class 9: growth cone navigation

Peripheral Domain (P-zone)

composed primarily of actin filaments, organized as parallel F-actin bundles in filopodia and branched F-actin meshwork in lamellipodia.  

Transition zone (T-zone)

contains actomyosin structures that regulate microtubule entry and couple actin retrograde flow to substrate adhesions.  

Central Domain (C-zone)

rich in bundled and dynamic microtubules, along w/ organelles, vesicles, and transported cargo.

what evidence shows that a growth cone can navigate locally without immediate input from the nucleus?  

axon severing, gradient turning in isolated cones, embryonic tissue rotation

axon severing / enucleation

retinal ganglion cell axons continue to extend and navigate accurately toward the optic tectum even after being cut / isolated from their cell bodies (soma) containing nucleus. 

gradient turning in isolated cones

isolated growth cones severed from cell soma retain the ability to turn directionally within molecular guidance gradients for limited periods.  

embryonic tissue rotation

rotating presumptive hindbrain embryonic tissue 180 degrees before Mauthner axons extend cause growth cones to initially follow the reversed local signals before course-correcting when encountering unrouted tissue, demonstrating sequential decisions driven by local tissue cues.  

why can removal of an intermediate target cause an axon to stall before reaching its final target? 

deprives the growth cone of its necessary next local cue, causing axon to stall and fail to advance toward its final target.  

how to pioneer axons contribute to formation of an axon tract

explore unmapped embryonic environment first, directly interpreting local guidance cues to establish initial path. 

how to follower axons contribute to formation of axon tract

extend along the established pioneer pathways via selective fasciculation (bundling along existing axon shafts via cell adhesion molecules) relying on pioneer’s oath rather than independently navigating raw tissue.  

first mechanical steps that produce growth-cone advance: protrusion (extension)

actin polymerizes near the leading edge (F-actin assembly at filopodial tips & lamellipodial veils) to push membrane forward.  

second mechanical steps that produce growth-cone advance: adhesion / substrate engagement

surface guidance receptors and adhesion complexes attach to extracellular matrix or cell-surface ligands.  

third mechanical steps that produce growth-cone advance: traction / mechanical coupling

adhesions engage a “molecular clutch” that connects actin filaments to stationary substrate anchors, reducing retrograde actin slippage so actomyosin contraction draws growth cone body forward.  

fourth mechanical steps that produce growth-cone advance: consolidation / microtubule invasion

dynamic microtubules invade the newly stabilized protrusion, delivering organelles, membranes, and cargo to consolidate the segment into a new axon shaft.  

integrins

bind extracellular matrix proteins to provide substrate traction.  

NCAM, L1, Cadherins

bind cell-surface proteins on neighboring cells or axons to mediate cell-cell or axon-axon interactions

NCAM & L1

engage in homophilic or heterophilic binding on neural cell membranes to support neurite extension.  

Cadherins

mediate calcium-dependent, typically homophilic cell-to-cell adhesions and link intracellularly to actin via catenin.  

How can an explant coculture assay distinguish a chemoattractant from a chemorepellent? 

In an explant coculture assay, neural tissue is placed near a candidate source in culture.  

chemoattractions

produce preferential or asymmetric axon growth directed toward the source (dorsal spinal cord growing toward netrin-1)  

chemorepellents

cause asymmetric avoidance or growth away from the source or induce growth-cone collapse on the side facing the tissue. (olfactory bulb axons repelled by septal tissue) 

How can netrin produce attraction in one growth cone and repulsion in another? 

• Attraction: When growth cones express DCC (neogenin) receptors alone. (pre-crossing commissural axons toward midline) 

• Repulsion: when growth cones express UNC5 family receptors (either alone or complexed w/ DCC) (repelling trochlear motor axons away from floor plate) 

what receptors mediate the major effects of netrin, Slit, and class 3 semaphorins?

• Netrin: DCC & neogenin (attraction) and UNC5 family receptors (repulsion) 

• Slit: Robo family receptors (Robo1 – Robo4)  

• Class 3 semaphorins: neuropilin (neuropilin-½) & plexin receptor complexes 

what experimental observations support Wnt4 and Frizzled3 in anterior guidance of post-crossing commissural axons? 

post-crossing commissural interneurons turn anteriorly along anterior-high to posterior-low Wnt4 concentration gradient in spinal cord.  

Frizzled3 receptor knockout / mutation

in commissural interneurons block their anterior longitudinal growth 

ectopic expression / cell transplantation

transplanting ectopic cells expressing Wnt4 (transfected COS cels) adjacent to the path, diverts post-crossing axons toward new Wnt source. 

why must commissural axons change their responses after crossing the midline? 

Pre-crossing commissural axons are attracted to midline floor plates by cues like netrin-1 and SHH

• if axons did not change their responsiveness after crossing, the persistent attraction and lack of repulsion would trap / stick to the growth cones at the floor plate

• exit the midline, avoid recrossing, and project anteriorly.  

how can local protein translation and protein degradation change growth-cone responses to guidance cues? 

axonal growth cones contain localized mRNAs, ribosomes, and translation.

• together, they rapidly shift surface receptor abundance and internal signaling, providing growth cone with a biochemical memory of recent encounters, enabling swift behavioral switches.  

local protein translation

• extracellular guidance cues trigger rapid local mRNA translation directly inside growth cone within minutes

• synthesizing new receptors or cytoskeletal receptors locally without waiting for transcriptional signals or transport from distant soma. 

local protein degradation

guidance signals induce targeted ubiquitin-dependent proteolysis to rapidly degrade existing surface receptors or signaling components.