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First published online 12 November 2008
doi: 10.1242/dev.028209
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MRC Centre for Developmental Neurobiology, New Hunt's House, Guy's Campus, King's College London, SE1 1UL, UK.
e-mail: julian.ng{at}kcl.ac.uk
Accepted 15 October 2008
Proper nerve connections form when growing axons terminate at the correct postsynaptic target. Here I show that Transforming growth factor beta (TGFβ) signals regulate axon growth. In most contexts, TGFβ signals are tightly linked to Smad transcriptional activity. Although known to exist, how Smad-independent pathways mediate TGFβ responses in vivo is unclear. In Drosophila mushroom body (MB) neurons, loss of the TGFβ receptor Baboon (Babo) results in axon overextension. Conversely, misexpression of constitutively active Babo results in premature axon termination. Smad activity is not required for these phenotypes. This study shows that Babo signals require the Rho GTPases Rho1 and Rac, and LIM kinase1 (LIMK1), which regulate the actin cytoskeleton. Contrary to the well-established receptor activation model, in which type 1 receptors act downstream of type 2 receptors, this study shows that the type 2 receptors Wishful thinking (Wit) and Punt act downstream of the Babo type 1 receptor. Wit and Punt regulate axon growth independently, and interchangeably, through LIMK1-dependent and -independent mechanisms. Thus, novel TGFβ receptor interactions control non-Smad signals and regulate multiple aspects of axonal development in vivo.
Key words: Neural development, Signal transduction, Cytoskeleton, Drosophila
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