Among the complex and interwoven cell signaling pathways integral to our understanding of the biological world around us, there are some forms of cellular communication so seemingly simple they escape notice. Consider a case of mistaken identity and a presumed waste-disposal organelle. Exosomes—small (30 – 120 nm), membrane-bound vesicles found to be enriched in conditioned cell culture media and bodily fluids—are in fact, tiny packages of bioactive molecules capable of inducing particular states in cells that incorporate them.
Exosomes are now known to be valuable biomarkers of health and disease, and are currently under development as nanoparticle carriers of therapeutic agents. Since exosomes are released by most, if not all, cell types, they are easily collected from bodily fluids and are also released from cells in vitro—making culture medium a valuable source of these message-bearing vesicles. These non-invasive sources for exosome biomarkers are known as liquid biopsies, and are rapidly becoming a method of choice for the next generation of molecular diagnostics. Due to their lipid bilayer, pinched from the membrane of their cells of origin, they can target their delivery to specific organs and cell types, without eliciting an immune response. This feature makes them particularly well suited to delivering drug products.
The biology of exosomes was not fully appreciated until quite recently, when research demonstrated the nature of exosomal cargoes, the uptake of exosomes by target cells and the induction of particular states by cargo molecules. These exosome processing facets remain the focus of basic and applied research across applications and disciplines.