Exosomes form from the inward budding of vesicles into endosomes. If an endosome later fuses with the plasma membrane, its exosome packets can be released into the extracellular space. In this way, exosomes have been shown to send signals and present antigens to recipient cells. Proteins within the exosomes' bellies and on their surfaces have been the main focus of study, but nobody had looked to see whether exosomes contained nucleic acids, explains Lötvall.
His team now finds that exosomes isolated from immune cells known as mast cells contain large amounts of RNA, including translatable mRNA. DNA, however, was not found in the packets.
Microarray analysis revealed the presence of approximately 1,300 different mRNAs within exosomes, which also contained miRNAs. Many of the exosomal RNAs were absent from the mast cell cytoplasm, suggesting that these might be ferried rapidly from the nucleus to their exosome transporters. Of the exosome-specific mRNAs, nearly one-fifth are implicated in pathways that regulate cellular development, protein synthesis, and posttranscriptional RNA modification.
Mast cells transferred exosomal RNA efficiently to other mast cells but not to T cells. Although it is not yet clear how dispatched exosomes interact with recipient cells, this preference suggests that exosomes have specific destinations as well as particular RNA contents.