Axon Guidance. Multiple decision points along a growing axon s trajectory Different types of axon guidance cues:

Transcription

1 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 of cues at different points along their trajectories to navigate toward their ultimate targets Sperry s chemoaffinity hypothesis

2 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

3 The retinotectal projection: a point-to-point topographic map inverted over A/P and D/V axes Retina Tectum Tessier-Lavigne, Cell 1995

4 Roger Sperry: rotation of retina - RGCs still make connections according to their original (intrinsic) positions Normal Eye rotated 180 o

5 Sperry: Chemoaffinity Hypothesis (1963) 1. Each position in the optic tectum has a unique molecular address or molecular tag 2. Each retinal ganglion cell (RGC) has a unique set of receptors for these tags (i.e., an identity), resulting in a positiondependent response dependent on differential affinities or differential intracellular responses 3. Unlikely to be unique molecules for each position (would be insufficient information in the genome to wire up entire brain this way) - rather, information is probably encoded in orthogonal gradients (A/P, D/V) => Positional identities of axons and targets are matched up to establish the point-to-point topographic map

6 Axons are guided by different types of guidance mechanisms Extracellular matrix adhesion Cell surface adhesion Fasciculation Contact inhibition Contact mediated Chemoattraction Chemorepulsion Long Range

7 Axon growth cone is a sensory-motor structure that recognizes and responds to guidance cues Santiago Ramón y Cajal

8 Structure of the growth cone lamellipodium filopodium Actin is concentrated in filopodia and lamellipodia Microtubules are concentrated in the central core of the growth cone Filopodia sensory ability of the GC.

9 Growth cone extension Filopodia extended Microtubules from central core advance Cytoplasm collapses to create new segment of axon

10 Extracellular Matrix Molecules (Contact Mediated) Laminins: major components of basal laminae and account for much of the axon outgrowth promoting ability of the extracellular matrix. Heterotrimers of related α,β,and γ subunits (5 α, 4 β and 3 γ genes at least 11 trimers has been identified). Integrins: Expressed on the growth cone, interacts with laminin (16 α and 8 β genes). Heterodimers of α and β, recognize different laminins.

11 Cell Adhesion Molecules (Contact Mediated) Cadherin superfamily Immunoglobulin superfamily Binds Ca 2+ Binds catenins, links to cytoskeletons

12 Contact-Mediated Repulsion Friedrich Bonhoeffer s stripe assay demonstrates sensitivity of posterior (temporal) retinal axons to a repellent activity in posterior tectal membranes Repulsive signaling mediated by the Ephrins (ligand) and Eph receptors

13 Wilkinson, Nat. Rev. Neurosci. 2001

14 Manipulations of Ephrin Expression Alter Topographic Map Normal: A -> P P -> A Overexpress ephrin in spots in tectum: A -> P P avoids areas w/ephrin Reduced ephrin in tectum: A -> P P -> A, P

15 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

16 Chemoattraction (Long Range) Netrin-1: secreted by the floorplate gradient guides axon turning Netrin-1 receptor mediating attractive response: DCC - vertebrates, is evolutionarily conserved = unc-40 - C. elegans (worms) = frazzled - Drosophila (flies)

17 Netrin-1 can act as both chemoattractant and chemorepellent Response to netrin-1 is dependent on two factors 1) Netrin receptors: DCC/unc-40/frazzled: attractive response DCC/unc-40 + unc5: repulsive response 2) camp levels and protein kinase A (PKA) activation High camp low camp

18 Robo receptors transduce a midline repulsive signal encoded in slit proteins and silence netrin-mediated attraction