Week 6 Lecture 1 exocytosis(1)

Note on Presynaptic Proteins and Synaptic Vesicle Cycling

Page 1: Overview of Presynaptic Proteins

  • Key Proteins Involved in Synaptic Vesicle Cycling

    • Transmitter loading

    • Uncoating

    • Transporters (e.g., Clathrin, Endophilin)

    • Budding and Mobilization (e.g., Dynamin, Synapsins)

    • Docking and Fusion (e.g., SNAREs, Synaptotagmins)

Page 4: Specialized Transporter Proteins

  • Function of Transporter Proteins

    • Load neurotransmitters into vesicles.

    • Monoamine Transporter

      • Utilizes a proton gradient (pH 5.5 inside vesicles, pH 7.4 outside) to drive monoamines into the vesicle lumen.

    • Key Discoveries

      • Rob Edwards identified the first vesicular transporter, vMAT, in 1992.

Page 6: Mobilization of Vesicle Reserve Pool

  • Role of Synapsin

    • Interacts with vesicles and actin in a dephosphorylated state.

    • CamKII Function

      • Phosphorylates synapsin, releasing vesicles from the reserve pool.

    • Experimental Evidence

      • Injection of synapsin antibodies disrupts synaptic transmission during high release rates.

Page 8-9: SNARE Complex Assembly

  • Priming Process

    • Requires assembly of SNARE complex (syntaxin, synaptobrevin, SNAP-25).

    • Primed vesicles are fusion competent.

    • SNARE complex brings vesicle close to presynaptic membrane.

Page 10-12: Tetanus and Botulinum Toxins

  • Tetanus Toxin

    • Cleaves synaptobrevin, causing painful muscle spasms.

  • Botulinum Toxin

    • Disrupts SNARE proteins, leading to flaccid paralysis.

    • Botox (botulinum toxin A) specifically cleaves SNAP-25.

Page 14-15: Knockout Studies

  • SNAP-25 Knockout

    • No evoked transmission observed in SNAP-25 knockout mutants.

  • Synaptobrevin Knockout

    • Abolishes evoked synaptic transmission.

Page 17-19: Syntaxin Configuration

  • Syntaxin's Role in SNARE Complex Formation

    • Closed syntaxin is incompatible with SNARE complex formation.

    • UNC-13 is required to open syntaxin for complex formation.

Page 25: Vesicle Fusion Regulation

  • Calcium's Role

    • Voltage-gated calcium channels open in response to action potentials.

    • Calcium influx triggers vesicle fusion within 200 µsec.

Page 27-30: Synaptotagmin Function

  • Calcium-Binding Protein

    • Interacts with SNARE complexes to regulate fusion.

    • Proposed Actions

      • Holds SNARE complexes in a fusion-competent state until Ca2+ increases.

      • Ca2+ binding causes insertion into the target membrane, facilitating fusion.

This note summarizes the roles of presynaptic proteins in synaptic vesicle cycling, highlighting key processes such as transmitter loading, vesicle mobilization, SNARE complex assembly, and the