Netrins are a small family of secreted proteins that guide growing axons during neural development by binding to the receptor DCC (Deleted in colorectal cancer). Our lab and others have previously shown that the activity of the Rho family GTPases Rac1 and Cdc42 are essential for DCC- mediated neurite outgrowth. Rac1 and Cdc42 act as molecular switches, mediating cytoskeleton remodelling when they are active and bound to GTP. Rac1 and Cdc42 are regulated positively by GEFs (guanine exchange factors) and negatively by GAPs (GTPase-activating proteins). Since DCC does not interact directly with Rac1, there should be an indirect link between DCC and Rac1. Trio is a GEF that activates Rac1 and RhoA. The orthologs of Trio in C.elegans (unc-73) and in D. melanogaster have been shown to play important roles in axon guidance, suggesting that mammalian Trio may link DCC to Rac1 activation. Here, we investigated how netrin-1 and its respective guidance receptor DCC are linked to Rac1 through studying the role of Trio in this signaling pathway. We found that Trio, Nck1, PAK1, and DCC are present in the same signaling complex, and that netrin-1-induced Rac1 activation is impaired in the absence of Trio. Trio -/- cortical neurons fail to extend neurites in response to netrin-1, while they are able to respond to glutamate. Accordingly, netrin-1-induced commissural axon outgrowth is severely impaired in Trio -/- spinal cord explants and commissural axon projections are defective in Trio -/- embryos. In addition to defects in spinal cord development, the anterior commissure is absent in Trio-null embryos, and netrin-1/DCC-dependent axonal projections that form the internal capsule and the corpus callosum are also defective in Trio -/- embryos. Thus, Trio through its ability to activate Rac1 mediates netrin-1 signaling in axon growth and guidance. / Le facteur de guidage chémotropique nétrine-1 favorise la croissance axonale à travers son récepteur DCC (Deleted in Colorectal Cancer) via l'activation de Rac1. Cependant, le facteur d'échange nucléotidique (GEF) qui lie nétrin-1/DCC à Rac1 n'a pas encore été identifié. Nous démontrons que Trio est la protéine GEF impliquée dans ce phénomène. Nous avons trouvé que Trio, Nck1, PAK1, et DCC sont présents dans le même complexe de signalisation et que l'activation de Rac1 induite par nétrine-1 est inhibée en absence de Trio. Les neurones corticaux Trio-/- échouent dans l'extension de neurites en réponse à la nétrine-1 alors qu'ils répondent à la stimulation par le glutamate. Par conséquent, l'induction de la croissance des axones commissuraux est sérieusement entravée dans les explants de moelle épinière des embryons Trio -/-. En plus du défaut dans le développement de la moelle épinière, les commissures antérieures sont absentes dans les embryons Trio-/-, et les projections axonales, qui forment la capsule interne et le corpus callosum, sont aussi affectées dans les embryons Trio -/-. Donc, par sa capacité d'activer Rac1, Trio favorise la signalisation par la nétrine-1 dans la croissance et le guidage axonale.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.18811 |
| Date | January 2008 |
| Creators | Ghogha, Atefeh |
| Contributors | Nathalie Lamarche (Supervisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Master of Science (Department of Anatomy and Cell Biology) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | Electronically-submitted theses. |
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