Zinc co-crystallizes with insulin in dense core secretory granules, but its role in insulin biosynthesis, storage and secretion is unknown. In this study we assessed the role of the zinc transporter ZnT8 using ZnT8-knockout (ZnT8 ؊/؊ ) mice. Absence of ZnT8 expression caused loss of zinc release upon stimulation of exocytosis, but normal rates of insulin biosynthesis, normal insulin content and preserved glucose-induced insulin release. Ultrastructurally, mature dense core insulin granules were rare in ZnT8 ؊/؊ beta cells and were replaced by immature, pale insulin ''progranules,'' which were larger than in ZnT8 ؉/؉ islets. When mice were fed a control diet, glucose tolerance and insulin sensitivity were normal. However, after high-fat diet feeding, the ZnT8 ؊/؊ mice became glucose intolerant or diabetic, and islets became less responsive to glucose. Our data show that the ZnT8 transporter is essential for the formation of insulin crystals in beta cells, contributing to the packaging efficiency of stored insulin. Interaction between the ZnT8 ؊/؊ genotype and diet to induce diabetes is a model for further studies of the mechanism of disease of human ZNT8 gene mutations.dense core granule ͉ diabetes ͉ zinc

We report on a hitherto poorly characterized class of genes that are expressed in all tissues, except in one. Often, these genes have been classified as housekeeping genes, based on their nearly ubiquitous expression. However, the specific repression in one tissue defines a special class of “disallowed genes.” In this paper, we used the intersection-union test to screen for such genes in a multi-tissue panel of genome-wide mRNA expression data. We propose that disallowed genes need to be repressed in the specific target tissue to ensure correct tissue function. We provide mechanistic data of repression with two metabolic examples, exercise-induced inappropriate insulin release and interference with ketogenesis in liver. Developmentally, this repression is established during tissue maturation in the early postnatal period involving epigenetic changes in histone methylation. In addition, tissue-specific expression of microRNAs can further diminish these repressed mRNAs. Together, we provide a systematic analysis of tissue-specific repression of housekeeping genes, a phenomenon that has not been studied so far on a genome-wide basis and, when perturbed, can lead to human disease.

SUMMARY The human growth hormone (hGH) minigene is frequently used in the derivation of transgenic mouse lines to enhance transgene expression. Although this minigene is present in the transgenes as a second-cistron, and thus not thought to be expressed, we found that three commonly used lines, Pdx1-CreLate, RIP-Cre, and MIP-GFP, each expressed significant amounts of hGH in pancreatic islets. Locally secreted hGH binds to prolactin receptors on β cells, activates STAT5 signaling, and induces pregnancy-like changes in gene expression, thereby augmenting pancreatic β cell mass and insulin content. In addition, islets of Pdx1-CreLate mice have lower GLUT2 expression and reduced glucose-induced insulin release and are protected against the β cell toxin streptozotocin. These findings may be important when interpreting results obtained when these and other hGH minigene-containing transgenic mice are used.

Aims/hypothesisUpregulation of the functional beta cell mass is required to match the physiological demands of mother and fetus during pregnancy. This increase is dependent on placental lactogens (PLs) and prolactin receptors, but the mechanisms underlying these events are only partially understood. We studied the mRNA expression profile of mouse islets during pregnancy to gain a better insight into these changes.MethodsRNA expression was measured ex vivo via microarrays and quantitative RT-PCR. In vivo observations were extended by in vitro models in which ovine PL was added to cultured mouse islets and MIN6 cells.ResultsmRNA encoding both isoforms of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), i.e. Tph1 and Tph2, were strongly induced (fold change 25- to 200-fold) during pregnancy. This induction was mimicked by exposing islets or MIN6 cells to ovine PLs for 24 h and was dependent on janus kinase 2 and signal transducer and activator of transcription 5. Parallel to Tph1 mRNA and protein induction, islet serotonin content increased to a peak level that was 200-fold higher than basal. Interestingly, only a subpopulation of the beta cells was serotonin-positive in vitro and in vivo. The stored serotonin pool in pregnant islets and PL-treated MIN6 cells was rapidly released (turnover once every 2 h).Conclusions/interpretationA very strong lactogen-dependent upregulation of serotonin biosynthesis occurs in a subpopulation of mouse islet beta cells during pregnancy. Since the newly formed serotonin is rapidly released, this lactogen-induced beta cell function may serve local or endocrine tasks, the nature of which remains to be identified.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-010-1913-7) contains supplementary material, which is available to authorised users.

Glucose homeostasis is critically dependent on insulin release from pancreatic β-cells, which is strictly regulated by glucose-induced oscillations in membrane potential (V m ) and the cytosolic calcium level ([Ca 2+ ] cyt ] cyt ) (1-4). This pattern, consisting of slow waves of depolarized plateaus on which bursts of action potentials are superimposed and separated by electrically silent intervals, plays a critical role in the regulation of insulin secretion. Indeed, in the absence of depolarization, no insulin is released, and the "extent" of electrical activity largely determines the amount of released insulin (1,5 ] cyt ), and the cellular metabolism of the β-cell (1, 6). The increase in [Ca 2+ ] cyt originates from glucoseinduced Ca 2+ influx through voltage-gated L-type Ca 2+ channels and, possibly, Ca 2+ mobilization from intracellular stores, the latter promoted by activation of the phospholipase C system and generation of inositol 1,4,5-trisphosphate (1,7,8). Despite intensive investigation, several aspects of the rhythmic electrical activity of β-cells, such as the origin of the variability in oscillation pattern, remain unclear. Indeed, glucose stimulation can result in highfrequency short bursts, low-frequency long bursts, or a combination of these two patterns, also known as compound bursts (9, 10).In this study we identified TRPM5 as a player in the electrical activity of glucose-stimulated pancreatic β-cells. TRPM5 is one of 28 members of the large transient receptor potential (TRP) superfamily (11-13). TRPM5, and its close homologue TRPM4, are Ca 2+ -activated cation channels that are permeable for monovalent cations, but not divalent cations, with a conductance of approximately 20 to 25 pS (14-16). Using Trpm5−/− mice we show here that this channel promotes high-frequency glucose-induced oscillations in V m and [Ca 2+ ] cyt in pancreatic β-cells. Loss of TRPM5 expression, and high-frequency bursting, is functionally relevant as this leads to reduced glucose-induced insulin release from isolated islets and impaired glucose tolerance. Results and DiscussionExpression of the Trpm5 Gene in Pancreatic β-Cells. Previously, Trpm5 expression was shown on the mRNA level in several β-cell lines and in human and mouse tissues, including taste buds, intestine, and pancreatic islets (13,(17)(18)(19)(20)(21)(22). Here we describe immunostaining of TRPM5 protein in pancreatic islets with a specific antibody (Fig. 1). TRPM5 is colocalized with insulin in WT islets, strongly suggesting expression of TRPM5 in insulin secreting β-cells. Specific staining with the TRPM5 antibody is absent in Trpm5 −/− islets. Quantitative PCR experiments in a purified β-cell sample confirmed expression of Trpm5 in the β-cells. Expression of Trpm5 could also be detected in purified α-cells, although to a lower level compared with β-cells (Fig. S1). Characterization of a Ca2+ Release-Activated Cation Current in Pancreatic Islet Cells. To determine whether TRPM5 is part of the Ca 2+-activated monovalent cation current descri...