The endocrine cells of the pancreatic islets develop from the pancreatic bud by commitment of pluripotent endodermal cells to endocrine stem cells (nesidioblasts) which proliferate before terminally differentiating into mature islet cells. In addition to insulin and glucagon, developing islet cells also transiently express genes not active in adult islets which permit growth of fetal nesidioblasts. One of these transiently expressed genes, gastrin, is an autocrine growth factor for some islet cell lines. Activity of the gastrin promoter in islet cells depends on an islet specific, cis-acting DNA element similar to the insulin enhancer, DNA transfection studies aim to show that the gastrin promoter is activated in islet cells by the same cell specific transcription factor (transactivator) which activates the insulin gene. This will be confirmed by analysing DNA/protein interactions in vitro using DNAase footprinting and methylation interference studies. Gastrin transgenes containing deletions of the insulin enhancer domain will be inserted into the germline of mice to see if this deletion abolishes pancreatic expression of the gastrin transgene during fetal development in vivo. Terminal differentiation of islet cells after birth results in both loss of proliferative capacity and extinction of gastrin gene expression. Islet cells express a specific transcriptional repressor which binds to a negative DNA element if the gastrin promoter adjacent to the insulin enhancer-like positive element. Cloning of the cDNA encoding the gastrin repressor will be initiated by screening an islet cDNA library using the gastrin negative element as a DNA probe. The cellular signals which regulate gastrin repressor activity will be analysed to elucidate potential mechanisms controlling gastrin gene expression in islet development. Analysing the expression of gastrin transgenes containing deletions of the negative element aim to demonstrate that this negative element mediates the postnatal extinction of pancreatic gastrin gene expression which in turn contributes the postnatal cessation of islet cell growth. These studies on the regulation of gastrin promoter aim to analyse the molecular events that regulate islet growth during pancreatic development. Understanding these events may lead to ways of improving islet cell regeneration in type I diabetes.
