Light-triggered protein secretion in neurons. (A) Localization of VSVG-2×UVR8 in neurons before UV-B treatment. Note the localization of clusters in the soma and in dendritic processes (arrowheads). The same cell was imaged 30 min after UV-B treatment (right). Note the massive redistribution of signal to the somatic Golgi (arrow). Bar, 4 µm. (B) Hippocampal neurons expressing VSVG-YFP-2×UVR8 (green) and mCherry (red) were labeled with an antibody against an extracellular epitope of VSVG (bottom panels) at various times after UV-B treatment. In the dark, VSVG surface labeling was not significantly greater than background values, but robustly increased after UV-B treatment. The inset in the bottom right panel shows VSVG surface label (green) overlaid with mCh cell fill (red). Bar, 10 µm (inset, 4 µm). (C) Quantification of data from B. 150 min after UV-B treatment, we observed a >30-fold increase in VSVG surface expression. Data are expressed as a normalized ratio of surface (anti-VSVG) signal to the total YFP signal (n = at least 4 cells for each time point). Error bars represent the standard deviation of the mean. (D) Formation of dendritic trafficking outposts. VSVG-YFP-2×UVR8 clusters were dissolved with UV-B (middle); we often observed a delayed accumulation of cargo in Golgi outposts positioned near dendritic branch points (right). Bar, 2 µm.