The endocrine pancreas is organized in islets of Langerhans comprising five original cell subtypes, α-, β-, δ-, ε-, and PP-cells, secreting glucagon, insulin, somatostatin, ghrelin, and pancreatic polypeptide (PP), respectively (Collombat et al., 2006). The identification and characterization of the genetic determinants underlying endocrine pancreas morphogenesis and regeneration may potentially aid designing cell replacement therapies to treat type 1 and 2 diabetes. In this context, a number of studies have demonstrated that, during development, the cooperation of several transcription factors successively specifies progenitor cells towards the pancreatic-, endocrine- and ultimately islet-cell fates. Hence, Pdx1 is required for pancreatic epithelium determination (Ahlgren et al., 1996; Ahlgren et al., 1998; Grapin-Botton et al., 2001; Jonsson et al., 1994; Offield et al., 1996) and subsequently Neurogenin3 (Ngn3) for endocrine lineage specification (Gradwohl et al., 2000; Gu et al., 2002; Jensen et al., 2000; Johansson et al., 2007). Next to Ngn3 induction, a complex network of transcription factors progressively and differentially promotes the particular endocrine fates, including Arx and Pax4 (Collombat et al., 2003; Sosa-Pineda et al., 1997). In mice lacking Arx, the β- and δ-cell fates were found favored at the expense of α-cell genesis, while the total endocrine cell content remained normal (Collombat et al., 2003). Conversely, in the absence of Pax4, the opposite phenotype was observed (Sosa-Pineda et al., 1997), indicating an inhibitory, cross-regulatory circuit between Arx and Pax4 (Collombat et al., 2005). Additional findings supported these conclusions and suggested that, firstly, Arx and Pax4 instruct endocrine precursor cells towards either an α-cell or a β-/δ-cell fate, respectively. Next, through the analysis of double-mutant mice, a secondary function of Pax4 in specifying the β-cell lineage in β-/δ-precursor-cells was uncovered (Collombat et al., 2005). Recent evidence have demonstrated that the forced expression of Arx in early pancreatic cells drives endocrine progenitors towards either an α- or, surprisingly, a PP-cell fate (Collombat et al., 2007). It was therefore concluded that Arx is not only necessary, but also sufficient to instruct the α- and PP-cell lineages.
Of particular interest was the finding that the forced expression of Arx triggered into adult β-cells induced their conversion into cells exhibiting α- or PP-cell phenotypes (Collombat et al., 2007). This discovery was of fundamental importance in the context of β-cell-based therapy and implied that the opposite conversion might be achieved, that is, to generate β-cells from other endocrine cells. To test this hypothesis, we generated mice conditionally and ectopically expressing the Pax4 gene. Our data indicates that the ectopic expression of Pax4 in early pancreatic cells, but also in α-cells, induces their respecification towards a β-cell fate or identity. As a consequence of the ensuing glucagon deficiency, an ongoing neogenesis of α-cells occurs. However, such α-cells are continuously converted into β-cells upon Pax4 ectopic expression, resulting in the development of oversized islets of Langerhans. Importantly, a prominent expression of the proendocrine transcription factor Ngn3 in the pancreas of such animals is highlighted. Our results are consistent with the recently reported notion of facultative adult stem cells that reactivate Ngn3 expression in injured pancreas (Xu et al., 2008). Finally, following streptozotocin-induced depletion of β-cells in young mice ectopically expressing Pax4, an α-cell-mediated regeneration of the β-cell mass, a progressive normalization of the glycemia and an extended lifespan are observed.