the agonist PRE-084 and in aortas as the antagonist NE-100.

Conclusions

S1R ligands regulate cellular functions and local blood circulation by influencing arachidonic acid metabolism. In diabetes mellitus, the cell-specific effects of S1R ligands have a compensatory role and aid in restoring physiological balance between the platelet and vessel.

Effects of sub-chronic, in vivo administration of sigma non-opioid intracellular receptor 1 ligands on platelet and aortic arachidonate cascade in rats

Platelets regulate cell-cell interactions and local circulation through eicosanoids from arachidonic acid. Sigma non-opioid intracellular receptor 1 (sigma-1 receptor) expressed in platelets and endothelial cells can regulate intracellular signalization. Our aim was to examine the influence of sub-chronic, in vivo-administered sigma-1 receptor ligands 2-morpholin-4-ylethyl 1-phenylcyclohexane-1-carboxylate (PRE-084); N-benzyl-2-[(1S)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-1-yl]ethan-1-amine; dihydrochloride, a new compound ((S)-L1); and N-[2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethyl]-N-propylpropan-1-amine (NE-100) on the ex vivo arachidonic acid metabolism of the platelets and aorta of male rats. The serum level of sigma-1 receptor ligands was determined by liquid chromatography-mass spectrometry. Sigma-1 receptor and cyclooxygenase gene expression in the platelets were determined by a reverse transcription-coupled quantitative polymerase chain reaction. The eicosanoid synthesis was examined using a radiolabeled arachidonic acid substrate and enzyme-linked immunosorbent assay. We confirmed the absorption of sigma-1 receptor ligands and confirmed that the ligands were not present during the ex vivo studies, so their acute effect could be excluded. We detected no changes in either sigma-1 receptor or cyclooxygenase mRNA levels in the platelets. Nevertheless, (S)-L1 and NE-100 increased the quantity of cyclooxygenases there. Both platelet and aortic eicosanoid synthesis was modified by the ligands, although in different ways. The effect of the new sigma-1 receptor ligand, (S)-L1, was similar to that of PRE-084 in most of the parameters studied but was found to be more potent. Our results suggest that sigma-1 receptor ligands may act at multiple points in arachidonic acid metabolism and play an important role in the control of the microcirculation by modulating the eicosanoid synthesis of the platelets and vessels.

S1R agonist modulates rat platelet eicosanoid synthesis and aggregation

Sigma-1 receptor (S1R) is detected in different cell types and can regulate intracellular signaling pathways. S1R plays a role in the pathomechanism of diseases and the regulation of neurotransmitters. Fluvoxamine can bind to S1R and reduce the serotonin uptake of neurons and platelets. We therefore hypothesized that platelets express S1R, which can modify platelet function. The expression of the SIGMAR1 gene in rat platelets was examined with a reverse transcription polymerase chain reaction and a quantitative polymerase chain reaction. The receptor was also visualized by immunostaining and confocal laser scanning microscopy. The effect of S1R agonist PRE-084 on the eicosanoid synthesis of isolated rat platelets and ADP- and AA-induced platelet aggregation was examined. S1R was detected in rat platelets both at gene and protein levels. Pretreatment with PRE-084 of resting platelets induced elevation of eicosanoid synthesis. The rate of elevation in thromboxane B₂ and prostaglandin D₂ synthesis was similar, but the production of prostaglandin E₂ was higher. The concentration-response curve showed a sigmoidal form. The most effective concentration of the agonist was 2 µM. PRE-084 increased the quantity of cyclooxygenase-1 as detected by ELISA. PRE-084 also elevated the ADP- and AA-induced platelet aggregation. S1R of platelets might regulate physiological or pathological functions.

Recent Expansions on Cellular Models to Uncover the Scientific Barriers Towards Drug Development for Alzheimer's Disease

Globally, the central nervous system (CNS) disorders appear as the most critical pathological threat with no proper cure. Alzheimer's disease (AD) is one such condition frequently observed with the aged population and sometimes in youth too. Most of the research utilizes different animal models for in vivo study of AD pathophysiology and to investigate the potency of the newly developed therapy. These in vivo models undoubtably provide a powerful investigation tool to study human brain. Although, it sometime fails to mimic the exact environment and responses as the human brain owing to the distinctive genetic and anatomical features of human and rodent brain. In such condition, the in vitro cell model derived from patient specific cell or human cell lines can recapitulate the human brain environment. In addition, the frequent use of animals in research increases the cost of study and creates various ethical issues. Instead, the use of in vitro cellular models along with animal models can enhance the translational values of in vivo models and represent a better and effective mean to investigate the potency of therapeutics. This strategy also limits the excessive use of laboratory animal during the drug development process. Generally, the in vitro cell lines are cultured from AD rat brain endothelial cells, the rodent models, human astrocytes, human brain capillary endothelial cells, patient derived iPSCs (induced pluripotent stem cells) and also from the non-neuronal cells. During the literature review process, we observed that there are very few reviews available which describe the significance and characteristics of in vitro cell lines, for AD investigation. Thus, in the present review article, we have compiled the various in vitro cell lines used in AD investigation including HBMEC, BCECs, SHSY-5Y, hCMEC/D3, PC-2 cell line, bEND3 cells, HEK293, hNPCs, RBE4 cells, SK-N-MC, BMVECs, CALU-3, 7W CHO, iPSCs and cerebral organoids cell lines and different types of culture media such as SCM, EMEM, DMEM/F12, RPMI, EBM and 3D-cell culture.

