Methylation of histone H3 lysine 4 is required for maintenance of beta cell function in adult mice.

AIMS/HYPOTHESIS: Beta cells control glucose homeostasis via regulated production and secretion of insulin. This function arises from a highly specialised gene expression programme that is established during development and then sustained, with limited flexibility, in terminally differentiated cells. Dysregulation of this programme is seen in type 2 diabetes but mechanisms that preserve gene expression or underlie its dysregulation in mature cells are not well resolved. This study investigated whether methylation of histone H3 lysine 4 (H3K4), a marker of gene promoters with unresolved functional importance, is necessary for the maintenance of mature beta cell function. METHODS: Beta cell function, gene expression and chromatin modifications were analysed in conditional Dpy30 knockout mice, in which H3K4 methyltransferase activity is impaired, and in a mouse model of diabetes. RESULTS: H3K4 methylation maintains expression of genes that are important for insulin biosynthesis and glucose responsiveness. Deficient methylation of H3K4 leads to a less active and more repressed epigenome profile that locally correlates with gene expression deficits but does not globally reduce gene expression. Instead, developmentally regulated genes and genes in weakly active or suppressed states particularly rely on H3K4 methylation. We further show that H3K4 trimethylation (H3K4me3) is reorganised in islets from the Leprdb/db mouse model of diabetes in favour of weakly active and disallowed genes at the expense of terminal beta cell markers with broad H3K4me3 peaks. CONCLUSIONS/INTERPRETATION: Sustained methylation of H3K4 is critical for the maintenance of beta cell function. Redistribution of H3K4me3 is linked to gene expression changes that are implicated in diabetes pathology.

Type 2 diabetes susceptibility gene GRK5 regulates physiological pancreatic ß-cell proliferation via phosphorylation of HDAC5.

Restoring functional ß cell mass is a potential therapy for those with diabetes. However, the pathways regulating ß cell mass are not fully understood. Previously, we demonstrated that Sox4 is required for ß cell proliferation during prediabetes. Here, we report that Sox4 regulates ß cell mass through modulating expression of the type 2 diabetes (T2D) susceptibility gene GRK5. ß cell-specific Grk5 knockout mice showed impaired glucose tolerance with reduced ß cell mass, which was accompanied by upregulation of cell cycle inhibitor gene Cdkn1a. Furthermore, we found that Grk5 may drive ß cell proliferation through a pathway that includes phosphorylation of HDAC5 and subsequent transcription of immediate-early genes (IEGs) such as Nr4a1, Fosb, Junb, Arc, Egr1, and Srf. Together, these studies suggest GRK5 is linked to T2D through regulation of ß cell growth and that it may be a target to preserve ß cells during the development of T2D.

Bax and Bak jointly control survival and dampen the early unfolded protein response in pancreatic ß-cells under glucolipotoxic stress.

ER stress and apoptosis contribute to the loss of pancreatic ß-cells under pro-diabetic conditions of glucolipotoxicity. Although activation of canonical intrinsic apoptosis is known to require pro-apoptotic Bcl-2 family proteins Bax and Bak, their individual and combined involvement in glucolipotoxic ß-cell death are not known. It has also remained an open question if Bax and Bak in ß-cells have non-apoptotic roles in mitochondrial function and ER stress signaling, as suggested in other cell types. Using mice with individual or combined ß-cell deletion of Bax and Bak, we demonstrated that glucolipotoxic ß-cell death in vitro occurs by both non-apoptotic and apoptotic mechanisms, and the apoptosis could be triggered by either Bax or Bak alone. In contrast, they had non-redundant roles in mediating staurosporine-induced apoptosis. We further established that Bax and Bak do not affect normal glucose-stimulated ß-cell Ca2+ responses, insulin secretion, or in vivo glucose tolerance. Finally, our experiments revealed that combined deletion of Bax and Bak amplified the unfolded protein response in islets during the early stages of chemical- or glucolipotoxicity-induced ER stress. These findings shed new light on roles of the core apoptosis machinery in ß-cell survival and stress signals of importance for the pathobiology of diabetes.

Wnt ligand-dependent activation of the negative feedback regulator Nkd1.

Misregulation of Wnt signaling is at the root of many diseases, most notably colorectal cancer, and although we understand the activation of the pathway, we have a very poor understanding of the circumstances under which Wnt signaling turns itself off. There are numerous negative feedback regulators of Wnt signaling, but two stand out as constitutive and obligate Wnt-induced regulators: Axin2 and Nkd1. Whereas Axin2 behaves similarly to Axin in the destruction complex, Nkd1 is more enigmatic. Here we use zebrafish blastula cells that are responsive Wnt signaling to demonstrate that Nkd1 activity is specifically dependent on Wnt ligand activation of the receptor. Furthermore, our results support the hypothesis that Nkd1 is recruited to the Wnt signalosome with Dvl2, where it becomes activated to move into the cytoplasm to interact with ß-catenin, inhibiting its nuclear accumulation. Comparison of these results with Nkd function in Drosophila generates a unified and conserved model for the role of this negative feedback regulator in the modulation of Wnt signaling.