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Central and Peripheral Axon Branches from One Neuron Are Guided Differentially by Semaphorin3D and Transient Axonal Glycoprotein-1

For multiple axons from one neuron to extend in different directions to unique targets, the growth cones of each axon must have distinct responses to guidance cues. However, the mechanisms by which separate axon branches are guided along different pathways are mainly unknown. Zebrafish Rohon-Beard (R-B) sensory neurons extend central axon branches in the spinal cord and peripheral axons to the epidermis. To investigate the differential guidance mechanisms of the central versus peripheral R-B axon branches, we used live-growth cone imaging in vivo combined with manipulation of individual guidance molecules. We show that a semaphorin expressed at the dorsal spinal cord midline, Semaphorin3D (Sema3D), may act to repel the peripheral axons out of the spinal cord. Sema3D knock-down reduces the number of peripheral axons. Remarkably, Sema3D ectopic expression repels and induces branching of peripheral axons in vivo but has no effect on central axons from the same neurons. Conversely, central axons require a growth-promoting molecule, transient axonal glycoprotein-1 (TAG-1), to advance, whereas peripheral axons do not. After TAG-1 knock-down, central growth cones display extensive protrusive activity but make little forward advance. TAG-1 knock-down has no effect on the motility or advance of peripheral growth cones. These experiments show how Sema3D and TAG-1 regulate the motility and behavior of growth cones extending in their natural in vivo environment and demonstrate that two different axon branches from one neuron respond differently to guidance cues in vivo.