Comparison of a Rat Primary Cell-Based Blood-Brain Barrier Model With Epithelial and Brain Endothelial Cell Lines: Gene Expression and Drug Transport

Cell culture-based blood-brain barrier (BBB) models are useful tools for screening of CNS drug candidates. Cell sources for BBB models include primary brain endothelial cells or immortalized brain endothelial cell lines. Despite their well-known differences, epithelial cell lines are also used as surrogate models for testing neuropharmaceuticals. The aim of the present study was to compare the expression of selected BBB related genes including tight junction proteins, solute carriers (SLC), ABC transporters, metabolic enzymes and to describe the paracellular properties of nine different culture models. To establish a primary BBB model rat brain capillary endothelial cells were co-cultured with rat pericytes and astrocytes (EPA). As other BBB and surrogate models four brain endothelial cells lines, rat GP8 and RBE4 cells, and human hCMEC/D3 cells with or without lithium treatment (D3 and D3L), and four epithelial cell lines, native human intestinal Caco-2 and high P-glycoprotein expressing vinblastine-selected VB-Caco-2 cells, native MDCK and MDR1 transfected MDCK canine kidney cells were used. To test transporter functionality, the permeability of 12 molecules, glucopyranose, valproate, baclofen, gabapentin, probenecid, salicylate, rosuvastatin, pravastatin, atorvastatin, tacrine, donepezil, was also measured in the EPA and epithelial models. Among the junctional protein genes, the expression level of occludin was high in all models except the GP8 and RBE4 cells, and each model expressed a unique claudin pattern. Major BBB efflux (P-glycoprotein or ABCB1) and influx transporters (GLUT-1, LAT-1) were present in all models at mRNA levels. The transcript of BCRP (ABCG2) was not expressed in MDCK, GP8 and RBE4 cells. The absence of gene expression of important BBB efflux and influx transporters BCRP, MRP6, -9, MCT6, -8, PHT2, OATPs in one or both types of epithelial models suggests that Caco-2 or MDCK models are not suitable to test drug candidates which are substrates of these transporters. Brain endothelial cell lines GP8, RBE4, D3 and D3L did not form a restrictive paracellular barrier necessary for screening small molecular weight pharmacons. Therefore, among the tested culture models, the primary cell-based EPA model is suitable for the functional analysis of the BBB.

Kynurenic acid and its derivatives are able to modulate the adhesion and locomotion of brain endothelial cells

The neuroprotective actions of kynurenic acid (KYNA) and its derivatives in several neurodegenerative disorders [characterized by damage to the cerebral endothelium and to the blood-brain barrier (BBB)] are well established. Cell-extracellular matrix (ECM) adhesion is supposedly involved in recovery of impaired cerebral endothelium integrity (endothelial repair). The present work aimed to investigate the effects of KYNA and its synthetic derivatives on cellular behaviour (e.g. adhesion and locomotion) and on morphology of the GP8 rat brain endothelial cell line, modeling the BBB endothelium. The effects of KYNA and its derivatives on cell adhesion were measured using an impedance-based technique, the xCELLigence SP system. Holographic microscopy (Holomonitor^™ M4) was used to analyse both chemokinetic responses and morphometry. The GP8 cells proved to be a suitable model cell line for investigating cell adhesion and the locomotion modulator effects of kynurenines. KYNA enhanced cell adhesion and spreading, and also decreased the migration/motility of GP8 cells at physiological concentrations (10^-9 and 10^-7 mol/L). The derivatives containing an amide side-chain at the C2 position (KYNA-A1 and A2) had lower adhesion inducer effects compared to KYNA. All synthetic analogues (except KYNA-A5) had a time-dependent inhibitory effect on GP8 cell adhesion at a supraphysiological concentration (10^-3 mol/L). The immobilization promoting effect of KYNA and the adhesion inducer activity of its derivatives indicate that these compounds could contribute to maintaining or restoring the protective function of brain endothelium; they also suggest that cell-ECM adhesion and related cell responses (e.g. migration/motility) could be potential new targets of KYNA.

Exposure to lipopolysaccharide and/or unconjugated bilirubin impair the integrity and function of brain microvascular endothelial cells

Background

Sepsis and jaundice are common conditions in newborns that can lead to brain damage. Though lipopolysaccharide (LPS) is known to alter the integrity of the blood-brain barrier (BBB), little is known on the effects of unconjugated bilirubin (UCB) and even less on the joint effects of UCB and LPS on brain microvascular endothelial cells (BMEC).

Methodology/Principal Findings

Monolayers of primary rat BMEC were treated with 1 µg/ml LPS and/or 50 µM UCB, in the presence of 100 µM human serum albumin, for 4 or 24 h. Co-cultures of BMEC with astroglial cells, a more complex BBB model, were used in selected experiments. LPS led to apoptosis and UCB induced both apoptotic and necrotic-like cell death. LPS and UCB led to inhibition of P-glycoprotein and activation of matrix metalloproteinases-2 and -9 in mono-cultures. Transmission electron microscopy evidenced apoptotic bodies, as well as damaged mitochondria and rough endoplasmic reticulum in BMEC by either insult. Shorter cell contacts and increased caveolae-like invaginations were noticeable in LPS-treated cells and loss of intercellular junctions was observed upon treatment with UCB. Both compounds triggered impairment of endothelial permeability and transendothelial electrical resistance both in mono- and co-cultures. The functional changes were confirmed by alterations in immunostaining for junctional proteins β-catenin, ZO-1 and claudin-5. Enlargement of intercellular spaces, and redistribution of junctional proteins were found in BMEC after exposure to LPS and UCB.

Conclusions

LPS and/or UCB exert direct toxic effects on BMEC, with distinct temporal profiles and mechanisms of action. Therefore, the impairment of brain endothelial integrity upon exposure to these neurotoxins may favor their access to the brain, thus increasing the risk of injury and requiring adequate clinical management of sepsis and jaundice in the neonatal period.

