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First published online August 9, 2006
doi: 10.1242/10.1242/jcs.03084

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RhoA-kinase coordinates F-actin organization and myosin II activity during semaphorin-3A-induced axon retraction

Drexel University College of Medicine, Department of Neurobiology and Anatomy, 2900 Queen Lane, Philadelphia, PA 19129, USA

e-mail: GGallo{at}drexelmed.edu

Accepted 6 June 2006

Axon guidance is mediated by the effects of attractant and repellent guidance cues on the cytoskeleton of growth cones and axons. During development, axon retraction is an important aspect of the pruning of inappropriately targeted axons in response to repellent guidance cues. I investigated the roles of RhoA-kinase and myosin II in semaphorin-3A-induced growth cone collapse and axon retraction. I report that semaphorin 3A activates myosin II in growth cones and axons. Myosin II activity is required for axon retraction but not growth cone collapse. Furthermore, semaphorin 3A promotes the formation of intra-axonal F-actin bundles in concert with the loss of F-actin in growth cone lamellipodia and filopodia. Formation of axonal F-actin bundles was independent of myosin II, but partially required RhoA-kinase activity. Conversely, RhoA-kinase activity was required to shut down F-actin polymerization underlying protrusive activity. Collectively, these observations suggest that guidance cues cause axon retraction through the coordinated activation of myosin II and the formation of intra-axonal F-actin bundles for myosin-II-based force generation. I suggest that in the context of semaphorin 3A signaling, RhoA-kinase serves as a switch to change the function of the F-actin cytoskeleton from promoting protrusive activity to generating contractile forces.

Key words: RhoA, RhoA-kinase, Cellular contractility, Axon guidance, Growth cone collapse, Cytoskeleton

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