Determining how glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP1) synergistically regulate beta cell function

Type 2 diabetes (T2DM) is a big healthcare challenge facing society. In the UK, 4.7 million people have diabetes. Currently, the NHS spends £1 million per hour treating this disease; the majority spent on its complications (e.g. blindness, amputations, heart disease, kidney failure). Despite this budget, 40% of people with T2DM in England do not meet the target for blood sugar control needed to reduce the risk of these complications.

In the past decade, new drugs have proven successful at improving blood sugar control and reducing weight. These drugs target natural body hormones called "incretins" which help insulin to be released from the pancreas organ. Insulin is a hormone that lowers blood sugar. Incretins include glucagon-like peptide 1 (GLP1) and glucose-dependent insulinotropic peptide (GIP). The advantage of incretin-based drugs is that they only release insulin when blood sugar levels are high, therefore reducing the risk of blood sugar levels dipping too low which can cause dangerous effects e.g. passing out.

One of the main types of incretin-based diabetes treatment activates the GLP1 receptor (GLP1R) found in many body cells. This is relevant to diabetes and obesity as activation of the GLP1R leads to insulin release and reduced appetite by slowing stomach emptying and deactivating hunger messages from the brain. In contrast, drugs that work on the GIP receptor (GIPR) are ineffective in treating diabetes or obesity, perhaps because GIPR are found in fewer cells or because of poor communication, or signalling, within these cells. There may also be differences in how the GIPR are recycled in the cell.

In the last few years, however, researchers have found that a single drug acting on both the GIPR and GLP1R, "twincretin", lead to an even better response upon blood sugar control and weight loss that either drug alone i.e. "the whole is greater than the sum of the parts". Due to these findings, "twincretin" drugs are currently in clinical trials.

However, the reason behind GIP-based therapy working so well with GLP1-based therapy remains unclear and understanding this is the goal of my project. By uncovering the reasons behind this, we can improve how these new drugs are delivered (e.g. will some people benefit more or less from them?) and also open the door to developing other methods of manipulating the incretins to improve outcomes for people with diabetes.

The aim of this study is to unveil these mechanisms by investigating the effect of GIP given with GLP1 upon the function and activity of beta cells in the human pancreas. Beta cells release insulin and are found grouped together with alpha and delta cells to form "islets" in the pancreas. In T2DM, these islets become dysfunctional and fail to produce enough insulin to control blood glucose levels. Working with international research collaborators, I will obtain donor human islets from people with and without T2DM and treat them with either glucose, one incretin, a twincretin or both incretins as separate hormones (to see if combining the two hormones into a single twincretin drug is what makes a difference). Experiments will determine differences in a) the amount of insulin subsequently released by the islets, b) the number of GIPR and GLP1R expressed on islet cells and c) how the GIPR and GLP1R are recycled after they are stimulated. These 3 sets of experiments will allow me to identify the key mechanisms by which GIP and GLP1 work harmoniously.

As described above, results from these experiments will be relevant to the delivery of the twincretin medications currently in clinical trials. However, the findings from these experiments will also be relevant to other researchers investigating treatments for T2DM and those looking at how the incretins work in other parts of the body (e.g. the brain, liver).

Background: Developing novel therapeutics to treat diabetes is crucial in the setting of epidemic prevalence and devastating, costly complications. The incretin axis has proven to be a key therapeutic target due to remarkable effects upon weight and glycaemic control. GLP1 receptor agonists (GLP1RA) are in widespread clinical use but GIP receptor agonism (GIPRA) has only recently gained traction as a therapy, and only in conjunction with GLP1RA as a dual agonist. The mechanisms behind the profound synergism of GIPRA with GLP1RA therapy is unclear but critical to further manipulation of the incretin pathway to help patients with Type 2 diabetes or obesity.

Objective: Utilise advanced imaging and chemical biology to confirm mechanisms by which GIP and GLP1 synergistically influence beta cell signalling (Ca2+, ATP/ADP and cAMP) and function (insulin secretion), receptor expression levels and trafficking.

Aim 1: Determine the impact of GIPR + GLP1R activation upon human beta cell stimulus-secretion coupling through a) measurement of insulin release using a perifusion system combined with highly sensitive HTRF assays; b) multicellular Ca2+ imaging and c) imaging of ATP and cAMP signalling.

Aim 2: Assess the effects of GIP + GLP1 co-administration on GIPR and GLP1R expression/localisation and beta cell differentiation through QPCR and immuno-staining cryostat sections of fixed-embedded human islets. This will also require the development of a fluorescent GIPR chemical probe, since specific antibodies do not exist and I will contribute to probe development and validation.

Aim 3: Compare GLPR and GIPR trafficking in response to activation by their cognate agonists and GIP + GLP1 through confocal and super-resolution imaging to understand receptor dynamics at the cell surface. Further staining with early/late endosomal or lysosomal markers will allow me to understand the propensity for recycling and degradation.