Pinocembrin attenuates blood-brain barrier injury induced by global cerebral ischemia-reperfusion in rats

Blood-brain barrier (BBB) disruption is a major consequence of cerebral ischemia/reperfusion. Several studies have reported the neuroprotection of pinocembrin on cerebral ischemia in vivo and in vitro, but the effects of pinocembrin on BBB and its underlying mechanisms are not clear. In this study, we investigated the effects of pinocembrin on BBB functions in the global cerebral ischemia/reperfusion (GCI/R) model in rats. Neurological scores and brain edema were evaluated. BBB permeability was assessed by detecting the concentrations of Evan's blue (EB) and fluorescein sodium (NaF) in brain tissue. The pathological changes of BBB ultrastructure were observed by transmission electron microscopy. Cerebral blood flow (CBF) was measured by laser Doppler flowmetry. The effects of pinocembrin on primary cultured rat cerebral microvascular endothelial cells (RCMECs) against oxygen-glucose deprivation/reoxygenation (OGD/R) were also investigated. The results showed pinocembrin decreased neurological score and lessened brain edema induced by GCI/R. Pinocembrin also reduced the concentrations of EB and NaF in brain tissue of the GCI/R rats. And pinocembrin alleviated the ultrastructural changes of cerebral microvessels, astrocyte end-feet and neurons, and improved CBF in the GCI/R rats. In addition, pinocembrin increased the viability and mitochondrial membrane potential of cultured RCMECs induced by OGD/R. In conclusion, these data demonstrate that pinocembrin alleviates blood-brain barrier injury induced by GCI/R in rats.

Adaptation of the hypothalamic blood flow to chronic nitric oxide deficiency is independent of vasodilator prostanoids

The aim of our study was to investigate the adaptation of the hypothalamic circulation to chronic nitric oxide (NO) deficiency in rats. Hypothalamic blood flow (HBF) remained unaltered during chronic oral administration of the NO synthase (NOS) inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME, 1 mg/ml drinking water) although acute NOS blockade by intravenous l-NAME injection (50 mg/kg) induced a dramatic HBF decrease. In chronically NOS blocked animals, however, acute l-NAME administration failed to influence the HBF. Reversal of chronic NOS blockade by intravenous l-arginine infusion evoked significant hypothalamic hyperemia suggesting the appearance of a compensatory vasodilator mechanism in the absence of NO. In order to clarify the potential involvement of vasodilator prostanoids in this adaptation, cyclooxygenase (COX) mRNA and protein levels were determined in the hypothalamus, but none of the known isoenzymes (COX-1, COX-2, COX-3) showed upregulation after chronic NOS blockade. Furthermore, levels of vasodilator prostanoid (PGI(2), PGE(2) and PGD(2)) metabolites were also not elevated. Interestingly, however, hypothalamic levels of vasoconstrictor prostanoids (TXA(2) and PGF(2alpha)) decreased after chronic NOS blockade. COX inhibition by indomethacin but not by diclofenac decreased the HBF in control animals. However, neither indomethacin nor diclofenac induced an altered HBF-response after chronic l-NAME treatment. Although urinary excretion of PGI(2) and PGE(2) metabolites markedly increased during chronic NOS blockade, indicating COX activation in the systemic circulation, we conclude that the adaptation of the hypothalamic circulation to the reduction of NO synthesis is independent of vasodilator prostanoids. Reduced release of vasoconstrictor prostanoids, however, may contribute to the normalization of HBF after chronic loss of NO.

Nicotiflorin reduces cerebral ischemic damage and upregulates endothelial nitric oxide synthase in primarily cultured rat cerebral blood vessel endothelial cells

Nicotiflorin is a flavonoid glycoside extracted from a traditional Chinese medicine Flos Carthami. In the current study, we investigated the neuroprotective effect of nicotiflorin on a transient focal cerebral ischemia-reperfusion model in rats. Nicotiflorin (2.5-10 mg/kg) administered after onset of ischemia markedly reduced brain infarct volume by 24.5-63.2% and neurological deficits. Also the effect of nicotiflorin on endothelial nitric oxide synthase (eNOS) activity, mRNA and protein expression after hypoxia-reoxygenation (H-R) treatment was investigated in an in vitro model mimic cerebrum ischemia-reperfusion in vivo. After total 4 h hypoxia and 12 h reoxygenation, eNOS activity, mRNA and protein levels in the primarily cultured rat cerebral blood vessel endothelial cells treated with nicotiflorin (25-100 microg/ml) 2 h after onset of hypoxia were significantly higher than eNOS activity, mRNA and protein levels in the pure H-R cells and also higher than eNOS activity, mRNA and protein levels in cells cultured under normoxic conditions. The results demonstrated that nicotiflorin had a protective effect against cerebral ischemic damage. The results also gave an important elucidation for the mechanism underlying the protective effect at the cellular level.

Adrenomedullin 2 protects rat cerebral endothelial cells from oxidative damage in vitro

Adrenomedullin 2 (AM2, intermedin) is a recently identified new member of the calcitonin gene-related peptide family. We examined whether AM2 can attenuate the increased blood-brain barrier permeability and cerebral endothelial cell (CEC) death induced by oxidative stress in vitro. Hydrogen peroxide (H(2)O(2), 0.5 mM) induced a continuous decrease of the transendothelial electrical resistance (TEER) and resulted in intercellular gap formations in rat CECs co-cultured with astrocytes. AM2 induced cAMP and nitric oxide production, increased TEER, enhanced peripheral localization of F-actin bands, and attenuated the increased permeability induced by H(2)O(2). AM2 treatment preserved mitochondrial membrane potential and improved CEC viability in H(2)O(2) treated cultures. These effects of AM2 were similar to those what were reported for adrenomedullin. These results suggest that AM2 protects CECs against oxidative injury in vitro.

