Science for Health
23 November 2012
Intercellular communication is essential for embryonic development and adult homeostasis. Signalling proteins, such as Wnts, spread within tissues to coordinate cell growth, differentiation and survival. Wnt proteins are closely associated with cellular membranes. They are nevertheless released from cells and have an effect at a distance of up to 20 cell diameters away. Several proteins that are required for Wnt secretion, such as the multipass transmembrane protein Evi, have been identified. However, it is not yet known in what form Wnt proteins are packaged for release from producing cells. One possibility is that Wnt remains associated with membranes during transport. For example, Wnts could be released in the extracellular space on exosomes, membranous microvesicles that are produced in multivesicular bodies (MVBs).
Karen Beckett and others in the group of Jean-Paul Vincent (pictured) in the division of Developmental Biology have tested this hypothesis for Wingless, the main Drosophila Wnt, which organises patterning and growth in wing imaginal discs. They found that both Wingless and Evi are present in an exosome fraction obtained from culture medium. Using mass spectrometry they identified Drosophila exosome markers. Many are homologs of mammalian proteins known to be on exosomes and include proteins involved in trafficking, cell adhesion, the cytoskeleton and metabolism. They also developed a cell-based assay to test the potential role of candidate proteins in exosome production and thus showed the essential role of the small GTPase, rab11. They also found, however, that in imaginal discs rab11 knockdown does not impair the spread of Wingless, suggesting that exosomes are not required for Wingless gradient formation. Since Evi is present in haemolymph (fly blood), it is conceivable that exosomes could contribute to the systemic release of a Wnt, a possibility that is currently being investigated.
We have found that Wingless is present on exosome-like vesicles in medium conditioned by Wingless-expressing Drosophila S2 cell and that this fraction activates signal transduction. This suggests that exosomes carrying Wg have a physiological role although this is not in Wingless gradient formation in imaginal discs. Our current challenge is to design a genetic approach to uncover the physiological function of exosomes.
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Wingless is secreted on exosomes in Drosophila S2 cells, but not in the wing imaginal disc.
Karen Beckett, Solange Monier, Lucy Palmer, Cyrille Alexandre, Hannah Green, Eric Bonneil, Graca Raposo, Pierre Thibault, Roland Le Borgne, Jean-Paul Vincent (2012)
Drosophila S2 Cells Secrete Wingless on Exosome-Like Vesicles but the Wingless Gradient Forms Independently of Exosomes
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