Directional cell movement through tissues is critical for multiple biological processes and requires maintenance of polarity in the face of complex environmental cues. Here we use intravital imaging to demonstrate that secretion of exosomes from late endosomes is required for directionally persistent and efficient in vivo movement of cancer cells. Inhibiting exosome secretion or biogenesis leads to defective tumour cell migration associated with increased formation of unstable protrusions and excessive directional switching. In vitro rescue experiments with purified exosomes and matrix coating identify adhesion assembly as a critical exosome function that promotes efficient cell motility. Live-cell imaging reveals that exosome secretion directly precedes and promotes adhesion assembly. Fibronectin is found to be a critical motility-promoting cargo whose sorting into exosomes depends on binding to integrins. We propose that autocrine secretion of exosomes powerfully promotes directionally persistent and effective cell motility by reinforcing otherwise transient polarization states and promoting adhesion assembly.
Unconventional secretion of exosome vesicles from multivesicular endosomes (MVEs) occurs across a broad set of systems and is reported to be upregulated in cancer, where it promotes aggressive behavior. However, regulatory control of exosome secretion is poorly understood. Using cancer cells, we identified specialized invasive actin structures called invadopodia as specific and critical docking and secretion sites for CD63- and Rab27a-positive MVEs. Thus, inhibition of invadopodia formation greatly reduced exosome secretion into conditioned media. Functionally, addition of purified exosomes or inhibition of exosome biogenesis or secretion greatly affected multiple invadopodia life cycle steps, including invadopodia formation, stabilization, and exocytosis of proteinases, indicating a key role for exosome cargoes in promoting invasive activity and providing in situ signaling feedback. Exosome secretion also controlled cellular invasion through three-dimensional matrix. These data identify a synergistic interaction between invadopodia biogenesis and exosome secretion and reveal a fundamental role for exosomes in promoting cancer cell invasiveness. Exosomes are small extracellular vesicles that carry bioactive cargo and promote tumor aggressiveness and survival. In this study, Weaver and colleagues identify invasive subcellular structures known as invadopodia as critical docking sites for exosome carriers. The number of exosomes secreted from cells was dependent on invadopodia, and conversely, exosome secretion contributed strongly to invadopodia biogenesis and function. These data reveal an intriguing connection between acquisition of the invasive phenotype and other aggressive tumor behaviors regulated by exosomes.
Exosomes are secreted membrane vesicles that share structural and biochemical characteristics with intraluminal vesicles of multivesicular endosomes (MVEs). Exosomes could be involved in intercellular communication and in the pathogenesis of infectious and degenerative diseases. The molecular mechanisms of exosome biogenesis and secretion are, however, poorly understood. Using an RNA interference (RNAi) screen, we identified five Rab GTPases that promote exosome secretion in HeLa cells. Among these, Rab27a and Rab27b were found to function in MVE docking at the plasma membrane. The size of MVEs was strongly increased by Rab27a silencing, whereas MVEs were redistributed towards the perinuclear region upon Rab27b silencing. Thus, the two Rab27 isoforms have different roles in the exosomal pathway. In addition, silencing two known Rab27 effectors, Slp4 (also known as SYTL4, synaptotagmin-like 4) and Slac2b (also known as EXPH5, exophilin 5), inhibited exosome secretion and phenocopied silencing of Rab27a and Rab27b, respectively. Our results therefore strengthen the link between MVEs and exosomes, and introduce ways of manipulating exosome secretion in vivo.
The NAD −dependent deacetylase Sirtuin 1 (SIRT1) regulates cell metabolism, proliferation, and DNA repair and acts as a tumor suppressor in breast cancer. In this issue of , Latifkar et al. show that SIRT1 controls lysosomal acidification and its loss enhances the secretion of pro-tumorigenic exosomes for breast cancer invasion. The NAD −dependent deacetylase Sirtuin 1 (SIRT1) regulates cell metabolism, proliferation, and DNA repair and acts as a tumor suppressor in breast cancer. In this issue of , Latifkar et al. show that SIRT1 controls lysosomal acidification and its loss enhances the secretion of pro-tumorigenic exosomes for breast cancer invasion.