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First published online November 26, 2007
doi: 10.1242/10.1242/dev.011452

Development 134, 4491-4501 (2007)
Published by The Company of Biologists 2007
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Semaphorin and neuropilin co-expression in motoneurons sets axon sensitivity to environmental semaphorin sources during motor axon pathfinding

Frédéric Moret, Christelle Renaudot, Muriel Bozon and Valérie Castellani*

Université de Lyon, F-69003, France, Université Lyon1, F-69003, France. CNRS, UMR5534, Centre de Génétique Moléculaire et Cellulaire, Villeurbanne, F-69622, France.

* Author for correspondence (e-mail: castellani{at}cgmc.univ-lyon1.fr)

Accepted 19 September 2007

Class III semaphorins (SemaIIIs) are intercellular cues secreted by surrounding tissues to guide migrating cells and axons in the developing organism. This chemotropic activity is crucial for the formation of nerves and vasculature. Intriguingly, SemaIIIs are also synthesized by neurons during axon pathfinding, but their function as intrinsic cues remains unknown. We have explored the role of Sema3A expression in motoneurons during spinal nerve development. Loss- and gain-of-function in the neural tube of the chick embryo were undertaken to target Sema3A expression in motoneurons while preserving Sema3A sources localized in peripheral tissues, known to provide important repulsive information for delineating the routes of motor axons towards their ventral or dorsal targets. Strikingly, Sema3A overexpression induced defasciculation and exuberant growth of motor axon projections into these normally non-permissive territories. Moreover, knockdown studies showed that motoneuronal Sema3A is required for correct spinal nerve compaction and dorsal motor axon extension. Further analysis of Sema3A gain- and loss-of-function in ex vivo models revealed that Sema3A in motoneurons sets the level of sensitivity of their growth cones to exogenous Sema3A exposure. This regulation is associated with post-transcriptional and local control of the availability of the Sema3A receptor neuropilin 1 at the growth cone surface. Thus, by modulating the strength of Sema3A-mediated environmental repulsive constraints, Sema3A in motoneurons enables axons to extend more or less far away from these repulsive sources. Such interplay between intrinsic and extrinsic Sema3A may represent a fundamental mechanism in the accurate specification of axon pathways.

Key words: Semaphorin, Motoneuron, Axon guidance


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Inside story on Sema3A and axonal pathfinding

Development 2007 134: e2404. [Full Text]  





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