Combined ectopic expression of Pdx1 and Ptf1a/p48 results in the stable conversion of posterior endoderm into endocrine and exocrine pancreatic tissue

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...

Pancreas organogenesis: From bud to plexus to gland

“…More evidence for an essential role for Ptf1a in committing cells with the MPC competence to produce all pancreatic lineages come from studies in Xenopus embryos. Afelik et al (2006) suggested that mis-expression of Ptf1a together with Pdx1 converts duodenum to pancreas, producing a ''giant pancreas'' that contained relatively normal acinar and endocrine proportions. One important role of Ptf1a after initial specification of the pancreas fate might be to ensure the rapid maintenance and/or upregulation of Pdx1 expression, possibly mediated by its transactivation via Area III of the Pdx1 promoter (Wiebe et al, 2007), with the idea being that Ptf1a/Pdx1 coexpression provides a more definitive pancreatic status to the MPC.…”

Murine Embryonic Stem Cell–Derived Pancreatic Acinar Cells Recapitulate Features of Early Pancreatic Differentiation

“…In its absence, foregut endoderm precursors assume an intestinal fate (14). Ptf1a also promotes ectopic pancreas fates from endoderm tissue (16)(17)(18). Mist1 is expressed in a wide array of secretory tissues and, in the adult pancreas, is only detected in acinar cells (19;20).…”

Homeoprotein hhex-induced conversion of intestinal to ventral pancreatic precursors results in the formation of giant pancreata in Xenopus embryos

“…Within the endoderm, expression of XPtf1a/p48 is specifically restricted to the dorsal and ventral pancreatic buds (28,30). As for XlHbox8, the ventral expression domain of XPtf1a/p48 was found to be expanded in hhex-injected embryos, although to a lesser extent (Fig.…”

“…XlHbox8 expression demarcates dorsal and ventral pancreatic buds, and part of stomach and duodenum at tail bud stages of development (27,28). The enlargement of the ventral foregut induced by ectopic hhex coincides with a specific expansion of ventral XlHbox8 expression domain (Fig.…”