Ultrastructural membrane dynamics of mouse and human cortical synapses

Live human brain tissues provide unique opportunities for understanding synaptic transmission. Investigations have been limited to anatomy, electrophysiology, and protein localization, while crucial parameters, such as synaptic vesicle dynamics, were not visualized. Here, we utilize zap-and-freeze time-resolved electron microscopy to overcome this hurdle. First, we validate the approach with calcium imaging in acute mouse brain slices. Next, we show that synaptic vesicle endocytosis is induced in both mouse and human brain slices. Crucially, clathrin-free pits appear immediately next to the active zone, where ultrafast endocytosis normally occurs, and can be trapped at this location by a dynamin inhibitor. In both species, a protein essential for ultrafast endocytosis, dynamin 1xA, localizes to the region peripheral to the active zone, the putative endocytic zone, indicating a possible conserved mechanism between mouse and human. This approach has the potential to reveal dynamic, high-resolution information about synaptic membrane trafficking in intact human brain slices.