Targeting GLP-1 receptor trafficking to improve therapies for type 2 diabetes

Type 2 diabetes (T2D) kills around 4 million people worldwide every year and causes ill health in many more. A major underlying problem in T2D is that specialised cells in the pancreas known as beta cells start to fail and do not produce or secrete enough insulin, which is needed to control blood sugar. Beta cells can be stimulated to release more insulin by activating a surface protein known as glucagon-like peptide-1 receptor (GLP-1R). GLP-1R treatments, known as GLP-1R agonists, are already used in T2D. We have found that a process known as "receptor trafficking" limits the effectiveness of GLP-1R agonists. When the agonist activates the receptor, it also "internalises" it into the cell where it is hidden and can no longer be activated by agonists outside the cell. We showed this using a modified version of a GLP-1R agonist currently used in T2D (exendin-4, also known as "Exenatide"), which we engineered to reduce the internalisation process. This new agonist called "Ex-phe1" was much more effective than unmodified exendin-4 in mice with T2D, as it allowed the receptor to remain longer at the cell surface and be continually reactivated, leading to more insulin release. Also, despite being more effective in beta cells, Ex-phe1 did not cause more nausea, a well-known side effect of GLP-1R agonists. This project has two goals. The first is to understand how changes in GLP-1R trafficking modify insulin release from beta cells. The second, to be performed in parallel, is to optimise the compounds we generated in our preliminary work to produce new, better, treatments for T2D. Our approach includes experiments in beta cells and pancreatic tissue ("islets"), which allow us to understand events in molecular detail, and in mice with T2D, which tell us what happens at the whole body level. We place a lot of emphasis on using human islets (provided for research when surplus to requirements for organ donation), as we believe this is essential to understand and treat human disease. We highlight below some key experiments: 1) We will use a combination of computer modelling and genetic modification of the GLP-1R to identify specific interactions between the receptor and Ex-phe1 or other biased GLP-1R agonists which underpin their different trafficking and therapeutic effects. This knowledge will be critical for the design of even more effective drugs in the future. 2) We will genetically modify human islets to reduce levels of "beta-arrestin-2", a protein in beta cells that we believe is a key link between GLP-1R activation, trafficking, and beneficial insulin release. We will also breed genetically modified mice lacking beta-arrestin-2 in beta cells. We will use these systems to look at complex patterns of protein and gene activation within beta cells in response to GLP-1R agonist treatment. From this we hope to identify molecules that could in the future be targeted as new T2D treatments. 3) We will study the long-term effects of our new, improved GLP-1R agonist Ex-phe1 on human islets transplanted into diabetic mice. Importantly, we will use a new approach in which the islets are actually transplanted in the eye, where changes to islet shape, behaviour and overall health can be directly and repeatedly viewed in a microscope. The mice will be treated with a long-lasting formulation of Ex-phe1 or exendin-4 for several months, and we will monitor changes to blood sugar, allowing us to compare the treatments for their effects on overall islet function. Due to well-known differences between mouse and human islets, we believe that using this "humanised" mice is critical to understanding long-term treatment effects. In summary, we have identified an exciting new approach based on GLP-1R trafficking for improving the treatment of T2D. In this project we will establish the molecular details of why this approach works, identify new potential drug targets and demonstrate the long-term effects of this new treatment in unique humanised mice.