Lecture 20: Growth Cones and Axon Pathfinding

What is neuronal differentiation?

the process by which a cell acquires the appearance and characteristics of a neuron after reaching its destination, although developmental programming occurs much earlier

What creates the cortical protomap?

migration of neurons through radial glial guides

Besides radial migration, what other migration can occur during cortical development?

lateral migration of some neurons

What external input contributes to cortical differentiation?

thalamic input

What is a growth cone?

the growing tip of a neurite that explores the environment and directs axon extension

What are the two major cytoskeletal components of the growth cone?

actin filaments and microtubules

Where is actin concentrated in the growth cone?

• leading edge

• filopodia

• lamellipodia

Where are microtubules concentrated in the growth cone?

most centrally within the advancing neurite

What are the three steps of axon growth?

1. filopodial protrusion and adhesion to a permissive surface

2. microtubule advance along the elongating neurite

3. expansion of the growth cone plasma membrane

How do microtubules lengthen during axon growth?

tubulin monomers are added to the distal (+) end

What is actin treadmilling?

continuous actin polymerization at the distal end and depolymerization at the proximal end

Where does actin polymerization occur in a filopodium?

at the distal (front) end

Where does actin depolymerization occur?

at the proximal (rear) end

What produces retrograde flow in the growth cone?

myosin activity

What happens if there is no permissive substrate?

retrograde flow occurs, but there is no net forward advance

What extracellular matrix proteins can serve as permissve substrates?

• laminin

• fibronectin

Which cell surface receptors link the growth cone to permissive substrates?

• integrins

• cadherins

• Ig-family molecules

Why is linking actin to a permissive substrate essential?

it prevents rearward movement of the actin network and follows forward extension

What drives advancement once the actin meshwork is anchored?

newly polymerized actin at the leading edge

What are commissural interneurons?

neurons that relay signals between sensory afferents and descending motor pathways

What functions do commissural interneurons serve?

locomotion control and reflex mediation

What three ways can growth cones be guided according to the lecture?

• pulled by attractants

• pushed by repellents

• hemmed in by both

What are guidepost cells?

cells that mark pathway segments and coordinate axon movement

What are the four general guidance mechanisms?

• chemoattraction

• chemo repulsion

• contact attraction

• contract repulsion

Which cues originate from the dorsal spinal cord?

chemo repellents

Which cues originate from the ventral spinal cord?

chemo attractants

What guides commissural axons anteriorly toward brain targets after midline crossing?

chemo attractants

What role do chemo repellants play after crossing?

prevent re-crossing of the midline

What are the four canonical cue families?

• netrin

• semaphorin

• slit

• ephrin

What receptors bind Netrin?

DCC and UNC5

What receptors bind Semaphorins?

plexin and neuropilin

What receptor binds Slit?

Robo

What receptors bind Ephrins?

EphA and EphB receptors

Is netrin generally attractive or repulsive?

classically an axon attractant

What axons does netrin guide in the spinal cord?

commissural axons from the dorsal spinal cord to the ventral midline

Is netrin evolutionarily conserved?

yes, it is phylogenetically conserved

What did the COS-cell experiment demonstrate?

Netrin-1 attracts commissural axons

What happens in Netrin-1 knockout animals?

guidance of commissural axons to the ventral midline is impaired

Why is netrin considered bifunctional?

it can cause either attraction or repulsion

How dos receptor composition affect netrin responses?

DCC alone mediates attraction, whereas DCC and UNC5 mediates repulsion

Can intracellular signaling alter netrin responses?

yes

What signaling manipulation show in the lecture converted attraction to repulsion?

PKA inhibition

Are semaphorins attractants or repellants?

secreted axon repellants

What process do semaphorins mediate?

surround repulsion and axon-axon fasciculation

What happens in Sema3A knockout mice?

axon defasciculation and axon growth beyond normal target regions

What type of cue is Slit?

a potent axon repellant

What receptor binds Slit?

Robo

Which Robo protein suppresses repulsion before midline crossing?

Robo3

What does Robo3 do?

suppresses Robo1/2 signaling to permit midline crossing

What happens after crossing the midline?

Robo3 is downregulated, allowing Robo1/2 and Slit-mediated repulsion

Why don’t commissural axons re-cross the midline?

Slit-Robo repulsive signaling pushes them away after crossing

What major developmental process do ephrins regulate?

formation of topographic maps in the visual system

Where do axons from the nasal retina project?

through the anterior tectum to the posterior tectum

Why can nasal retinal axons reach posterior tectum?

they lack Eph receptor-mediated repulsion

Where do temporal retinal axons terminate?

anterior tectum

Why do temporal retinal axons avoid posterior tectum?

they are repelled by ephrin-containing posterior tectal membranes

Which morphogens can act as guidance cues?

• Wnts

• Hedgehog (Hh)

• BMPs

Which growth factors can serve as guidance cues?

• GDNF

• FGFs

• HGF

• neurotrophins

What is another name for Hh?

hedgehog

What does BMP stand for?

bone morphogenic protein

What happens to corticospinal tract axons after crossing the midline?

they normally turn rostrally (anteriorly/upward)

What happens when the Wnt receptor Frizzled-3 is absent?

axons become disorganzed

What major pathway is discussed in this example?

the corticospinal tract (CST)

What are two major roles of Ig superfamily molecules in axon guidance?

axon fasciculation and axon branching

What is fasciculation?

bundling of axons together

How do guidance cues regulate growth cones?

through interactions between guidance receptors and the cytoskeleton

What three cytoskeletal processes are regulated by guidance cues?

• actin polymerization/depolymerization

• myosin-mediated F-actin translocation

• microtubule dynamics

What does APC stand for?

adenomatous polyposis coli

What does APC bind?

the plus end of microtubules

What effect does NGF signaling have on growth cones?

promotes microtubule assembly and axon extension

Where can protein synthesis occur during axon guidance?

directly within the growth cone

What happens when filopodia detect netrin?

localized actin synthesis is stimulated

What is the functional consequence of local actin synthesis?

propels the growth cone toward the netrin source