2.1. Cell Culture and Reagents
All reagents were purchased from Millipore-Sigma (Burlington, MA, USA), unless otherwise stated. Human brain microvascular endothelial cells (hBMECs) were obtained from Neuromics (Minneapolis, MN; catalog number: #HEC02). These cells have been proved to be the most suitable human cell line for an in vitro blood–brain barrier (BBB) model [29].
Cells were cultured in a standard humidified atmosphere (37°C) containing 5% CO2. In some experiments, cells were transfected with pcDNA3.1-Neuropilin-1 plasmids (GenScript, Piscataway, NJ).
2.2. Identification of miRNAs targeting Neuropilin-1
To identify miRNAs targeting the 3′-UTR of Neuropilin-1, we used the online target prediction tool Targetscan 7.2, as previously described by our research group [28, 30-33].
2.3. Biological Validation of miR-24 as a Regulator of Neuropilin-1
To evaluate the effects of miR-24 on Neuropilin-1 gene transcription, we used a luciferase reporter containing the 3’-UTR of the predicted miRNA interaction site, both wild-type and mutated, in hBMECs cells. The mutant construct of Neuropilin-1 3′-UTR (Neuropilin-1 MUT, as shown in Figure 1), harboring a substitution of two nucleotides within the predicted miR-24 binding sites of Neuropilin-1 3′-UTR was obtained through means of the NEBaseChanger and Q5 site-directed mutagenesis kit (New England Biolabs, Ipswich, MA, USA) as we described [30, 32].
We transfected hBMECs with the 3′-UTR reporter plasmid (0.05 μg) and miR-24 mirVanaTM mimics (ThermoFisher Scientific, Waltham MA, USA) or miR-24 miRIDIAN hairpin inhibitors (PerkinElmer, Waltham MA, USA), as well as a non-targeting negative control (scramble), all used at a final concentration of 50 nMol/L, using Lipofectamine RNAiMAX (ThermoFisher Scientific) [32]. Firefly and Renilla luciferase activities were measured 48 hours after transfection, using a Luciferase Reporter Assay System (Promega, Madison, WI, USA), normalizing Firefly luciferase to Renilla luciferase activity [32]. Levels of miR-24 were measured via TaqMan miRNA assays (ThermoFisher Scientific), according to the manufacturer’s instructions, and normalized to the level of U6 as we previously described and validated [28, 30]. Cellular expression of Neuropilin-1 was determined by RT-qPCR, as we described [28, 30, 33], normalizing to endogenous glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Sequences of oligonucleotide primers (Merck KGaA, Darmstadt, Germany) are reported in Table 1.
Table 1. Sequences of oligonucleotide primers and product sizes.
|
Primer
|
Sequence (5’-3’)
|
Amplicon (bp)
|
Neuropilin-1
|
Forward
|
CCA CAG TGG AAC AGG TGA TG
|
114
|
Reverse
|
ACA CAC ACA GGC GTT AGC TG
|
GAPDH
|
Forward
|
GGC TCC CTT GGG TAT ATG GT
|
94
|
Reverse
|
TTG ATT TTG GAG GGA TCT CG
|
GAPDH: glyceraldehyde 3-phosphate dehydrogenase.
2.4. Endothelial permeability assay
The in vitro permeability assay was performed as we previously described [34]. Briefly, hBMECs transfected with miR-24 mimic or miR scramble were grown on 0.4-mm fibronectin-coated (R&D Systems) Transwell filters (Corning Inc., Corning, NY, USA). After 48 hours, the medium in the upper well was replaced by FITC-dextran 70 kD (0.5 mg/ml in PB).
Cells were stimulated in the lower well with PBS alone or PBS containing 50 ng/ml VEGF-A165 (R&D Systems, Inc., Minneapolis, MN). The entity of endothelial permeabilization was determined measuring at 520 nm the fluorescence of Dextran that passed in the bottom chamber through the cell monolayer.
2.5. Statistical Analysis
All data are expressed as means ± standard error of means (SEM). Statistical analyses were carried out using GraphPad 8 (Prism, San Diego, CA, USA). Statistical significance, set at p < 0.05, was tested using the two-way ANOVA followed by Tukey–Kramer multiple comparison test or the non-parametric Mann–Whitney U test, as appropriate.