Neurotransmitter Release Mechanism

Key Proteins Involved in Synaptic Vesicle Fusion

  • SNARE Complex: Composed of three proteins: syntaxin, synaptobrevin, and SNAP-25.

  • Vesicle Priming: Requires assembly of the SNARE complex, making vesicles fusion competent.

Syntaxin Configuration

  • Syntaxin has a default closed state where its binding domain is hidden.

  • This closed state is incompatible with SNARE complex formation.

Opening Syntaxin

  • The transition from closed to open syntaxin is crucial for SNARE complex assembly.

    • How to Open Syntaxin: Factors like UNC-13 play a critical role.

Role of UNC-13

  • UNC-13 binds to the N-terminus of syntaxin, promoting SNARE complex assembly.

    • Research Reference: Betz et al. 1997 highlights the role of UNC-13 in modulating syntaxin.

Synaptic Vesicle Docking

  • Textual inquiries: Are syntaxin and UNC-13 essential for docking?

  • Mutants of Syntaxin and UNC-13 show defects in synaptic vesicle docking.

  • High-resolution imaging shows the organization of protein complexes supporting vesicle docking.

    • Techniques Used: High-pressure freezing preserves synapse in its physiological state.

Detailed Structures

  • Images Display: Side view and top view of docked synaptic vesicles showcasing protein complexes.

    • Each view depicts distances significant for molecular interactions (e.g., 50 nm apart).

Munc13/UNC-13 Structure

  • Crystal structure reveals conformational states regulated by DAG and Ca2+.

  • Three Stages Identified:

    • State 1: Unassembled SNAREs, referred to as "Captured".

    • State 2: Pre-primed state of unassembled SNAREs.

    • State 3: Primed state with clamped SNAREpins.

Calcium Regulation in Vesicle Fusion

  • Fusion is dictated by a calcium influx via voltage-gated calcium channels in response to action potentials.

  • Timing: Fusion occurs within 200 milliseconds of calcium entry due to spatial proximity to primed vesicles.

Synaptotagmin's Role

  • Synaptotagmin I: Knockout mice studies show loss of synchronous release, emphasizing its essential role.

  • Structure: Synaptotagmin interacts with SNARE complexes and responds to calcium.

Mechanism of Action**

  1. Binding: Synaptotagmin binds to the SNARE complex, holding the structure stable until Ca2+ is present.

  2. Insertion: Ca2+ binding to C2B domain induces insertion into the target membrane which curves the membrane, reducing energy requirements for fusion.

  3. SNARE Activation: Ca2+-synaptotagmin interactions lead to the zippering of SNARE complexes, initiating vesicle fusion.

Summary of Exocytosis Process

  • Munc13 facilitates the docking of synaptic vesicles and promotes SNARE complex assembly (priming).

  • The synaptotagmin ring stabilizes primed vesicles until Ca2+ triggers a release.

  • When calcium channels open in response to action potentials, vesicle fusion is ultimately triggered.