A common molecular basis for membrane docking and functional priming of synaptic vesicles

Area of Science:

• Neuroscience
• Cell Biology
• Biochemistry

Background:

• Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes mediate synaptic vesicle (SV) fusion.
• Munc13 proteins are known to facilitate SNARE complex assembly, a process called priming, which is essential for SV fusion competence.

Purpose of the Study:

• To clarify the role of Munc13 proteins in SV docking to the plasma membrane.
• To investigate whether SV docking is a prerequisite for Munc13-dependent priming.

Main Methods:

• Utilized high-pressure freezing (HPF) to immobilize cultured hippocampal slices, preserving near-native ultrastructure.
• Employed electron tomography (ET) to achieve high-resolution imaging of SVs and their association with the plasma membrane.
• Examined slice cultures from both wild-type and Munc13-deficient mice.

Main Results:

• In control slices, docked SVs were observed linked to the plasma membrane and active zone material by fine strands, not hemifused.
• In Munc13-deficient slice cultures, SVs were notably absent from the docked positions at the plasma membrane.
• HPF-ET imaging circumvented aldehyde fixation artifacts, revealing distinct SV localization patterns.

Conclusions:

• SV docking at the plasma membrane is not independent of Munc13s, contrary to previous findings with aldehyde fixation.
• Munc13s are indispensable for the morphological process of SV docking.
• SV docking and functional priming represent the same molecular event mediated by SNARE complexes and Munc13s.