RESEARCH ARTICLE
Vitamin D Prevents Hypoxia/ReoxygenationInduced Blood-Brain Barrier Disruption via Vitamin D Receptor-Mediated NF-kB Signaling Pathways Soonmi Won, Iqbal Sayeed*, Bethany L. Peterson, Bushra Wali, Jared S. Kahn, Donald G. Stein Department of Emergency Medicine Brain Research Laboratory, Emory University, Atlanta, Georgia, United States of America * [email protected]
Abstract OPEN ACCESS Citation: Won S, Sayeed I, Peterson BL, Wali B, Kahn JS, Stein DG (2015) Vitamin D Prevents Hypoxia/Reoxygenation-Induced Blood-Brain Barrier Disruption via Vitamin D Receptor-Mediated NF-kB Signaling Pathways. PLoS ONE 10(3): e0122821. doi:10.1371/journal.pone.0122821 Academic Editor: Robyn Klein, Washington University, UNITED STATES Received: November 21, 2014 Accepted: February 21, 2015 Published: March 27, 2015 Copyright: © 2015 Won et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Maintaining blood-brain barrier integrity and minimizing neuronal injury are critical components of any therapeutic intervention following ischemic stroke. However, a low level of vitamin D hormone is a risk factor for many vascular diseases including stroke. The neuroprotective effects of 1,25(OH)2D3 (vitamin D) after ischemic stroke have been studied, but it is not known whether it prevents ischemic injury to brain endothelial cells, a key component of the neurovascular unit. We analyzed the effect of 1,25(OH)2D3 on brain endothelial cell barrier integrity and tight junction proteins after hypoxia/reoxygenation in a mouse brain endothelial cell culture model that closely mimics many of the features of the blood-brain barrier in vitro. Following hypoxic injury in bEnd.3 cells, 1,25(OH)2D3 treatment prevented the decrease in barrier function as measured by transendothelial electrical resistance and permeability of FITC-dextran (40 kDa), the decrease in the expression of the tight junction proteins zonula occludin-1, claudin-5, and occludin, the activation of NF—kB, and the increase in matrix metalloproteinase-9 expression. These responses were blocked when the interaction of 1,25(OH) )2D3 with the vitamin D receptor (VDR) was inhibited by pyridoxal 5’-phosphate treatment. Our findings show a direct, VDR-mediated, protective effect of 1,25(OH) )2D3 against ischemic injury-induced blood-brain barrier dysfunction in cerebral endothelial cells.
Data Availability Statement: All relevant data are within the paper. Funding: This work was supported by American Heart Association SDG grant 11SDG5430002 to IS and National Health Institute R01HD061971 to DGS. Partial funding for some of the assays used in this project was provided by the Marcus Foundation of Atlanta and BHR Pharma. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Introduction Cerebrovascular disorders including ischemic stroke are among the main causes of death and disability in industrialized countries. A large number of studies have shown that cerebral ischemia-reperfusion injury causes structural and functional breakdown of the blood–brain barrier (BBB), resulting in increased BBB permeability [1], and the extent of disruption is directly correlated with the severity and duration of the insult [2]. BBB breakdown not only facilitates
PLOS ONE | DOI:10.1371/journal.pone.0122821 March 27, 2015
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Vitamin D Prevents Blood-Brain Barrier Disruption
Competing Interests: This study was partly funded by BHR Pharma. There are no patents, products in development or marketed products to declare. This does not alter the authors adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.
