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Ongoing Notch signaling maintains phenotypic fidelity in the adult exocrine pancreas.

ABSTRACT: The Notch signaling pathway regulates embryonic development of the pancreas, inhibiting progenitor differentiation into exocrine acinar and endocrine islet cells. The adult pancreas appears to lack progenitor cells, and its mature cell types are maintained by the proliferation of pre-existing differentiated cells. Nonetheless, Notch remains active in adult duct and terminal duct/centroacinar cells (CACs), in which its function is unknown. We previously developed mice in which cells expressing the Notch target gene Hes1 can be labeled and manipulated, by expression of Cre recombinase, and demonstrated that Hes1(+) CACs do not behave as acinar or islet progenitors in the uninjured pancreas, or as islet progenitors after pancreatic duct ligation. In the current study, we assessed the function of Notch signaling in the adult pancreas by deleting the transcription factor partner of Notch, Rbpj, specifically in Hes1(+) cells. We find that loss of Rbpj depletes the pancreas of Hes1-expressing CACs, abrogating their ongoing contribution to growth and homeostasis of more proximal duct structures. Upon Rbpj deletion, CACs undergo a rapid transformation into acinar cells, suggesting that constitutive Notch activity suppresses the acinar differentiation potential of CACs. Together, our data provide direct evidence of an endogenous genetic program to control interconversion of cell fates in the adult pancreas.

SUBMITTER: Kopinke D

PROVIDER: S-EPMC3254730 | biostudies-other | 2012 Feb

REPOSITORIES: biostudies-other

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Ongoing Notch signaling maintains phenotypic fidelity in the adult exocrine pancreas.

Kopinke Daniel D Brailsford Marisa M Pan Fong Cheng FC Magnuson Mark A MA Wright Christopher V E CV Murtaugh L Charles LC

Developmental biology 20111129 1

The Notch signaling pathway regulates embryonic development of the pancreas, inhibiting progenitor differentiation into exocrine acinar and endocrine islet cells. The adult pancreas appears to lack progenitor cells, and its mature cell types are maintained by the proliferation of pre-existing differentiated cells. Nonetheless, Notch remains active in adult duct and terminal duct/centroacinar cells (CACs), in which its function is unknown. We previously developed mice in which cells expressing th ...[more]

PMID: 22146645

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Project description:BackgroundReports of exocrine-to-endocrine reprogramming through expression or stabilization of the transcription factor neurogenin 3 (NGN3) have generated renewed interest in harnessing pancreatic plasticity for therapeutic applications. NGN3 is expressed by a population of endocrine progenitor cells that give rise exclusively to hormone-secreting cells within pancreatic islets and is necessary and sufficient for endocrine differentiation during development. In the adult human pancreas, NGN3 is expressed by dedifferentiating exocrine cells with a phenotype resembling endocrine progenitor cells and the capacity for endocrine differentiation in vitro. Neurotrophic tyrosine kinase receptor type 2 (TRKB), which regulates neuronal cell survival, differentiation and plasticity, was identified as highly overexpressed in the NGN3 positive cell transcriptome compared to NGN3 negative exocrine cells. This study was designed to determine if NGN3 is regulated by TRKB signaling in the adult human exocrine pancreas.MethodsTranscriptome analysis, quantitative reverse transcriptase polymerase chain reaction (RTPCR) and immunochemistry were used to identify TRKB isoform expression in primary cultures of human islet-depleted exocrine tissue and human cadaveric pancreas biopsies. The effects of pharmacological modulation of TRKB signaling on the expression of NGN3 were assessed by Student's t-test and ANOVA.ResultsApproximately 30 % of cultured exocrine cells and 95 % of NGN3+ cells express TRKB on their cell surface. Transcriptome-based exon splicing analyses, isoform-specific quantitative RTPCR and immunochemical staining demonstrate that TRKB-T1, which lacks a tyrosine kinase domain, is the predominant isoform expressed in cultured exocrine tissue and is expressed in histologically normal cadaveric pancreas biopsies. Pharmacological inhibition of TRKB significantly decreased the percentage of NGN3+ cells, while a TRKB agonist significantly increased this percentage. Inhibition of protein kinase B (AKT) blocked the effect of the TRKB agonist, while inhibition of tyrosine kinase had no effect. Modulation of TRKB and AKT signaling did not significantly affect the level of NGN3 mRNA.ConclusionsIn the adult human exocrine pancreas, TRKB-T1 positively regulates NGN3 independent of effects on NGN3 transcription. Targeting mechanisms controlling the NGN3+ cell population size and endocrine cell fate commitment represent a potential new approach to understand pancreas pathobiology and means whereby cell populations could be expanded for therapeutic purposes.

Project description:BackgroundBeta-catenin is an essential mediator of canonical Wnt signaling and a central component of the cadherin-catenin epithelial adhesion complex. Dysregulation of beta-catenin expression has been described in pancreatic neoplasia. Newly published studies have suggested that beta-catenin is critical for normal pancreatic development although these reports reached somewhat different conclusions. In addition, the molecular mechanisms by which loss of beta-catenin affects pancreas development are not well understood. The goals of this study then were; 1] to further investigate the role of beta-catenin in pancreatic development using a conditional knockout approach and 2] to identify possible mechanisms by which loss of beta-catenin disrupts pancreatic development. A Pdx1-cre mouse line was used to delete a floxed beta-catenin allele specifically in the developing pancreas, and embryonic pancreata were studied by immunohistochemistry and microarray analysis.ResultsPdx1-cre floxed beta-catenin animals were viable but demonstrated small body size and shortened median survival. The pancreata from knockout mice were hypoplastic and histologically demonstrated a striking paucity of exocrine pancreas, acinar to duct metaplasia, but generally intact pancreatic islets containing all lineages of endocrine cells. In animals with extensive acinar hypoplasia, putative hepatocyte transdifferention was occasionally observed. Obvious and uniform pancreatic hypoplasia was observed by embryonic day E16.5. Transcriptional profiling of Pdx1-cre floxed beta-catenin embryonic pancreata at E14.5, before there was a morphological phenotype, revealed significant decreases in the beta-catenin target gene N-myc, and the basic HLH transcription factor PTF1, and an increase of several pancreatic zymogens compared to control animals. By E16.5, there was a dramatic loss of exocrine markers and an increase in Hoxb4, which is normally expressed anterior to the pancreas.ConclusionWe conclude that beta-catenin expression is required for development of the exocrine pancreas, but is not required for development of the endocrine compartment. In contrast, beta-catenin/Wnt signaling appears to be critical for proliferation of PTF1+ nascent acinar cells and may also function, in part, to maintain an undifferentiated state in exocrine/acinar cell precursors. Finally, beta-catenin may be required to maintain positional identity of the pancreatic endoderm along the anterior-posterior axis. This data is consistent with the findings of frequent beta-catenin mutations in carcinomas of acinar cell lineage seen in humans.

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Ongoing Notch signaling maintains phenotypic fidelity in the adult exocrine pancreas.

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Ongoing Notch signaling maintains phenotypic fidelity in the adult exocrine pancreas.

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