1. The Guidance of Axons to Their Targets
서울대학교 어린이병원 신경외과
왕 규 창

2. Axon Growth: Two Views molecular view
random growth with selective survival
stereotropism
mechanical guidance
along scratches, blood vessels or cartilage
resonance
congruent activity

3. Retina-Tectum Experiment
Roger Sperry, 1940s
lower vertebrates
regeneration of cut retinal axons
frog, cut optic nerve, rotation of the eye

4. Retina-Tectum Experiment
chemical matching rather than functional validation of random connection
chemospecificity hypothesis
recognition molecules
Molecular matching predominates during embryonic development.
Activity (experience) modifies the circuits once they have been established.

5. Axon Guidance Cues from the target (wrong) series of discrete steps
retina – optic fiber layer – retinal basal lamina and end-feet of glia – optic nerve head – optic stalk (‘pioneer’ axons) – optic chiasm - ventral diencephalon – superior colliculus at different subregions – - radial glial cells - synaptic partner – a specific layer – specific area of dendrites

6. Axon Guidance Cues optic chiasm terminal arbor
different responses to special midline cells
intermediate targets
terminal arbor
interaction with target
patterns of neural activity

7. Positional Cues correction of ‘mistaken’ fibers
ephrins
rotation of neural tube between the chiasm and the tectum
markers of position or polarity

8. Motor Axon Guidance series of discrete steps
motor pool in the spinal cord – segmental ventral roots by barriers in the somites – rearrangement in plexus region – large nerve – target muscle – synapse on a muscle fiber
similar pattern to the retinal fibers

9. Axon Growth: Options grow / turn / stop positive and negative cues
finer control over the direction of growth

10. Pioneers short distance in a small embryo
the first axons to exit the retina

11. Intermediate Targets
decision points
optic chiasm, limb plexus

12. Gradients
cell surface molecules
soluble molecules

13. Ranges of Cues short-range cues long-range cues cell membranes
extracellular matrix
precise contact guidance
long-range cues
soluble molecules
less precise guidance

14. Growth Cone Santiago Ramon y Cajal, 1890s
“both a sensory structure and a motor structure”
transduces positive and negative cues into signals that regulate the cytoskeleton and thereby determine the course and rate of axon outgrowth
coupling between the sensory and motor capabilities

15. Growth Cone central core lamellipodia filopodia
microtubules, mitochondria, etc
lamellipodia
motile, ruffled appearance
filopodia
long slender extensions

16. Filopodia sensory capability rod-like, actin-rich, membrane-limited
highly motile: advance, retract, turn
length
rapid movement
flexibility

17. Filopodia Second Messengers
calcium
set point: optimal concentration
gradient of calcium: change of direction
cyclic nucleotides
modulate protein kinases, protein phosphatases, rho-family GTPases

18. Pathway Guidance Cues promotion / inhibition
cell surface / extracellular matrix / soluble form

19. Pathway Guidance Cues extracellular matrix adhesion
cell surface adhesion
fasciculation
chemoattraction
contact inhibition
chemorepulsion

20. ECM Adhesion collagen, fibronectin, proteogylcans, etc laminins
heterotrimer, at least 14 trimers, unique distribution, position- or stage-dependent signals
integrins
heterodimer, at least 16 alpha and 8 beta chains, specific ligands
all cells in the body: at least one integrin

21. Cell-Cell Adhesion selective adhesive interaction
cadherin: calcium dependent
immunoglobulin-like adhesion molecules: calcium independent
cell-cell binding
short-range promoter of neurite growth

22. Cadherins at least 100 related membrane-spanning glycoproteins
extracellular calcium binding segments
N-cadherin, proto-cadherins, cadherin-related neural receptors
cells throughout the body

23. Cadherins homophilic interaction
prefers to bind to its own kind
selective adhesion
Cytoplasmic domain binds catenins, then affects cytoskeletal elements.

24. Adhesion Molecules adhesion assay
Initial adhesion triggers a cytoplasmic reaction that strengthens the adhesion.
not just adhesion molecules but ‘signaling molecules’ activated by membrane receptors

25. Ig Superfamily disulfide bridges
less ligand-specificity than cadherins
intracellular domain: protein tyrosine phosphatase or protein tyrosine kinase

26. Chemoattractants soluble growth factors
trophic factor
chemotaxis: tropism
no clear examples of trophic and tropic factors in vivo
neuronal chemoattractants
two glycoproteins: netrins

27. Vertebrate vs. C. elegans
netrin unc-6
unc-5H unc-5
DCC, neogenin unc-40
Ig superfamily
remarkably conserved during evolution

28. Ephrins stripe assay axons from temporal retina to the anterior tectum
heat treatment of each membranes
heat treatment of posterior membrane: random growth
presence of inhibitory material in posterior membranes

29. Ephrins repulsive axon guidance signal (RAGS) = ephrin A5
eph kinases: receptor tyrosine kinases
major group of inhibitory ligands and receptors in the developing nervous system

30. Ephrins ephrin A2 and ephrin A5
low-to high gradients in the rostral direction in the tectum
eph A3 (a kinase receptor which binds ephrin A2 and A5)
low-to-high gradient in the temporal direction in retinal ganglion cells

31. Somaphorins an inhibitory molecule at least 15 somaphorins
in distinct types of neurons and nonneural cells
key receptor: neuropillins, plexins

32. Chemorepulsion semaphorins netrins some: membrane-bound
others: soluble factor
netrins
DDC or neogenin: attraction
unc-5H: repellant

33. Chemorepulsion
attract or inhibit according to the receptors or the level of intracellular messengers
some neurotransmitters
One synaptic transmission inhibits formation of another.