Figure 5A. Representative image of Syt antibody uptake at axons of hippocampal neurons (DIV18) co-expressing RFP and pSuper empty vector, SCRN1 shRNA or SCRN1 shRNA with GFP-SCRN1-F402A. Yellow and grey arrowheads mark presynaptic boutons with and without internalized Syt, respectively. Zooms represent typical boutons. Scale bars: 5 µm (full size) and 2 µm (zoom). B. Quantifications of fluorescence intensity of internalized endogenous Syt at individual presynaptic boutons of hippocampal neurons (DIV18) co-expressing RFP and pSuper empty vector, SCRN1 shRNA or SCRN1 shRNA with GFP-SCRN1-F402A. N=2, n=201-300 boutons. Data information: Data represent Mean ± SEM; NS: not significant; *P < 0.05; **P < 0.01; ***P < 0.001, by Mann-Whitney U test. C. Representative time-lapse of cytosolic GCaMP6f upon electric field stimulation (50 APs, 20 Hz) in axons of hippocampal neurons (DIV21) co-expressed with RIM1a-mCherry to visualize presynaptic sites (arrowheads). Scale bar: 5 µm. D. Time-lapses of cytosolic GCaMP6f upon electric field stimulation (50 APs, 20 Hz) in axons of hippocampal neurons (DIV21) co-expressing pSuper control, SCRN1 shRNA or VAPA/B shRNAs. Arrowheads mark presynaptic boutons. Scale bar: 5 µm. E. Average normalized response of GCaMP6f fluorescent intensity at presynaptic boutons upon stimulation (50 APs, 20 Hz) in hippocampal neurons (DIV21) co-expressing pSuper empty vector, SCRN1 shRNA or VAPA/B shRNAs. N=5-6, n=15-27. F. Average normalized peak response of GCaMP6f at presynaptic boutons upon stimulation (50 APs, 20 Hz) in hippocampal neurons (DIV21) co-expressing pSuper empty vector, SCRN1 shRNA or VAPA/B shRNAs. N=5-6, n=15-27. Data information: Data represent Mean ± SEM; NS: not significant; *P < 0.05; **P < 0.01; ***P < 0.001, by Mann-Whitney U test. G. Representative time-lapse of cytosolic GCaMP6f before (F0) and after (Fmax) ionomycin treatment at axons of hippocampal neurons (DIV18) co-expressed with mRFP and pSuper empty vector or VAPA/B shRNAs. Arrowheads mark presynaptic boutons. Scale bar: 5 µm. H. Basal GCaMP6f fluorescence (F) normalized to the maximum GCaMP6f fluorescence (Fmax) after ionomycin treatment at presynaptic boutons of hippocampal neurons (DIV18) co-expressing mRFP with pSuper empty vector or VAPA/B shRNAs. N=2, n=47-50. I. Average basal GCaMP6f fluorescence (F0) normalized to the max GCaMP6f fluorescence intensity (Fmax) after ionomycin treatment at presynaptic boutons of hippocampal neurons (DIV18) co-expressing mRFP with pSuper empty vector or VAPA/B shRNAs. N=2, n=47-50. Data information: Data represent Mean ± SEM; NS: not significant; *P < 0.05; **P < 0.01; ***P < 0.001, by Mann-Whitney U test. J. Representative time-lapse of cytosolic R-GECO1 before (F0) and after (Fmax) ionomycin treatment at axons of hippocampal neurons (DIV18) co-expressing GFP, GFP-SCRN1 or GFP-SCRN1-F402A. Arrowheads mark presynaptic boutons. Scale bar: 5 µm. K. Basal R-GECO1 fluorescence (F) normalized to the maximum R-GECO1 fluorescence (Fmax) after ionomycin treatment at presynaptic boutons of hippocampal neurons (DIV18) co-expressing GFP, GFP-SCRN1 or GFP-SCRN1-F402A. N=3, n=70-89. L. Average basal R-GECO1 fluorescence (F0) normalized to the maximum R-GECO1 fluorescence intensity (Fmax) after ionomycin treatment at presynaptic boutons of hippocampal neurons (DIV18) co-expressing GFP, GFP-SCRN1 or GFP-SCRN1-F402A. N=3, n=70-89. Data information: Data represent Mean ± SEM; NS: not significant; *P < 0.05; **P < 0.01; ***P < 0.001, by Mann-Whitney U test.