The Guidance of Axons

to Their Targets

서울대학교 어린이병원 신경외과

왕 규 창

Axon Growth: Two Views

molecular view random growth with selective survival

stereotropism• mechanical guidance• along scratches, blood vessels or cartilage

resonance• congruent activity

Retina-Tectum Experiment

Roger Sperry, 1940s lower vertebrates regeneration of cut retinal axons frog, cut optic nerve, rotation of

the eye

Retina-Tectum Experiment chemical matching rather than functional valid

ation of random connection chemospecificity hypothesis

• recognition molecules• Molecular matching predominates during embryon

ic development.• Activity (experience) modifies the circuits once the

y have been established.

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

Axon Guidance Cues

optic chiasm• different responses to special midline

cells• intermediate targets

terminal arbor• interaction with target• patterns of neural activity

Positional Cues

correction of ‘mistaken’ fibers• ephrins

rotation of neural tube between the chiasm and the tectum• markers of position or polarity

Motor Axon Guidance

series of discrete steps • motor pool in the spinal cord – segmental vent

ral 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

Axon Growth: Options

grow / turn / stop positive and negative cues

• finer control over the direction of growth

Pioneers

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

Intermediate Targets

decision points optic chiasm, limb plexus

Gradients

cell surface molecules soluble molecules

Ranges of Cues

short-range cues• cell membranes• extracellular matrix• precise contact guidance

long-range cues• soluble molecules• less precise guidance

Growth Cone

Santiago Ramon y Cajal, 1890s “both a sensory structure and a motor structu

re” transduces positive and negative cues into si

gnals that regulate the cytoskeleton and thereby determine the course and rate of axon outgrowth

coupling between the sensory and motor capabilities

Growth Cone

central core• microtubules, mitochondria, etc

lamellipodia• motile, ruffled appearance

filopodia• long slender extensions

Filopodia

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

FilopodiaSecond Messengers

calcium• set point: optimal concentration• gradient of calcium: change of direction

cyclic nucleotides

modulate protein kinases, protein phosphatases, rho-family GTPases

Pathway Guidance Cues

promotion / inhibition cell surface / extracellular matrix / s

oluble form

Pathway Guidance Cues

extracellular matrix adhesion cell surface adhesion fasciculation chemoattraction contact inhibition chemorepulsion

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, s

pecific ligands• all cells in the body: at least one integrin

Cell-Cell Adhesion selective adhesive interaction

cadherin: calcium dependent immunoglobulin-like adhesion molecules: calci

um independent

cell-cell binding short-range promoter of neurite growth

Cadherins

at least 100 related membrane-spanning glycoproteins

extracellular calcium binding segments N-cadherin, proto-cadherins, cadherin-relate

d neural receptors cells throughout the body

Cadherins

homophilic interaction• prefers to bind to its own kind• selective adhesion

Cytoplasmic domain binds catenins, then affects cytoskeletal elements.

Adhesion Molecules

adhesion assay Initial adhesion triggers a cytoplasmic rea

ction that strengthens the adhesion. not just adhesion molecules but ‘signalin

g molecules’ activated by membrane receptors

Ig Superfamily

disulfide bridges less ligand-specificity than cadherins intracellular domain: protein tyrosine phos

phatase or protein tyrosine kinase

Chemoattractants

soluble growth factors• trophic factor• chemotaxis: tropism

no clear examples of trophic and tropic factors in vivo

neuronal chemoattractants• two glycoproteins: netrins

Vertebrate vs. C. elegans netrin unc-6 unc-5H unc-5 DCC, neogenin unc-40

• Ig superfamily

remarkably conserved during evolution

Ephrins stripe assay axons from temporal retina to the anterior te

ctum heat treatment of each membranes

• heat treatment of posterior membrane: random growth

presence of inhibitory material in posterior membranes

Ephrins

repulsive axon guidance signal (RAGS) = ephrin A5

eph kinases: receptor tyrosine kinases major group of inhibitory ligands and rece

ptors in the developing nervous system

Ephrins ephrin A2 and ephrin A5 low-to high gradients in the rostral directi

on in the tectum eph A3 (a kinase receptor which binds ep

hrin A2 and A5) low-to-high gradient in the temporal direc

tion in retinal ganglion cells

Somaphorins

an inhibitory molecule at least 15 somaphorins in distinct types of neurons and non

neural cells key receptor: neuropillins, plexins

Chemorepulsion semaphorins

• some: membrane-bound• others: soluble factor

netrins• DDC or neogenin: attraction• unc-5H: repellant

Chemorepulsion

attract or inhibit according to the receptors or the level of intracellular messengers

some neurotransmitters• One synaptic transmission inhibits

formation of another.