Insm1 cooperates with Neurod1 and Foxa2 to maintain mature pancreatic β-cell function.

Key transcription factors control the gene expression program in mature pancreatic beta-cells, but their integration into regulatory networks is little understood. Here, we show that Insm1, Neurod1 and Foxa2 directly interact and together bind regulatory sequences in the genome of mature pancreatic beta-cells. We used Insm1 ablation in mature beta-cells in mice and found pronounced deficits in insulin secretion and gene expression. Insm1-dependent genes identified previously in developing beta-cells markedly differ from the ones identified in the adult. In particular, adult mutant beta-cells resemble immature beta-cells of newborn mice in gene expression and functional properties. We defined Insm1, Neurod1 and Foxa2 binding sites associated with genes deregulated in Insm1 mutant beta-cells. Remarkably, combinatorial binding of Insm1, Neurod1 and Foxa2 but not binding of Insm1 alone explained a significant fraction of gene expression changes. Human genomic sequences corresponding to the murine sites occupied by Insm1/Neurod1/Foxa2 were enriched in single nucleotide polymorphisms associated with glycolytic traits. Thus, our data explain part of the mechanisms by which beta-cells maintain maturity: Combinatorial Insm1/Neurod1/Foxa2 binding identifies regulatory sequences that maintain the mature gene expression program in beta-cells, and disruption of this network results in functional failure.