Award details

The role of SUR1 in synaptic and secretory vesicle function

Reference	BB/R017220/1
Principal Investigator / Supervisor	Professor Dame Frances Ashcroft
Co-Investigators / Co-Supervisors	Dr Maike Glitsch, Dr Gregor Sachse
Institution	University of Oxford
Department	Physiology Anatomy and Genetics
Funding type	Research
Value (£)	480,917
Status	Completed
Type	Research Grant
Start date	01/09/2018
End date	30/06/2022
Duration	46 months

Abstract

The membrane potential (Vm) serves to integrate signals and ion transport across the cell surface. However, its role in intracellular membranes, and the interplay of ion channels/transporters that generate it, is less well understood. This is partly because it is difficult to measure. We have adapted pHoenix, a novel archaerhodopsin/GFP-based optogenetic tool, to measure Vm in synaptic and secretory vesicles directly. Simultaneously, we can optically control and measure vesicle acidification, which is required for cargo loading and modification, enabling us to study its interaction with vesicle Vm. We will use this approach to define the role of the sulfonylurea receptor SUR1 in synaptic and secretory vesicles. SUR1 is a subunit of the plasmalemmal KATP channel, which couples the metabolism of the cell to its excitability. It is abundant in insulin granules and modulates their release via a cAMP/Epac2-mediated pathway. cAMP/Epac2 regulation of exocytosis is equally important in brain, but whether neuronal SUR1 is involved is unknown and a mechanistic understanding is lacking. We recently found SUR1 localizes to synaptosomes and interacts with synaptic vesicle proteins, suggesting it may have a novel non-canonical function in neurones. We will address the role of SUR1 in vesicle function by (i) characterising the brain and beta-cell SUR1 interactomes with high-resolution proteomics; (ii) determining the SUR1 influence on vesicle Vm and acidification using pHoenix in primary cells; (iii) clarifying the role of SUR1 in cAMP-regulated vesicle release. In a complementary approach, we will study both neuronal and endocrine cells. Direct measurement of vesicle Vm and pH will not only help us understand the non-canonical functions of SUR1 in vesicles. It will also provide mechanistic insight into the regulation of neurotransmitter/hormone loading and release, fundamental processes that underlie cognition, memory and homeostatic control.

Summary

We're all familiar with the fact that machines are powered by electricity, but it's perhaps not so widely appreciated that the same is true for ourselves. Your ability to read and understand this page, to see and hear, to think and speak, and to move your arms and legs is due to the electrical events taking place in the nerve cells in your brain and the muscle cells in your limbs. And, in turn, that electrical activity is initiated and regulated by tiny protein pores embedded in the membranes of each one of your cells, known as ion channels. Nerve cells are used to transmit electrical signals round the body. Within our brains, billions of nerve cells also engage in a constant electrical conversation, directing all our thoughts and actions. But nerve cells are not physically connected to one another and the electrical impulse cannot jump the gap between them. Instead a chemical messenger, known as a neurotransmitter, is used to send signals from one cells to another. Transmission takes place at specialised junctions called synapses, where the two nerve cells come close together and the gap between them is very tiny. At the synapse, the tip of the nerve cell is densely packed with small membrane-bound vesicles filled with neurotransmitter, and when an electrical impulse arrives at the nerve terminal it causes the vesicles to release their contents into the gap between the two cells. The neurotransmitter then diffuses across the gap and stimulates an electrical impulse in the next cell. A similar process takes place in gland cells, which release hormones into the blood stream. This process also involves the packaging of the hormone into tiny vesicles which then fuse with the surface membrane of the cell and empty their contents into the bloodstream when the gland cell is stimulated. The hormones then travel around the body in the blood to their target organs. The way in which nerves work, how they talk to one another at synapses, and the role of ion channels in this process is explained in The Spark of Life, a book for the general reader written by one of the applicants of this grant (Frances Ashcroft). This project is focused on the precise way in which the release of vesicles from nerve endings and gland cells is controlled. We are particularly interested in an ion channel known as KATP channel. It plays a very important role in the regulation of blood glucose levels because it controls the release of the hormone insulin from the beta-cells of the pancreas. It also is important in the nerve cells of the brain, and people with mutations in KATP channel genes not only get diabetes but may also have delayed development. Our preliminary data suggests that one of the proteins that makes up the KATP channel has a novel role in regulating vesicle function and release in both nerve and gland cells. Although we have known for some years that this protein (called SUR1) is present in the insulin secretory vesicles, we still don't fully understand what it does there. Recently, the mystery has deepened, as we discovered SUR1 is also present in vesicles at nerve endings in the brain. The aim of the grant is therefore to understand the role of SUR1 in both the synaptic and secretory vesicles. This question is of considerable scientific importance as it addresses a fundamental topic - how do cells communicate with one another? It is also of importance to the pharmaceutical industry because many clinically important drugs influence synaptic function and hormonal release. Elucidating the molecular pathways in which SUR1 is involved may lead to new targets for drug development. Finally, the methods that we propose to use are novel and will lead to the development of a new tool for scientists studying vesicle function. In addition, we are collaborating with a UK company to generate new applications for their microscopes.

Impact Summary

The aim of this project is to understand the role of SUR1 in intracellular vesicles at the synapse and in neuroendocrine cells, such as the pancreatic beta-cell. SUR1 regulates plasma membrane KATP channel activity, but is also found in insulin secretory granules, where it influences exocytosis. Its presence in synaptosomes suggests it may play a similar role at the synapse. In support of this idea, activating mutations in SUR1 cause a range of neurological complications (including developmental delay, muscle weakness of neuronal origin, ADHD, and behavioural problems). A detailed understanding of the biophysical and biochemical mechanisms underlying the novel non-canonical role of SUR1 is needed to underpin therapeutic strategies targeted at the devastating neurological problems of these patients. Such fundamental knowledge is currently lacking but has the potential to transform therapy. We have a track record of successfully translating the fruits of our basic science studies into the clinic. Over 90% of patients with neonatal diabetes caused by activating KATP channel mutations have now transferred from insulin to oral sulphonylurea (SU) drugs (which close their open KATP channels). This has resulted in dramatic improvements in their clinical condition and quality of life. It also costs less. These patients consider themselves 'cured'. However, the situation is very different for patients with neurological problems in addition to their diabetes, as SU drugs often have very limited effects of the neurological symptoms. A new approach to therapy is urgently required and would be life-transforming. The long-term socio-economic benefits of our study will also include improvements in the design and development of drugs targeted at vesicular release from neuroendocrine cells, such the pancreatic beta-cell. This is expected to have a major impact upon the rational design of drugs. By impacting the early stages of drug design and development it has the potential to reduce both the time and the overall costs involved in drug development. Our results will also shed light on the fundamental question of synaptic transmission and its regulation by cAMP, which is involved in synaptic plasticity, learning and memory. This will be of considerable interest to a wide scientific community. Finally, as the general public has a tremendous curiosity about science, we intend to host an outreach programme of public engagement. This will engage with a wide section of the community and is expected to have a major impact on the public perception of science and public trust in UK-based science. Our previous experience is that it will also have the added benefit of stimulating interest in STEM subjects within the next generation of potential leaders in both science and health-related sectors. This has especially been the case for FMA's popular science books.

Committee	Research Committee D (Molecules, cells and industrial biotechnology)
Research Topics	Neuroscience and Behaviour
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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