class 9: growth cone navigation dox

growth cones consist of two distinctive protrusions:

• filopodia

• lamellipodia

filopodia

thin, finger-like extensions rich in bundled actin filaments

• serve as sensory organelles, probing environment for guidance cues

lamellipodia

broad, sheet-like regions filled with branched actin networks that generate protrusive forces and support exploratory movements

growth cone cytoskeleton is dominated by

actin filaments and microtubules

actin filaments (F-actin)

drive protrusion

• polymerization at leading edge pushes membrane forward

• depolymerization at rear allows recycling of actin monomers

microtubules

composed of tubulin, extend into growth cone from axon shaft

• provide structural support and deliver organelles and proteins through axonal transport.

myosin motors

connect actin to microtubules generating traction forces

balance of actin polymerization and depolymerization determines:

whether a filopodium extends or retracts

• attractive guidance cues enhance actin assembly, while repulsive cues promote disassembly

growth cones are capable of

navigating independently of their cell bodies

• experiments demonstrated that axons continue to elongate and find appropriate targets even when served from soma.

retinal axons

can still project to tectum after optic nerve is cut

• shows that navigation depends on local interactions rather than nuclear transcription

• relies on local protein synthesis and degradation within growth cone

adhesion

for forward movement, growth cones must adhere to substrate. two main adheasion systems are used

integrins bind to extracellular matrix (ECM)

such as laminin, fibronectin, and collagen

• interactions both anchor the growth cone and activate intracellular signaling pathways that regulate motility

cell adhesion molecules (CAMs)

such as NCAM, L1, and cadherins mediate adhesion to other cells

• molecules also transduce signals that influence actin polymerization

force generation movement results from

interplay between adhesion and cytoskeletal contraction

• actin polymerization pushes filopodia forward

• actomyosin contraction provides the pulling force necessary to translocate the growth cone body

explant culture assays

central in identifying axon guidance molecules

explant assays: dorsal root ganglion (DRG)

extend into dorsal spinal cord tissue but avoid ventral spinal cord

explant assays: olfactory bulb axons

repelled by septal tissue

explant assays: netrin-1

discovered as bifunctional guidance cue, capable of attracting or repelling axons depending on receptor context

ablation experiments

laser ablation of intermediate target halts axon growth, demonstrating that intermediate waypoints are essential for navigation

• pioneer axons often establish initial trajectories which follower axons then use as guides

nerve growth factor (NGF)

first discovered by Rita Levi-Montalcini

• provides trophic support and promotes axon extension

netrins

secreted cues that can attract commissural axons to ventral midline or repel others, such as trochlear motor neurons

semaphorins

typically repulsive signals that prevent axons from entering inappropriate regions

slits

repulsive signals at the midline, preventing axons from lingering

Wnts

gradient molecules

• Wnt4 directs commissural internneurons anteriorly in spinal cord

sonic hedgehog (SHH)

serves both as a morphogen in patterning as a guidance cue

intermediate targets and pathway formation

• axons navigate through intermediate targets

• ablation experiments show removal of checkpoints cause axons to stall

• pioneer axons can also meet from opposite direction, forming pathway that axons follow

• ensures reliable wiring even across embryonic distances