A combined C/EBP binding site-E-box mediates Fos induction by BDNF/TrkB signaling in primary cortical neurons. (a) Time course of luciferase activity based upon BDNF stimulation (50 ng/ml) of E15.5 wild-type (wt) cortical neurons transfected with a human Fos (hFos) promoter luciferase reporter plasmid (the proximal region of the Fos promoter in mouse and human is identical; see supplemental Figure 2a in Additional data file 1) and a pRSV-β-gal plasmid. Maximum luciferase activity was found between 2 and 4 hours of BDNF stimulation. (b) The combined C/EBP and Ebox sites are fundamental for the activation of the Fos promoter downstream of BDNF/TrkB. E15.5 wild-type primary cortical neurons were transfected with either wild-type or mutated Fos promoter luciferase reporter plasmids and pRSV-β-gal, and stimulated with 50 ng/ml of BDNF for 2 hours. Data shown are the average of three independent experiments, and results are indicated as the mean ± standard error. Cells transfected with a Fos promoter mutated in both the C/EBP and the Ebox binding sites (pMcebp/ebox) showed a significant decrease in luciferase activity (*p < 0.005 relative to pWT). Instead, cortical neurons transfected with a plasmid mutated in the SRE and C/EBP binding site (pMSRF/cebp) revealed a significant increment in the activation of the promoter (*p < 0.005 relative to pWT). No significant difference in luciferase activity was observed between the wild-type Fos promoter (pWT) and the Fos promoter mutated at the Ebox alone (pMebox).