ObjectiveThe C57Bl/6J (Bl/6J) mouse is the most widely used strain in metabolic research. This strain carries a mutation in nicotinamide nucleotide transhydrogenase (Nnt), a mitochondrial enzyme involved in NADPH production, which has been suggested to lead to glucose intolerance and beta-cell dysfunction. However, recent reports comparing Bl/6J to Bl/6N (carrying the wild-type Nnt allele) under normal diet have led to conflicting results using glucose tolerance tests. Thus, we assessed glucose-stimulated insulin secretion (GSIS), insulin sensitivity, clearance and central glucose-induced insulin secretion in Bl/6J and N mice using gold-standard methodologies.MethodsGSIS was measured using complementary tests (oral and intravenous glucose tolerance tests) and hyperglycemic clamps. Whole-body insulin sensitivity was assessed using euglycemic-hyperinsulinemic clamps. Neurally-mediated insulin secretion was measured during central hyperglycemia.ResultsBl/6J mice have impaired GSIS compared to Bl/6N when glucose is administered intravenously during both a tolerance test and hyperglycemic clamp, but not in response to oral glucose. First and second phases of GSIS are altered without changes in whole body insulin sensitivity, insulin clearance, beta-cell mass or central response to glucose, thereby demonstrating defective beta-cell function in Bl/6J mice.ConclusionsThe Bl/6J mouse strain displays impaired insulin secretion. These results have important implications for choosing the appropriate test to assess beta-cell function and background strain in genetically modified mouse models.

Genetic analysis has shown that the slower than normal rhythmic defecation behavior of the clk-1 mutants of Caenorhabditis elegans is the result of altered lipoprotein metabolism. We show here that this phenotype can be suppressed by drugs that affect lipoprotein metabolism, including drugs that affect HMG-CoA reductase activity, reverse cholesterol transport, or HDL levels. These pharmacological effects are highly specific, as these drugs affect defecation only in clk-1 mutants and not in the wild-type and do not affect other behaviors of the mutants. Furthermore, drugs that affect processes not directly related to lipid metabolism show no or minimal activity. Based on these findings, we carried out a compound screen that identified 190 novel molecules that are active on clk-1 mutants, 15 of which also specifically decrease the secretion of apolipoprotein B (apoB) from HepG2 hepatoma cells. The other 175 compounds are potentially active on lipid-related processes that cannot be targeted in cell culture. One compound, CHGN005, was tested and found to be active at reducing apoB secretion in intestinal Caco-2 cells as well as in HepG2 cells. This compound was also tested in a mouse model of dyslipidemia and found to decrease plasma cholesterol and triglyceride levels.Thus, target processes for pharmacological intervention on lipoprotein synthesis, transport, and metabolism are conserved between nematodes and vertebrates, which allows the use of C. elegans for drug discovery.

The molecular mechanisms of β-cell compensation to metabolic stress are poorly understood. We previously observed that nutrient-induced β-cell proliferation in rats is dependent on epidermal growth factor receptor (EGFR) signaling. The aim of this study was to determine the role of the EGFR ligand heparin-binding EGF-like growth factor (HB-EGF) in the β-cell proliferative response to glucose, a β-cell mitogen and key regulator of β-cell mass in response to increased insulin demand. We show that exposure of isolated rat and human islets to HB-EGF stimulates β-cell proliferation. In rat islets, inhibition of EGFR or HB-EGF blocks the proliferative response not only to HB-EGF but also to glucose. Furthermore, knockdown of HB-EGF in rat islets blocks β-cell proliferation in response to glucose ex vivo and in vivo in transplanted glucose-infused rats. Mechanistically, we demonstrate that HB-EGF mRNA levels are increased in β-cells in response to glucose in a carbohydrate-response element–binding protein (ChREBP)–dependent manner. In addition, chromatin immunoprecipitation studies identified ChREBP binding sites in proximity to the HB-EGF gene. Finally, inhibition of Src family kinases, known to be involved in HB-EGF processing, abrogated glucose-induced β-cell proliferation. Our findings identify a novel glucose/HB-EGF/EGFR axis implicated in β-cell compensation to increased metabolic demand.

Puberty is associated with transient insulin resistance that normally recedes at the end of puberty; however, in overweight children insulin resistance persists leading to an increased risk of type 2 diabetes. The mechanisms whereby pancreatic b cells adapt to pubertal insulin resistance, and how they are affected by the metabolic status, have not been investigated. Here we show that puberty is associated with a transient increase in b-cell proliferation in rats and humans of both sexes. In rats, b-cell proliferation correlated with a rise in growth hormone (GH) levels. Serum from pubertal rats and humans promoted b-cell proliferation, suggesting the implication of a circulating factor. In pubertal rat islets, expression of genes of the GH/serotonin (5-HT) pathway underwent changes consistent with proliferative effect.Inhibition of the pro-proliferative 5-HT receptor isoform HTR2B blocked the increase in bcell proliferation in pubertal islets ex vivo and in vivo. Peri-pubertal metabolic stress blunted b-cell proliferation during puberty and led to altered glucose homeostasis later in life. This study identifies a role of GH/GHR/5-HT/HTR2B signaling in the control of b-cell mass expansion during puberty and a mechanistic link between pubertal obesity and the risk of developing type 2 diabetes.

The pancreatic β-cell responds to changes in the nutrient environment to maintain glucose homeostasis by adapting its function and mass. Nutrients can act directly on the β-cell and also indirectly through the brain via autonomic nerves innervating islets. Despite the importance of the brain-islet axis in insulin secretion, relatively little is known regarding its involvement in β-cell proliferation. We previously demonstrated that prolonged infusions of nutrients in rats provoke a dramatic increase in β-cell proliferation in part because of the direct action of nutrients. Here, we addressed the contribution of the autonomic nervous system. In isolated islets, muscarinic stimulation increased, whereas adrenergic stimulation decreased, glucose-induced β-cell proliferation. Blocking α-adrenergic receptors reversed the effect of epinephrine on glucose + nonesterified fatty acids (NEFA)-induced β-cell proliferation, whereas activation of β-adrenergic receptors was without effect. Infusion of glucose + NEFA toward the brain stimulated β-cell proliferation, and this effect was abrogated following celiac vagotomy. The increase in β-cell proliferation following peripheral infusions of glucose + NEFA was not inhibited by vagotomy or atropine treatment but was blocked by coinfusion of epinephrine. We conclude that β-cell proliferation is stimulated by parasympathetic and inhibited by sympathetic signals. Whereas glucose + NEFA in the brain stimulates β-cell proliferation through the vagus nerve, β-cell proliferation in response to systemic nutrient excess does not involve parasympathetic signals but may be associated with decreased sympathetic tone.