Relative roles of the different Pax6 domains for pancreatic alpha cell development

Abstract

BackgroundThe transcription factor Pax6 functions in the specification and maintenance of the differentiated cell lineages in the endocrine pancreas. It has two DNA binding domains, the paired domain and the homeodomain, in addition to a C-terminal transactivation domain. The phenotype of Pax6-/- knockout mice suggests non-redundant functions of the transcription factor in the development of glucagon-expressing α-cells as this cell type is absent in the mutants. We ask the question of how the differentiation of pancreatic endocrine cells, in particular that of α-cells, is affected by selective inactivation of either one of the three major domains of Pax6.ResultsThe Pax6Aey18 mutant mouse line, in which the paired domain is inactivated, showed a phenotype similar to that of Pax6-/- knockout mice with a near complete absence of glucagon-positive α-cells (0-4 cells/section; ≤1% of wt), reduced β-cell area (74% of wt) and disorganized islets. The proportion of ghrelin-positive ε-cells was expanded. In Pax6Sey-Neu mutants, which lack the transactivation domain, α-and β-cells where reduced to 25 and 40% of wt, respectively. We also studied two mouse lines with mutations in the homeodomain, Pax64Neu and Pax6132-14Neu. Neighboring amino acids are affected in the two lines and both point mutations abolish DNA binding of the classical P3 homeodomain target sequence. The pancreatic phenotype of the two mutants however was divergent. While Pax64Neu homozygotes showed a reduction of α- and β-cells to 59 and 61%, respectively, pancreatic endocrine development was unaltered in the Pax6132-14Neu mutant strain.ConclusionsWe show that inactivation of the Pax6 paired domain leads to a more severe phenotype with regards to the differentiation of pancreatic α-cells than the loss of the transactivation domain. The analysis of two different homeodomain mutants suggests that the binding of Pax6 to P3 homeodomain consensus sequences is not required for α-cell development. It rather seems that the homeodomain has a modulating role in Pax6 function, possibly by facilitating a PH0-like binding confirmation on paired domain target genes like proglucagon. This function is differentially affected by the two homeodomain mutations analyzed in this study.

Pancreas-enriched miRNA refines endocrine cell differentiation

“…Although Pax6 positively regulates insulin and glucagon expression, it negatively regulates ghrelin expression and differentiation of -cells (Heller et al, 2005;Dames et al, 2010). Accordingly, miR-7 KD resulted in reduced numbers of ghrelinpositive cells (Fig.…”

“…However, Pax6 negatively regulates the formation of ghrelin-expressing -cells (Sander et al, 1997;StOnge et al, 1997;Ashery-Padan et al, 2004;Heller et al, 2005;Dames et al, 2010). In both humans and mice, two Pax6 alleles are required in order to maintain proper glucose homeostasis, i.e.…”

Pax6 in Collembola: Adaptive evolution of eye regression

“…It was further confirmed that the highly conserved Pax6 is a universal master control gene for eye development in the animal kingdom. Similar to the studied Pax6 genes in zebrafish 42 , Drosophila 36 , quail 43 , mouse [44][45][46] , and human 21,47,48 , the non-conserved C-terminus of the collembolan Pax6 is vital for protein transactivation ability. In our study, gradual deletions from the C-terminal end of collembolan Pax6 led to a gradual loss of activity in inducing ectopic eyes on Drosophila (Fig.…”

A common variant upstream of the PAX6 gene influences islet function in man

“…However, more detailed investigation is required to determine whether these genes are targets or regulators of PAX6 expression or regulated by common upstream factors. If the PAX6 polymorphism changes the islet cell ratio, as is seen in mouse models with mutated Pax6 [34], this could also contribute to the many correlations. The most common cause for an increase in fasting insulin concentration is insulin resistance, also evidenced by an increase in the HOMA-IR.…”

“…However, these studies were performed in individuals with more severe mutations making generalisation of the findings to potential effects of common variants in the gene difficult. Further, there is evidence from mouse models that the effect of mutations and deletions in Pax6 on pancreatic development depends upon which functional domain of the protein is affected [34].…”