Inflammatory mediators of cerebral endothelium: a role in ischemic brain inflammation

Brain inflammation has been implicated in the development of brain edema and secondary brain damage in ischemia and trauma. Adhesion molecules, cytokines and leukocyte chemoattractants released/presented at the site of blood-brain barrier (BBB) play an important role in mobilizing peripheral inflammatory cells into the brain. Cerebral endothelial cells (CEC) are actively engaged in processes of microvascular stasis and leukocyte infiltration by producing a plethora of pro-inflammatory mediators. When challenged by external stimuli including cytokines and hypoxia, CEC have been shown to release/express various products of arachidonic acid cascade with both vasoactive and pro-inflammatory properties, including prostaglandins, leukotrienes, and platelet-activating factor (PAF). These metabolites induce platelet and neutrophil activation and adhesion, changes in local cerebral blood flow and blood rheology, and increases in BBB permeability. Ischemic CEC have also been shown to express and release bioactive inflammatory cytokines and chemokines, including IL-1beta, IL-8 and MCP-1. Many of these mediators and ischemia in vitro and in vivo have been shown to up-regulate the expression of both selectin and Ig-families of adhesion molecules in CEC and to facilitate leukocyte adhesion and transmigration into the brain. Collectively, these studies demonstrate a pivotal role of CEC in initiating and regulating inflammatory responses in cerebral ischemia.

Effects of LPS stimulation on the expression of prostaglandin carriers in the cells of the blood-brain and blood-cerebrospinal fluid barriers

Prostaglandins produced in cerebral endothelial cells (CECs) are the final signal transduction mediators from the periphery to the brain during fever response. However, prostaglandins are organic anions at physiological pH, and they enter cells poorly using simple diffusion. Several transporters have been described that specifically transport prostaglandins across cell membranes. We examined the expression of the two principal prostaglandin carriers, prostaglandin transporter (PGT), and multidrug resistance-associated protein 4 (MRP4) in cells of the blood-brain barrier and in choroid epithelial cells in vitro as well as in vivo in rat brain in control conditions and after lipopolysaccharide (LPS) challenge. We detected PGT in primary cultures of rat CECs, astrocytes, pericytes, and choroid epithelial cells. LPS stimulation had no effect on the expression level of PGT in these cells; however, after LPS stimulation the polarized, dominantly luminal, expression pattern of PGT significantly changed. MRP4 is also expressed in CECs, and its level was not influenced by LPS treatment. In rat brain, PGT was highly expressed in the supraoptic and paraventricular nuclei of the hypothalamus, in the ependymal cell layer of the third ventricle, and in the choroid plexus. LPS treatment increased the expression of PGT in the supraoptic and paraventricular nuclei. Our results suggest that PGT and MRP4 likely play a role in transporting prostaglandins through the blood-brain and blood-cerebrospinal fluid barriers and may be involved in the maintenance of prostaglandin homeostasis in the brain and in the initiation of fever response.

Autocrine peptide mediators of cerebral endothelial cells and their role in the regulation of blood-brain barrier

A unique feature of cerebral endothelial cells (CECs) is the formation of the blood-brain barrier (BBB), which contributes to the stability of the brain microenvironment. CECs are capable of producing several substances mediating endothelium-dependent vasorelaxation or vasoconstriction, regulating BBB permeability, and participating in the regulation of cell-cell interactions during inflammatory and immunological processes. The chemical nature of these mediators produced by CECs ranges from gaseous anorganic molecules (e.g. nitric oxide) through lipid mediators (e.g. prostaglandins) to peptides. Peptide mediators are a large and diverse family of bioactive molecules which can elicit multiple effects on cerebral endothelial functions. In this review, we summarize current knowledge of peptide mediators produced by CECs, such as adrenomedullin, angiotensin, endothelin and several others and their role in the regulation of BBB functions.

Adrenomedullin protects rat cerebral endothelial cells from oxidant damage in vitro

Increased permeability and reduced cerebral endothelial cell (CEC) viability induced by oxidative stress are the hallmarks of the blood-brain barrier disruption. In our experiments hydrogen peroxide (H2O2, 0.5 mM) induced a continuous decrease of the transendothelial electrical resistance (TEER) and resulted in intercellular gap formations in cultured rat CECs. Adrenomedullin (AM) increased TEER, enhanced peripheral localization of F-actin bands and attenuated the increased permeability induced by H2O2. Furthermore, AM treatment preserved mitochondrial membrane potential, attenuated cytochrome c release, and consequently improved CEC viability in H2O2 treated cultures. These results suggest that AM treatment protects CECs against oxidative injury.

Diazoxide preconditioning attenuates global cerebral ischemia-induced blood-brain barrier permeability

Brain edema formation due to blood-brain barrier (BBB) disruption is a major consequence of cerebral ischemia. Previously, we demonstrated that targeting mitochondrial ATP-sensitive potassium channels (mitoK(ATP)) protects neuronal tissues in vivo and in vitro, however, the effects of mitoK(ATP) openers on cerebral endothelial cells and on BBB functions have never been examined. We investigated the effects of mitoK(ATP) channel opener diazoxide on BBB functions during ischemia/reperfusion injury (I/R). Rats were treated with 6, 20 or 40 mg/kg diazoxide ip for 3 days then exposed to global cerebral ischemia for 30 min. BBB permeability was assessed by administering Evan's-blue (EB) and Na-fluorescein (NaF) at the beginning of the 30 min reperfusion. I/R increased BBB permeability for the large molecular weight EB (ng/mg) in the cortex (control: 146 +/- 12, n = 7; I/R: 1049 +/- 152, n = 11) which was significantly attenuated in diazoxide-treated rats (575 +/- 99, n = 9; 582 +/- 104, n = 8; 20 and 40 mg/kg doses). Diazoxide pretreatment also significantly inhibited the extravasation of the low molecular weight NaF. Edema formation in the cortex was also decreased after diazoxide pretreatment. In cultured cerebral endothelial cells, diazoxide depolarized the mitochondrial membrane, suggesting a direct diazoxide effect on the endothelial mitochondria. Our results demonstrate that preconditioning of cerebral endothelium with diazoxide protects the BBB against ischemic stress.

