Rho-GTPases in Morphogenesis and Differentiation of the Developing Pancreas

Sammanfattning: Branched tubular networks constitute the basic structure of many organs to increase surface area and to transport fluids and gases. Glandular organs such as the pancreas arise from multilayered epithelium, which undergoes de novo lumen formation and extensive remodeling and branching. Endocrine cells originate from endocrine progenitors within the ductal epithelium. Upon differentiation into hormone-producing cells such cells delaminate, aggregate and migrate away from the ductal epithelium to form the islet of Langerhans. The cellular and molecular mechanisms regulating these processes are poorly understood. Extensive biochemical and cell biological studies have brought the attention to Rho-GTPases as key regulators of various cellular processes such as cell polarisation, cell-cell adhesion and cell migration and thus serve as likely candidates for regulating various aspects of pancreatic morphogenesis. The aim of this thesis was to identify signalling pathways regulating pancreatic morphogenesis and specifically to address the role of Rac1 and Cdc42. To achieve this task we have used three mouse models: In the first model we examined the role of Rac1 in islet morphogenesis by expressing dominant negative Rac1 in ?-cells. Blocking Rac1 function lead to an increase in cell-cell contact E-cadherin and inhibited the migration of islets away from the ductal epithelium. In the second model we addressed the role of Rac1 during pancreatic morphogenesis and differentiation by ablating Rac1 specifically in the pancreatic epithelial cells. Knocking out Rac1 perturbed epithelial remodelling and branching. Furthermore, differentiation into beta cells and islet organisation was disturbed leading to perturbed glucose sensing and increase in blood glucose. In the third model the role of Cdc42 during islet formation, including cell delamination and migration was investigated by expressing dominant active Cdc42 in beta cells. Activating Cdc42 inhibited delamination of newly formed beta cells and migration away from the ductal epithelium. Cells expressing dominant active Cdc42 showed high levels of polarised F-actin and E-cadherin. Consequently, insulin was lost, resulting in decreased number of beta cells, leading to diabetes. In conclusion our results provide new insight into how Rac1 and Cdc42 control morphogenetic processes and differentiation during organogenesis of a tubular epithelial organ. Furthermore, we show the significance of controlling morphogenetic processes for proper differentiation and function of the mouse pancreas.