Pax4 is not essential for beta-cell differentiation in zebrafish embryos but modulates alpha-cell generation by repressing arx gene expression.
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in BMC Developmental Biology (2012), 12
BACKGROUND: Genetic studies in mouse have demonstrated the crucial function of PAX4 in pancreatic cell differentiation. This transcription factor specifies beta- and delta-cell fate at the expense of ... [more ▼]
BACKGROUND: Genetic studies in mouse have demonstrated the crucial function of PAX4 in pancreatic cell differentiation. This transcription factor specifies beta- and delta-cell fate at the expense of alpha-cell identity by repressing Arx gene expression and ectopic expression of PAX4 in alpha-cells is sufficient to convert them into beta-cells. Surprisingly, no Pax4 orthologous gene can be found in chicken and Xenopus tropicalis raising the question of the function of pax4 gene in lower vertebrates such as in fish. In the present study, we have analyzed the expression and the function of the orthologous pax4 gene in zebrafish. RESULTS: pax4 gene is transiently expressed in the pancreas of zebrafish embryos and is mostly restricted to endocrine precursors as well as to some differentiating delta- and epsilon-cells but was not detected in differentiating beta-cells. pax4 knock-down in zebrafish embryos caused a significant increase in alpha-cells number while having no apparent effect on beta- and delta-cell differentiation. This rise of alpha-cells is due to an up-regulation of the Arx transcription factor. Conversely, knock-down of arx caused to a complete loss of alpha-cells and a concomitant increase of pax4 expression but had no effect on the number of beta- and delta-cells. In addition to the mutual repression between Arx and Pax4, these two transcription factors negatively regulate the transcription of their own gene. Interestingly, disruption of pax4 RNA splicing or of arx RNA splicing by morpholinos targeting exon-intron junction sites caused a blockage of the altered transcripts in cell nuclei allowing an easy characterization of the arx- and pax4-deficient cells. Such analyses demonstrated that arx knock-down in zebrafish does not lead to a switch of cell fate, as reported in mouse, but rather blocks the cells in their differentiation process towards alpha-cells. CONCLUSIONS: In zebrafish, pax4 is not required for the generation of the first beta- and delta-cells deriving from the dorsal pancreatic bud, unlike its crucial role in the differentiation of these cell types in mouse. On the other hand, the mutual repression between Arx and Pax4 is observed in both mouse and zebrafish. These data suggests that the main original function of Pax4 during vertebrate evolution was to modulate the number of pancreatic alpha-cells and its role in beta-cells differentiation appeared later in vertebrate evolution. [less ▲]Detailed reference viewed: 28 (7 ULg)
The Pax6b homeodomain is dispensable for pancreatic endocrine cell differentiation in zebrafish.
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in Journal of Biological Chemistry (2010), 285(18), 13863-73
Pax6 is a well conserved transcription factor that contains two DNA-binding domains, a paired domain and a homeodomain, and plays a key role in the development of eye, brain, and pancreas in vertebrates ... [more ▼]
Pax6 is a well conserved transcription factor that contains two DNA-binding domains, a paired domain and a homeodomain, and plays a key role in the development of eye, brain, and pancreas in vertebrates. The recent identification of the zebrafish sunrise mutant, harboring a mutation in the pax6b homeobox and presenting eye abnormalities but no obvious pancreatic defects, raised a question about the role of pax6b in zebrafish pancreas. We show here that pax6b does play an essential role in pancreatic endocrine cell differentiation, as revealed by the phenotype of a novel zebrafish pax6b null mutant and of embryos injected with pax6b morpholinos. Pax6b-depleted embryos have almost no beta cells, a strongly reduced number of delta cells, and a significant increase of epsilon cells. Through the use of various morpholinos targeting intron-exon junctions, pax6b RNA splicing was perturbed at several sites, leading either to retention of intronic sequences or to deletion of exonic sequences in the pax6b transcript. By this strategy, we show that deletion of the Pax6b homeodomain in zebrafish embryos does not disturb pancreas development, whereas lens formation is strongly affected. These data thus provide the explanation for the lack of pancreatic defects in the sunrise pax6b mutants. In addition, partial reduction of Pax6b function in zebrafish embryos performed by injection of small amounts of pax6b morpholinos caused a clear rise in alpha cell number and in glucagon expression, emphasizing the importance of the fine tuning of the Pax6b level to its biological activity. [less ▲]Detailed reference viewed: 19 (3 ULg)