Rat brain endothelial cell lines for the study of blood-brain barrier permeability and transport functions

(1) In vitro models of the BBB have been developed from cocultures between bovine, porcine, rodent or human brain capillary endothelial cells with rodent or human astrocytes. Since most in vivo BBB studies have been performed with small laboratory animals, especially rats, it is important to establish a rat brain endothelial (RBE) cell culture system that will allow correlations between in vitro and in vivo results. The present review will constitute a brief description of the best characterized RBE cell lines (RBE4, GP8/3.9, GPNT, RBEC1, TR-BBBs and rBCEC4 cell lines) and will summarize their recent and important contribution to our current knowledge of the BBB transport functions and permeability to blood-borne solutes, drugs, and cells. (2) In most cases, primary cultures of RBE cells were transduced with an immortalizing gene (SV40 or polyoma virus large T-antigen or adenovirus E1A), either by transfection of plasmid DNA or by infection using retroviral vectors. In one case however, the conditionally immortalized TR-BBB cell line was derived from primary cultures of brain endothelial cells of SV40-T-expressing transgenic rats. (3) All cell lines appear to have an endothelial morphology. The absence of foci formation would mean that the cells are not transformed. The endothelial origin is shown by the expression of Factor VIII-related antigen. Immortalized RBE cells express all the enzymes and transporters that are considered as specific for the blood-brain barrier endothelium, with similar characteristics to those expected from in vivo analyses, but at a significantly lower level. Some RBE cell lines are responsive to astroglial factors, such as RBE4 cells, rBEC4, and TR-BBB cells. None of the immortalized RBE cell lines appear to generate the necessary restrictive paracellular barrier properties that would allow to use them in transendothelial permeability screening. (4) RBE cell lines have been used to demonstrate that transporters such as organic cation transporter/carnitine transporter, serotonin transporter, and the ATA2 system A isoform are expressed in rat brain endothelium. When the transporter is shown to be expressed with the same properties in the immortalized RBE cells as in vivo, regulation studies may be initiated even if the transporter is down-regulated. Pharmacological applications have been proposed with well-characterized transporters such as monocarboxylic acid transporter-1, large neutral amino acid tansporter-1, nucleoside carrier systems, and P-glycoprotein. RBE cell monolayers have also been used to investigate the mechanism of the transendothelial transport of large molecules, such as immunoliposomes or nanoparticles, potentially useful as drug delivery vectors to the brain. (5) RBE4 and GP8 cell lines have been extensively used to demonstrate that intercellular adhesion molecule-1 (ICAM-1) engagement in brain endothelial cells triggers multiple signal transduction pathways. Using functional assays, it was established that ICAM-1 signaling is intimately and actively involved in facilitating lymphocyte infiltration. (6) Several RBE cell lines have been described, which constitute tentative in vitro models of the rat BBB. The major limitation of these models generally appears to be due to their relatively high paracellular permeability to small molecules, thus limiting their use for permeability studies. The strategies developed for the production of these RBE cell lines will enable the characterization of still more efficient permeability models, as well as the immortalization of human brain endothelial cells.

Acetaminophen-sensitive prostaglandin production in rat cerebral endothelial cells

Acetaminophen is a widely used antipyretic and analgesic drug whose mechanism of action has recently been suggested to involve inhibitory effects on prostaglandin synthesis via a newly discovered cyclooxygenase variant (COX-3). Because COX-3 expression is high in cerebral endothelium, we investigated the effect of acetaminophen on the prostaglandin production of cultured rat cerebral endothelial cells (CECs). Acetaminophen dose-dependently inhibited both basal and LPS-induced PGE2 production in CECs with IC50 values of 15.5 and 6.9 μM, respectively. Acetaminophen also similarly inhibited the synthesis of 6-keto-PGF1α and thromboxane B2. LPS stimulation increased the expression of COX-2 but not COX-1 or COX-3. In addition, the selective COX-2 inhibitor NS398 (1 μM) was equally as effective as acetaminophen in blocking LPS-induced PGE2 production. Acetaminophen did not influence the expression of the three COX isoforms and the inducible nitric oxide synthase. In LPS-stimulated isolated cerebral microvessels, acetaminophen also significantly inhibited PGE2 production. Our results show that prostaglandin production in CECs during basal and stimulated conditions is very sensitive to inhibition by acetaminophen and suggest that acetaminophen acts against COX-2 and not COX-1 or COX-3. Furthermore, our findings support a critical role for cerebral endothelium in the therapeutic actions of acetaminophen in the central nervous system.

Cloning and Characterization of Cyclooxygenase-1b (Putative Cyclooxygenase-3) in Rat

A splice variant of cyclooxygenase-1 (COX-1), COX-1b (previously termed as COX-3), has been identified in canine tissues as an acetaminophen-sensitive isoform, but the sequence of COX-1b mRNA and the encoded protein are not known in rats. We cloned and sequenced rat COX-1b mRNA from cerebral endothelial cells. Sequence analysis indicated that the 98-base pair intron-1 of COX-1 gene remains unprocessed in the COX-1b mRNA, causing a frameshift mutation and a 127-amino acid open reading frame with no sequence similarity with known cyclooxygenases. Transient and permanent transfection of COS-7 cells with a vector containing the rat COX-1b cDNA resulted in synthesis of a protein of the expected size. We generated an affinity-purified polyclonal antibody against the rat COX-1b protein. Western blot analysis of rat tissues using this antibody demonstrated the likely existence of rat COX-1b protein in vivo with the highest expression in heart, kidney, and neuronal tissues. Our results on both stable and on transiently transfected COS-7 cells suggest that rat COX-1b does not have cyclooxygenase activity and does not have any effect on the inhibition of prostaglandin production by acetaminophen. Because this protein has a completely different amino acid sequence than COX-1 and COX-2 and it does not have cyclooxygenase activity, we suggest a name cyclooxygenase variant protein to distinguish it from the known prostaglandin-synthesizing cyclooxygenase isoforms.

