axon guidance - department of molecular & cell...

Axon Guidance• Multiple decision points along a growing axon’s

trajectory• Different types of axon guidance cues:

– Contact mediated - requires direct contact by growth cone– Long range - growth cone responds to chemical gradient

• Growth cones respond to these different types ofcues at different points along their trajectories tonavigate toward their ultimate targets

• Sperry’s chemoaffinity hypothesis

A number discrete steps and decision points along the way…A. To leave the retina at the otic nerve head (ONH)B. To cross (or not) at the optic chiasm (and where)C. A/P and D/V positions

The retinotectal projection: a point-to-point topographic mapinverted over A/P and D/V axes

Retina Tectum

Tessier-Lavigne, Cell 1995

Roger Sperry: rotation of retina - RGCs still make connectionsaccording to their original (intrinsic) positions

Normal Eye rotated 180o

Sperry: Chemoaffinity Hypothesis (1963)

1. Each position in the optic tectum has a unique “molecularaddress” or “molecular tag”

2. Each retinal ganglion cell (RGC) has a unique set of receptorsfor these tags (i.e., an identity), resulting in a position-dependent response dependent on differential affinities ordifferential intracellular responses

3. Unlikely to be unique molecules for each position (would beinsufficient information in the genome to wire up entire brainthis way) - rather, information is probably encoded inorthogonal gradients (A/P, D/V)

=> Positional identities of axons and targets are matched up toestablish the point-to-point topographic map

Axons are guided by different types of guidance mechanismsExtracellular matrix adhesion Cell surface adhesion Fasciculation

Chemoattraction

Contact inhibition

Chemorepulsion

Con

tact

med

iate

dLo

ng R

ange

Axon growth cone is a sensory-motor structure thatrecognizes and responds to guidance cues

Santiago Ramón y Cajal1852-1934

Structure of the growth cone

Actin is concentrated infilopodia and lamellipodia

filopodiumlamellipodium

Microtubules areconcentrated in the centralcore of the growth cone

Filopodia – sensory ability of the GC.

Growth cone extension

Filopodia extended

Microtubules fromcentral core advance

Cytoplasm collapsesto create newsegment of axon

Extracellular Matrix Molecules (Contact Mediated)

Laminins: major components ofbasal laminae and account formuch of the axon outgrowthpromoting ability of theextracellular matrix.

Heterotrimers of related α,β,andγ subunits (5 α, 4 β and 3 γgenes – at least 11 trimers hasbeen identified).

Integrins: Expressed on thegrowth cone, interacts withlaminin (16 α and 8 β genes).

Heterodimers of α and β,recognize different laminins.

Cell Adhesion Molecules (Contact Mediated)

Binds Ca2+

Binds catenins, linksto cytoskeletons

Cadherinsuperfamily

Immunoglobulin superfamily

Contact-Mediated RepulsionFriedrich Bonhoeffer’s stripe assay demonstrates sensitivity of posterior (temporal) retinal

axons to a repellent activity in posterior tectal membranes

Repulsive signalingmediated by theEphrins (ligand)and “Eph” receptors

Wilkinson, Nat. Rev. Neurosci. 2001

Manipulations of Ephrin Expression Alter Topographic Map

Normal:A -> PP -> A

Overexpress ephrinin spots in tectum:A -> PP avoids areasw/ephrin

Reduced ephrin intectum:A -> PP -> A, P

Chemorepulsion (Long Range)Growth cone collapse upon exposure to a chemorepellent

Growth cone collapse from point source (gradient)leads to axon turning away from chemorepellent source

The Semaphorins and their receptors

Chemoattraction (Long Range)

Netrin-1: • secreted by the floorplate • gradient guides axon turning

Netrin-1 receptor mediatingattractive response:

• “DCC” - vertebrates, isevolutionarily conserved

= “unc-40” - C. elegans (worms) = “frazzled” - Drosophila (flies)

Response to netrin-1 is dependent on two factors1) Netrin receptors: DCC/unc-40/frazzled: attractive response DCC/unc-40 + unc5: repulsive response

2) cAMP levels and protein kinase A (PKA) activation

Netrin-1 can act as both chemoattractant and chemorepellent…

High cAMP

low cAMP

Robo receptors transduce a midline repulsive signal encoded in slitproteins and silence netrin-mediated attraction