Mediation and modulation of transmitter release 3 lecture

Introduction

  • Objective of the lecture: To complete the discussion on the fusion mechanism of neurotransmitter release and introduce G-protein coupled receptors (GPCRs).

Fusion Mechanism

  • Base Concept: Vesicles are central to neurotransmitter release, following Katz's ideas.

    • Focus on distinct biochemical states involved:

    • Tethering

    • Priming

    • Docking

    • Fusion

    • Retrieval

  • Importance of Collaboration: Individual molecules collaborate, enabling the transition between distinct states.

    • Biochemical Events Leading to Fusion Event:

    • The vesicle approaches the fusion event which is triggered by calcium influx.

    • Calcium is a crucial element for the activation of the fusion process.

Regulation of the Fusion Event

  • Key Internal Interactions: Internal protein-protein interactions regulate calcium-dependent events, with synaptotagmin serving as an example.

    • Synaptotagmin is a calcium sensor that interacts with other proteins.

  • Activation Mechanism:

    • Syntaxin: A regulatory protein that modulates its conformation to control interactions with other proteins.

    • Examples of Modulatory Proteins:

    • Mention of monk 13 and its role in priming and activation.

  • Coordination of Ions and Proteins: Requires synchronization of ions and proteins to facilitate fusion where lipid bilayers merge.

    • Important to understand lipid involvement and the speed of vesicle exocytosis events, estimated at 100 microseconds.

Membrane Fusion Dynamics

  • Dual Aspects of Reaction: Consider the impact of the calcium influx on the fusion reaction and the diffusion of vesicles.

  • Calcium Sensor:

    • Synaptotagmin: Functions through calcium binding domains (C2A and C2B) integral for membrane fusion events.

  • Snap Proteins: Three key proteins identified:

    • Synaptobrevin: Primarily in synaptic vesicles.

    • Snap 25: Membrane-associated, interacts with synaptobrevin.

    • Syntaxin: Embedded in the presynaptic membrane.

  • Mechanism of Fusion:

    • Discuss the physical process and energy dynamics in membrane approach and fusion events.

    • Concept of semi-fused state discussed: Intermediate fusion state where lipid bilayers mix without total fusion.

    • The temperature and conditions under which this reaction occurs.

    • Address physical interaction and repulsion barriers during fusion.

Factors Influencing Fusion Event

  • Steps to Achieve Final Fusion:

    • The proteins work together progressively to bring vesicles close enough to overcome repulsion forces.

    • The role of calcium ion influx in conjunction with synaptotagmin to facilitate final fusion.

  • Thought Experiment:

    • Investigating the necessity of synaptotagmin regarding neurotransmitter release by using knockout preparations as a model.

    • Conduct experiments comparing knockout studies to those with intact proteins, observing synaptic potential responses.

Observational Evidence via Experiments

  • Knockout Observations:

    • Differences in neurotransmitter release in synaptic tag knockouts versus functional ones.

    • Observational evidence that synaptotagmin's absence slows synaptic transmission.

  • Noteworthy Induction:

    • Propose experiments based on synaptotagmin's role in neurotransmitter release, primarily through calcium dependency and interaction with Snap proteins.

Complexity of Release Mechanisms

  • Complexity Proteins: Discuss the existence and function in regulating neurotransmitter release.

    • Explanation of how complexity inhibits synaptic release, creating a balance between activation and inhibition during extreme conditions.

  • Bioluminescence Analogy: Illustrated precision of interactions whereby calcium binding alters protein structures, thus determining functional efficiency in neurotransmission.

G-Protein Coupled Receptors (GPCRs)

  • Introduction of GPCRs and their importance in neuronal signaling.

    • Discuss primary structure: seven transmembrane domains & mechanisms of action via G-proteins (alpha, beta, gamma subunits).

  • GPCR Activation: Ligand binding leads to G-protein dissociation, triggering downstream signaling events affecting neurotransmitter release.

    • Exploration of both pre- and post-synaptic modulation potential influenced by GPCRs.

    • The necessity for diversity and complexity in GPCR types, impacting various neurotransmitter systems.

Additional Insights

  • G protein-related pharmacology: Impact on drug development and pharmacological applications.

  • Overview of GPCR relevance in neurobiology and necessary functions in the modulation of neuronal activity.

Summary and Further Study

  • Emphasize the integrated nature of neurotransmitter release and GPCR signaling.

  • Preview of subsequent lectures focusing on GPCR modulation in influence on synaptic transmission, especially in relation to drug interactions.

  • Introduction to synaptic tag mean one and its association with autism in a follow-up discussion. Especially relevant papers will be provided for further reading.

Conclusion

  • Encouragement for students to reflect upon the interconnectedness of cellular mechanisms in signal transduction and its relevance in clinical contexts.

  • Preparation for future discussions on receptor types following GPCRs, particularly emphasizing a widespread relevance in various biological systems.