SNARE complex formation is triggered by Ca2+ and drives membrane fusion

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Neurotransmitter exocytosis relies on SNARE protein complexes (syntaxin, SNAP-25, VAMP).
• SNARE-cleaving neurotoxins, like Botulinum neurotoxin E, inhibit this essential cellular process.
• PC12 cells are a widely used model system for studying neuronal function and exocytosis.

Purpose of the Study:

• To investigate the mechanism by which Botulinum neurotoxin E inhibits norepinephrine release.
• To determine the role of the SNAP-25 C-terminal fragment (S25-C) in rescuing exocytosis.
• To elucidate the influence of calcium ions (Ca2+) on SNARE complex formation and exocytosis.

Main Methods:

• Utilizing permeabilized PC12 cells to study norepinephrine release.
• Employing a 65 amino acid C-terminal fragment of SNAP-25 (S25-C) for rescue experiments.
• Introducing mutations to the hydrophobic face of the S25-C helix to assess their impact on SNARE complex stability and function.
• Monitoring exocytosis rescue in the presence and absence of S25-C and Ca2+.

Main Results:

• Botulinum neurotoxin E inhibition of norepinephrine release was rescued by adding S25-C.
• Mutations in S25-C altered SNARE complex thermostability and rescue efficiency.
• Successful rescue required the continuous presence of both S25-C and Ca2+.
• Exocytosis rescue correlated directly with SNARE complex formation.

Conclusions:

• Calcium ions likely trigger the initial low-affinity binding of S25-C, initiating trans-SNARE complex formation.
• The formation of high-affinity cis-SNARE complexes on apposing membranes drives bilayer fusion and neurotransmitter release.
• S25-C plays a critical role in the SNARE complex assembly required for regulated exocytosis.