brain edema and hemorrhage, but has also been linked to an increase in the expression of various cytokines and chemokines, predisposing the brain to a secondary cascade of ischemic injury. Following a brain injury, rapid protection against BBB disruption would be a critical component of any therapeutic intervention to minimize secondary neuronal injury. The BBB regulates the transport of molecules to the central nervous system and restricts permeability across brain endothelium [3]. Tight junction (TJ) proteins are the most prominent feature of brain endothelium for maintaining BBB integrity and a critical component of the paracellular pathway, which is vulnerable to ischemic injury [4]. Many studies using in vitro BBB models have shown that hypoxia and hypoxia/reoxygenation (H/R) induce an increase in BBB permeability and/or TJ disturbance [5]. Factors leading to BBB disruption during and after stroke include alteration in intracellular calcium, production of vascular endothelial growth factor (VEGF), and increased production of nitric oxide and reactive oxygen species (ROS) [6]. ROS contribute to brain injury by inflicting damage on proteins, lipids, and nucleic acids, as well as by activating a number of redox-sensitive signaling pathways. Many laboratories have shown that the production of ROS influences BBB permeability through the upregulation of cell-disrupting enzymes and protein complexes such as matrix metalloproteinases (MMPs), nuclear factor kappa B (NF-kB), VEGF and activator protein 1 (AP-1), among others [7,8]. Vitamin D is a steroid hormone synthesized in human skin from 7-dehydrocholesterol by UV light, and is primarily metabolized in the liver and then in the kidney into calcitriol (1,25 (OH)2D3). Calcitriol is the most biologically active metabolite of vitamin D and an FDA-approved molecule whose neuroprotective and neurotrophic actions are being increasingly recognized, and it may prove to be a candidate intervention for stroke. Calcitriol exerts steroid-like effects throughout the body, directly affecting the expression of multiple genes [9] through the nuclear vitamin D receptor (VDR). While 1,25(OH)2D3 has classically been associated with systemic calcium homeostasis, there is now substantial evidence that it is also a potent modulator of the immune system that can regulate inflammation, neuromuscular function, cell-cycle control, and neuroprotective functions by regulating neurotrophin production [10]. Low levels of 25-hydroxyvitamin D3 (25(OH)D) and 1,25-dihydroxyvitamin D (1,25(OH)2D) are independently predictive for fatal strokes, while 1,25(OH)2D3 deficiency exacerbates experimental stroke injury in rats [11]. Although the neuroprotective effects of 1,25(OH)2D3 after ischemic stroke are well documented, less is known about the effects of vitamin D and its underlying mechanisms on brain endothelial cells after hypoxic injury. We do know that the actions of 1,25(OH)2D3 are mediated primarily by the VDR, a member of the nuclear receptor family [12]. Vitamin D has been shown to exert its immuno-modulatory activity on both mononuclear and polynuclear cell lines through its effects on the VDR [13]. The important role of the VDR in mediating the effect of vitamin D has been demonstrated in VDR-knockout mice, in which cardiomyocytes developed cardiac hypertrophy [14] in the face of this depletion. Using bEnd.3 cells as an in vitro BBB model, we investigated the effects of 1,25(OH)2D3 and the role of the VDR in BBB permeability, cerebral endothelial cell death, and increased production of ROS-induced NF-kB—MMP-2/9 expression after H/R. Previous studies have found brain endothelial cells to be a suitable in vitro BBB model for studying TJ protein and gene expression, sucrose permeability, transendothelial electrical resistance (TEER), and solute transport [15–18]. This model allowed us to study the mechanisms of action of 1,25(OH)2D3 by directly testing the effects of the VDR antagonist pyridoxal 5’-phosphate (P5P) (the active form of vitamin B), which in in vivo testing could have confounding mechanistic effects independent of the VDR [19,20]. We hypothesized that 1,25(OH)2D3 blocks NF-kB-mediated hypoxia-induced BBB disruption and cerebral endothelial cell death by binding to the VDR, and
PLOS ONE | DOI:10.1371/journal.pone.0122821 March 27, 2015
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Vitamin D Prevents Blood-Brain Barrier Disruption
this beneficial effect is blocked by P5P. Our findings in this study show that the hormonal form of vitamin D can play an important role repairing BBB damaged by stroke.
Materials and Methods Endothelial cell culture An immortalized mouse brain endothelial cell line (bEnd.3) was purchased from American Type Culture Collection (ATCC, Manassas, VA) and cultured in Dulbecco’s modified Eagle medium (DMEM; ATCC) supplemented with 10% fetal bovine serum (ATCC). Cell cultures were incubated at 37°C in a 5/95% mixture of CO2 and atmospheric air, and the medium was replaced every 2 days (see below for measurement details).
