UBC Theses and Dissertations
Investigating the role of transcriptional coactivator MED15 in beta cell maturation Kadhim, Alexandre Zacharie
The Mediator complex, a coregulator required for RNA pol II activity, interacts with specific transcription factors through its distinct subunits. These interactions promote the expression of defined gene sets both during development and for tissue homeostasis. Transcriptional regulatory networks are critical for the development and function of pancreatic β-cells. To date, few studies examining how transcription factors interact with co-activators, such a Mediator, have been performed in the pancreas. Tail module subunit MED15 is highly expressed in nascent β-cells and required for lipid metabolism, various stress responses, and TGF-β signalling, all of which are important for β-cell function. As such, we hypothesized that MED15 plays a role in β-cells. We found MED15 to be expressed during mouse pancreatogenesis and in mature β-cells. Expression of MED15 was impaired in human T2D islets suggesting it is important for mature β-cell function. After generating a β-cell specific knockout mouse (IM15KO), we observed defects in maturation as assessed by loss of β-cell maturation markers UCN3, MAFA, and GLUT2. In agreement with reduced GLUT2 expression, IM15KO cells had impaired glucose uptake and reduced glucose-stimulated insulin secretion, the hallmark process of β-cell maturation. ChIP-seq analysis determined that MED15 is bound to key GSIS related genes and Co-IP found transcription factors NEUROD1 and NKX6-1 to bind MED15. As the pancreas contains among the highest levels of Zn²⁺ in the body, we also found a role for MED15 in heavy metal stress response. Through this thesis, I provide evidence of the importance of Mediator in β-cell maturation and demonstrate an additional layer of control that modulates transcription factor function. A greater understanding of how Mediator and MED15 regulate β-cell maturation could help refine the generation of cell-based therapies for diabetes.
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