FIGURE 7.

Model of membrane fusion between a VLC-SFA-rich synaptic vesicle and presynaptic active zone membrane. In this cartoon, a synaptic vesicle (SV) containing lipids with common fatty acids (phosphatidylcholine or phosphatidylserine molecules with palmitic, 16:0, and stearic acids, 18:0) is used as a model to illustrate the three stages of synaptic vesicular exocytosis, namely, (1) undocked vesicle; (2) docked and hemifused vesicle stage, with deformation tilt of the membranes and stalk formation; and (3) membrane fusion and contents release stage. In (2b), an SV is depicted with lipids containing, in addition, VLC-SFA (typically 28:0 or 30:0 fatty acid chains, see inset). We have shown that the normal presence of these fatty acids in the SV membrane renders it more stable than do the shorter LC fatty acids, thereby fusing and releasing at comparatively lower rates (Hopiavuori et al., 2018) (PS, phosphatidylserine; PC, phosphatidylcholine; SM, sphingomyelin; SL, sphingolipids; see text for more details).