Ethics statement
All procedures involving laboratory mice were in accordance with national (Portaria 1005/92) and European regulations (European Directive 86/609/CEE) on animal experimentation and were approved by the Instituto Gulbenkian de Ciência (IGC) Ethics Committee and the Direcção-Geral de Alimentação e Veterinária, the national authority for animal welfare.
Isolation of brain vessels and culture in inserts
C57BL/6 mice with 6–8 weeks of age were used to obtain the brain vessels. Immediately after euthanasia, mice were exsanguinated by cardiac puncture. The brains were removed from the skull and kept in a petri dish with HBSS medium (Biowest) on ice. Then, the brains were transferred to a freezer block covered with gaze and the meninges removed by rolling a cotton swab over them. Brain vessels were then isolated adapting a previously described procedure (Howland et al. 2015). Briefly, brains were minced with a blade in 1 ml of HBSS medium and aspirated with a syringe. The tissue was then homogenized pushing it five times through a 23G needle. This lysate was mixed with equal volume of cold 30% (wt/vol) dextran (MW ~ 60 kDa) (Sigma) solution and centrifuged at 10000g for 15 min. After centrifugation the myelin layer in the top was removed and the pellet containing the vessels was suspended in DMEM and passed through a 40-µm cell strainer followed by 5 ml of medium to wash away other cells and myelin debris. The strainer was back-flushed with 5 ml of DMEM into a 60 mm Petri dish to collect the brain vessels of 2–3 brains supplemented with 1mg/ml of collagenase type IV (Millipore), 10 µg/ml DNAse (Roche) and 2% FBS and followed by incubation for 2–3 hours at 36.5 ºC in an orbital shaker. The digested vessels were then centrifuged and suspended in EGM™-2 Endothelial Cell Growth Medium-2 BulletKit™ (LONZA) containing 4 µg/ml of puromycin and plated on hanging cell culture inserts with polyethylene terephthalate (PET) filters with pore size of 5 µm (cat. nr. MCMP24H48, Millipore) at a ratio of one brain for 2.5 inserts. The inserts were previously coated with ECM Gel from Engelbreth-Holm-Swarm murine sarcoma (matrigel matrix) (SIGMA) for 2 hours in 24-well plates at 37 ºC (37°C, 5% CO2). Every four days, the medium in the top and bottom of the insert (0.5 ml in each compartment) was changed for fresh medium.
Measurement of transendothelial electrical resistance (TEER)
The 24-well plates containing the cells cultured on inserts were removed from the tissue culture incubator (37°C, 5% CO2) and allowed to equilibrate to room temperature. TEER values were measured before changing the medium using an EVOM2 voltmeter with STX-2 electrodes (Millipore). To calculate TEER (Ω (Ohms) × cm2) in each insert culture, electrical resistance across a matrigel-coated insert without cells was subtracted from the readings obtained on inserts with cells and then multiplied by the surface area of the insert (0.3 cm2).
Preparation of synchronized iRBCs and extracellular particles (EPs)
C57BL/6 mice with 8 weeks of age were intraperitoneally injected with 106 Plasmodium berghei ANKA-GFP (PbA)- iRBCs. Parasitemia was determined as the percentage of GFP+ RBCs, using flow cytometry analysis (FACScan Cell Analyzer; Becton Dickinson). At day 5–6 post-infection when parasitemia was 10–20%, blood was collected (between 13:00h and 15:00h) by cardiac puncture with a syringe containing 100 µl of heparin. The collected blood was suspended in 5 ml of RPMI1640 medium and centrifuged at 500g for 10 min. iRBCs were then synchronized in RPMI1640 medium supplemented with FBS, to a final concentration of 20%, and neomycin (100 µg/ml) in 250 ml culture flasks flushed with a gas mixture of 5% CO2, 5% O2, 90% N2 using a 0.2-µm filter unit connected to the gas hose as previously described (Janse et al. 2006). After 16 hours, RBCs and iRBCs were collected by centrifugation and the supernatant used to collect EPs. The pellet was resuspended in PBS containing 2% BSA and mature-stage iRBC were selected and concentrated using automatic magnetically activated cell sorting (MACS) as previously described (de Moraes et al. 2016). Supernatant from RBCs and iRBCs was further centrifuged at 1,600g for 20 min to remove lysed cells. The resultant supernatant was further centrifuged at 20,000g for 20 minutes to obtain a pellet with EPs. Medium and PBS solutions used to prepare EPs were previously filtered through a 0.2 µm filter.
EPs flow cytometry analysis
EPs, RBCs and iRBCs were suspended in 0. 2 µm filtered PBS and analyzed by flow cytometry (Fortessa, BD) using side scatter from the blue violet laser (SSC-BV) to detect smaller particles. PBS was used to set the background cytometric signal.
EPs were also stained with Phycoerythrin (PE) labeled anti-mouse TER119 antibody (Biolegend) and PerCP/Cy5.5 labeled Annexin V (Biolegend) in calcium binding buffer (0.01 M HEPES, pH 7.4; 0.14 M NaCl; 2.5 mM CaCl 2). After staining, the preparation were washed in PBS by centrifuging at 20,000g.
Monolayer permeability to Lucifer yellow (LY)
After 22–24 hours of endothelial cell culture with the different stimuli, the inserts were removed to a new 24-well plate containing 0.5 ml of HBSS medium without phenol red and with 10 mM HEPES and 2% of FBS (transport buffer) per well. LY (Sigma-Aldrich) at 100 µM was added in 0.5 ml of transport buffer to the insert and incubated at 37 ºC for 1 and 2 hours. At these time points, 100 µl were collected from the lower chamber and the amount of LY detected by optical absorbance in a plate reader (Glomax Explorer) (Excitation: 405 nm and Emission: 500–550 nm).
Immunofluorescence staining
Inserts with cell cultures were fixed with 4% paraformaldehyde in PBS for 20 min at RT and washed with PBS. The inserts were then incubated with CF-488A phalloidin (1:400) (Biotum) for 30 min and washed with PBS. To analyze the distribution of the ZO-1 protein on the cell monolayers, the inserts were incubated in PBS with 0,25% Triton X-100 for 10 min and blocked with 10% goat serum, 1% BSA and 100 mM glycine in PBS. The inserts were then incubated over night at 4ºC in blocking buffer containing anti-ZO-1 polycolonal antibody (2.5 µg/ ml) (Invitrogen). After washing, the inserts were incubated in PBS with 5 µM of DRAQ5 to stain the nuclei (Biostatus). The filters were carefully removed from the insert and mounted with mounting medium (Ibidi). Images were acquired with a Leica SP5 Inverted Confocal, and processed and analyzed using ImageJ software.
Gene expression analysis and protein quantification
Real-time qRT-PCR analysis was performed with Taqman probes after cDNA preparation from cell lysate using Cell-to-CT kit (Thermo Fisher Scientific) according with manufacturer’s instructions. Results are expressed as fold increase as compared to controls.
Protein was quantified in the EPs, RBCs and iRBCs preparations using the Bradford method.
Statistical analysis
Data were expressed as mean ± standard deviation (SD). Statistical analysis was carried out by one-way or two-way analysis of variance (ANOVA) as indicated in the figure legends. All statistical tests were performed using GraphPad prism 6 Software.