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The Journal of Neuroscience, September 5, 2007, 27(36):9653-9663; doi:10.1523/JNEUROSCI.1741-07.2007

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Development/Plasticity/Repair
Semaphorin3D Regulates Axon–Axon Interactions by Modulating Levels of L1 Cell Adhesion Molecule

Marc A. Wolman,^1,2 Ann M. Regnery,¹ Thomas Becker,³ Catherina G. Becker,³ and Mary C. Halloran^1,2

¹Departments of Zoology and Anatomy and ²Neuroscience Training Program, University of Wisconsin, Madison, Wisconsin 53706, and ³Centre for Neuroscience Research, Royal (Dick) School of Veterinary Studies, Summerhall, Edinburgh EH9 1QH, United Kingdom

Correspondence should be addressed to Mary C. Halloran, Department of Zoology, 1117 West Johnson Street, Madison, WI 53706. Email: mchalloran{at}wisc.edu

The decision of a growing axon to selectively fasciculate with and defasciculate from other axons is critical for axon pathfinding and target innervation. Fasciculation can be regulated by cell adhesion molecules that modulate interaxonal adhesion and repulsive molecules, expressed by surrounding tissues that channel axons together. Here we describe crosstalk between molecules that mediate these mechanisms. We show that Semaphorin3D (Sema3D), a classic repulsive molecule, promotes fasciculation by regulating L1 CAM levels and axon–axon interactions rather than by creating a repulsive surround. Knockdown experiments show that Sema3D and L1 genetically interact to promote fasciculation. Sema3D overexpression increases and Sema3D knockdown decreases levels of axonal L1 protein. Moreover, excess L1 rescues defasciculation caused by the loss of Sema3D. In vivo time-lapse imaging reveals that Sema3D or L1 knockdown cause identical defects in growth cone behaviors during axon–axon interactions, consistent with a loss of adhesion. These results reveal a novel mechanism by which a semaphorin promotes fasciculation and modulates axon–axon interactions by regulating an adhesion molecule.

Key words: semaphorin; L1; axon guidance; fasciculation; adhesion; zebrafish

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Received April 17, 2007; revised June 21, 2007; accepted July 14, 2007.

Correspondence should be addressed to Mary C. Halloran, Department of Zoology, 1117 West Johnson Street, Madison, WI 53706. Email: mchalloran{at}wisc.edu

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