Tissue-specific regulation and function of Pancreatic and duodenal homeobox 1
Boyer, Daniel Frederick
The transcription factor Pancreatic and duodenal homeobox 1 (Pdx1) is essential for formation of the pancreas and for glucose-responsive insulin secretion from mature pancreatic beta cells. In humans and mice, heterozygosity for inactivating mutations in the Pdx1 gene confers a high risk of early-onset type II diabetes. Additional genes that have been linked to heritable type II diabetes encode transactivating factors that bind to the upstream promoter region of Pdx1, suggesting that Pdx1 may play a central role in the maintenance of mature beta cell function. These findings prompted close scrutiny of cis-regulatory elements in the Pdx1 promoter, leading to the identification of proximal and distal regions of phylogenetically conserved sequences with enhancer-like activity. In the first part of my dissertation, I present experimental data employing a transgene-based genetic complementation strategy to demonstrate that the distal conserved region of the Pdx1 promoter is required for high-level expression in postnatal duodenal and stomach mucosal epithelia of mice. Reduced expression of Pdx1 in these tissues leads to decreased numbers of specific enteroendocrine cell types. The second part of the dissertation presents evidence that reduced expression of Pdx1 in pancreatic endocrine progenitor cells during embryogenesis results in decreased production of pancreatic beta cells and increased numbers of glucagon- and pancreatic-polypeptide-expressing cells. The third experimental chapter of the dissertation provides evidence that heterozygosity for Pancreatic transcription factor 1a (Ptf1a), which encodes a critical regulator of pancreatic gene expression, alters the timing of endocrine and exocrine pancreatic differentiation during early stages of pancreatic organogenesis in mice. Collectively, these experiments demonstrate that the discrete developmental transitions and specific cell fates that arise during pancreatic organogenesis require different levels of key transcription factors. This information is useful for understanding the regulation of differentiation of diverse daughter cells from common progenitor cell populations in the embryonic pancreas. Future experiments will apply this knowledge to improve techniques for promoting differentiation of cultured stem cells to produce mature pancreatic beta cells.