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Hedgehog signaling in the pancreas epithelium


Current studies of Hedgehog (Hh) signaling in the pancreas have demonstrated that Hh signaling regulates expansion of pancreas area and must be excluded from the early developing pancreas during embryogenesis. However, contraindicating evidence suggests that there may be some possible functional activity and requirement in the developing pancreas and postnatal islet. Thus the role of Hh signaling in the pancreas remains unclear.

In this dissertation, we investigate the role of Hh signaling specifically within the pancreatic epithelium during development and in the postnatal islet. First, we characterized Patched1–lacZ mice to identify Hh active cells in the pancreatic epithelium. Characterization of Patched1–lacZmice showed that Hh active cells reside in the developing pancreatic epithelium as early as embryonic day 10.5, predominate in ductal and endocrine regions, and postnatally have increased Hh activity.

Secondly, we analyzed mice that have downregulated Hh function in the pancreatic epithelium. To accomplish this, we generated mice that carried a floxed-over-null allele of Smoothened (Smo), a key upstream activator of Hh signaling, in Pdx1–Creearly mice. Pdx1–Creearly;Smolox/null mice have downregulated Smoothened expression and Hh activity, and show a temporary early loss of pancreatic epithelial and insulin area. Furthermore, adult Pdx1–Creearly;Smolox/null mice are deficient in insulin production resulting in impaired insulin secretion and glucose intolerance.

Finally, we analyzed activated Hh signaling in the pancreatic epithelium. To accomplish this, we generated mice that conditionally overexpress a transgene, CLEG2, that is a constitutively active form of Gli2 in Pdx1–Creearly mice. Surprisingly, Pdx1–Creearly;CLEG2 mice showed only low level Hh activation. Recent studies have shown that primary cilia are modulators of Hh signaling. Thus, we generated Pdx1–Creearly;CLEG2 mice in combination with a conditional model of cilia ablation (Kif3alox/lox) to achieve high levels of Hh activation in the pancreas epithelium. Pdx1–Creearly;CLEG2; Kif3alox/lox mice show strong Hh activation, development and expansion of an unusual population of cells that express foregut progenitor markers, and loss of endocrine epithelium. Thus, this body of work furthers the understanding of Hh signaling in the pancreas epithelium by demonstrating its role in regulating expansion of early pancreas epithelium and modulating postnatal epithelial activity.

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