How does EFR3 control insulin-stimulated plasma membrane dispersal of GLUT4?

All cells are surrounded by a structure called the plasma membrane. This provides protection for a cell and helps keep the environment inside the cell stable. The plasma membrane has several functions, including regulating what gets in and out of the cell, and helping the cell sense and respond to external events.

Nutrients like sugars are transported across the plasma membrane by specialised proteins called 'transporters'. Understanding how these nutrient transporters are controlled is important, as depending on the conditions a cell may need to increase or decrease the rate of entry of a particular nutrient, perhaps in response to a specific cue.

Our group studies how sugar (glucose) transport is regulated, and in particular how this is regulated by a hormone called insulin. Insulin plays an important role in helping keep blood sugar levels constant by controlling the rate by which sugar crosses the plasma membrane of fat and muscle. Defective glucose transport is associated with diseases such as diabetes and also some neurological disorders, hence understanding how transporter proteins are regulated may have considerable impact.

Our group has uncovered a new way in which membrane transporters are controlled. We have found that insulin acts to 'disperse' glucose transporters. In the absence of insulin transporters are clustered together (rather like sheep in a pen). Insulin acts to open the gates to the pen and allow the transporters to rapidly move around in the plasma membrane so that they can function effectively and allow glucose transport.

We have been able to gain some clues to how a cell does this, and here we seek to further understand how this dispersal is controlled, to ask whether this is specific for sugar transporters or if it is displayed by other nutrient transporters (or happens to all plasma membrane proteins), and we will look to understand how hormones like insulin induce a change in clustering.

Our work will, we believe, be of wide interest to colleagues who study cell membrane function from both a basic biology and a translational standpoint. To address these questions, we have created a team of biologists, physicists, and electronics experts to bring their skills to bear on a common problem of fundamental importance.

Controlling the rate of solute entry across the plasma membrane allows cells to adapt to different environmental situations. Understanding the regulatory events that control solute transport offers potential for understanding and exploiting basic cell biology and underpinning disease mechanisms.

One example of such control is the ability of insulin to increase glucose transport across the plasma membrane of adipocytes. Insulin stimulates the delivery of a pool of intracellular vesicles containing GLUT4 glucose transporters to the plasma membrane. Recent work has revealed that insulin also increases the dispersal of GLUT4 away from the site of vesicle fusion with the plasma membrane.

We have shown that the plasma membrane localised protein EFR3A and phosphatidylinositol 4-kinase type IIIalpha are required for insulin-stimulated glucose transport and that knockdown of EFR3A inhibits insulin-stimulated GLUT4 dispersal in the plasma membrane. Our working model proposes that EFR3A/PI4K-IIIalpha controls the dynamics of GLUT4 movement within the plasma membrane in an insulin and PI4K-dependent manner and that this controls GLUT4 activity by regulating release of GLUT4 from clusters.

Here we aim to understand how protein EFR3A and phosphatidylinositol 4-kinase type IIIalpha impacts the cluster behaviour of GLUT4 and the ability of insulin to regulate glucose transport/GLUT4 clustering. We shall compare this with the behaviour of other plasma membrane proteins, including those delivered selectively to the cell surface in response to a signal, nutrient transporters in general and to other recycling membrane proteins. This information is important as it will define how widespread the mechanism of regulation is and identify other systems that are similarly regulated. We will also determine how this regulatory mechanism is controlled in space and time.