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Dames, Petra (2012): Neue molekulare Regulatoren für die Differenzierung und den Funktionserhalt des endokrinen Pankreas. Dissertation, LMU München: Faculty of Biology
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Abstract

Abstract Considering the globally increasing rate of incidence, Type 2 diabetes mellitus belongs to the most frequent endocrine metabolic disorders today. In addition to insulin resistance of peripheral tissues, this disease is the result of dysfunction of the endocrine pancreas and in particular of the functional failure of β-cells. The progress of therapeutical strategies is based on the research of the underlying mechanisms. The aim of the present dissertation was the analysis of new molecular regulators which might improve our underdstanding of the differentiation of pancreatic islets and the functional maintenance of adult β-cells. The first part of this work concerned the role of the transcription factor Pax6 and especially the role of its transactivation domain (TA) and of its two DNA binding domains, the paired domain (PD) and the homeodomain (HD), in differentiation of pancreatic endocrine cells. By analyzing four different mouse lines with specific mutations in one of these three domains, we found that the PD of Pax6 is essential for differentiation of glucagon producing α-cells. Inactivation of this domain resulted in a phenotype similar to that of Pax6 knockout mice (Pax6-/-) with a near complete absence of glucagon positive α-cells, a markedly reduced number of insulin producing β-cells, and a disorganized islet structure. Mutations of HD or TA showed a less severe pancreatic phenotype. Islets either exhibited no morphological changes or they showed a reduction of α- and β-cells. Intraperitoneal glucose tolerance tests demonstrated the utmost importance of the transcription factor Pax6 for maintenance of normal pancreatic endocrine function in adult animals. In the second part of this study we identified new genes and proteins, respectively, which could play a regulatory role in normal function of β-cells. In particular it was possible to show that Eny2, hitherto a protein only described in yeast or invertebrates like drosophila, is involved in the regulation of insulin secreting vertebrate cells. si-RNA mediated knockdown of Eny2 resulted in markedly increased glucose and incretin-induced insulin secretion. This could be at least in part attributed to a higher glucose-dependent cellular metabolism and an enhanced signal transduction via protein kinase A and is accompanied by elevated levels of intracellular calcium. Taken together, these results indicate that Eny2 functions as a negative regulator of glucose-stimulated and incretin-mediated insulin secretion, at least in vitro. However, a gap of knowledge still remains between the established nuclear functions of Eny2 and the cellular phenotype we observed upon its suppression. Nevertheless, the effects of an Eny2-knockdown are glucose dependent and additive to the incretin signaling. This feature makes this model attractive to obtain new insights in how insulin secretion of β- cells proceeds and how to find new therapeutical strategies to treat type 2 diabetes mellitus.