No abstract

Patterning of the embryonic endoderm into distinct sets of precursor cells involves the precisely regulated activities of key transcription regulators. Ectopic, pan-endodermal activation of XPtf1a/p48 during pancreas precursor cell stages of Xenopus embryogenesis results in an expansion of the pancreatic territory, precisely within the borders of XlHbox8 expression. A combination of both activities is sufficient to expand the pancreatic precursor cell population also into more posterior portions of the endoderm. Both treatments result in the formation of a giant pancreas that persists up to late tadpole stages of development and carries both supernumerary endocrine and exocrine cells. A combination of XPtf1a/ p48 and XlHbox8 is thus sufficient to convert nonpancreatic endodermal cells into pancreatic precursor cells. The vertebrate pancreas develops from one dorsal and two ventral evaginations in the endodermal epithelium (Slack 1995). Early pancreatic precursor cells express Pdx1, a homeodomain protein that is also expressed in the adjacent presumptive stomach and duodenum (Ohlsson et al. 1993;Jonsson et al. 1994), as well as Ptf1a/p48, a basic helix-loop-helix (bHLH) transcription factor (Kawaguchi et al. 2002). Lineage tracing studies have indicated that the Pdx1-expressing cells represent precursor cells that contribute to the formation of both endocrine and exocrine lineages of the mature pancreas (Gu et al. 2002). In Pdx1-homozygous mutant mice, pancreatic buds seem to be arrested in their development; however, early glucagon-and insulin-expressing cells can still be detected (Ahlgren et al. 1996;Offield et al. 1996). Such mice exhibit additional malformations in stomach and duodenum, where Pdx1 is normally expressed. Interestingly, and of primary relevance for the results reported in this communication, ectopic expression of Pdx1 in nonpancreatic chicken endoderm resulted in the initiation of pancreatic budding, but it was not sufficient to promote differentiation of either exocrine or endocrine cells (Grapin-Botton et al. 2001). Ptf1a/p48 is a bHLH transcription factor, which was originally identified as a part of a heterotrimeric pancreas transcription factor complex, referred to as PTF1 that activates transcription of exocrine specific pancreatic genes in the mature pancreas (Cockell et al. 1989;Beres et al. 2006). Mice bearing a null mutation of Ptf1a/ p48 are completely devoid of exocrine pancreas, while endocrine pancreatic cells still form, but are found to be translocated to the spleen (Krapp et al. 1998). More recent studies have revealed that Ptf1a/p48 is already expressed in pancreatic precursor cells which contribute to all pancreatic cell types, and that, in the absence of Ptf1a/p48, pancreatic precursor cells adopt a duodenal fate (Kawaguchi et al. 2002). These findings suggest a role for Ptf1a/p48 that is not solely in exocrine differentiation, but that is also relevant for cells of the endocrine lineages.Here, we report on observations that are in strong support of the concept that a combi...

In eukaryotic cells, newly synthesized secretory proteins require COPII (coat protein complex II) to exit the endoplasmic reticulum (ER). COPII contains five core components: SAR1, SEC23, SEC24, SEC13, and SEC31. SEC23 is a GTPase-activating protein that activates the SAR1 GTPase and also plays a role in cargo recognition. Missense mutations in the human COPII paralogues SEC23A and SEC23B result in craniolenticulosutural dysplasia and congenital dyserythropoietic anemia type II, respectively. We now report that mice completely deficient for SEC23B are born with no apparent anemia phenotype, but die shortly after birth, with degeneration of professional secretory tissues. In SEC23B-deficient embryonic pancreas, defects occur in exocrine and endocrine tissues shortly after differentiation. Pancreatic acini are completely devoid of zymogen granules, and the ER is severely distended. Similar ultrastructural alterations are also observed in salivary glands, but not in liver. Accumulation of proteins in the ER lumen activates the proapoptotic pathway of the unfolded protein response, suggesting a central role for apoptosis in the degeneration of these tissues in SEC23B-deficient embryos. Although maintenance of the secretory pathway should be required by all cells, our findings reveal a surprising tissue-specific dependence on SEC23B for the ER exit of highly abundant cargo, with high levels of SEC23B expression observed in professional secretory tissues. The disparate phenotypes in mouse and human could result from residual SEC23B function associated with the hypomorphic mutations observed in humans, or alternatively, might be explained by a species-specific shift in function between the closely related SEC23 paralogues. mammalian embryo abnormalities | vesicular transport protein | genetics | secretory granules | pancreatitis I n eukaryotic cells, secreted proteins and proteins that are targeted to the plasma membrane and internal organelles are synthesized in the endoplasmic reticulum (ER) and sorted through the secretory pathway. This process has been extensively studied, particularly in budding yeast (1). Proteins destined to traffic from the ER to the Golgi are packaged into COPII (coat protein complex II)-coated vesicles (2-4). COPII is composed of at least five proteins, a small GTPase SAR1 and two cytosolic protein complexes, SEC23-SEC24 and SEC13-SEC31 (5). The GTP-bound form of SAR1 binds to the ER membrane and recruits the SEC23-SEC24 heterodimer to form the "prebudding complex," which in turn recruits the outer coat composed of SEC13-SEC31 heterotetramers to complete the COPII coat structure (6).The COPII complex captures cargo into vesicles and mediates vesicle budding from the ER. Cargo recognition appears to be mediated primarily by the SEC24 subunit, which recognizes divergent export signals located in the cytosolic domain of cargo proteins (7,8). SEC23 and SAR1 also play a role in the recognition of at least a subset of cargos (9, 10). SEC23 is a GTPaseactivating protein (GAP) that activates the SAR1...

SOX9 encodes a transcription factor that presides over the specification and differentiation of numerous progenitor and differentiated cell types, and although SOX9 haploinsufficiency and overexpression cause severe diseases in humans, including campomelic dysplasia, sex reversal and cancer, the mechanisms underlying SOX9 transcription remain largely unsolved. We identify here an evolutionarily conserved enhancer located 70-kb upstream of mouse Sox9 and call it SOM because it specifically activates a Sox9 promoter reporter in most Sox9-expressing somatic tissues in transgenic mice. Moreover, SOM-null fetuses and pups reduce Sox9 expression by 18–37% in the pancreas, lung, kidney, salivary gland, gut and liver. Weanlings exhibit half-size pancreatic islets and underproduce insulin and glucagon, and adults slowly recover from acute pancreatitis due to a 2-fold impairment in Sox9 upregulation. Molecular and genetic experiments reveal that Sox9 protein dimers bind to multiple recognition sites in the SOM sequence and are thereby both necessary and sufficient for enhancer activity. These findings thus uncover that Sox9 directly enhances its functions in somatic tissue development and adult regeneration through SOM-mediated positive auto-regulation. They provide thereby novel insights on molecular mechanisms controlling developmental and disease processes and suggest new strategies to improve disease treatments.

This review summarizes our current understanding of exocrine pancreas development, including the formation of acinar, ductal and centroacinar cells. We discuss the transcription factors associated with various stages of exocrine differentiation, from multipotent progenitor cells to fully differentiated acinar and ductal cells. Within the branching epithelial tree of the embryonic pancreas, this involves the progressive restriction of multipotent pancreatic progenitor cells to either a central “trunk” domain giving rise to the islet and ductal lineages, or a peripheral “tip” domain giving rise to acinar cells. This review also discusses the soluble morphogens and other signaling pathways that influence these events. Finally, we examine centroacinar cells as an enigmatic pancreatic cell type whose lineage remains uncertain, and whose possible progenitor capacities continue to be explored.