Intra- and Inter-cellular Signals Regulate Human Islet Cell Function in Health and Diabetes

Abstract

Pancreatic islet dysfunction is central to all forms of diabetes; however, the mechanisms of pancreatic islet dysfunction in diabetes are not well understood. Hormone secretion from endocrine cells within the islet is tightly regulated by gene regulatory networks that maintain cell identity and function. Additionally, endocrine cells integrate multiple extrinsic signals from other cells within the islet to tune hormone secretion. The primary goal of this Dissertation was to advance our understanding of how adult human islet cell function is regulated and how alterations in these regulatory mechanisms contribute to islet dysfunction. To accomplish this, I took a multi-pronged approach that included a statistical analysis of hormone secretion by normal human islets, manipulation of human islet cell gene expression, and alteration of islet composition to understand how β and α cell-intrinsic and cell-extrinsic signals regulate cell function. In the first study, we found that insulin and glucagon secretion by human islets isolated from 299 organ donors without diabetes was highly heterogeneous and predicted by multiple donor traits, most notably islet cell composition, which was associated with donor sex, genetic ancestry, and genetic risk for type 2 diabetes. In our second study, we explored regulation at the intracellular level, where we investigated the role of the islet-enriched transcription factor NKX2.2, which is known to have both repressor and activator functions. We found that, distinct from findings in mice, NKX2.2 is expressed in all three major endocrine cell types in human, including δ cells; further, NKX2.2 was highly co-expressed with other important islet-enriched transcription factors throughout the human lifespan, suggesting an important role in coordinating gene regulatory networks. Surprisingly, reduced expression of NKX2.2 in human islet cells leads to increased hormone secretion by β cells but not α cells, indicating a cell type-specific function. Finally, we investigated the role of altered regulation in the α cell dysfunction seen in type 1 diabetes. We found that, in the absence of β cells, α cells have greater glucagon secretion, while repeated exposure to low glucose leads to impaired glucagon secretion. Overall, the studies presented in this Dissertation highlight the complex nature of coordinated human islet function and have implications for diabetes pathophysiology and treatment.