Download Axonal Guidance

Postal Clerks & the Neurobiology
of Sadism
Axonal Growth Cone Guidance:
stories in Chemoattraction &
Repulsion
Ashim Malhotra
December 1, 2003
Neurophysiology
What Me
Worry?
Organization of Nerve Cell Connections
C. Golgi. Riv. sper. freniat. Reggio-Emilia, 1875
Conceptual Beginnings
Cajal
Harrison
Compensation for small Dislocations of Axonal
Initiation Position in chick Embryo
Lance-Jones & Landmesser. J. Physiol. 1980
Early Hypothesis Explaining Neuronal Path
Finding
Chemotaxis?
Chemotactic cues
guide commissural
neurons, 1892
Pioneer Hypothesis
Neurons that are born first,
send out axons that later-born
neuronal projections follow
Phenomenon of Haptotaxis
Haptotaxis:
Outgrowth of A Sensory Neuron on parallel stripes of Laminin
on a background of type IV Collagen
Gundersen. Dev Biol. 1987
Haptotaxis on Chemical Gradients: Examples
 Retinal axons in embryonic mice brains- laminin on
neuroepithelial cells & astrocytes. Liesi & Silver, Dev Biol, 1988
 Cell Adhesion Molecules (CAMs)- L1 & N-CAM.
Mutations in L1 cause defects in neuronal migrations in
mice. (V. few cross the dorsal midline) Kenwrick & Doherty,
Bioessays, 1998
The Labeled Pathway Hypothesis
Goodman & Bastiani, Sci Am, 1984
What Part of the Neuron Senses Migratory
Specificity?
Transient, actin-tubulin microfilament based extensions of
axons in the developing NS: The Growth Cone
TEM
Letourneau, Exp Cell Res, 1979
GC of Manduca, Levin & Luedemanan
Growth Cones
Time lapse of a neurite GC
Growth Cones of the Future?!
How do GCs Lend Axonal Specificity?
Diffusible Molecules
Chemoattractants
Chemorepulsives
Connexin
Netrin
Fasciclin
Semaphorin
Slit-Robo
Netrins
• Hinted at by Cajal, discovered in C. elegans, but first
understood in the vertebral system in 1994
• Surface proteins on axonal GCs that cause chemoattraction
• The Netrin receptor, unc-5, is present on cell surfaces in
substrata.
• Although usually chemoattractive, Netrins may also, at
times guide axons through repulsion.
Netrin based Guidance of Commissural
Growth Cones in the rat Spinal Cord
Developmental Biology, 6th Edition, Gilbert
Experimental Evidence
Experimental Evidence
COS cells secreting netrins elicit axon outgrowth from
11-day embryonic rat DSC explants
Dorsal Spinal Cord Explants
Kennedy, Cell, 1994
Semaphorins
Cell surface proteins that are present on the ECM
cells and guide axons through repulsion
Their receptors (made of NP1 & 2 & perhaps L1) are
on axon GCs
The collapse assay
Semaphorin based GC “turn” in Developing
Grasshopper Limb
The Molecular Biology of Chemoattraction
Netrin
Unc-5 receptor
?
Slit-Robo
In Drosophila Slit is secreted by midline neurons
& prevents most neurons from crossing the midline
from either side.
The receptor for Slit, Roundabout or Robo, is
present on GCs.
Commisural neurons that traverse, downregulate
Robo.
Diagram Depicting Slit-Robo Action
Thomas. Curr. Biol. 1998
Experimental Evidence
Kidd, Bland & Goodman, Cell, 1999
Summary
• Axon GCs project to specific targets & follow
gradients of same, diffusible molecules.
• Axon projections follow, either pioneer
neurons, or Laminin gradients for pathway
selection, but for address selection, they
follow more diffuse chemical cues
• Netrins (on the whole), along with Fasciclines
& a host of other proteins mediate
chemoattraction.
• Semaphorins, Slit-Robo & others lead to a
repulsion based GC guidance.
Watch out, Mr. Clerk! I am
making Netrin anti-bodies!
Thank you!