Interrogating the Insulin Granule, Investigating the Composition and Function of Insulin Secretory Granules

Abstract

Glucose homeostasis relies on the pancreatic beta-cells to produce and secrete insulin, a peptide hormone that facilitates glucose uptake into insulin-sensitive tissues. Insulin is stored within specialised vesicles termed insulin secretory granules, which play a critical role in controlling insulin turnover in beta-cells through exocytosis and degradation. Type 2 diabetes is characterised by chronic hyperglycaemia and insulin resistance which negatively impact insulin production and secretion from pancreatic beta-cells. Recent research has shifted towards understanding how insulin granules are affected during the progression of type 2 diabetes and exploring the heterogeneity of insulin granule populations within beta-cells. Current diabetes therapies primarily target the distal stages of exocytosis to enhance insulin secretion, overlooking the distinct functional properties of discrete granule populations within. This thesis focuses on the insulin secretory granules as a key regulator of beta-cell function, aiming to elucidate its composition and the impact of metabolic stress on granule properties. Furthermore, it investigates the role of insulin granule aging and its response to pharmacological stimuli. A key outcome of this work is identifying the murine (MIN6) insulin granule proteome and validating novel granule proteins that regulate beta-cell function. This thesis also shows that metabolic stress, induced by elevated fatty acids, disrupts the docking machinery of insulin granules. Using a fluorescent protein timer and anti-diabetic drugs, we demonstrate that the preferential release of younger insulin granules is glucose-dependent. These studies offer new insights into insulin granule biology, emphasizing the adaptive role of beta-cells in response to metabolic stress and highlighting the need to study the proximal stages of granule regulation for future diabetes therapies.