PROJECT DESCRIPTION/ABSTRACT Exosomes are secreted small extracellular vesicles with the size 30-130nm. They contain DNA fragments, mRNAs and proteins, and emerge as a new category of messengers that facilitate cross-talk among cells, tissues, and organs. Tracking the distribution and dynamic nature of these small extracellular vesicles, and quantifying their kinetics are essential. A critical demand arises for the development of a sensitive and noninvasive tracking system for endogenous exosomes; such a system will provide a powerful tool for the exploration of the normal biological functions of exosomes. To meet such as a demand, a unique exosome reporter mouse is proposed, through which to reveal communication networks of endogenous exosomes among organs/tissues or cells in physiological conditions and investigate normal exosome biology. A modified bioluminescent reporter will be knocked in the mice, which will provide an ultra-sensitive and stable labeling and tracking system. The specific inductions of the reporter will be tested in cardiomyocytes and cardiac fibroblasts as two examples. The luminescent signaling will be detected non-invasively. In summary, the study will provide a powerful and versatile genetic mouse model to enable location tracking, quantifying endogenous exosomes derived from almost any tissues, organs or cell lineages. The proposed genetic mice are superior to the current ex vivo and in vivo labeling.