IDENTIFYING MECHANISMS OF INSULIN PRODUCTION AND SECRETION IN SMALL AND LARGE RAT ISLETS

Abstract

The existence of islet subpopulations according to size difference has been described since 1869 when Dr. Paul Langerhans first discovered the islets in the pancreas. Unfortunately, little is known about the functional differences between islet subpopulations until recently. Small islets have been shown to secret more insulin than large islets per volume (islet equivalent; IE) and led to better transplantation outcome both in rodents and in humans. Insulin is produced and released from the beta cells in islets through a cascading pathway from insulin gene transcription to proinsulin biosynthesis to insulin secretion. The central hypothesis of this dissertation is that small and large islets have different characteristics in insulin production and secretion that lead to different transplantation outcomes. More than ten thousands small (diameter less or equal 100µm) and large (diameter above or equal 200µm) islets from healthy rats were investigated. First, the same percentage of beta cells was identified in small and large islets, but small islets had higher density both in vitro and in situ. Next, a new regression model was established to better estimate the islet volume by cell number based on size (diameter), since an overestimation was seen when using conventional IE measurement to normalize islet volume. By applying this new normalization method, a superior glucose-stimulated proinsulin biosynthesis was identified in large islets. However, when normalized to cell number, insulin secretion was not different between small and large islets, unlike the results in literature when normalized to IE. While small and large islets showed no difference in total protein content per cell, large islets showed higher protein levels of prosinulin, NeuroD/Beta2 and MafA with a lower PDX-1 level under basal conditions suggesting that the different characteristics between small and large islets in the insulin production pathway may not correspond to measured insulin secretion. All the findings will not only elucidate new intricacies concerning islet biology research, but also will have significant implications to current islet transplantation research to optimize the success for curing type 1 diabetes.