Drug treatment Cells were exposed to various concentrations (0, 5, 20, or 200 nmol/L) of 1,25(OH)2D3 (SigmaAldrich, St Louis, MO) for 24 h before H/R. In all cases, the treatments continued throughout the H/R period. 1,25(OH)2D3 was dissolved in ethanol (Sigma) and further dilutions were made with culture medium. To investigate the specificity of the functional effects of 1,25 (OH)2D3, we blocked the VDR with the VDR antagonist P5P (1 mM), administering it for 24 h before H/R and continuously over the H/R period with or without 1,25(OH)2D3.
Normoxia and H/R study Normoxic cells were transferred into a serum-free medium of glucose-containing (4.5 g/L) phenol red-free DMEM. To measure BBB permeability without endothelial cell death under ischemic conditions in vitro, bEnd.3 cells were exposed to 6-h hypoxia and 3-h reoxygenation as previously described [5]. To measure and quantify cerebral endothelial cell death, bEnd.3 cells were subjected to 16-h hypoxia and 10-min reoxygenation. Based on our preliminary parametric testing, we determined that this prolonged hypoxia was necessary for the cell viability assay. In brief, confluent bEnd.3 cells were subjected to an ischemic injury by transferring cultures to glucose-free medium (DMEM without glucose) pre-equilibrated with 95% N2 and 5% CO2. The oxygen concentration was 0.1% as monitored by an oxygen analyzer (Biospherix, Redfield, NY). Reoxygenation was initiated by adding glucose-containing (4.5 g/L) phenol red-free DMEM for 3h at 37° C in 95% air and 5% CO2.
BBB permeability assay Monolayer bEnd.3 cells were seeded on the luminal side of the filter (0.4μ m pore size; Corning, Lowell, MA) coated with fibronectin (5μg/mL) in 12-well plates. TEER was measured with a millicell ERS-2 volt/ohm meter (Millipore, Billarica, MA). TEERs of cell-free fibronectin-coated filters were subtracted from the measured TEERs shown as O x cm2 as previously described [5]. Paracellular BBB permeability with TEER measurement was confirmed using FITC-dextran fluorescein. FITC-dextran (40 μL, 1 mg/ml final concentration) (Sigma) was filled to the insert before 3-h reoxygenation. Fifty μL of medium was collected from the lower chamber. The aliquots were diluted to 1 ml with 1x PBS. We transferred 100 μL of each diluted sample into 96-well black plates and measured the fluorescent content for FITC-dextran at 492/520 nm absorption/emission wavelengths.
Immunofluorescence staining Monolayer bEnd.3 cells were grown directly on cover slips in 6-well plates (4 x 104; Corning) and cultured for 5 days. After being washed in phosphate-buffered saline (PBS; pH 7.4), the
PLOS ONE | DOI:10.1371/journal.pone.0122821 March 27, 2015
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Vitamin D Prevents Blood-Brain Barrier Disruption
cells were fixed in 4% paraformaldehyde for 10 min at RT. After permeabilization with 0.5% Triton X-100 or 0.1% saponin followed by blocking with 5% bovine albumin (BSA), cells were incubated with anti-ZO-1, anti-claudin-5, anti-occludin (1:200; Invitrogen), or NF-kB p65 (1:200; Cell Signaling, Danvers, MA) in PBS with 5% BSA at 4° C overnight and followed by Alexa fluor 488 conjugated anti-rabbit or Alexa fluor 594 conjugated anti-mouse IgG (1:400; Molecular Probes, Eugene, OR) in 5% BSA for 1h at RT. The cell nuclei were counterstained with DAPI.
Western blot analysis Endothelial (bEnd.3) cell homogenate 8–12% was separated in SDS-PAGE gels, transferred to a polyvinylidene difluoride (PVDF) membrane, and incubated with primary antibodies against claudin-5 (1:1000; Invitrogen), occludin (1:1000; Invitrogen), phospho-IkB (1:1000; Cell Signaling), MMP2/9 (1;1000; Abcam, Cambridge, MA), and β-actin (1:3000; Sigma). Enhanced chemiluminescence (Denville Scientific, Metuchen, NJ) was used for visualization. Intensity of the bands was measured using Image Gauge 4.0 (FUJI Film, Tokyo, Japan).