Putative cyclooxygenase-3 expression in rat brain cells

Cyclooxygenase-3 (COX-3), a new acetaminophen-sensitive isoform of the COX family, has recently been cloned from canine tissues. Canine COX-3 apparently is identical to the full-length form of COX-1, with the exception that the COX-3 mRNA retains intron 1. Additionally, COX-3 mRNA expression is high in the brain. We investigated the expression of the putative rat COX-3 mRNA in primary cultures of neurons, astrocytes, endothelial cells, pericytes, and choroidal epithelial cells from the rat brain. Specific RT-PCR primers were designed to detect putative rat COX-3 mRNA, and the RT-PCR products were sequenced and compared to the known sequence of the rat COX-1 gene. Our results demonstrate that the mRNA of the putative COX-3 is expressed in all of the cell types except neurons. Cerebral endothelial cells showed the highest COX-3 expression. Whereas COX-2 expression increased several-fold after lipopolysaccharide (LPS) challenge, COX-1 and COX-3 expression did not change significantly, suggesting that cells constitutively express COX-3. In summary, we report, for the first time to our knowledge, that the putative COX-3 mRNA is detectable in rats and is differentially expressed in various cell types from rat brain, as well as that its expression is not stimulated by LPS.

Adrenomedullin, an autocrine mediator of blood-brain barrier function

Since the discovery that adrenomedullin gene expression is 20- to 40-fold higher in endothelial cells than even in the adrenal medulla, this peptide has been regarded as an important secretory product of the vascular endothelium, together with nitric oxide, eicosanoids, endothelin-1, and other vasoactive metabolites. Cerebral endothelial cells secrete an exceptionally large amount of adrenomedullin, and the adrenomedullin concentration is about 50% higher in the cerebral circulation than in the peripheral vasculature. The adrenomedullin production of cerebral endothelial cells is induced by astrocyte-derived factors. Adrenomedullin causes vasodilation in the cerebral circulation, may participate in the maintenance of the resting cerebral blood flow, and may be protective against ischemic brain injury. Recent data from our laboratory indicate that adrenomedullin, as an endothelium-derived autocrine/paracrine hormone, plays an important role in the regulation of specific blood-brain barrier properties. Adrenomedullin is suggested to be one of the physiological links between astrocyte-derived factors, cyclic adenosine 3'5'-monophosphate (cAMP), and the induction and maintenance of the blood-brain barrier. Moreover, the role of adrenomedullin in the differentiation and proliferation of endothelial cells and in angiogenesis suggests a more complex function for adrenomedullin in the cerebral circulation and in the development of the blood-brain barrier.

Chronic adrenomedullin treatment improves blood-brain barrier function but has no effects on expression of tight junction proteins

We previously found that the production of adrenomedullin (AM) is one magnitude higher in cerebral endothelial cells (CECs) than in the peripheral endothelium and the AM concentration in the cerebral circulation is significantly higher than in other tested parts of the circulation. We also showed that CECs express AM receptors, and AM as an autocrine hormone is important to regulate the intracellular cAMP level in CECs. Further we reported that acute AM treatment has cAMP-like effects on specific BBB functions: AM decreased endothelial fluid phase endocytosis, activated the P-glycoprotein, increased transendothelial electrical resistance (TEER) and reduced endothelial permeability for sodium fluorescein, which suggests a tightening of intercellular junctions. In the present study, we found chronic AM exposure also increased TEER. In contrast, we could not detect significant effect of AM on the expression of tight junction proteins (claudin-1, occludin and zonula occludens-1). While not affecting expression of tight junction proteins, chronic AM treatment may influence the localization of these proteins which has been reported to correlate with functional changes of the BBB without a change in protein expression.

Estrogen blocks 3-nitropropionic acid-induced Ca2+i increase and cell damage in cultured rat cerebral endothelial cells

Systemic administration of 3-nitropropionic acid (3-NPA, a mycotoxin) induces brain damage accompanied by disturbance in the blood-brain barrier (BBB). Since the endothelial cells are important components of the BBB and the first target of a systemic intoxication, in the present study, the effect of 3-NPA on primary cultured rat brain endothelial cells (rBECs) was examined by studying intracellular Ca(2+) ([Ca(2+)](i)) response using imaging techniques with fura-2. rBECs were prepared using a method of Kis et al. [Eur. J. Pharmacol. 368 (1999) 35-42] and Szabo et al. [Neurobiology 5 (1997) 1-16]. Almost all cells were immunoreactive to antibody against the factor VIII-related antigen (von-Willebrand factor). They showed a typical dose-dependent increase of [Ca(2+)](i) in response to ATP or bradykinin. Low concentrations of 3-NPA (1.7 mM, 3.4 mM) caused no changes, and a medium concentration (6.8 mM) increased the [Ca(2+)](i) gradually and progressively, and the increase was reversed incompletely back to the resting level after washing. A high concentration (13.6 mM) increased the [Ca(2+)](i) irreversibly. These elevations of [Ca(2+)](i) were absent in a Ca(2+)-free medium. In endothelial cells treated with 17beta-estradiol (above 10(-5) M) or with a selective estrogen receptor modulator, tamoxifen (5 x 10(-7) M), no elevation of [Ca(2+)](i) was observed with 3-NPA treatment. The response to ATP was impaired after application of 3-NPA, but it was preserved by cotreatment with 17beta-estradiol or tamoxifen. An estrogen receptor antagonist ICI 182,780 inhibited these effects by 17beta-estradiol or tamoxifen. Lysosomal neutral red uptake and TUNEL experiments revealed the necrotic but not apoptotic cell death at least in this acute stage. Data indicate that a medium to high concentration of 3-NPA induces damage on rBECs as revealed by an accumulation of [Ca(2+)](i), but the damage was protected by cotreatment with 17beta-estradiol or tamoxifen, suggesting that estrogen may be protective for the brain vascular damage via estrogen receptor.

