XB-ART-37993Proc Natl Acad Sci U S A July 1, 2008; 105 (26): 8962-7.
Remodeling the exocrine pancreas at metamorphosis in Xenopus laevis.
At metamorphosis the Xenopus laevis tadpole exocrine pancreas remodels in two stages. At the climax of metamorphosis thyroid hormone (TH) induces dedifferentiation of the entire exocrine pancreas to a progenitor state. The organ shrinks to 20% of its size, and approximately 40% of its cells die. The acinar cells lose their zymogen granules and approximately 75% of their RNA. The mRNAs that encode exocrine-specific proteins (including the transcription factor Ptf1a) undergo almost complete extinction at climax, whereas PDX-1, Notch-1, and Hes-1, genes implicated in differentiation of the progenitor cells, are activated. At the end of spontaneous metamorphosis when the endogenous TH has reached a low level, the pancreas begins to redifferentiate. Exogenous TH induces the dedifferentiation phase but not the redifferentation phase. The tadpole pancreas lacks the mature ductal system that is found in adult vertebrate pancreases, including the frog. Exocrine pancreases of transgenic tadpoles expressing a dominant negative form of the TH receptor controlled by the elastase promoter are resistant to TH. They do not shrink when subjected to TH. Their acinar cells do not dedifferentiate at climax, nor do they down-regulate exocrine-specific genes or activate Notch-1 and Hes-1. Even 2 months after metamorphosis these frogs have not developed a mature ductal system and the acinar cells are abnormally arranged. The TH-dependent dedifferentiation of the tadpole acinar cells at climax is a necessary step in the formation of a mature frog pancreas.
PubMed ID: 18574144
PMC ID: PMC2449347
Article link: Proc Natl Acad Sci U S A
Genes referenced: amy2a cela1.6 hes1 notch1 pdx1 ptf1a trdn trhd
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|Fig. 1. Morphological and histological changes of the X. laevis pancreas at different stages expressing GFP driven by the rat elastase promoter. (A–D) Stages of X. laevis metamorphosis. (E–H) Abdominal view of the pancreas observed by fluorescent microscope. (I–L) H&E-stained sections of pancreases at the corresponding stages. Expression of the elastase-GFP transgene had no effect on normal development and remodeling of pancreas. (Scale bars: A–D, 4 mm; E–H, 1 mm; I–L, 40 μm.)|
|Fig. 2. Change in gene expression in the exocrine pancreas during metamorphosis. (A–D and I–L) In situ hybridization with amylase (A–D) and Ptf1a (I–L) probes. Every mRNA encoding a “terminally differentiated” exocrine protein that we have tested is extinguished at metamorphic climax (NF62) to the same extent as these 2 mRNAs (see discussion). (E–H) Although amylase mRNA is extinguished at climax, amylase protein was still detected by immunohistochemistry, confirming the dedifferentiation stages of previously differentiated exocrine cells. (Scale bar: 40 μm.)|
|Fig. 3. Change in gene expression in the exocrine pancreas during metamorphosis. (A–C) In situ hybridization with PDX-1 probes. PDX-1 is expressed exclusively in islet cells in the tadpole (A) and frog pancreases (C), and at climax expression is extended to the dedifferentiated exocrine cells (B). (D–I) Notch-1 (D–F) and Hes-1 (G–I) genes are activated at climax, although low level of expression of Notch-1 and Hes-1 has been observed at premetamorphic and adult frogs. (Scale bar: 40 μm.)|
|Fig. 4. The TRDN-transgene prevents TH-induced exocrine dedifferentiation. (A, D, and G) WT control NF54 tadpole. (B, E, and H) NF54 tadpole after 4 days of 10 nM T3 exposure. (C, F, and I), pElastase-TRDN-GFP NF54 transgenic tadpole after 4 days of 10 nM T3 exposure. (A–C) Abdominal view of the pancreas. The thin white line outlines each pancreas. (D–F) H&E-stained sections. (G–I) In situ hybridization for amylase. Exogenous TH induces the dedifferentiation of the exocrine pancreas, and the TRDN transgene prevents the dedifferentiation process. (Scale bars: A–C, 1 mm; D–I, 40 μm.)|
|Fig. 5. The TRDN transgene prevents dedifferentiation of acinar cells in spontaneous metamorphosis. NF66 WT (A–D) and NF66 pElastase-TRDN (E–H) pancreas sections. (A and E) Stained with H&E. (B–D and F–H) In situ hybridization. (B and F) Amylase. (C and G) Notch-1. (D and H) Hes-1. (Scale bar: 40 μm.)|
|Fig. 6. Differences in ductal systems between tadpole and frog. (A) Premetamorphic tadpoles have very few ducts compared with (B) adult pancreas. The complex ductal system arises after metamorphosis. (C) Sections of a 2-month-old pElastase-TRDN-GFP frog showing reduced number of ducts. (D) All types of duct cells in control animals (n = 5 per group) were counted in a unit area from H&E-stained sections of NF54 tadpoles (PreMM), 2 month-old frog (2M), and an adult frog. (Scale bar: 40 μm.)|
|Fig. 7. Schematic representation of morphological and gene expression changes in the exocrine pancreas during X. laevis metamorphosis. The adult frog pancreas has all of the types of ducts that have been described for adult vertebrates. Tadpoles lack the intercalated ducts. At the climax of metamorphosis, TH induces dedifferentiation of the exocrine pancreas to a progenitor state that has extinguished the mRNAs encoding terminally differentiated proteins (represented by amylase). PDX-1, Notch-1, and Hes-1 are up-regulated. Late in climax Ptf1a is reactivated followed by the differentiation of acinar and duct cells. Abbreviations: ac-acinar; ld-lobular duct, icd-intercalated duct. Figure is not drawn to scale.|