Conditional ablation of Notch signaling in pancreatic development

The role of the Notch signaling members Notch1, Notch2 and Rbpj in exocrine pancreatic development is not well defined. We therefore analyzed conditional pancreas-specific Rbpj and combined Notch1/Notch2 knockout mice using Ptf1a(+/Cre(ex1)) mice crossed with floxed Rbpj or Notch1/Notch2 mice. Mice were analyzed at different embryonic stages for pancreatic exocrine and endocrine development. The absence of Rbpj in pancreatic progenitor cells impaired exocrine pancreas development up to embryonic day 18.5 and led to premature differentiation of pancreatic progenitors into endocrine cells. In Rbpj-deficient pancreata, amylase-expressing acini and islets formed during late embryonic and postnatal development, suggesting an essential role of Rbpj in early but not late development. Contrary to this severe phenotype, the concomitant inactivation of Notch1 and Notch2 only moderately disturbed the proliferation of pancreatic epithelial cells during early embryonic development, and did not inhibit pancreatic development. Our results show that, in contrast to Rbpj, Notch1 and Notch2 are not essential for pancreatogenesis. These data favor a Notch-independent role of Rbpj in the development of the exocrine pancreas. Furthermore, our findings suggest that in late stages of pancreatic development exocrine cell differentiation and maintenance are independent of Rbpj

Liver-specific inactivation of Notch2, but not Notch1, compromises intrahepatic bile duct development in mice

Freddy Radtke

The Notch pathway is an evolutionary conserved, intercellular signaling pathway that plays an important role in cell fate specification and the embryonic development of many organs, including the liver. In humans, mutations in the Notch receptor ligand Jagged1 gene result in defective intrahepatic bile duct (IHBD) development in Alagille syndrome. Developmental abnormalities of IHBD in mice doubly heterozygous for Jagged1 and Notch2 mutations propose that interactions of Jagged1 and its receptor Notch2 are crucial for normal IHBD development. Because different cell types in the liver are involved in IHBD development and morphogenesis, the cell-specific role of Notch signaling is not entirely understood. We investigated the effect of combined or single targeted disruption of Notch1 and Notch2 specifically in hepatoblasts and hepatoblast-derived lineage cells on liver development using AlbCre transgenic mice. Hepatocyte differentiation and homeostasis were not impaired in mice after combined deletion of Notch1 and Notch2 (N1N2(F/F)AlbCre). However, we detected irregular ductal plate structures in N1N2(F/F)AlbCre newborns, and further postnatal development of IHBD was severely impaired characterized by disorganized ductular structures accompanied by portal inflammation, portal fibrosis, and foci of hepatocyte feathery degeneration in adulthood. Further characterization of mutant mice with single deletion of Notch1 (N1(F/F)AlbCre) or Notch2 (N2(F/F)AlbCre) showed that Notch2 but not Notch1 is indispensable for normal perinatal and postnatal IHBD development. Further reduction of Notch2 gene dosage in Notch2 conditional/mutant (N2(F/LacZ)AlbCre) animals further enhanced IHBD abnormalities and concomitant liver pathology. CONCLUSION: Notch2 is required for proper IHBD development and morphogenesis

2008

The Cell Differentiation Status Influences the Outcome of Notch-Induced Malignancy

Caroline Poisson

The Notch signaling cascade is a well conserved pathway that regulates many aspects of embryonic development and plays a crucial role in the differentiation process and tissue homeostasis in multiple organs of adult species. Several human pathological disorders, including cancer, arise as a consequence of the deregulation of the Notch signaling pathway. Notch deregulation, up to date, has been best characterized in acute T-cell lymphoblastic leukemia (T-ALL). Indeed, in T-ALL pediatric patients activating Notch1 mutations have been identified in more than 50% of the cases. Increased knowledge of the mechanisms underlying T-ALL development and maintenance is clearly needed as one out of 5 patients with T-ALL has a very poor prognosis. Moreover, the high toxicity of chemotherapeutic regimens has been associated with elevated risks of developing severe secondary health problems. In the present study, we characterized the role of Notch in T-ALL induction and maintenance. Using transgenic mouse models, we could demonstrate that the oncogenic potential of Notch is dependent on RBP-Jκ transcriptional activity. Indeed, the canonical Notch signaling pathway, mediated by the RBP-Jκ transcription factor, is essential for both Notch-mediated T-ALL induction and maintenance, as well as for the induction of a Notch-mediated lymphoproliferative disorder. Moreover, constitutive overexpression of Notch1 at discrete developmental stages within the hematopoietic system, led us to identify a correlation between the differentiation status of a cell, within the hierarchy of the hematopoietic tree, and its sensitivity to Notch1-induced transformation. We demonstrated that forced Notch1 expression in hematopoietic stem cells was not sufficient to induce T-ALL. Nevertheless, aberrant Notch1 expression in the bone marrow resulted in the development of a lethal lymphoproliferative disorder. Overexpression of Notch1 in thmus at the DN2 to DN3 stage of T-cell delvelopment generated an aggressive transplantable T-ALL with important metastatic potential. This suggested that progenitors already committed to the T-cell lineage are highly susceptible to Notch-mediated transformation, which is relevant to the human disease. Nevertheless, forced Notch1 expression after the β-selection checkpoint in the thymys, was no longer sufficient to induce transformation of late DN3 to DN4 thymocytes. We therefore established a murine genetic system that allowed us to dissect the mechanisms of aberrant Notch1 signaling in a lymphoproliferative disorder and a true T-ALL model. We were able to assess the genetic signature that is involved in the transplantability and metastatic potential of leukemic cells and generated a list of candidate genes implicated in these processes. In addition, we correlated genes identified in our murine models with genes associated as well in human T-ALL profiles to identify genes implicated in T-ALL pathogenesis in both mouse and human.

EPFL2011

The Cell Differentiation Status Influences the Outcome of Notch-Induced Malignancy

Caroline Poisson

EPFL2011