112124 lecture

Recap from Last Session

  • Focus on neuronal function, emphasizing polarity in dendrites and axons.

    • Dendritic Polarization:

      • Mixed polarity microtubule structures, organelle distribution, receptor presence.

    • Axonal Polarization:

      • Unipolar microtubule structures, organelle distribution, involvement of synaptic vesicles.

Microtubule Structure

  • Comprised of α-β tubulin heterodimers.

  • Heterodimers alternate in a single protofilament which further forms a microtubule from 13 protofilaments.

  • Protofilaments form a helical tube due to the angle of interaction.

  • Plus end: fast-growing; Minus end: slow-growing.

Differences in Microtubule Structure

  • Dendrites:

    • Have mixed polarity in microtubules.

  • Axons:

    • Feature unipolar microtubules (plus end facing distal processes).

  • Growth cones have highly dynamic microtubules, which affect function.

Recap: Actin Protofilaments

  • Subunits assemble into F-actin filaments.

  • Filaments exhibit a + (barbed) end and – (pointed) end.

    • + End: fast-growing.

    • – End: slow-growing.

Recap: Actin Bundling and Cross-Linking

  • Nucleated by Formins and Arp2/3.

  • Formins create branched networks in filamentous actin.

Recap: Actin Treadmilling and Growth Cone Mobility

  • Vital for growth cone motility:

    • Polymerization and recycling of actin filaments.

    • Attachment to the substrate through focal adhesions.

Recap: Actin-Mediated Growth Cone Motility

  • Mechanism:

    • Retrograde actin flow exerts force on the growth cone.

    • Monomers are added at the filopodia edge.

    • Actin attaches to the substrate via transmembrane proteins (focal adhesions).

    • Monomers are removed from the filament’s trailing part.

Navigating the Developing Brain: Growth Cone Guidance

  • Retinal Ganglion Cell Innervation of the LGN serves as a model for connection establishment:

    • Axons enter the CNS via the optic nerve, project to LGN or superior colliculus, synapsing to the visual cortex.

Steps of Axon Outgrowth

  1. Pathway Selection:

    • Axons choose between ipsilateral or contralateral pathways at the optic chiasm.

  2. Target Selection:

    • Axons innervate specific areas in the optic tract, including the lateral geniculate nucleus.

  3. Address Selection:

    • Establish precise retinotopy and innervate the correct LGN layer.

Extracellular Cues for Axon Outgrowth

  • Type of Cues:

    • Cell-cell contacts, local extracellular cues, and diffusible signals.

Growth Cone Structure and Function

  • Growth cone interacts with extracellular components for guidance:

    • Stimulation stabilizes filopodia, allowing advance; repulsive signals cause retraction.

Three Stages of Growth Cone Advance

  1. Protrusion:

    • Filopodia and lamellipodia extend.

  2. Engorgement:

    • Microtubules extend closer to the peripheral region fixing growth direction.

  3. Consolidation:

    • Actin filaments depolymerize, shrinking the membrane to form a shaft.

Permissive Signals for Axon Outgrowth

  • Originate from:

    • Extracellular matrix.

    • Long-distance cell interactions.

    • Typically chemoattractants or chemorepellents.

Extracellular Matrix and Axon Guidance

  • Composed of glycoproteins and proteoglycans:

    • Glycoproteins have oligosaccharide chains attached,

    • Proteoglycans are glycoproteins with larger polysaccharide side chains.

ECM Components Influencing Axon Guidance

  • Examples:

    • Laminin

    • Fibronectin

    • Collagen

    • Tenascin

    • Heparin sulfate proteoglycans.

Laminin Role in Axon Outgrowth

  • Heterotrimeric Glycoprotein:

    • Composed of alpha, beta, and gamma subunits, forms a cruciform shape.

    • Key component of ECM in the nervous system.

Effects of Laminin Mutations

  • Defects in laminin alpha 1 gene relate to issues in axon pathways (e.g., retinal ganglion cells).

Fibronectin's Role in Axon Outgrowth

  • Expressed during morphogenesis, including dynamic patterns.

  • Influences proliferation and migration in neurons.This interaction is crucial for proper neuronal connectivity and the establishment of functional neural circuits. Additionally, fibronectin provides a substrate for axonal guidance, facilitating the directional growth of axons towards their target cells.

  • This guidance is essential for the formation of synapses and the overall organization of the nervous system, ultimately impacting sensory and motor function.

  • Disruptions in fibronectin signaling can lead to impaired axon growth and result in neurological disorders, highlighting its importance in neural development.

Chemoattractants and Chemorepellents

  • Guide axon growth using molecules like netrin (attractant) and slit (chemorepellent).

    • Netrin components attract dorsal horn neurons towards the spinal cord midline.

Attraction and Repulsion in Visual System

  • Ephrins serve to pattern visual development at synapse levels, influencing retinotopic mapping.

Focal Adhesions and Integrin Signaling

  • Integrin receptors bind to ECM components like laminin and fibronectin, modulating axon outgrowth through signaling cascades.

Summary

  • Signals from the ECM, other cells, and guidance cues like netrins and ephrins are crucial in directing neuronal growth cones during development, with focal adhesion proteins linking the cell's cytoskeleton to extracellular signals.