The uptake of manganese in brain endothelial cultures

The present study focused on central nervous system (CNS) transport kinetics of manganese phosphate and manganese sulfate; these findings were correlated with the transport kinetics of manganese chloride (MnCl2), a model Mn compound that has been previously studied. A series of studies was performed to address the transport of Mn salts in confluent cultured endothelial cells. The initial rate of uptake (5 min) of Mn salts (chloride, sulfate, and phosphate) in rat brain endothelial (RBE4) cell cultures is salt-dependent, with the highest rates of uptake for Mn chloride and Mn sulfate (as reflected by the greatest displacement of 54Mn compared with control). Mn phosphate had a lower rate of uptake than the other two Mn salts. These data show that brain endothelial cells efficiently transport Mn sulfate.

Prostanoid synthesis in the cerebral blood vessels of asphyxiated piglets

The aim of this study was to examine the effects of asphyxia-reventilation and hyperoxia on the cerebral blood perfusion and prostanoid production of the brain arteries and microvessels in piglets. After 10 min of asphyxia, animals were ventilated with room air, or with 100% O2. Following 4 hours of recovery, the brains were perfused, cerebral arteries were removed and microvessels were isolated from the cortex. The microvessels and the arteries were incubated with 1-14C-arachidonic acid, and the 1-14C-prostanoids were then separated by means of overpressure thin-layer chromatography and were quantitatively determined. Under control conditions, the synthesis of dilatory prostanoids dominated the arachidonate cascade both in the microvessels and in the arteries. Asphyxia and reventilation with room air did not modify the prostanoid production. O2 ventilation greatly affected the prostanoid synthesis of the microvessels, with an enhancement of PGD2 up to 247 +/- 27%. In the arteries, the production of PGI2 and of PGE2 was elevated to 272 +/- 15% and to 148 +/- 13%, respectively. These findings indicate that O2 ventilation after asphyxia substantially increases the extent of prostanoid synthesis in the cerebral blood vessels.

Cerebral endothelial cells are a major source of adrenomedullin

Adrenomedullin is a peptide hormone with multifunctional biological properties. Its most characteristic effects are the regulation of circulation and the control of fluid and electrolyte homeostasis through peripheral and central nervous system actions. Although adrenomedullin is a vasodilator of cerebral vasculature, and it may be implicated in the pathomechanism of cerebrovascular diseases, the source of adrenomedullin in the cerebral circulation has not been investigated thus far. We measured the secretion of adrenomedullin by radioimmunoassay and detected adrenomedullin mRNA expression by Northern blot analysis in primary cultures of rat cerebral endothelial cells (RCECs), pericytes and astrocytes. We also investigated the expression of specific adrenomedullin receptor components by reverse transcriptase-polymerase chain reaction and intracellular cAMP concentrations in RCECs and pericytes. RCECs had approximately one magnitude higher adrenomedullin production (135 +/- 13 fmol/10(5) cells per 12 h; mean +/- SD, n = 10) compared to that previously reported for other cell types. RCECs secreted adrenomedullin mostly at their luminal cell membrane. Adrenomedullin production was not increased by thrombin, lipopolysaccharide or cytokines, which are known inducers of adrenomedullin release in peripheral endothelial cells, although it was stimulated by astrocyte-derived factors. Pericytes had moderate, while astrocytes had very low basal adrenomedullin secretion. In vivo experiments showed that adrenomedullin plasma concentration in the jugular vein of rats was approximately 50% higher than that in the carotid artery or in the vena cava. Both RCECs and pericytes, which are potential targets of adrenomedullin in cerebral microcirculation, expressed adrenomedullin receptor components, and exhibited a dose-dependent increase in intracellular cAMP concentrations after exogenous adrenomedullin administration. Antisense oligonucleotide treatment significantly reduced adrenomedullin production by RCECs and tended to decrease intraendothelial cAMP concentrations. These findings may suggest an important autocrine and paracrine role for adrenomedullin in the regulation of cerebral circulation and blood-brain barrier functions. Cerebral endothelial cells are a potential source of adrenomedullin in the central nervous system, where adrenomedullin can also be involved in the regulation of neuroendocrine functions.

Adrenomedullin regulates blood-brain barrier functions in vitro

Adrenomedullin (AM) is an important vasodilator in cerebral circulation, and cerebral endothelial cells are a major source of AM. This in vitro study aimed to determine the AM-induced changes in blood-brain barrier (BBB) functions. AM administration increased, whereas AM antisense oligonucleotide treatment decreased transendothelial electrical resistance. AM incubation decreased BBB permeability for sodium fluorescein (mol. wt 376 Da) but not for Evan's blue albumin (mol. wt 67 kDa), and it also attenuated fluid-phase endocytosis. AM treatment resulted in functional activation of P-glycoprotein efflux pump in vitro. Our results indicate that AM as an autocrine mediator plays an important role in the regulation of BBB properties of the cerebral endothelial cells.

