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 validation of random connection chemospecificity hypothesis recognition molecules Molecular matching predominates during embryonic development. Activity (experience) modifies the circuits once they 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 terminal arbor 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 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

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 long-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 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

Growth Cone central core lamellipodia filopodia 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

Filopodia Second 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 / soluble 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, specific ligands all cells in the body: at least one integrin

Cell-Cell Adhesion selective adhesive interaction cadherin: calcium dependent immunoglobulin-like adhesion molecules: calcium 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-related 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 reaction that strengthens the adhesion. not just adhesion molecules but ‘signaling molecules’ activated by membrane receptors

Ig Superfamily disulfide bridges less ligand-specificity than cadherins intracellular domain: protein tyrosine phosphatase 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 tectum 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 receptors in the developing nervous system

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

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

Chemorepulsion semaphorins netrins 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.