Study Notes on Circuitry Development and Related Neuroscience Concepts

Circuitry Development Learning Objectives

  • Intrinsic and extrinsic factors involved in axonal pathfinding

  • Synaptogenesis

  • Trophic support and synapse elimination

Axonal Pathfinding

General Overview

  • Polarity in Neurons:

    • Polarity allows regions of cells to acquire specific functions; in neurons:

    • Dendrites: Receive signals

    • Soma: Contains the cell nucleus

    • Axons: Responsible for communicating with targets

  • Growth Cone:

    • Integral in axonal pathfinding; facilitates the establishment of neuronal connections.

    • Distinct structures within growth cones:

    • Filopodia: Long, slender projections

    • Lamellipodia:

The Structure and Action of Growth Cones

  • Cytoskeletal Components:

    • Actin and tubulin are dispersed in specific regions of the growth cone:

    • Filamentous Actin (F-actin): Found in both lamellipodia and filopodia (red visualization)

    • Tyrosinated Microtubules: Present in lamellipodia (green visualization)

    • Acetylated Microtubules: Identified in the axon (blue visualization)

Classes of Guidance Cues in Axonal Pathfinding

General Types of Guidance Cues

  • Guidance cues can act:

    • At a distance

    • Locally by contact

  • Cues can be:

    • Positive (chemoattraction)

    • Negative (chemorepulsion)

Growth Cone Behavior

Pioneer Growth Cones

  • Pioneer Growth Cones: Lead the way for subsequent growth cones by fasciculating with the pioneer axons.

    • Chemoattraction: Attracts following growth cones to specific targets.

    • Contact-mediated Attraction/ Repulsion: Growth cones navigate through positive and negative interactions with guidance molecules.

Changes in Growth Cone Shape

  • Growth cone morphology changes based on guidance cue interactions at decision points, influencing directionality towards targets.

Cytoskeleton Dynamics within Growth Cones

Treadmilling of Actin Filaments

  • Actin Dynamics:

    • Involves ATP/ADP interchange mechanisms:

    • At Rest:

      • Net assembly occurs at the plus ends

      • Net disassembly at the minus ends

      • Resulting in a net rate of filament growth of zero (Treadmilling)

Structural Domains of Growth Cones

  • Domains of Growth Cone:

    1. P-Domain: Contains F-actin bundles and binds pioneer microtubules

    2. C-Domain: Bundled microtubules, organelles, vesicles, and central actin bundles

    3. T-Domain: Transitional area with acto-myosin contractile structures (actin arcs)

Steps of Axonal Outgrowth

  1. Encounter Substrate: Involves engagement of clutch mechanisms

  2. Protrusion:

    • Attenuation of F-actin retrograde flow

    • Continued F-actin polymerization at clutch site leading to forward movement of P-domain

  3. Engorgement:

    • Reorientation of F-actin arcs from C-domain towards growth site

    • Advancement of C-domain microtubules

  4. Consolidation:

    • Formation of a new axonal shaft via suppression of MTs in the C-domain by myosin arcs

Axonal Guidance Molecules

Families of Ligands and Receptors

  • Major Classes: Includes extracellular matrix molecules, cell adhesion molecules, cadherins, ephrins, netrins, and slits.

Extracellular Matrix Molecules (ECM)

  • Integrin Receptors:

    • Key ECM components include laminin, fibronectin, and collagen.

    • Integrins activate cytoplasmic protein kinases and Ca2+ channels.

Cell Adhesion Molecules (CAMs)

  • Homophilic Binding:

    • Ca2+-independent

    • Membrane-bound

    • Activates intracellular kinases leading to cytoskeletal alterations

Cadherins

  • Homophilic Interaction:

    • Ca2+-dependent

    • Membrane-bound, associated with catenins

    • β-catenin influences gene expression

Ephrins & EPH Receptors

  • Functionality:

    • EPHs are tyrosine kinases; ephrins are their ligands

    • Engaged through bidirectional signaling (forward and reverse)

    • Play roles as either chemorepulsive or chemoattractant based on intracellular signaling molecules present

Signals from the Embryonic Environment

Major Families of Instructive Diffusible Cues

  1. Netrin with receptors DCC, neurogenin, UNC-5

  2. Slit binding to ROBO receptors

  3. Semaphorin with neuropilins, plexins

  4. Ephrin interacting with EPH receptors

Netrins and Their Receptors

  • Netrin Subfamilies:

    • DCC (Deleted in Colorectal Cancer)

    • Unc5H

  • Commissural Neurons:

    • Extend projections to floorplate where they cross over; disrupted in netrin and DCC mutants

Mechanism of Netrin Action

  • Attractant or Repellent:

    • Netrin acts differently based on receptor types on axons.

    • DCC only: attraction, DCC + Unc5H: repulsion

Robo & Slit Interaction

  • Robo Signaling:

    • Slits interact with Robo receptors, inducing axonal repulsion, characterized in Drosophila

  • DCC & Robo Cooperation:

    • DCC's P3 domain interacts with Robo's CC1 domain under the presence of slit/netrin, causing silencing and linear forward axon movement.

Semaphorins, Plexins & Neuropilin

  • Characteristics of Semaphorins:

    • At least 15 types recognized, include both soluble and membrane-bound forms.

    • Activation and signaling aim for growth cone repulsion or attraction via binding with plexin receptors.

Polarized Dendritic Growth

  • Mechanisms of Polarization:

    • Dependent on secreted signals guiding cells to develop axons and dendrites.

    • Dendritic branching occurs in multiple directions initially, later refined to extend away from the axon.

Molecular Mechanisms of Synapse Formation

  • Initial Steps:

    • Synapsis begins via recognition and adhesion facilitated by cadherins and protocadherins.

  • Induction:

    • Additional adhesion molecules participate, driving presynaptic active zone differentiation.

  • Important Interactions:

    • Neurexin and Neuroligin:

    • Neurexin: recruits microtubules and synaptic vesicle proteins, linking presynaptic sites to voltage-gated Ca2+ channels.

    • Neuroligins: localize neurotransmitter receptors at the postsynaptic density (PSD95) zone.

Target-Derived Trophic Support

Importance of Trophic Factors

  • Survival Signals:

    • Following synaptic connections establishment, target sources provide trophic support for neuronal survival.

  • Neurotrophins:

    • First described in experiments by Hamburger and Levi-Montalcini, showing roles of limb bud removal on motor neuron survival.

Effects of Manipulating Limb Buds

  • Extra Limb Bud Experiment:

    • Addition results in increased motor neuron numbers.

    • Identified substance named Nerve Growth Factor (NGF).

Neurotrophic Hypothesis

  • Concept Overview:

  • Neuronal survival is influenced by target-secreted factors, balancing the number of target areas and incoming neurons.

  • Primarily applies to peripheral neurons, with central nervous system neurons demanding more survival factors.

Synapse Elimination

Process of Monoinnervation

  • Each muscle fiber innervated by one neuron to enhance efficiency, initiated through initial polysynaptic connections during development.

  • Efficiency of Synaptic Connections:

    • Synaptic elimination enhances the overall synaptic efficiency.

Occurrence in CNS

  • Similar polynodal synapse eliminations transpire in the central nervous system, refining to monoinnervation with increasing connection complexity.