Reactive oxygen species assays Fluorescence detection of mitochondrial superoxide and hydrogen peroxide was measured using the fluorogenic probe MitoSOX Red (Invitrogen). MitoSOX Red selectively enters mitochondria within living cells. When oxidized by superoxide anions in the mitochondria, the dye emits red fluorescence. After 60-min hypoxia, cells were switched to reoxygenation conditions and incubated for 20 min at 37° C with MitoSOX Red (5 μmol/L). Cells were then washed with Hanks’ Balanced Salt Solution to remove excess dye. DAPI staining was used to confirm the localization of nucleus to cells. The intensity of MitoSOX Red fluorescence was quantified by ImageJ 1.45s software (NIH). H2O2 was measured using the HRP-linked Amplex Red (Invitrogen) fluorometric assay. The cell media was reacted with working solution (100 μmol/L Amplex Red reagent and 2 U/mL HRP in a 1X reaction buffer) in 24-well plates and incubated at 37° C for 20 min. Absorbance of the color reaction was determined spectrophotometrically (BioTek, Winooski, VT) at Ex530/Em590. The results were expressed as fluorescence per mg protein.
MTT assay For cell viability determination, bEnd.3 cells (2 x 105 cells/mL) were seeded in 96-well plates and subjected to16-h hypoxia and 10 min reoxygenation. Medium containing 5 mg/ml 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was then added to the cells for a final concentration of 0.5 mg/ml and incubated at 37° C for 4 h. The medium was aspirated, and the formazan product was solubilized with dimethyl sulfoxide. The absorbance of the sample was read directly in the wells at a wavelength of 570 nm. Data were normalized to respective controls and represented as percent viability compared to controls.
Statistical analysis Significant differences between treatment groups were determined by one-way analysis of variance (ANOVA) followed by the post-hoc Tukey test for independent group comparison using GraphPad Prism 5. Data were expressed as mean ± s.e.m. For all our statistical testing, we considered a p-value of
Vitamin D Prevents Hypoxia/ReoxygenationInduced Blood-Brain Barrier Disruption via Vitamin D Receptor-Mediated NF-kB Signaling Pathways Soonmi Won, Iqbal Sayeed*, Bethany L. Peterson, Bushra Wali, Jared S. Kahn, Donald G. Stein Department of Emergency Medicine Brain Research Laboratory, Emory University, Atlanta, Georgia, United States of America * [email protected]
Abstract OPEN ACCESS Citation: Won S, Sayeed I, Peterson BL, Wali B, Kahn JS, Stein DG (2015) Vitamin D Prevents Hypoxia/Reoxygenation-Induced Blood-Brain Barrier Disruption via Vitamin D Receptor-Mediated NF-kB Signaling Pathways. PLoS ONE 10(3): e0122821. doi:10.1371/journal.pone.0122821 Academic Editor: Robyn Klein, Washington University, UNITED STATES Received: November 21, 2014 Accepted: February 21, 2015 Published: March 27, 2015 Copyright: © 2015 Won et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Maintaining blood-brain barrier integrity and minimizing neuronal injury are critical components of any therapeutic intervention following ischemic stroke. However, a low level of vitamin D hormone is a risk factor for many vascular diseases including stroke. The neuroprotective effects of 1,25(OH)2D3 (vitamin D) after ischemic stroke have been studied, but it is not known whether it prevents ischemic injury to brain endothelial cells, a key component of the neurovascular unit. We analyzed the effect of 1,25(OH)2D3 on brain endothelial cell barrier integrity and tight junction proteins after hypoxia/reoxygenation in a mouse brain endothelial cell culture model that closely mimics many of the features of the blood-brain barrier in vitro. Following hypoxic injury in bEnd.3 cells, 1,25(OH)2D3 treatment prevented the decrease in barrier function as measured by transendothelial electrical resistance and permeability of FITC-dextran (40 kDa), the decrease in the expression of the tight junction proteins zonula occludin-1, claudin-5, and occludin, the activation of NF—kB, and the increase in matrix metalloproteinase-9 expression. These responses were blocked when the interaction of 1,25(OH) )2D3 with the vitamin D receptor (VDR) was inhibited by pyridoxal 5’-phosphate treatment. Our findings show a direct, VDR-mediated, protective effect of 1,25(OH) )2D3 against ischemic injury-induced blood-brain barrier dysfunction in cerebral endothelial cells.