Adrenomedullin in the cerebral circulation

The central nervous system requires an effective autoregulation of cerebral circulation in order to meet the critical and unusual demands of the brain. In addition, cerebral microvessels has a unique feature, the formation of the blood-brain barrier, which contributes to the stability of the brain parenchymal microenvironment. Many factors are known to be involved in the regulation of cerebral circulation and blood-brain barrier functions. In the last few years a new potential candidate, adrenomedullin, a hypotensive peptide was added to this list. Adrenomedullin has a potent vasodilator effect on the cerebral vasculature, and it may be implicated in the pathologic mechanism of cerebrovascular diseases. In this review, we describe current knowledge about the origin and possible role of adrenomedullin in the regulation of cerebral circulation and blood-brain barrier functions.

Pentosan polysulfate regulates scavenger receptor-mediated, but not fluid-phase, endocytosis in immortalized cerebral endothelial cells

1. Effects of pentosan polysulfate (PPS) and the structurally related sulfated polyanions dextran sulfate, fucoidan, and heparin on the scavenger receptor-mediated and fluidphase endocytosis in GP8 immortalized rat brain endothelial cells were investigated. 2. Using 1,1'-dioctadecyl-3,3,3,3'-tetramethylindocarboxyamine perchlorate-labeled acetylated low-density lipoprotein (DiI-AcLDL), we found a binding site with high affinity and low binding capacity, and another one with low affinity and high binding capacity. Increasing ligand concentrations could not saturate DiI-AcLDL uptake. DiI-AcLDL uptake, but not binding, was sensitive to pretreatment with filipin, an inhibitor of caveola formation. 3. PPS (20-200 microg/ml) significantly reduced the binding of DiI-AcLDL after coincubation for 3 hr, though this effect was less expressed after 18 hr. Among other polyanions, only fucoidan decreased the DiI-AcLDL binding after 3 hr, whereas dextran sulfate significantly increased it after 18 hr. PPS treatment induced an increase in DiI-AcLDL uptake, whereas other polysulfated compounds caused a significant reduction. 4. Fluid-phase endocytosis determined by the accumulation of Lucifer yellow was concentration and time dependent in GP8 cells. Coincubation with PPS or other sulfated polyanions could not significantly alter the rate of Lucifer yellow uptake. 5. In conclusion. PPS decreased the binding and increased the uptake of DiI-AcLDL in cerebral endothelial cells, an effect not mimicked by the other polyanions investigated.

Effects of eicosanoids and nitric oxide on the noradrenaline-induced contractions in the rat mesenteric bed

1. The effects of the inhibition of the metabolism of arachidonic acid (AA) on the constrictor responses to noradrenaline (NA) were studied in the rat perfused mesenteric bed. The inhibitor of all the pathways of AA metabolism, 10 microM eicosatetraynoic acid (ETYA), reduced the constrictor responses to all the concentrations of NA assayed. 2. The constrictor responses to NA were also reduced by the cyclooxygenase (COX) inhibitor, indomethacin (10 microM), as well as by the lipoxygenase inhibitor, nordihidroguaiaretic acid (1 microM; NDGA), whereas they were unmodified by the cytochrome P450 monooxigenase inhibitors, clotrimazole (10 microM), metyrapone (10 microM) and proadifen (10 microM). 3. The reduction in NA contractility induced by indomethacin was reverted with a decreasing order of potency by the thromboxane A2 analogue, U-46619 > prostaglandin (PG) E2 > PGF2alpha. The exposure of the mesenteric bed to NA increased the production of PGF2alpha, whereas it did not modify the production of the remaining AA metabolites. 4. The increase in the NA-induced contractions caused by endothelium removal, as well as by the inhibition of nitric oxide synthase (NOs) with NG-nitro-L-arginine methyl ester (400 microM; L-NAME), was suppressed by indomethacin but not by NDGA. These observations suggest that the lipoxygenase-derived metabolites are formed in the endothelium, whereas the COX-derived metabolites are formed in the vascular smooth muscle. 5. The TP receptor antagonist, SQ29548, did not modify the NA-induced contractions, either in the presence or in the absence of the endothelium. 6. Contractions elicited by KCI (60-100 mM) were unmodified by the AA metabolism inhibitors, ETYA, NDGA and indomethacin. 7. In summary, these results show that metabolites of AA, through both the COX and the lipoxygenase pathways, are involved in the NA-induced contractions in the rat mesenteric bed. The lipoxygenase metabolites are likely to be formed in the vascular endothelium, whereas the COX metabolite, which could be PGF2alpha, is apparently formed within the vascular smooth muscle.

Effects of N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine on the blood-brain barrier permeability in the rat

Histamine plays a role in the regulation of the blood-brain barrier function. In this study, effects of N, N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine (DPPE), an intracellular histamine binding site antagonist on the cerebrovascular permeability were investigated in control and post-ischemic male Wistar rats. Intravenous administration of DPPE, in a dose of 1 and 5 mg/kg, was not followed by any major clinical change, but 20 mg/kg proved to be toxic. A significantly (P<0.05) increased permeability for sodium fluorescein (MW=376) was seen in hippocampus, striatum, and cerebellum, but not in parietal cortex, of rats 2 h after the injection of 5 mg/kg DPPE, whereas no increase was measured later. There was a more intense (5- to 12-fold) and prolonged elevation in Evan's blue-labeled albumin (MW=67,000) extravasation 2, 4, and 8 h after 5 mg/kg DPPE administration in each brain region. In parietal cortex, a dose-dependent increase in albumin extravasation developed 4 h after intravenous injection of 1, 5, and 20 mg/kg DPPE, but doses applied resulted in no significant change in sodium fluorescein permeability. Cerebral ischemia-reperfusion evoked by four-vessel occlusion caused a significant (P<0.05) increase in the permeability for albumin in each region, but few changes in that of sodium fluorescein. DPPE treatment failed to prevent the ischemia-reperfusion-induced changes in the blood-brain barrier permeability. In conclusion, DPPE induced an increased permeability in the rat, which supports a role for histamine, as an intracellular messenger, in the regulation of the blood-brain barrier characteristics.