Mechanism of Lupus La:miR-122 ribonucleoprotein sorting into breast cancer-derived exosomes

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PROJECT SUMMARY / ABSTRACT Extracellular vesicles (EVs) comprise a heterogeneous pool of membrane-enclosed compartments secreted to the extracellular milieu of cells. Eukaryotic cells release a variety of EVs subpopulations that can be classified broadly into two categories on the basis of their membrane of origin. Microvesicles are EVs that form by direct outwards budding from the plasma membrane. Exosomes are EVs that originate from the endocytic pathway. Upon fusion of a multivesicular body (MVB) with the plasma membrane, the intraluminal vesicles (ILVs) are exported to the extracellular space as exosomes. Exosomes have elicited broad interest as their intraluminal contents (e.g. miRNAs, yRNAs, and tRNAs) are distinct from their progenitor cells. An accumulating repertoire of evidence has suggested that EV-mediated intercellular propagation of miRNAs plays important roles in various aspects of cancer biology. In particular, several studies have suggested that the intercellular propagation of miR-122 through breast cancer-derived EVs promotes breast cancer metastasis by reprogramming glucose metabolism in the pre-metastatic niche in vivo. Consistent with this line of experimental evidence, our lab demonstrated previously, both in cells and a cell-free reaction, that the Lupus La antigen (La) mediates the selective sorting and enrichment of miR-122 into exosomes derived from a metastatic breast cancer cell line. However, the molecular mechanism(s) by which the La:miR-122 ribonucleoprotein (RNP) complex itself is selectively incorporated into breast cancer-derived exosomes remains unknown. In this proposed research project, Jordan Ngo seeks to elucidate the molecular mechanism(s) by which the La:miR-122 RNP is selectively sorted into ILVs. In Specific Aim #1, Jordan will assess the efficacy by which a panel of candidate endosomal receptor proteins (identified by unbiased proximity labeling proteomics) are able to invoke the capture of cytoplasmic La into ILVs prior to exosome secretion. In Specific Aim #2, Jordan will identify the domains of La required for (i) its unconventional secretion within exosomes and (ii) its interaction with miR-122. In Specific Aim #3, Jordan will establish a cell-free reaction that recapitulates the selective sorting of La into ILVs, allowing further biochemical dissection of this high-fidelity sorting mechanism. Completion of the proposed research will provide novel insights into the molecular mechanisms by which specific cytoplasmic constituents are efficiently and selectively packaged into exosomes, and by extension, identify putative therapeutic targets for metastatic breast cancer and inform efforts to engineer exosomes into efficacious delivery vehicles for therapeutic compounds and nucleic acids. At the conclusion of Jordan’s NRSA-sponsored training, Jordan will have a rigorous intellectual foundation, scientific independence, and a uniquely broad technical toolkit that will make him an outstanding candidate for an independent investigator position in the future.