Exosome biogenesis and signalling
Using secondary cells, we have been able to image the intraluminal vesicles that become exosomes in their compartment of origin, employing high-resolution fluorescence microscopy. These exosomes are formed in compartments labelled by the recycling endosomal marker, Rab11, which is also incorporated into these vesicles as a signature for this exosome subtype. These exosomes are loaded with cargos different from those made in late endosomes, but they require some of the same regulatory molecules, such as the ESCRTs (endosomal sorting complexes required for transport) to make them.
Collaborating with the research teams of Prof Deborah Goberdhan and Prof Adrian Harris, we have shown that exosomes are also made in recycling endosomes of cancer cells, so-called Rab11a-exosomes, and that they are secreted in response to stresses, such as the nutrient depletion induced by tumour growth. These exosomes have pro-tumorigenic properties and may therefore drive adaptive changes in tumours subjected to growth-inhibitory stresses, including those associated with cancer therapies.
By analysing the contents of stress-induced extracellular vesicles secreted from cancer cells, we have identified putative regulators of Rab11a-exosomes, which we have shown are required for Rab11-exosome biogenesis and secretion in secondary cells. These experiments are revealing that the generation of Rab11-exosomes is a highly evolutionarily conserved process that we can dissect out in the fly to identify selective mechanisms to block their production. They have also shown that the intraluminal vesicles in Rab11-compartments regulate the formation of dense-core granules in these structures.
Rab11-exosomes are an evolutionarily conserved exosome subtype formed in compartments containing recycling endosomal cargos. They are involved both in intercellular signalling and in controlling secretory processes intrinsic to the cell.
Rab11a-exosomes appear to have important roles in tumour adaptation. We believe they are secreted by many or all cell types and carry bioactive cargos that are not present in exosomes from late endosomes. Furthermore, formation of these exosomes and dense-core granules is inter-dependent; we have evidence that this is relevant to other secretory cells, such as pancreatic beta-cells and in amyloidogenesis, implicating these vesicles in diseases diabetes and neurodegenerative disorders. Understanding Rab11-exosome regulation and functions in flies may reveal ways of blocking them in disease or detecting them in diagnostic and prognostic applications.
image description: Exosome formation in secondary cells
Super-resolution image of single living secondary cells (dashed circle) expressing exosome marker CD63-GFP (yellow) and stained with LysoTracker Red (magenta) to label acidic compartments. Box marks enlarged compartment containing intraluminal vesicles that will be secreted as exosomes. Scale bars = 5 µm and 2 µm. From Fan et al., 2020.