A shared pathway of exosome biogenesis operates at plasma and endosome membranes
Eukaryotic cells secrete exosomes, which are small (~30-200 nm dia.), single membrane-bound organelles that transmit signals and molecules to other cells. Exosome-mediated signaling contributes to diverse physiological and disease processes, rendering their biogenesis of high biomedical importance. The prevailing hypothesis is that exosomes bud exclusively at endosome membranes and are released only upon endosome fusion with the plasma membrane. Here we tested this hypothesis by examining the intracellular sorting and exosomal secretion of the exosome cargo proteins CD63, CD9, and CD81. We report here that CD9 and CD81 are both localized to the plasma membrane and bud >5-fold more efficiently than endosome-localized CD63. Furthermore, we show that redirecting CD63 from endosomes to the plasma membrane by mutating its endocytosis signal (CD63/Y235A) increased its exosomal secretion ~6-fold, whereas redirecting CD9 to endosomes by adding an endosome targeting signal (CD9/YEVM) reduced its exosomal secretion ~5-fold. These data demonstrate that the plasma membrane is a major site of exosome biogenesis, and more importantly, that cells possess a common pathway for exosome protein budding that operates at both plasma and endosome membranes. Using a combination of single-particle interferometry reflectance (SPIR) imaging and immunofluorescence (IF) microscopy, we also show that variations in exosome composition are controlled by differential intracellular protein trafficking rather than by separate mechanisms of exosome biogenesis. This new view of exosome biogenesis offers a simple explanation for the pronounced compositional heterogeneity of exosomes and a validated roadmap for exosome engineering. SummaryThis study of exosome cargo protein budding reveals that cells use a common pathway for budding exosomes from plasma and endosome membranes, providing a new mechanistic explanation for exosome heterogeneity and a rational roadmap for exosome engineering.