Exosomes et sécrétion non-conventionnelle (M. Vidal)

We are interested in unconventional secretion and more specifically in exosome secretion (study of the molecular sorting mechanisms, the physiologic functions of the phenomenon and its hijacking by various pathogens). In contrast to other kinds of extracellular vesicles released by plasma membrane shedding (ectosomes), exosomes are defined as small vesicles (60-80 nm) originating from the endosomal compartment, by membrane budding toward the endosomal lumen. These intraluminal vesicles are released upon fusion of multivesicular endosomes (MVE) with the plasma membrane.


Our previous data obtained on the secretion of reticulocyte exosomes enabled us to release several ideas which constitute the principal screen of our project. These results converge (i) in mechanistic terms, on a sorting process which likely involves the cellular machinery ESCRT, as well as the molecular state of the components in the membrane (aggregation state, association with lipid rafts). We had especially noticed some points occurring during viral budding: e.g. sorting of Gag by the ESCRT complexes, viral proteins gathering in lipid rafts, oligomerization of Gag, routing of Gag towards the plasma membrane depending on rab11, reminiscent of data that we have obtained on protein sorting in the exosomal pathway; (ii) in functional terms, our data are consistent with a role of the secreted vesicles in relation with clearance of undesirable or obsolete molecules. This is perfectly consistent with the major function of MVE: to direct membrane proteins towards lysosomal degradation. The vesicles secreted in the extracellular medium are then likely, at least for part of them, degraded by recipient cells defining what we called an externalized degradation process. This concept of material transfer from one cell to another one via membrane vesicles has probably allowed the selection of pathogenic agents (retrovirus, PrPsc) hijacking this pathway for their benefit.
Activités de recherche

Unconventional secretion of proteasomal particules

In a second project, we investigated the global exosomal response of cells to serum starvation by analyzing the total protein content of secreted exosomes using iTRAQ proteomics. We compared the protein composition of purified exosomes secreted by cells impaired or not for macroautophagy by Atg5 depletion, during serum starvation conditions or complete medium culture. We show that the absence of serum modifies exosomal content, especially inducing secretion of two cytoplasmic protein complexes, namely proteasomal 19S regulatory particle (RP) and components of non-canonical translation preinitiation complex (PIC). This process is enhanced when autophagy is impaired by Atg5 depletion. Moreover, we show that the proteasome 20S core particle (CP) is released in the extracellular space. However, in striking contrast to what is seen for its 19S RP regulator, release is independent of the exosomal vesicles, Atg5 expression and cell culture conditions. Exosome secretion can thus be considered as a cell process that participates in and reflects cell homeostasis, and care must be taken when studying potential extracellular function of exosomes due to the possible co-purification of proteasome 20S CP.

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Michel VIDAL
Directeur de Recherche (DR) CNRS
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