Lecture 20: Growth Cones and Axon Pathfinding

What happens once migration is complete?

cell takes the appearance and characteristics of a neuron after reaching its destination

What is the differentiation of cortical neurons?

• cortical protomap of neurons is made by their migration through radial glial guide

• some neurons migrate laterally

• thalamic input contributes to cortical differentiation

What is the growth cone?

growing tip of a neurite

How does the axon grow?

The axon grows as:

• filopodial protrusion and adhesion to the permissive surface of the growth cone occur

• microtubules of the growth cone advance along the elongating neurite

  • microtubules lengthen by tubulin monomers adding to the distal (+) ends

• the plasma membrane of the growth cone expands

What happens at the filopodium of the growth cone?

• polarized polymerization (assembly) of actin monomers at distal (+) end

• depolymerization of actin monomers at the proximal (-) end

What occurs due to myosin?

retrograde flow

Why is there no advances at the filopodium?

there is no permissive substrate

What is required for forward advances of the growth cone?

linking between the actin cytoskeleton to permissive substrate

What attaches to the receptor, and therefore links to the permissive substrate?

actin meshwork

With the meshwork in place, what helps advance the leading edge?

newly polymerized actin in front

What do commissural interneurons do?

relay signals between sensory afferent neurons and descending motor pathway

What is the critical role of commissural interneurons?

controlling muscle activity during locomotion and mediating reflex

What facilitates midline crossing and prevents recrossing?

chemorepellents

Where do chemorepellents come from?

the dorsal spinal cord

Where do chemoattractants come from?

ventral spinal cord

What do chemoattractants do?

guide axons anteriorly toward targets in the brain

What do guidepost cells do?

mark the end of the segment of the pathway

What can growth cones do in relation to chemorepellents and chemoattractants?

can be pushed behind by chemorepellents, pulled from in front by chemoattractants, and hemmed in by both cues

What is the overlying process of guidance mechnaisms?

mediate simultaneous and coordinate movement of axons

What are the canonical guidance cue molecules?

• Netrin

• Semaphorin

• Slit

• Ephrin

What are the receptors for Netrin?

UNC-5 and DCC

What are the receptors for Semaphorin?

Plexin and Neurophillin

What are the receptors for Slit?

Robo

What are the receptors for Ephrin?

EphA or EphB

What domains do the UNC-5 and DCC receptors have?

UNC-5: Ig domain and TSP

DCC: Ig domain and FNIII domain

What domains do the Plexin receptors have?

Sema domain

What domains does the Robo receptor have?

Ig domain and FNIII domain

What domains does the EphA or EphB receptors have?

Eph LBD and FNIII domain

What is true of Netrin and Slit?

they both have EGF repeats

What domains does Semaphorin have?

Sema domain and Ig domain

What is Netrin and what does it do?

• phylogenetically conserved axon attractants

• guide commissural axons from the dorsal spinal cord to the ventral midline

What was an experiment done with Netrin?

• pieces of the dorsal half of the spinal cord were placed in culture for 40h with either control COS cells or COS cells secreting Netrin-1

• Netrin-1 is required for guidance of commissural axons to the ventral midline

What does receptor composition have to do with the bi-fucntionality of Netrin?

depending on the receptor composition, you can get either attraction or repulsion

What happens if you have the DCC receptor for Netrin?

you get attraction

What happens if you have DCC alongside a PKA inhibitor?

you get repulsion

What is Semaphorins and what do they do?

• secreted axon repellents

• mediate ‘surround repulsion’ (ex. axon-axon fasciculation)

If you have Sema3A^-/-, what will happen?

axon de-fasciculation and axon growth beyond normal target region

What is Slit and what does it do?

• potent axon repellent that binds to Robo

• guide commissural axons to enter and then leave CNS midline

What happens when axons project to the midline through Netrin/DCC?

Robo3 suppresses Robo-1/2 activity

What is true of Robo3 after axons have crossed the midline?

it is downregulated, allowing Robo-1/2 and Slit to mediate their repulsion from the midline

What do Ephrins do?

establish topographic maps of retinal projections in the visual system

What do the axons from the nasal retina do?

they have no ephrin receptor, so they cross the anterior part of the tectum and innervate the neurons in the posterior part

What do axons from the temporal retina do?

repelled by membranes from posterior tectum and grow only into the anterior tectum

What are the proteins that specify cell fate?

Morphogens: Wnts, HH (hedgehog), Bmps (bone morphogenic protein)

What are proteins that regualtion cell survival?

Growth factors: GDNF, FGFs, HGF (hepatocyte), Neurotrophins

With Morphogens, after crossing the midline, what do their axons look like in wildtype and with the Wnt receptor frizzled-3^-/-?

WT: axons turn rostrally (up)

Frizzled-3^-/-: axons are disorganized

What does the Ig superfamily do in non-canonical guidance?

• regulate axon fasciculation (bundling)

• regulate branching of axons

How do guidance cues control growth cones?

via interactions between the cytoskeleton and guidance receptors in the growth cone

What do guidance cues regulate with growth cones?

• actin polymerization and depolymerization

• the rate of myosin-mediated F-actin translocation, thereby regulating filopodia and leading edge

• microtubule dynamics

What is an example of microtubule dynamics?

APC binds to the + end of microtubules for axon extension. This is inhibited by NGF

What is true in the detection of Netrin by its receptors?

in filopodia it directs localized synthesis of actin that propels the growth cone forward toward the Netrin