Data Availability Statement: All relevant data are within the paper. Funding: This work was supported by American Heart Association SDG grant 11SDG5430002 to IS and National Health Institute R01HD061971 to DGS. Partial funding for some of the assays used in this project was provided by the Marcus Foundation of Atlanta and BHR Pharma. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Introduction Cerebrovascular disorders including ischemic stroke are among the main causes of death and disability in industrialized countries. A large number of studies have shown that cerebral ischemia-reperfusion injury causes structural and functional breakdown of the blood–brain barrier (BBB), resulting in increased BBB permeability [1], and the extent of disruption is directly correlated with the severity and duration of the insult [2]. BBB breakdown not only facilitates
PLOS ONE | DOI:10.1371/journal.pone.0122821 March 27, 2015
1 / 17
Vitamin D Prevents Blood-Brain Barrier Disruption
Competing Interests: This study was partly funded by BHR Pharma. There are no patents, products in development or marketed products to declare. This does not alter the authors adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.
brain edema and hemorrhage, but has also been linked to an increase in the expression of various cytokines and chemokines, predisposing the brain to a secondary cascade of ischemic injury. Following a brain injury, rapid protection against BBB disruption would be a critical component of any therapeutic intervention to minimize secondary neuronal injury. The BBB regulates the transport of molecules to the central nervous system and restricts permeability across brain endothelium [3]. Tight junction (TJ) proteins are the most prominent feature of brain endothelium for maintaining BBB integrity and a critical component of the paracellular pathway, which is vulnerable to ischemic injury [4]. Many studies using in vitro BBB models have shown that hypoxia and hypoxia/reoxygenation (H/R) induce an increase in BBB permeability and/or TJ disturbance [5]. Factors leading to BBB disruption during and after stroke include alteration in intracellular calcium, production of vascular endothelial growth factor (VEGF), and increased production of nitric oxide and reactive oxygen species (ROS) [6]. ROS contribute to brain injury by inflicting damage on proteins, lipids, and nucleic acids, as well as by activating a number of redox-sensitive signaling pathways. Many laboratories have shown that the production of ROS influences BBB permeability through the upregulation of cell-disrupting enzymes and protein complexes such as matrix metalloproteinases (MMPs), nuclear factor kappa B (NF-kB), VEGF and activator protein 1 (AP-1), among others [7,8]. Vitamin D is a steroid hormone synthesized in human skin from 7-dehydrocholesterol by UV light, and is primarily metabolized in the liver and then in the kidney into calcitriol (1,25 (OH)2D3). Calcitriol is the most biologically active metabolite of vitamin D and an FDA-approved molecule whose neuroprotective and neurotrophic actions are being increasingly recognized, and it may prove to be a candidate intervention for stroke. Calcitriol exerts steroid-like effects throughout the body, directly affecting the expression of multiple genes [9] through the nuclear vitamin D receptor (VDR). While 1,25(OH)2D3 has classically been associated with systemic calcium homeostasis, there is now substantial evidence that it is also a potent modulator of the immune system that can regulate inflammation, neuromuscular function, cell-cycle control, and neuroprotective functions by regulating neurotrophin production [10]. Low levels of 25-hydroxyvitamin D3 (25(OH)D) and 1,25-dihydroxyvitamin D (1,25(OH)2D) are independently predictive for fatal strokes, while 1,25(OH)2D3 deficiency exacerbates experimental stroke injury in rats [11]. Although the neuroprotective effects of 1,25(OH)2D3 after ischemic stroke are well documented, less is known about the effects of vitamin D and its underlying mechanisms on brain endothelial cells after hypoxic injury. We do know that the actions of 1,25(OH)2D3 are mediated primarily by the VDR, a member of the nuclear receptor family [12]. Vitamin D has been shown to exert its immuno-modulatory activity on both mononuclear