Exosomal Trafficking in Xenopus Development

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Development of the embryonic body plan is a complex process, requiring integration of autonomous cellular activities with both local and long-range intercellular signaling to specify various tissues and organs. This project investigates whether the early embryo uses extracellular microvesicles (exosomes), as tissue-specifying signaling agents. Exosomes are a recently discovered class of small, membrane-bound particles secreted from cells. It is now known that exosomes carry proteins involved with cell-cell recognition and signaling on their surfaces, and also that they contain microRNAs and mRNAs with the potential to alter gene expression patterns of cells receiving them. The ability of exosomes secreted from one cell type to "reprogram" other cells at a distance has been demonstrated in tissue culture, but whether this exciting property is utilized during normal embryonic development is not known. The current project investigates this possibility by pursuing the observation that blood formation requires exosomes. The goals of this research are to investigate the cell biology of exosomal exchange in during embryogenesis in frogs. The project will address whether qualitative or quantitative changes in the exosomal proteins occur during early development? Which tissues secrete exosomes? Are specific tissues in the early frog embryo targeted by exosomes; if so, how do those tissues respond? Can altering the secretion or specificity of exosome uptake alter embryonic blood development? Answering these questions will advance the nascent, rapidly growing field of exosomal cell biology, and expand our understanding of the repertoire of signaling mechanisms used by embryonic cells early in normal development. Many aspects of this research present excellent hands-on opportunities for students and trainees at all levels, and thus can serve as a critical starting point or advancement step for developing careers in biology or biomedicine.This project explores the novel hypothesis that the early Xenopus embryo uses extracellular microvesicles, specifically exosomes, for embryonic tissue patterning. Exosomes have recently been recognized as potent conveyors of membrane-associated signaling ligands and receptors, functional microRNAs and mRNAs, but most such demonstrations have been in limited tissue-culture situations. Recent work demonstrated that an extracellular compartment in the intact early Xenopus embryo is highly enriched with exosomes. Mass spectroscopic proteomic analysis confirms an abundance of exosomal markers, and electron microscopy confirms an abundance of particles in the size range typical of exosomes. Embryos reared deprived of microvesicles develop with a characteristic set of morphological defects, particularly a profound reduction in hematopoiesis, suggesting a fundamental role for exosome-dependent signaling during tissue specification steps of early development. This significant cell-biological activity has not previously been explored in a vertebrate embryo, so the goals of this project are to characterize exosomes, including the time course of their appearance, their cargo and the surface proteins that potentially define their tissue target specificity. This information will be used to develop probes and reagents to investigate their tissue origins and potential tissue targets.