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

Pioneer Hypothesis  Neurons that are born first, send out axons that later-born neuronal projections follow  Phenomenon of Haptotaxis Early Hypothesis Explaining Neuronal Path Finding Chemotaxis?  Chemotactic cues guide commissural neurons, 1892

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 Semaphorin Slit-Robo NetrinFasciclin Connexin

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, 6 th Edition, Gilbert

Experimental Evidence

COS cells secreting netrins elicit axon outgrowth from 11-day embryonic rat DSC explants Experimental Evidence 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

Netrin Unc-5 receptor ? The Molecular Biology of Chemoattraction

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!