and polynuclear cell lines through its effects on the VDR [13]. The important role of the VDR in mediating the effect of vitamin D has been demonstrated in VDR-knockout mice, in which cardiomyocytes developed cardiac hypertrophy [14] in the face of this depletion. Using bEnd.3 cells as an in vitro BBB model, we investigated the effects of 1,25(OH)2D3 and the role of the VDR in BBB permeability, cerebral endothelial cell death, and increased production of ROS-induced NF-kB—MMP-2/9 expression after H/R. Previous studies have found brain endothelial cells to be a suitable in vitro BBB model for studying TJ protein and gene expression, sucrose permeability, transendothelial electrical resistance (TEER), and solute transport [15–18]. This model allowed us to study the mechanisms of action of 1,25(OH)2D3 by directly testing the effects of the VDR antagonist pyridoxal 5’-phosphate (P5P) (the active form of vitamin B), which in in vivo testing could have confounding mechanistic effects independent of the VDR [19,20]. We hypothesized that 1,25(OH)2D3 blocks NF-kB-mediated hypoxia-induced BBB disruption and cerebral endothelial cell death by binding to the VDR, and
PLOS ONE | DOI:10.1371/journal.pone.0122821 March 27, 2015
2 / 17
Vitamin D Prevents Blood-Brain Barrier Disruption
this beneficial effect is blocked by P5P. Our findings in this study show that the hormonal form of vitamin D can play an important role repairing BBB damaged by stroke.
Materials and Methods Endothelial cell culture An immortalized mouse brain endothelial cell line (bEnd.3) was purchased from American Type Culture Collection (ATCC, Manassas, VA) and cultured in Dulbecco’s modified Eagle medium (DMEM; ATCC) supplemented with 10% fetal bovine serum (ATCC). Cell cultures were incubated at 37°C in a 5/95% mixture of CO2 and atmospheric air, and the medium was replaced every 2 days (see below for measurement details).
Drug treatment Cells were exposed to various concentrations (0, 5, 20, or 200 nmol/L) of 1,25(OH)2D3 (SigmaAldrich, St Louis, MO) for 24 h before H/R. In all cases, the treatments continued throughout the H/R period. 1,25(OH)2D3 was dissolved in ethanol (Sigma) and further dilutions were made with culture medium. To investigate the specificity of the functional effects of 1,25 (OH)2D3, we blocked the VDR with the VDR antagonist P5P (1 mM), administering it for 24 h before H/R and continuously over the H/R period with or without 1,25(OH)2D3.
Normoxia and H/R study Normoxic cells were transferred into a serum-free medium of glucose-containing (4.5 g/L) phenol red-free DMEM. To measure BBB permeability without endothelial cell death under ischemic conditions in vitro, bEnd.3 cells were exposed to 6-h hypoxia and 3-h reoxygenation as previously described [5]. To measure and quantify cerebral endothelial cell death, bEnd.3 cells were subjected to 16-h hypoxia and 10-min reoxygenation. Based on our preliminary parametric testing, we determined that this prolonged hypoxia was necessary for the cell viability assay. In brief, confluent bEnd.3 cells were subjected to an ischemic injury by transferring cultures to glucose-free medium (DMEM without glucose) pre-equilibrated with 95% N2 and 5% CO2. The oxygen concentration was 0.1% as monitored by an oxygen analyzer (Biospherix, Redfield, NY). Reoxygenation was initiated by adding glucose-containing (4.5 g/L) phenol red-free DMEM for 3h at 37° C in 95% air and 5% CO2.
BBB permeability assay Monolayer bEnd.3 cells were seeded on the luminal side of the filter (0.4μ m pore size; Corning, Lowell, MA) coated with fibronectin (5μg/mL) in 12-well plates. TEER was measured with a millicell ERS-2 volt/ohm meter (Millipore, Billarica, MA). TEERs of cell-free fibronectin-coated filters were subtracted from the measured TEERs shown as O x cm2 as previously described [5]. Paracellular BBB permeability with TEER measurement was confirmed using FITC-dextran fluorescein. FITC-dextran (40 μL, 1 mg/ml final concentration) (Sigma) was filled to the insert before 3-h reoxygenation. Fifty μL of medium was collected from the lower chamber. The aliquots were diluted to 1 ml with 1x PBS. We transferred 100 μL of each diluted sample into 96-well black plates and measured the fluorescent content for FITC-dextran at 492/520 nm absorption/emission wavelengths.
