No abstract

Although important progress has been made recently in the elucidation of the molecular mechanisms that regulate differentiation and morphogenesis of endoderm-derived tissues such as pancreas and liver, less is known about the preliminary steps of early regional specification. Recent evidence supports the proposal that the early endoderm contains a bipotential precursor cell type for pancreas and liver. We have also previously shown that the activity of the homeobox gene Prox1 controls hepatocyte migration during liver morphogenesis. Using detailed comparative analysis of whole embryos and reverse transcriptase polymerase chain reaction of dissected embryonic endoderm, we have now determined that in the early endoderm, Prox1 expression is restricted to regions giving rise to the mammalian pancreas and liver. This finding indicates that Prox1 is one of the earliest specific markers of this commonly fated region of the mammalian endoderm.

The development of the mammalian pancreas is governed by various signaling processes and by a cascade of gene activation events controlled by different transcription factors. Here we show that the divergent homeodomain transcription factor Prox1 is a novel, crucial regulator of mouse pancreas organogenesis. Loss of Prox1 function severely disrupted epithelial pancreas morphology and hindered pancreatic growth without affecting significantly the genesis of endocrine cells before E11.5. Conversely, the lack of Prox1 activity substantially decreased the formation of islet cell precursors after E13.5, during a period known as the "secondary transition". Notably, this defect occurred concurrently with an abnormal increment of exocrine cells. Hence, it is possible that Prox1 contributes to the allocation of an adequate supply of islet cells throughout pancreas ontogeny by preventing exocrine cell differentiation of multipotent pancreatic progenitors. Prox1 thus appears to be an essential component of a genetic program destined to produce the cellular complexity of the mammalian pancreas.

Contributions of null and hypomorphic alleles of Apc in mice produce both developmental and pathophysiological phenotypes. To ascribe the resulting genotype-to-phenotype relationship unambiguously to the Wnt/β-catenin pathway, we challenged the allele combinations by genetically restricting intracellular β-catenin expression in the corresponding compound mutant mice. Subsequent evaluation of the extent of resulting Tcf4-reporter activity in mouse embryo fibroblasts enabled genetic measurement of Wnt/β-catenin signaling in the form of an allelic series of mouse mutants. Different permissive Wnt signaling thresholds appear to be required for the embryonic development of head structures, adult intestinal polyposis, hepatocellular carcinomas, liver zonation, and the development of natural killer cells. Furthermore, we identify a homozygous Apc allele combination with Wnt/β-catenin signaling capacity similar to that in the germline of the Apcmin mice, where somatic Apc loss-of-heterozygosity triggers intestinal polyposis, to distinguish whether co-morbidities in Apcmin mice arise independently of intestinal tumorigenesis. Together, the present genotype–phenotype analysis suggests tissue-specific response levels for the Wnt/β-catenin pathway that regulate both physiological and pathophysiological conditions.

Dysregulated Wnt signaling is seen in approximately 30% of hepatocellular carcinomas; thus, finding pathways downstream of the activation of Wnt signaling is key. Here, using cre-lox technology, we deleted the Apc gene in the adult mouse liver and observed a rapid increase in nuclear β-catenin and c-Myc, which is associated with an induction of proliferation that led to hepatomegaly within 4 days of gene deletion. To investigate the downstream pathways responsible for these phenotypes, we analyzed the impact of inactivating APC in the context of deficiency of the potentially key effectors β-catenin and c-Myc. β-catenin loss rescues both the proliferation and hepatomegaly phenotypes after APC loss. However, c-Myc deletion, which rescues the phenotypes of APC loss in the intestine, had no effect on the phenotypes of APC loss in the liver. The consequences of the deregulation of the Wnt pathway within the liver are therefore strikingly different from those observed within the intestine, with the vast majority of Wnt targets being β-catenin-dependent but c-Myc-independent in the liver.