Throughout life, cells communicate to coordinate the organism’s response to different stimuli. Cells release extracellular vesicles that carry signals to alter development or disease response. Released vesicles can also seal the cell membrane after damage. However, extracellular vesicles and other cellular debris need to be cleared from the environment and degraded intracellularly for normal physiology and to avoid an autoimmune response. The goal of our research is to discover how vesicles bud from the surface of cells, identify which signals extracellular vesicles send in animals, and determine how cells take up and process extracellular vesicles and other cellular debris. Defining how vesicles form is an essential first step to designing strategies to induce or suppress their formation and thereby determine their functional roles. This research could lead to new strategies to monitor or influence disease severity from cancer to inflammation and beyond.
Extracellular Vesicle Biogenesis
Most cells release extracellular vesicles (EVs) carrying lipid, protein, and nucleic acid signals. While much is known about their signaling potential, EV formation is poorly understood. We use the genetic model system C. elegans to discover evolutionarily conserved proteins that regulate EV budding. One example is the lipid flippase TAT-5, which regulates the distribution of specific lipids across the two layers of the plasma membrane. In tat-5 mutant worms, lipids are mislocalized and too many EVs are produced. This finding suggests that lipids have instructive roles in regulating membrane dynamics. Our research aims to define exactly how TAT-5 and lipid distribution regulate EV budding, which is likely to be conserved in human cells.
We are using the power of C. elegans genetics to identify additional proteins that regulate EV budding. We revealed that conserved regulators of viral budding also have a role in EV budding in C. elegans, including the membrane-sculpting complex known as ESCRT. Our studies are building a pathway of proteins that regulate TAT-5 localization and activity and thereby modulate EV release, which are likely to be co-opted by viruses. The proteins we identify may be used to alter EV production in other systems, which could impact the availability of non-invasive biomarkers and have the potential to influence viral spread or disease state.
Studying the mechanisms of EV production has provided us with techniques to induce or prevent their formation. This allows us to test which signaling pathways require EVs for signaling to occur, as well as define other functional roles for EVs. We are studying how changing EV production or uptake affects conserved developmental and immune signaling pathways in C. elegans. Thus, our research aims to define the diverse functional roles of EVs, which are likely to be similar in humans.
Clearance by Phagocytosis
Tissues contain cell fragments and dying cells in addition to healthy cells. For example, cells release a remnant of the intercellular bridge after cell division as a 1 µm EV. Cells need to clear these debris from their environment by phagocytosis. Furthermore, the immune system needs to degrade cell debris in phagolysosomes to avoid generating an autoimmune response. We use C. elegans to study the mechanisms of EV and cell corpse uptake by phagocytosis and the steps of phagolysosomal clearance.
C. elegans embryonic cells take up mitotic midbody remnants released during cell division in addition to dying cells such as the meiotic polar body. These phagosomes mature similar to mammalian phagosomes and gradually acidify and degrade their cargo. Thus, we can use C. elegans to study the conserved mechanisms of EV signaling, the pathways regulating EV uptake, and use time-lapse imaging to determine the ultimate fate of engulfed cargos in a developing animal.
Lipid asymmetry also regulates this dynamic process and we are interested in the signaling roles of lipids as well as proteins and metabolites. Analyzing defects in EV uptake complements our studies on EV budding and will allow us to elucidate the interplay of lipids and lipid regulators during dynamic remodeling of the membrane. Studying the fate of EVs also provides important insights into the functional roles of EV, which are likely to be conserved in humans. Furthermore, understanding these mechanisms will help to identify key modulators of the immune response, which can be disrupted during pathogen infection or autoimmune disease.