PDX-1 is Required for Posterior Poregut Patterning and Differentiation of the Pancreas and Duodenum

The Xenopus homeobox gene, XlHbox8, has been proposed to be involved in endodermal differentiation, specifically in pancreatic and duodenal development (Wright et al., 1988. Development 105, 787-794). To test this hypothesis directly, the mouse homolog, pdx-1, was cloned and gene targeting was used to produce two separate null alleles. In one, the second pdx-1 exon, including homeobox sequences, was replaced by a neomycin resistance cassette. In the second, a lacZ reporter was fused in-frame with the N-terminus of PDX-1, replacing most of the homeodomain. Neonatal $pdx{-}1\ {-}/{-}$ mice for both mutations are apancreatic, in confirmation of the report by Jonsson et al. (Jonsson. J., Carlsson, L., Edlund, T. and Edlund, H. 1994. Nature 371, 606-609.). However, the data presented in this dissertation show that the pancreatic buds form in homozygous mutants, with the dorsal bud undergoing limited proliferation and outgrowth to form a small, irregularly branched, ductular tree. No insulin or amylase-positive cells are found in these outgrowths, but glucagon-expressing GLUT2-positive cells are found. The rostral duodenum suffers a local absence of the normal columnar epithelial lining, villi, and Brunner\u27s glands, which are replaced by a GLUT2-positive cuboidal epithelium resembling the bile duct lining. The abundance of enteroendocrine cells in the rostral duodenal villi is greatly reduced in pdx-$1\ {-}/{-}$ embryos. The PDX-1/$\beta$-galactosidase fusion allele is expressed in the pancreatic and duodenal cells in the absence of functional PDX-1, and the majority of these cells express PDX-1/$\beta$-galactosidase fusion protein into perinatal stages without changes in the boundaries or levels of expression. These results are discussed in terms of a role for pdx-1 in posterior foregut patterning, specifically in the differentiation of the pancreas and rostral duodenum