Studies on the role of extracellular vesicles in intercellular communication network

Abstract

Communication between cells and the environment is an essential process in living organisms, and intercellular communication is believed to be mediated mainly by the secretion of soluble factors, cell-to-cell contacts, and tunneling machinery such as nanotubes. Recently, a mechanism mediated by extracellular vesicles, termed EVs, which are spherical, bilayered proteolipids with an average diameter of 0.03 to 1 m, has drawn much attention. Throughout evolution, both prokaryotic and eukaryotic cells have adapted to manipulate EVs for intercellular communication via (outer) membrane vesicles in the case of bacteria and exosomes in eukaryotic cells. Increasing evidence suggests that EVs act as potent communicasomes, that is, nano-sized extracellular organelles that play diverse roles in intercellular communication. The biogenesis and functions of EVs may share many features in different biological systems. Thus, the study of EVs provides crucial keys to understanding the intercellular communication network in living organisms and the evolutionary connections between prokaryotes and eukaryotes. A wide variety of Gram-negative bacteria constitutively secrete EVs during growth. Bacterial EVs are composed of lipopolysaccharide, proteins, genetic materials, and other factors associated with virulence. Studies of EVs from diverse bacterial strains suggest their roles in the delivery of toxins to host cells, the transfer of proteins and genetic material between bacterial cells, cell-to-cell signals, and the elimination of competing organisms. Although growing evidence suggests that Gram-negative bacterial EVs are essential for bacterial survival and pathogenesis in hosts, the mechanisms of vesicle formation and of protein sorting into EVs, as well as the pathophysiological roles of EVs, have not been clearly defined. To address these issues, vesicular proteins should be comprehensively identified. Proteomics offers a powerful approach to decode the protein components of EVs. Using a proteomics approach, a comprehensive proteome map of Escherichia coli-derived native EVs has been established with high confidence. This information helps elucidate the biogenesis and functions of EVs from nonpathogenic and pathogenic bacteria. Although archaea, Gram-negative bacteria, and mammalian cells constitutively secrete EVs as a mechanism for cell-free intercellular communication, this cellular process has been overlooked in Gram-positive bacteria. In the present study, I found for the first time that Gram-positive bacteria naturally produce EVs into the extracellular milieu. This observation suggests that the secretion of EVs is an evolutionally conserved, universal process that occurs from simple organisms to complex multicellular organisms. With a proteomics approach, a total of 90 protein components of Gram-positive Staphylococcus aureus-derived EVs were identified with high confidence. In the group of identified proteins, I found key proteins that facilitate the transfer of antibiotic resistance proteins to other bacteria and pathological functions in systemic infectious diseases including coagulation and thrombosis disorders. In-depth study on the role of these EV-associated proteins revealed that S. aureus EVs play diverse roles in polymicrobial community as well as multiple roles for pathogenesis in an inter-species world. Studies of EVs will help us not only to elucidate the biogenesis and functions of EVs but also to stimulate the development of diagnostic tools, novel vaccines, and therapeutic agents which will advance both basic and clinical sciences.