Altered growth and development of the endocrine pancreas is a frequent cause of the hyperglycemia associated with diabetes. Here we show that microRNA-375 (miR-375), which is highly expressed in pancreatic islets, is required for normal glucose homeostasis. Mice lacking miR-375 (375KO) are hyperglycemic, exhibit increased total pancreatic ␣-cell numbers, fasting and fed plasma glucagon levels, and increased gluconeogenesis and hepatic glucose output. Furthermore, pancreatic ␤-cell mass is decreased in 375KO mice as a result of impaired proliferation. In contrast, pancreatic islets of obese mice (ob/ob), a model of increased ␤-cell mass, exhibit increased expression of miR-375. Genetic deletion of miR-375 from these animals (375/ob) profoundly diminished the proliferative capacity of the endocrine pancreas and resulted in a severely diabetic state. Bioinformatic analysis of transcript data from 375KO islets revealed that miR-375 regulates a cluster of genes controlling cellular growth and proliferation. These data provide evidence that miR-375 is essential for normal glucose homeostasis, ␣-and ␤-cell turnover, and adaptive ␤-cell expansion in response to increasing insulin demand in insulin resistance.diabetes ͉ glucagon ͉ microRNA ͉ islet ͉ proliferation T he maintenance of ␤-cell mass during development and throughout life is a highly regulated process responsible for normal glucose homeostasis. Defects in the development of pancreatic islets lead to changes in islet composition, and they often result in the hyperglycemia that characterizes the diabetic state (1, 2). The dynamic adaptation of ␤-cell mass in adult life is influenced by various metabolic stresses, which control the balance between proliferation and apoptosis. These processes, known to be regulated at the transcriptional level, contribute to the development and maintenance of many tissues, including the pancreatic islet (3, 4). Recent studies have shown that microRNAs (miRNAs), which regulate gene expression at a posttranscriptional level, are powerful regulators of growth, differentiation, and organ function (5-7). For instance, mutant mice in which miRNAs are collectively silenced during endocrine pancreas development exhibit defects in all pancreatic lineages, including a dramatic reduction of insulin-producing ␤ cells (8). It is estimated that Ϸ30% of all protein coding genes are miRNA targets. Combining target prediction with experimental analysis of miRNA expression and production of loss of function mutants is beginning to improve our understanding of the roles that miRNAs play in normal and disease states (7-12). We have previously reported that miR-375, the highest expressed miRNA in pancreatic islets of humans and mice, regulates insulin secretion in isolated pancreatic ␤ cells (13). In this study we have investigated the effect of genetic ablation of miR-375 on pancreatic islet development and function and in the etiology of type 2 diabetes. Results Development of Hyperglycemia in miR-375-Null Mice.To elucidate the role of miR-375 in th...

OBJECTIVE-To characterize the voltage-gated ion channels in human ␤-cells from nondiabetic donors and their role in glucosestimulated insulin release.RESEARCH DESIGN AND METHODS-Insulin release was measured from intact islets. Whole-cell patch-clamp experiments and measurements of cell capacitance were performed on isolated ␤-cells. The ion channel complement was determined by quantitative PCR.RESULTS-Human ␤-cells express two types of voltage-gated K ϩ currents that flow through delayed rectifying (K V 2.1/2.2) and large-conductance Ca 2ϩ -activated K ϩ (BK) channels. Blockade of BK channels (using iberiotoxin) increased action potential amplitude and enhanced insulin secretion by 70%, whereas inhibition of K V 2.1/2.2 (with stromatoxin) was without stimulatory effect on electrical activity and secretion. Voltage-gated tetrodotoxin (TTX)-sensitive Na ϩ currents (Na V 1.6/1.7) contribute to the upstroke of action potentials. Inhibition of Na ϩ currents with TTX reduced glucose-stimulated (6 -20 mmol/l) insulin secretion by 55-70%. Human ␤-cells are equipped with L-(Ca V 1.3), P/Q-(Ca V 2.1), and T-(Ca V 3.2), but not N-or R-type Ca 2ϩ channels. Blockade of L-type channels abolished glucosestimulated insulin release, while inhibition of T-and P/Q-type Ca 2ϩ channels reduced glucose-induced (6 mmol/l) secretion by 60 -70%. Membrane potential recordings suggest that L-and T-type Ca 2ϩ channels participate in action potential generation. Blockade of P/Q-type Ca 2ϩ channels suppressed exocytosis (measured as an increase in cell capacitance) by Ͼ80%, whereas inhibition of L-type Ca 2ϩ channels only had a minor effect.CONCLUSIONS-Voltage-gated T-type and L-type Ca 2ϩ channels as well as Na ϩ channels participate in glucose-stimulated electrical activity and insulin secretion. Ca 2ϩ -activated BK channels are required for rapid membrane repolarization. Exocytosis of insulin-containing granules is principally triggered by Ca 2ϩ influx through P/Q-type Ca 2ϩ channels. Diabetes 57:1618-1628, 2008

Pancreatic β cells secrete insulin, the body's only hormone capable of lowering plasma glucose levels. Impaired or insufficient insulin secretion results in diabetes mellitus. The β cell is electrically excitable; in response to an elevation of glucose, it depolarizes and starts generating action potentials. The electrophysiology of mouse β cells and the cell's role in insulin secretion have been extensively investigated. More recently, similar studies have been performed on human β cells. These studies have revealed numerous and important differences between human and rodent β cells. Here we discuss the properties of human pancreatic β cells: their glucose sensing, the ion channel complement underlying glucose-induced electrical activity that culminates in exocytotic release of insulin, the cellular control of exocytosis, and the modulation of insulin secretion by circulating hormones and locally released neurotransmitters. Finally, we consider the pathophysiology of insulin secretion and the interactions between genetics and environmental factors that may explain the current diabetes epidemic.