Rab5-mediated endocytosis of activin is not required for gene activation or long-range signalling in Xenopus.

Wellcome Trust and Cancer Research UK Gurdon Institute & Department of Zoology, University of Cambridge, Cambridge, UK.
Development (Impact Factor: 6.27). 09/2009; 136(16):2803-13. DOI: 10.1242/dev.034124
Source: PubMed

ABSTRACT 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-beta 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.

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    • "This internalization results in clearance of ligand from the extracellular space and is thought to be a major regulator of signal stability. For example, overexpression of a receptor of the TGFβ ligand Activin increases the frequency of Activin internalization (Hagemann et al. 2009). The accompanying decrease in signaling range may be caused by the decreased stability of Activin or the sequestration of Activin by its receptor. "
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