Representative images were taken for each condition, using the same exposure time for each filter, to allow comparison of fluorescence intensity between different fields and conditions. Primary cultures of cortical neurons were obtained from C57BL/6 mice, at day 16 of gestation. After dissociation
and centrifugation of the dissected cortices, the tissue was resuspended in Neurobasal medium (Invitrogen, San Diego, CA), supplemented with 2% (v/v) B27 supplement (Invitrogen) and 100 U/ml penicillin/streptomycin (Invitrogen). Cells were plated at a density of 500 000 cells/well in six-well multi-well plates previously coated with poly-l-lysine. Characterization of the embryonic neuronal cultures confirmed the presence of 95% neurons, as determined by GFAP and NeuN-immunostaining. Primary cultures were kept at 37° in a humidified atmosphere Gefitinib mouse containing 5% CO2. After 8 days in culture, neurons were incubated for 24 hr with a 1 : 1 mixture of Neurobasal medium (500 μl) and conditioned medium (500 μl). The conditioned medium was obtained from untreated N9 cells, N9 cells exposed to LPS PKC412 chemical structure (0·1 μg/ml) for 24 hr, and N9 cells transfected with anti-miR-155 or control oligonucleotides 24 hr before exposure to LPS. In parallel experiments neurons were incubated
with LPS (0·1 μg/ml) for 24 hr. Cell viability of primary neuronal cultures was determined by a modified Alamar Blue assay. This assay measures the redox capacity of neurons and allows the determination of cell viability without the detachment
of the cells, so cell integrity is maintained. Briefly, 1 ml Neurobasal medium supplemented with 10% (v/v) of Alamar Blue dye was added to each well following the 24-hr incubation period with conditioned medium or LPS. After 3 hr of incubation at 37°, 150 μl supernatant was collected from each well and transferred to 96-well plates. The optical density of the supernatant was aminophylline measured at 570 and 600 nm in a microplate reader and cell viability was calculated as a percentage of control cells, using the formula: (A570–A600) of treated cells × 100/(A570–A600) of control cells. All data are presented as mean ± standard deviation (SD) and are the result of three independent experiments, each performed at least in triplicate. One-way analysis of variance combined with Tukey post-hoc test was used for multiple comparisons in cell culture experiments. Statistical differences are presented at probability levels of P < 0·05 (*), P < 0·01 (**) and P < 0·001 (***). Calculations were performed with standard statistical software (GraphPad Prism 5, GraphPad Software, La Jolla, USA). Since miR-155 has been described as being up-regulated in various cells of myeloid origin upon their activation and as contributing to the modulation of the immune response mediated by these cells, we first investigated the expression of this miRNA in mouse N9 microglia cells and primary microglia cultures employing qRT-PCR.