Mediation and modulation of transmitter release 2 lecture

Overview of Neurotransmitter Release and Exocytosis

  • The presentation highlights key aspects of neurotransmitter release, focusing on mechanisms of synaptic vesicle trafficking and the involvement of calcium channels and proteins.

Test Information

  • A test will occur at the end of the lecture cycle.

  • Consists of 15 questions based on the lectures.

  • Questions will be straightforward and require engagement with the material.

    • Example Questions:

    • "Is she the calcium sensor for transmitter release?"

    • "Is this enough to be the only calcium sensor big enough to…?"

    • These questions test the recognition of concepts, not trick questions.

  • Aim: To ensure students revise effectively post-Christmas.

Fundamental Concept of Neurotransmitter Release

  • Neurotransmitter release is primarily defined by the process of exocytosis.

  • The rapid process involves:

    1. An action potential that invades the nerve terminal.

    2. Activation of calcium channels, leading to calcium influx.

    3. Triggering of neurotransmitter from synaptic vesicles via exocytosis.

  • The neurotransmitter is released into the synaptic cleft, a critical step for communication between neurons.

Nerve Terminal Dynamics

  • The cell body of a neuron can be up to 2 meters away from the nerve terminal where neurotransmitter release occurs.

  • The delivery of proteins from the cell body to the nerve terminal happens at a max speed of 2 meters per second.

  • Therefore, it can take approximately 6 weeks for new proteins to travel this distance, highlighting the need for sustained neurotransmitter release mechanisms.

Experimental Insights into Neurotransmitter Release

  • Synaptic Preparation Experiment:

    • Isolated presynaptic terminals are stimulated in the presence of an impermeable dye.

    • Exocytosis occurs, exposing the inside of synaptic vesicles to the extracellular space.

    • After stimulation, washing out the dye reveals:

    • Dye-filled synaptic vesicles → Indicates active exocytosis.

    • Dye-filled internal membranes (larger structures known as M designs) → Indicates membrane retrieval processes.

  • The experiment showcases that synaptic vesicles undergo cycles of exocytosis followed by endocytosis (retrieval).

Types of Endocytosis and Vesicle Retrieval Processes

  • Two main routes for synaptic vesicle recycling:

    1. Direct Retrieval: Vesicles are recovered as whole entities.

    2. Indirect Retrieval: Vesicles are taken into larger internal structures before reforming new vesicles.

  • Stimulation levels influence which route is taken:

    • Moderate stimulation favors direct retrieval.

    • High stimulation favors indirect retrieval due to increased demand.

Types of Retrieval Mechanisms

  • **Endocytosis Mechanisms: **

    1. Kiss-and-Run Mechanism: Vesicle fuses with the membrane but does not fully collapse, allowing for a rapid return to the vesicle pool.

    2. Fast Retrieval Mechanism: Similar kinetic efficiency, where collapsed vesicles quickly reform without clathrin coating.

    3. Clathrin-Mediated Endocytosis: Involves well-characterized clathrin coats, slower, used to recover vesicles from the plasma membrane.

    4. Bulk Retrieval: Used during extensive stimulation, involves large membrane pieces, requires complex biogenesis.

Clathrin and Its Role

  • Clathrin Triskelion Structure:

    • Made from three heavy chains and three light chains.

    • Forms a cage-like structure crucial for vesicle formation and membrane retrieval.

  • Dynamin Protein:

    • Required for vesicle fission from the membrane.

    • Uses GTP to drive membrane pinching.

  • Heat Shock Protein 70:

    • Removes the clathrin coat post-retrieval, resulting in a naked vesicle ready for neurotransmitter storage.

Neurotransmitter Release Mechanism Overview

  • Active Zone: The site of neurotransmitter release characterized by:

    • Proximity to voltage-gated calcium channels.

    • Specialized biochemical environment facilitating exocytosis.

  • Key Players in Release:

    • Monk 13: A protein crucial for neurotransmitter release, identified through genetic studies in C. elegans.

    • Rab GTPases: Regulate vesicle trafficking by interacting with various proteins.

    • Syntaxins, SNAPs (SyNaptosomal Associated Proteins), and Synaptobrevin: Essential proteins for vesicle docking and fusion.

Protein Interactions and Fusion Process

  • Protein complexes assemble to facilitate:

    • Priming of vesicles: Preparation for calcium-induced exocytosis.

    • Calcium sensing: Molecules like synaptotagmin bind calcium and are essential for triggering fusion.

  • Importance of Spatial Organization:

    • Nearest neighbors among proteins allow efficient neurotransmitter release and calcium influx management.

  • Fusing Mechanism:

    • Involves hydrophobic interactions driving the membranes together, facilitated by the formation of a four-helix bundle composed of different proteins.

Conclusion and Student Engagement

  • Understanding of neurotransmitter release involves complex processes with significant interplay between structural and molecular components.

  • Students should engage deeply with the outlined mechanisms to prepare for assessments and future applications in neuroscience.