Cilia and extracellular vesicles (ECVs) are signaling organelles. Cilia act as a cellular antennae and function in sensation and adhesion, with defects resulting in human ciliopathies. ECVs act as intercellular signaling parcels that contain and deliver donor cell cargoes to recipient cells. In Chlamydomonas, C. elegans, and mammals, ECVs are closely associated with cilia, suggesting that cilia may be essential in ECV-mediated communication as both senders and receivers. Alternatively, ECVs may play a role in maintaining ciliary structure. In humans, the polycystin (PC)-encoding genes PKD1 and PKD2 are needed for kidney function; loss of polycystin function leads to Autosomal Dominant Polycystic Kidney Disease (ADPKD, frequency 1/400- 1:1000), one of the most common monogenic diseases. ECVs mediate a broad range of physiological processes. Urinary ECVs released from renal epithelial cells are enriched in PC1, PC2, and the ARPKD protein fibrocystin, and are a source of biomarkers for renal disease. In ARPKD patients and mice, ECVs are associated with renal primary cilia. In vitro, PC-containing ECVs interact with primary cilia of cultured renal cells. Consistent with a possible role for ECVs in PKD and other ciliopathies, ECVs also play sinister roles in the spread of toxic cargoes in cancer, infectious diseases, and neurodegenerative disorders. In C. elegans and mammals, the PCs act in the same genetic pathway, act in a sensory capacity, localize to cilia, and are contained in secreted ECVs, indicating ancient conserved functions. This application uses C. elegans as a springboard to study the fundamental biology of ECVs in vivo and the roles of the PCs in cilia and ECVs, which will advance frontiers of knowledge where very little is known. First, we will define the molecular signature of a PC-containing and ECV-releasing cell. This approach will provide a comprehensive picture of the molecules that influence PC function in ECVs and cilia, and will provide important insight to the fundamental biology of ciliary ECVs. Second, we will determine the mechanisms regulating PC ciliary localization and formation of PC-containing ECVs. By using novel approaches to visualize GFP-labeled ciliary ECVs released from living animals in real time and to measure ECV activity, we developed the first and only animal system to study ECVs in vivo and are uniquely poised to study their biogenesis, dynamics, and functions. Finally, we aim to understand the relationship between cilia and ECVs. This knowledge is essential for determining the biological significance of ECVs, for understanding their relationship to human cystic kidney disease, and for harnessing their potential therapeutic uses. A genetically tractable model can make inroads where other systems have not, and advance frontiers of knowledge where little is known.