Figure 6. Presynaptic Ca²⁺ influx is correlated with ER Ca²⁺ content in wild type but not in STIM1 KD neurons.

(A-B) Neurons expressing ER-GCaMP6-150 and cytosolic jRCaMP1b were co-transfected with or without an shRNA targeting STIM1 and were used to examine single AP-driven Ca²⁺ influx (jRCaMP1b) and axonal [Ca²⁺]_ER (ER-GCaMP6-150) in the same cells. Recordings from wild type (n=15) and STIM1 KD (n=12) neurons were grouped using a binning size of [Ca²⁺]_ER=25µM to better estimate fitting parameters (see methods; see Fig. S6 for unbinned data). These two variables are correlated only in wild type cells (A) and are well described by a generalized Hill equation (red line) with a K_1/2 of 105 µM ± 5 µM and a Hill coefficient of 5.4 ± 2.7(Adjusted R-square = 0.87). Fitting in the case of STIM1 KD was not possible (B, see methods). Grey and brown lines show the predicted impact of CPA on Ca²⁺ influx when the average value of [Ca²⁺]_ER (152 µM) decreases by 48 µM, as measured during SERCA block.