Control of intracellular membrane traffic

All eukaryotic cells are organised into discrete membrane bound compartments or organelles. Delivery of macromolecules to the correct location at the right time is critical in many physiological processes such as hormone and neurotransmitter secretion, antigen presentation and cell division, and is perturbed in many disease states including numerous types of cancer and autoimmune disease. In addition, many pathogenic agents such as viruses, bacteria, or parasites hijack protein trafficking machinery to ensure survival (both to facilitate their own proliferation and also to subvert host cell response to infection. Thus understanding the basic cell biology mechanisms that regulate protein trafficking has the potential to further our understanding of many human diseases.

While each intracellular organelle maintains its own unique complement of macromolecules necessary for its particular function, there is a high level of communication (exchange of material) between these various intracellular compartments. This is facilitated through membrane fusion events, in many cases by vesicular transport where cargo molecules, both membrane-bound and soluble, are packaged into vesicles that bud from the donor compartment. These vesicles then dock and subsequently fuse with the appropriate target organelle, delivering their contents.

Non-disruptive transportation of molecules between these organelles and to the plasma membrane is extremely important, and each trafficking event is tightly regulated both spatially and temporally – i.e. it is imperative that each transport vesicle docks and fuses with the right target compartment at the right time. SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins are central to this process, facilitating fusion by formation of specific complexes between SNAREs on opposing lipid bilayers, through their highly conserved α-helical cytosolic SNARE motifs.

As with many processes in eukaryotic cell biology, the molecular mechanisms that regulate membrane traffic are conserved through evolution from yeast to humans. We therefore use the genetically tractable model eukaryote Saccharomyces cerevisiae (Baker’s yeast) to study human diseases such as neutropenia (which we have mapped to a defect in the SNARE regulator Vps45).

We are particularly interested in how the hormone insulin regulates membrane traffic in fat and muscle cells. This is an important goal, as defects in this system underlie Type-2 Diabetes, a debilitating disease whose incidence is increasing worldwide at an exponential rate. One of the major actions of insulin is to bring about changes in the membrane trafficking of a glucose transporter called GLUT4 that deliver it to the cell surface. We have discovered that this is achieved by altering the way in which SNARE complexes are formed. As this system is dysfunctional in individuals suffering from Type-2 Diabetes, this discovery has diagnostic and therapeutic potential.

Another exciting project in our lab involves using our knowledge of membrane traffic to enhance the secretory capacity of cells that are used to produce monoclonal antibodies for therapeutic purposes. This has the potential to greatly reduce the cost of these medicines and thus contribute to healthcare worldwide.

This figure shows 3T3-L1 adipocytes expressing a version of GLUT4 harbouring an HA-epitope tag in its first exofacial loop treated with 100nM insulin (right-hand panel) or not (left-hand panel) for 30 minutes prior to labelling for surface (blue) and total (green) GLUT4.