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Pregnancy and obesity are frequently associated with diminished insulin sensitivity, which is normally compensated for by an expansion of the functional β cell mass that prevents chronic hyperglycemia and development of diabetes mellitus. The molecular basis underlying compensatory β cell mass expansion is largely unknown. We found in ents that β cell mass expansion during pregnancy and obesity is associated with changes in the expression of several islet microRNAs, including miR-338-3p. In isolated pancreatic islets, we recapitulated the decreased miR-338-3p level observed in gestation and obesity by activating the G protein-coupled estrogen receptor GPR30 and the glucagon-like peptide 1 (GLP1) receptor. Blockade of miR-338-3p in β cells using specific anti-miR molecules mimicked gene expression changes occurring during13 cell mass expansion and resulted in increased proliferation and improved survival both in vitro and in vivo. These findings point to a major role for miR-338-3p in compensatory0 cell mass expansion occurring under different insulin resistance states., Introduction Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia resulting from defective function and/or loss of insulin-secreting β cells (1), (2). These cells, located within the islets of [...]

Alteration of pancreatic [beta]-cell survival and Preproinsulin gene expression by prolonged hyperglycemia may result from increased c-MYC expression. However, it is unclear whether c-MYC effects on [beta]-cell function are compatible with its proposed role in glucotoxicity. We therefore tested the effects of short-term c-MYC activation on key [beta]-cell stimulus-secretion coupling events in islets isolated from mice expressing a tamoxifenswitchable form of c-MYC in [beta]-cells (MycER) and their wild-type littermates. Tamoxifen treatment of wild-type islets did not affect their cell survival, Preproinsulin gene expression, and glucose stimulussecretion coupling. In contrast, tamoxifen-mediated c-MYC activation for 2-3 days triggered cell apoptosis and decreased Preproinsulin gene expression in MycER islets. These effects were accompanied by mitochondrial membrane hyperpolarization at all glucose concentrations, a higher resting intracellular calcium concentration ([[[Ca.sup.2+].sub.i]], and lower glucose-induced [[[Ca.sup.2+]].sub.i] rise and islet insulin content, leading to a strong reduction of glucose-induced insulin secretion. Compared with these effects, 1-wk culture in 30 mmol/l glucose increased the islet sensitivity to glucose stimulation without reducing the maximal glucose effectiveness or the insulin content. In contrast, overnight exposure to a low [H.sub.2][O.sub.2] concentration increased the islet resting [[[Ca.sup.2+]].sub.i] and reduced the amplitude of the maximal glucose response as in tamoxifen-treated MycER islets. In conclusion, c-MYC activation rapidly stimulates apoptosis, reduces Preproinsulin gene expression and insulin content, and triggers functional alterations of [beta]-cells that are better mimicked by overnight exposure to a low [H.sub.2][O.sub.2] concentration than by prolonged culture in high glucose. [beta]-cell mass; apoptosis; cytosolic calcium concentration; insulin secretion; mitochondrial membrane potential

Stimulatory concentrations of glucose induce two patterns of cytosolic [Ca.sup.2+] concentration ([[[Ca.sup.2+]].sub.c]) oscillations in mouse islets: simple or mixed. In the mixed pattern, rapid oscillations are superimposed on slow ones. In the present study, we examined the role of the membrane potential in the mixed pattern and the impact of this pattern on insulin release. Simultaneous measurement of [[[Ca.sup.2+]].sub.c] and insulin release from single islets revealed that mixed [[[Ca.sup.2+]].sub.c] oscillations triggered synchronous oscillations of insulin secretion. Simultaneous recordings of membrane potential in a single [beta]-cell within an islet and of [[[Ca.sup.2+]].sub.c] in the whole islet demonstrated that the mixed pattern resulted from compound bursting (i.e., clusters of membrane potential oscillations separated by prolonged silent intervals) that was synchronized in most [beta]-cells of the islet. Each slow [[[Ca.sup.2+]].sub.c] increase during mixed oscillations was due to a progressive summation of rapid oscillations. Digital image analysis confirmed the good synchrony between subregions of an islet. By contrast, islets from sarco(endo)plasmic reticulum [Ca.sup.2+]-ATPase isoform 3 (SERCA3)-knockout mice did not display typical mixed [[[Ca.sup.2+]].sub.c] oscillations in response to glucose. This results from a lack of progressive summation of rapid oscillations and from altered spontaneous electrical activity, i.e., lack of compound bursting, and membrane potential oscillations characterized by lower-frequency but larger-depolarization phases than observed in SERCA[3.sup.+/+] [beta]-cells. We conclude that glucose-induced mixed [[[Ca.sup.2+]].sub.c] oscillations result from compound bursting in all [beta]-cells of the islet. Disruption of SERCA3 abolishes mixed [[[Ca.sup.2+]].sub.c] oscillations and augments [beta]-cell depolarization. This latter observation indicates that the endoplasmic reticulum participates in the control of the [beta]-cell membrane potential during glucose stimulation. electrical activity; insulin-secreting cell; thapsigargin