Exosome biogenesis and signalling
Using secondary cells, we have been able to image the intraluminal vesicles (ILVs) 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. Rab11-exosomes are loaded with cargos different from those made in late endosomes, but they require some of the same regulatory molecules, such as the ‘core’ 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, where they can promote drug resistance.
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. For example, accessory ESCRT-III proteins control Rab11-exosome biogenesis, but do not appear to be required to make ILVs in late endosomes. These experiments have also shown that the ILVs in Rab11-compartments interact with secreted proteins during DCG biogenesis and can play a role in DCG formation (see Objective 2).
Biological relevance
Rab11-exosomes are an evolutionarily conserved exosome subtype formed in compartments containing recycling endosomal cargos. These exosomes have subtype-specific functions and are involved both in intercellular signalling and in controlling secretory processes intrinsic to the cell.
Disease relevance
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 DCGs is inter-dependent, which is likely to be relevant to diseases of secretion like diabetes and Alzheimer’s Disease. Indeed, we are starting to identify important links in our studies of AD. 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.