Stewart - Cell biology, unconventional protein secretion, extracellular vesicles, inflammation, cancer
Eukaryotic cells have a highly evolved system of protein secretion, and dysfunction in this pathway is associated with many diseases including cancer, infection and neurological disorders. Most proteins in the human body are secreted using the endoplasmic reticulum (ER)/Golgi network, defining the “conventional secretory pathway”. However, several cytosolic proteins that lack a signal peptide for entry into the ER can be secreted by alternative routes, called unconventional protein secretion pathways. The extracellular functions of these proteins have been well documented, as are associations of their perturbed secretion with disease.
Despite the well described extracellular functions of unconventionally secreted proteins, the mechanism(s) of unconventional protein secretion and its regulation remain largely uncharacterised. This represents a major gap in our understanding of the fundamental mechanisms supporting protein trafficking and secretion. Our goal is to understand how cytosolic proteins are secreted from mammalian cells and describe the pathways and molecular regulators involved in unconventional protein secretion. We are investigating several major pathways including direct translocation across the plasma membrane and the role of extracellular vesicles in unconventional protein secretion. Understanding these fundamental cellular processes is extremely important and may lead to novel drug targets for a range of diseases.
Research Areas
Unconventional protein secretion: How do proteins cross membrane barriers?
Described over the last 20 years, there are many protein families with members that use unconventional protein secretion pathways. This illustrates that unconventional protein secretion traffics a diverse range of proteins that impact many fields of biology. The extracellular functions of these proteins have been well documented, as are associations of their perturbed secretion with disease. However, pathways of unconventional secretion remain poorly understood.
The Stewart Lab is investigating whether lipid remodelling regulates the unconventional secretion of proteins from mammalian cells, with a particular focus on inflammation and cancer. This research will provide further mechanistic insights and investigate commonalities in UPS more generally, as well as shedding light on unconventional protein secretion in specific desease models. Unravelling the mechanistic details of unconventional protein secretion will have profound implications for a range of diseases and provide significant insights into fundamental cell biology in processes.
Inside-out: Extracellular vesicle packaging and function
Cells communicate with each other by direct cell-to-cell contact and via the secretion of proteins. Recently, it has emerged that cells secrete small vesicles, which convey messages between cells. These extracellular vesicles (EVs) are a ‘snap shot’ of the secreting cell; they contain a range of proteins and nucleic acids including RNA from the cytosol, encapsulated by a membrane. Trillions of extracellular vesicles are found in blood under both physiological and pathological conditions and their function and composition is currently a topic of great interest.
Despite their important role in intercellular communication, the mechanisms of extracellular vesicle protein packaging are unclear. The current model for extracellular vesicles biogenesis dictates that the subcellular location of proteins is reflected in the extracellular vesicles architecture, with cytosolic proteins in the lumen and cell surface proteins on the outside of extracellular vesicles. However, the Stewart lab (and others) have reported that there are cytosolic proteins on the surface of extracellular vesicles, suggesting that certain proteins and receptors are in an inside-out orientation in extracellular vesicles. Therefore, we are investigating how proteins are packaged both into and onto extracellular vesicles and whether this has important functional implications in health and disease.
