The winged helix transcription factor Foxa2 regulates Pdx1 gene expression and fetal endocrine pancreas development. We show here by inducible gene ablation that Foxa2 inactivation in mature beta-cells induces hyperinsulinemic hypoglycemia in Foxa2loxP/loxP, Pdx1-CreERT2 adult mice. Mutant beta-cells exhibited a markedly increased pool of docked insulin granules, some of which were engaged in sequential or compound exocytosis, consistent with an increased first phase glucose-stimulated insulin secretion. Expression of multiple genes involved in vesicular trafficking, membrane targeting and fuel-secretion pathways is dependent on Foxa2. In addition, impaired cytosolic Ca2+ oscillations and elevated intracellular cAMP production accompanied this secretory defect, and were likely contributors to the sensitization of the exocytotic machinery. Thus, in the absence of Foxa2, alterations in intracellular second messenger signaling redistribute the insulin granules into the readily releasable pool. We conclude that Foxa2 is required both for the fetal pancreas development and for the function of mature beta-cells.

Previous studies have demonstrated that Foxa2 plays a critical role in the differentiation of fetal pancreatic alpha and beta cells. Expression of Foxa2 persists in adult beta cells but its role has been unclear. Conditional gene ablation was used to inactivate Foxa2 in mature beta cells to address this question.

Foxa2loxP/loxP,Pdx1-CreERT2 mice were generated and Cre activity was induced in 8- to 10-week-old Foxa2loxP/loxP control and Foxa2loxP/loxP,Pdx1-CreERT2 mice by subcutaneous administration of tamoxifen. Immunostaining of pancreatic sections for insulin and Foxa2 demonstrated Foxa2 deletion in 80% to 85% of beta cells in mutant islets while all control beta cells and non-beta mutant islet cells showed nuclear Foxa2 staining. Expression profiling was performed on RNA isolated from mutant and control islets using the PancChip 6.1 array.

In contrast to control littermates, tamoxifen-treated mutant mice showed a significant reduction in blood glucose levels under both fed and fasted conditions. Plasma insulin levels of fed mutant mice were more than 2-fold higher than those of controls. Mutant beta cells exhibited a markedly increased pool of docked insulin granules, some of which were engaged in sequential or compound exocytosis, consistent with an increase in first phase glucose-stimulated insulin secretion. In addition, impaired cytosolic Ca2+ oscillations and elevated intracellular cAMP production accompanied this secretory defect, and were likely contributors to the sensitization of the exocytotic machinery. Microarray analysis identified genes differentially expressed between 4 pairs of mutant and control islets. Multiple genes were identified involved in vesicular trafficking, membrane targeting and fuel-secretion pathways.

Foxa2 inactivation in mature beta cells induces hyperinsulinemic hypoglycemia in Foxa2loxP/loxP,Pdx1-CreERT2 adult mice through alterations in intracellular second-messenger signaling that redistribute the insulin granules into the readily releasable pool. Foxa2 is required for both fetal pancreas development and the function of mature beta cells.

• Foxa2: 550, 2040, 3042,
• insulin secretion: 550, 930, 990, 1030, 1050, 2160, 3042, 3546, 3650, 3980,