Figure 8. Syt-7 induces membrane-apposition of LDCVs.

(A, C) 2D-EM micrographs of ultrathin adrenal sections from WT (A) and Syt-7 KO (C) newborn mice. Nucleus is designated (N). Scale bar: 2 µm. (B, D) Magnification of selection in (A) and (C), respectively. Scale bar: 1 µm. (E) Frequency distribution of large dense core vesicles (LDCVs) within 2 µm from the plasma membrane (PM) in WT (persian green) and Syt-7 KO (vermilion) cells. (F) Cumulative frequency plot of (E). (G) Total number of LDCV per cell profile. (H) Number of LDCVs per cytosolic area (Density = LDCVs/µm²). (I) Number of PM-Proximal LDCVs per µm of PM circumference. 2D analysis revealed normal cell morphology and LDCV distribution in the absence of Syt-7. (J, M) 3D-EM reconstructed tomogram subvolume from WT (J) and Syt-7 KO (M) showing two cells with opposing membranes. Scale bar: 300 nm. K, L, N, O Magnifications of selected regions in (J) and (M); showing docked (L, N) and non-docked (K, O) LDCVs. Scale bar: 150 nm. (P) Frequency distribution of PM-proximal LDCVs where docked vesicles are accumulated in the 0–4 nm bin. Insert is a summation of 20–40 nm into single bins. (Q) Frequency distribution of LDCV diameter. (R) Diameter of LDCVs within 100 nm from the PM. (S) PM-Proximal LDCVs (0–40 nm) normalized to (0–100 nm) LDCVs (0–40 nm LDCVs/0–100 nm LDCVs). (T) Vesicles within 6 nm of the membrane (bin 0–6 nm) normalized to PM-Proximal LDCVs (0–6 nm LDCVs/0–40 nm LDCVs). Overall, the 3D analysis showed that LDCVs 0–6 nm from the PM are markedly reduced in the absence of Syt-7. Non-docked LDCVs in the Syt-7 KO accumulated at 20–40 nm from the PM. Data information: Values are mean ± SEM.*: p<0.05; **: p<0.01. Student’s t-test: (G): p=0.4240; (H): p=0.9711; (I): p=0.9241; (Insert in (P): 20–40 nm): p=0.0025; (R): p=0.0581; (S): p=0.6127. Mann Whitney test: (P): 4–6 nm: p=0.0039; (T): p=0.0184. Number of cells, 2D analysis: (WT) N = 60 cells, (Syt-7 KO) N = 46 cells; 3D analysis: (WT) N = 74 cells, (Syt-7 KO) N = 74 cells.