Immunofluorescence staining Monolayer bEnd.3 cells were grown directly on cover slips in 6-well plates (4 x 104; Corning) and cultured for 5 days. After being washed in phosphate-buffered saline (PBS; pH 7.4), the
PLOS ONE | DOI:10.1371/journal.pone.0122821 March 27, 2015
3 / 17
Vitamin D Prevents Blood-Brain Barrier Disruption
cells were fixed in 4% paraformaldehyde for 10 min at RT. After permeabilization with 0.5% Triton X-100 or 0.1% saponin followed by blocking with 5% bovine albumin (BSA), cells were incubated with anti-ZO-1, anti-claudin-5, anti-occludin (1:200; Invitrogen), or NF-kB p65 (1:200; Cell Signaling, Danvers, MA) in PBS with 5% BSA at 4° C overnight and followed by Alexa fluor 488 conjugated anti-rabbit or Alexa fluor 594 conjugated anti-mouse IgG (1:400; Molecular Probes, Eugene, OR) in 5% BSA for 1h at RT. The cell nuclei were counterstained with DAPI.
Western blot analysis Endothelial (bEnd.3) cell homogenate 8–12% was separated in SDS-PAGE gels, transferred to a polyvinylidene difluoride (PVDF) membrane, and incubated with primary antibodies against claudin-5 (1:1000; Invitrogen), occludin (1:1000; Invitrogen), phospho-IkB (1:1000; Cell Signaling), MMP2/9 (1;1000; Abcam, Cambridge, MA), and β-actin (1:3000; Sigma). Enhanced chemiluminescence (Denville Scientific, Metuchen, NJ) was used for visualization. Intensity of the bands was measured using Image Gauge 4.0 (FUJI Film, Tokyo, Japan).
Reactive oxygen species assays Fluorescence detection of mitochondrial superoxide and hydrogen peroxide was measured using the fluorogenic probe MitoSOX Red (Invitrogen). MitoSOX Red selectively enters mitochondria within living cells. When oxidized by superoxide anions in the mitochondria, the dye emits red fluorescence. After 60-min hypoxia, cells were switched to reoxygenation conditions and incubated for 20 min at 37° C with MitoSOX Red (5 μmol/L). Cells were then washed with Hanks’ Balanced Salt Solution to remove excess dye. DAPI staining was used to confirm the localization of nucleus to cells. The intensity of MitoSOX Red fluorescence was quantified by ImageJ 1.45s software (NIH). H2O2 was measured using the HRP-linked Amplex Red (Invitrogen) fluorometric assay. The cell media was reacted with working solution (100 μmol/L Amplex Red reagent and 2 U/mL HRP in a 1X reaction buffer) in 24-well plates and incubated at 37° C for 20 min. Absorbance of the color reaction was determined spectrophotometrically (BioTek, Winooski, VT) at Ex530/Em590. The results were expressed as fluorescence per mg protein.
MTT assay For cell viability determination, bEnd.3 cells (2 x 105 cells/mL) were seeded in 96-well plates and subjected to16-h hypoxia and 10 min reoxygenation. Medium containing 5 mg/ml 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was then added to the cells for a final concentration of 0.5 mg/ml and incubated at 37° C for 4 h. The medium was aspirated, and the formazan product was solubilized with dimethyl sulfoxide. The absorbance of the sample was read directly in the wells at a wavelength of 570 nm. Data were normalized to respective controls and represented as percent viability compared to controls.
Statistical analysis Significant differences between treatment groups were determined by one-way analysis of variance (ANOVA) followed by the post-hoc Tukey test for independent group comparison using GraphPad Prism 5. Data were expressed as mean ± s.e.m. For all our statistical testing, we considered a p-value of
