SNARE zippering and synaptic strength

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Synaptic transmission relies on neurotransmitter release, with significant variations in release probability across different synapse types.
• The Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor (SNARE) complex is crucial for vesicle fusion and neurotransmitter release.
• The degree of SNARE complex zippering is hypothesized to regulate the initial probability of neurotransmitter release.

Purpose of the Study:

• To investigate whether the zippered state of the trans-SNARE complex dictates the initial release probability at phasic and tonic synapses.
• To determine if specific sites on VAMP (vesicle-associated membrane protein) are accessible for cleavage by Clostridial neurotoxins, indicating the extent of SNARE complex zippering.

Main Methods:

• Presynaptic injection of three Clostridial neurotoxins (BoNT/B-LC, BoNT/D-LC, TeNT-LC) targeting different VAMP cleavage sites.
• Assessment of evoked neurotransmitter release under low stimulation conditions at phasic and tonic synapses.
• Analysis of VAMP cleavage to infer the structural state of the trans-SNARE complex.

Main Results:

• Botulinum B light-chain (BoNT/B-LC) inhibited release and cleaved VAMP at both synapse types, while BoNT/D-LC and tetanus neurotoxin light-chain (TeNT-LC) were ineffective.
• This selective VAMP susceptibility suggests that SNARE complexes are partially zippered at the N-terminal end (to the zero-layer) with the C-terminal end exposed in the resting state.
• The presence of this conserved partially zippered state in both high and low release probability synapses challenges the sole role of SNARE zippering in determining release probability.

Conclusions:

• The zippered state of the trans-SNARE complex, at least to the zero-layer, does not solely determine the wide range of neurotransmitter release probabilities observed.
• Synapses with vastly different release probabilities (phasic vs. tonic) exhibit a similar partially zippered trans-SNARE complex structure.
• Further mechanisms beyond the initial SNARE complex zippering must regulate synaptic release probability.