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First published online 15 July 2009
doi: 10.1242/dev.034124

Development 136, 2803-2813 (2009)
Published by The Company of Biologists 2009
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Rab5-mediated endocytosis of activin is not required for gene activation or long-range signalling in Xenopus

Anja I. Hagemann1, Xin Xu1, Oliver Nentwich1, Marko Hyvonen2 and James C. Smith1,3,*

1 Wellcome Trust and Cancer Research UK Gurdon Institute & Department of Zoology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.
2 Department of Biochemistry, University of Cambridge, 80 Tennis Court Road, Cambridge CB2 1GA, UK.
3 MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK.

* Author for correspondence (e-mail: jim.smith{at}nimr.mrc.ac.uk)

Accepted 12 June 2009

Morphogen gradients provide positional cues for cell fate specification and tissue patterning during embryonic development. One important aspect of morphogen function, the mechanism by which long-range signalling occurs, is still poorly understood. In Xenopus, members of the TGF-β family such as the nodal-related proteins and activin act as morphogens to induce mesoderm and endoderm. In an effort to understand the mechanisms and dynamics of morphogen gradient formation, we have used fluorescently labelled activin to study ligand distribution and Smad2/Smad4 bimolecular fluorescence complementation (BiFC) to analyse, in a quantitative manner, the cellular response to induction. Our results indicate that labelled activin travels exclusively through the extracellular space and that its range is influenced by numbers of type II activin receptors on responding cells. Inhibition of endocytosis, by means of a dominant-negative form of Rab5, blocks internalisation of labelled activin, but does not affect the ability of cells to respond to activin and does not significantly influence signalling range. Together, our data indicate that long-range signalling in the early Xenopus embryo, in contrast to some other developmental systems, occurs through extracellular movement of ligand. Signalling range is not regulated by endocytosis, but is influenced by numbers of cognate receptors on the surfaces of responding cells.

Key words: Xenopus, TGF-β, Smads, Activin, Nodal-related proteins, Bimolecular fluorescence complementation (BiFC), Morphogen


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