Axonal Outgrowth & Synapse formation lecture 2

Lecture Overview

  • Questions from Previous Lecture

    • Inquiry about the first lecture: No questions raised. The previous lecture was deemed easy.

Lecture Objectives

  • Mechanisms of Neural Specification

    • Dissection of neural specification mechanisms.

    • Introduction to the growth cone and discussion on axonal guidance.

    • Emphasis on continuous learning outcomes from previous lectures.

Key Concepts

Transcription Factors

  • Definition: Proteins that regulate the transcription of genes by binding to specific DNA sequences.

  • Reciprocal Regulation:

    • Transcription factors can activate or inhibit each other, driving cellular specification.

    • Essential for determining different populations of neurons.

  • Importance in Neurodevelopment:

    • Drives the specification and regionalization of the nervous system.

    • Establishes boundaries for layers in the brain; these boundaries are functional for neuron subtypes.

Morphogens and Ligands

  • Key Morphogens:

    • FGF (Fibroblast Growth Factor):

    • Plays a role in inhibiting BMP signaling.

    • Essential for dorsalization of ectoderm and neuron differentiation.

    • BMP (Bone Morphogenetic Protein):

    • Initially antagonized to promote the formation of the neural tube.

    • After neural tube formation, it contributes to the polarization of the nervous system.

    • Wnt: Another morphogen involved in neural specification processes.

Neural Tube and Body Axis Specification

  • Body Body Axis:

    • Establishment of anterior-posterior axis allows different cells to receive varying signals based on their positions.

  • Gradients of Morphogens:

    • Cells in different positions respond differently to morphogen concentrations, affecting their fate and development.

Interplay of Transcription Factors

  • Homeobox and Hox Genes:

    • Specific regulatory networks that control patterns of neural development.

  • Example: Pax6:

    • Essential transcription factor involved in the specification of the nervous system.

    • Exhibits reciprocal inhibition when influenced by BMP and FGF.

  • Reciprocal Repression: Explained with transcription factors being either activated or inhibited based on cells' positional context.

Effects of Dysregulation

  • Hypothetical Scenario: Downregulation of specific transcription factor (mix) could lead to over-specification of frontal cortex neurons and affect the overall numerical specification of neurons.

  • Neuron Number:

    • Neurons increase from initial stages to trillions through processes of specification and localization.

Case Study: Spinal Cord Development

Sonic Hedgehog (Shh) Morphogen

  • Role: Released from notochord, creating a gradient mitogenically affecting neural specification.

  • Mechanism:

    • Promotes transcription factor expression (e.g., Pax6) and inhibits others (e.g., DBX2) based on concentration.

  • End Result:

    • Specification of distinct layers of neurons, with patterns determined by morphogen levels and transcription factor activity.

Mechanisms of Axon Guidance

Growth Cone

  • Structure:

    • Hand-like structure guiding axonal growth. Composed of dynamic cytoskeletal elements (actin and microtubules).

  • Function:

    • Its mechanics are crucial for responding to attractive and repulsive signals during axon navigation.

Cytoskeletal Dynamics

  • Actin & Microtubules:

    • Actin: Forms protrusive structures like lamellipodia and filopodia.

    • Microtubules: Provide structural support and help transport materials within the neuron.

  • Crosstalk Between Actin and Microtubules:

    • Activating proteins coordinate the interactions between these components, influencing growth cone behavior.

Molecular Mechanisms of Cytoskeletal Dynamics

  • Small GTPases:

    • Proteins such as CDC42, RhoA, and RhoB play significant roles in integrating signals that regulate actin dynamics.

  • Mechanism of Action:

    • Their activity states (active GTP-bound vs. inactive GDP-bound) determine the extent of polymerization or depolymerization in the cytoskeleton.

Expansion Microscopy

  • Technique: A novel method allowing for enhanced imaging resolution of cellular structures, revealing intricate interactions between cytoskeletal components during growth cone dynamics.

Summary of Lecture Points

  • Key Learning Outcomes:

    • Mechanisms behind neural specification, growth cone dynamics, and axon guidance.

    • Emphasis on positional information and the reciprocal relationship between actin and microtubules in neural development.

  • Future Topics:

    • Further exploration of synapse formation in subsequent lectures.

Questions and Clarifications

  • A prompt for students to ask clarification questions at the end of the lecture before move on to the next subject.