TrkB localization and its activation upon BDNF stimulation are impaired in Smn-deficient axon terminals. (A) NMJs in the Transversus abdominis anterior (TVA) muscle in control and SMNΔ7 P10 animals stained against TrkB (gray), Synaptophysin-1 (SYP, magenta), and postsynaptic ACh receptors (Bungarotoxin [BTX], green); scale bar: 10 µm. Dotted line depicts outline of the presynapse (SYP signal). Normalized mean gray values of TrkB (N = 5, n = 109; U-Mann–Whitney, ****P ≤ 0.0001). (B) Control and Smn^−/−;SMN2 growth cones stained against TrkB (gray) and SYP (magenta); scale bar: 5 µm. Normalized mean gray values of TrkB (N = 6, n = 120; U-Mann–Whitney, *P = 0.0204). Bar represents the mean ± SEM. (C) Relative expression of TrkB in whole-cell lysates from control and Smn^−/−;SMN2 motoneurons. Quantification of TrkB relative to Gapdh (N = 3, n = 7; U-Mann–Whitney). Data are presented as mean ± SD. (D) NMJs in the TVA muscle in control and SMNΔ7 P10 animals unstimulated and 30 min BDNF stimulated stained against p-TrkB (gray), SYP (magenta), and BTX (green), scale bar: 10 µm. Dotted line depicts outline of the presynapse (SYP signal). Normalized mean gray values of p-TrkB (N = 4, n = 56; ANOVA Kruskal–Wallis, ***P ≤ 0.001; ****P ≤ 0.0001). (E) Control and Smn^−/−;SMN2 growth cones unstimulated and 15 min BDNF stimulated stained against p-TrkB (gray) and SYP (magenta); scale bar: 5 µm. Normalized mean gray values of p-TrkB (N = 6, n = 130; ANOVA Kruskal–Wallis, ****P ≤ 0.0001). (F) Control and Smn^−/−;SMN2 growth cones unstimulated and 15 min BDNF stimulated that were pre-treated with 0.5 µM Cytochalasin D (CytoD) stained against p-TrkB (gray) and SYP (magenta), scale bar: 5 µm. Normalized mean gray values of p-TrkB (N = 3, n = 105; ANOVA Kruskal–Wallis, *P ≤ 0.05; ****P ≤ 0.0001). Data are presented as scatter dot plot with bar. Bar represents the mean ± SEM (except for C).