Graphical Abstract Highlights d MemBright allows for bright and specific staining of EVs d The zebrafish embryo allows tracking of tumor EVs at high spatiotemporal resolution d Circulating tumor EVs are mostly taken up by endothelial cells and patrolling macrophages d Zebrafish melanoma EVs favor metastatic outgrowth in zebrafish embryos

Cancer extracellular vesicles (EVs) shuttle at distance and fertilize pre-metastatic niches facilitating subsequent seeding by tumor cells. However, the link between EV secretion mechanisms and their capacity to form pre-metastatic niches remains obscure. Using mouse models, we show that GTPases of the Ral family control, through the phospholipase D1, multi-vesicular bodies homeostasis and tune the biogenesis and secretion of pro-metastatic EVs. Importantly, EVs from RalA or RalB depleted cells have limited organotropic capacities in vivoand are less efficient in promoting metastasis. RalA and RalB reduce the EV levels of the adhesion molecule MCAM/CD146, which favors EV-mediated metastasis by allowing EVs targeting to the lungs. Finally, RalA, RalB, and MCAM/CD146, are factors of poor prognosis in breast cancer patients. Altogether, our study identifies RalGTPases as central molecules linking the mechanisms of EVs secretion and cargo loading to their capacity to disseminate and induce pre-metastatic niches in a CD146-dependent manner.

A range of cell types, including embryonic stem cells, neurons and astrocytes have been shown to release extracellular vesicles (EVs) containing molecular cargo. Across cell types, EVs facilitate transfer of mRNA, microRNA and proteins between cells. Here we describe the release kinetics and content of EVs from mouse retinal progenitor cells (mRPCs). Interestingly, mRPC derived EVs contain mRNA, miRNA and proteins associated with multipotency and retinal development. Transcripts enclosed in mRPC EVs, include the transcription factors Pax6, Hes1, and Sox2, a mitotic chromosome stabilizer Ki67, and the neural intermediate filaments Nestin and GFAP. Proteomic analysis of EV content revealed retinogenic growth factors and morphogen proteins. mRPC EVs were shown to transfer GFP mRNA between cell populations. Finally, analysis of EV mediated functional cargo delivery, using the Cre-loxP recombination system, revealed transfer and uptake of Cre+ EVs, which were then internalized by target mRPCs activating responder loxP GFP expression. In summary, the data supports a paradigm of EV genetic material encapsulation and transfer within RPC populations. RPC EV transfer may influence recipient RPC transcriptional and post-transcriptional regulation, representing a novel mechanism of differentiation and fate determination during retinal development.

Microvesicles (MVs) are lipid bilayer-covered cell fragments that range in diameter from 30 nm–1uM and are released from all cell types. An increasing number of studies reveal that MVs contain microRNA, mRNA and protein that can be detected in the extracellular space. In this study, we characterized induced pluripotent stem cell (iPSC) MV genesis, content and fusion to retinal progenitor cells (RPCs) in vitro. Nanoparticle tracking revealed that iPSCs released approximately 2200 MVs cell/hour in the first 12 hrs with an average diameter of 122 nm. Electron and light microscopic analysis of iPSCs showed MV release via lipid bilayer budding. The mRNA content of iPSC MVs was characterized and revealed the presence of the transcription factors Oct-3/4, Nanog, Klf4, and C-Myc. The protein content of iPSCs MVs, detected by immunogold electron microscopy, revealed the presence of the Oct-3/4 and Nanog. Isolated iPSC MVs were shown to fuse with RPCs in vitro at multiple points along the plasma membrane. These findings demonstrate that the mRNA and protein cargo in iPSC MVs have established roles in maintenance of pluripotency. Building on this work, iPSC derived MVs may be shown to be involved in maintaining cellular pluripotency and may have application in regenerative strategies for neural tissue.

| 1 www.fasebbioadvances.org | INTRODUCTIONCancer is among the most common causes of morbidity and mortality worldwide, and the vast majority of cancerrelated death is due to metastasis rather than primary tumors. 1 Thus, the limitations of anti-metastasic treatments require a deeper understanding of the complex stepwise process of tumor cell dissemination toward target organs in order to design innovative therapies. 2 Metastasis is a highly inefficient process as only a very

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Cancer extracellular vesicles (EVs) mainly exert pro-tumoral functions by changing the phenotypes of stromal cells to the benefit of tumor growth and metastasis. In particular, they shuttle to distant organs and seed pre-metastatic niches facilitating subsequent colonization by circulating tumor cells. The levels of tumor secreted EVs have been correlated with tumor aggressiveness, however, the link between EV secretion mechanisms and their capacity to form pre-metastatic niches remains obscure. Here, we show that GTPases of the Ral family control, through the phospholipase D1 (PLD1), multi-vesicular bodies homeostasis and thereby tune the biogenesis and secretion of pro-metastatic EVs. Mice experiments revealed that RalA and RalB promote lung metastasis of mammary carcinoma cells without affecting their invasive behaviors. Importantly, we demonstrate that EVs from RalA or RalB depleted cells have limited organotropic capacities in vivo and, as a consequence, are less efficient in seeding lung metastasis. Furthermore, we show that such EVs lack the adhesion molecule MCAM/CD146, which is responsible for EVs organotropism. Finally, we observe that RalA and RalB have increased expression in human breast cancer patients with lung metastasis. Altogether, our study identifies Ral GTPases as central molecules linking the mechanisms of EVs secretion, cargo loading to their capacity to disseminate and induce pre-metastatic niches.

The original version of this Article contained a typographical error in the spelling of the author M. Valeria Canto-Soler, which was incorrectly given as M. Valeria Canto-Solar. This has now been corrected in the PDF and HTML versions of the Article and in the accompanying supplementary material.