In vivo models of gestational and type 2 diabetes mellitus characterized by endocrine pancreas cell impairments.

Insulin resistance contributes to the development of various diseases, including type 2 diabetes and gestational diabetes. Even though gestational diabetes is specific to pregnancy, it can result in long-term glucose intolerance and type 2 diabetes after delivery. Given the substantial health and economic burdens associated with diabetes, it is imperative to better understand the mechanisms leading to insulin resistance and type 2 diabetes so that treatments targeted at reversing symptoms can be developed. Considering that the endocrine cells of the pancreas (islets of Langerhans) largely contribute to the pathogenesis of diabetes (beta-cell insufficiency and dysfunction), the elucidation of the various mechanisms of endocrine cell plasticity is important to understand. By better defining these mechanisms, targeted therapeutics can be developed to reverse symptoms of beta-cell deficiency and insulin resistance in diabetes. Animal models play an important role in better understanding these mechanisms, as techniques for in vivo imaging of endocrine cells in the pancreas are limited. Therefore, this review article will discuss the available rodent models of gestational and type 2 diabetes that are characterized by endocrine cell impairments in the pancreas, discuss the models with a comparison to human diabetes, and explore the potential mechanisms of endocrine cell plasticity that contribute to these phenotypes, as these mechanisms could ultimately be used to reverse blood glucose dysregulation in diabetes.

Paraxanthine safety and comparison to caffeine.

Introduction: Caffeine, one of the most ubiquitous ingredients found in beverages and other ingested food products, has a long history of safe use. As a member of the methylxanthine class of stimulants, caffeine is not devoid of unwanted side effects at any serving level. Caffeine safety has been the subject of a safety workshop by FDA and the Institute of Medicine in the past decade. Thus, investigation into an alternate stimulant with similar pharmacology but improved safety is warranted. Paraxanthine (1,7-dimethylxanthine) is the predominant metabolite of caffeine in humans with similar stimulant properties. The few toxicity studies that are available for paraxanthine suggest that the molecule is relatively safe, although thorough characterization of its safety is required prior to widespread incorporation into foods/beverages. Methods: The aim of this study was to evaluate the toxicity of paraxanthine (Rarebird, Inc.) relative to caffeine through a battery of toxicological studies conducted in accordance with international guidelines. These studies evaluated the potential mutagenicity (bacterial reverse mutation, in vitro mammalian chromosomal aberration), genetic toxicity (in vitro mammalian cell gene mutation) and acute, sub-acute and sub-chronic oral toxicity of paraxanthine in Sprague Dawley rats. Results/Discussion: There was no evidence of genetic toxicity or mutagenicity in the in vitro studies. An acute oral LD50 of 829.20 mg/kg body weight (bw) was established. There was no mortality or treatment-related adverse effects in the 14-day repeat dose oral toxicity study, wherein rats received low, mid, or high doses of paraxanthine (50, 100, or 150 mg/kg bw, n = 5 rats/sex/group). The same findings were observed in the subchronic repeat-dose 90-day oral toxicity study at daily doses of paraxanthine of 100, 150, or 185 mg/kg bw which were compared to caffeine at 150 or 185 mg/kg bw (n = 10 animals/sex/group). However, mortality was reported in two animals in the high dose caffeine-treated animals. Therefore, the no observed adverse effect level (NOAEL) from the 90-day study was determined to be 150 mg/kg bw for caffeine and 185 mg/kg bw for paraxanthine for both male and female Sprague Dawley rats. These findings may suggest that paraxanthine could be a safer alternative to caffeine in humans.

Pleiotrophin Expression and Actions in Pancreatic ß-Cells.

Pleiotrophin (PTN) is a heparin-binding cytokine that is widely expressed during early development and increases in maternal circulation during pregnancy.Aged PTN-deficient mice exhibit insulin resistance, suggesting a role in metabolic control. The objectives of this study were to determine if PTN is expressed in mouse pancreatic ß-cells in young vs. adult animals, and its effects on DNA synthesis, ß-cell gene expression and glucose-stimulated insulin secretion (GSIS). The Ptn gene was expressed in isolated fractions of young mouse ß-cells, especially within immature ß-cells with low glucose transporter 2 expression. Expression was retained in the adult pancreas but did not significantly change during pregnancy. PTN and its receptor, phosphotyrosine phosphatase-ß/ζ, were also expressed in the proliferative INS1E ß-cell line. Fluorescence immunohistochemistry showed that PTN peptide was present in islets of Langerhans in adult mice, associated predominantly with ß-cells. The percentage of ß-cells staining for PTN did not alter during mouse pregnancy, but intense staining was seen during ß-cell regeneration in young mice following depletion of ß-cells with streptozotocin. Incubation of INS1E cells with PTN resulted in an increased DNA synthesis as measured by Ki67 localization and increased expression of Pdx1 and insulin. However, both DNA synthesis and GSIS were not altered by PTN in isolated adult mouse islets. The findings show that Ptn is expressed in mouse ß-cells in young and adult life and could potentially contribute to adaptive increases in ß-cell mass during early life or pregnancy.

Acetone Ingestion Mimics a Fasting State to Improve Glucose Tolerance in a Mouse Model of Gestational Hyperglycemia.

Gestational diabetes mellitus results, in part, from a sub-optimal ß-cell mass (BCM) during pregnancy. Artemisinins were reported to increase BCM in models of diabetes by α- to ß-cell conversion leading to enhanced glucose tolerance. We used a mouse model of gestational glucose intolerance to compare the effects of an artemisinin (artesunate) on glycemia of pregnant mice with vehicle treatment (acetone) or no treatment. Animals were treated daily from gestational days (GD) 0.5 to 6.5. An intraperitoneal glucose tolerance test was performed prior to euthanasia at GD18.5 or post-partum. Glucose tolerance was significantly improved in both pregnant and non-pregnant mice with both artesunate and vehicle-alone treatment, suggesting the outcome was primarily due to the acetone vehicle. In non-pregnant, acetone-treated animals, improved glucose tolerance was associated with a higher BCM and a significant increase in bihormonal insulin and glucagon-containing pancreatic islet cells, suggesting α- to ß-cell conversion. BCM did not differ with treatment during pregnancy or post-partum. However, placental weight was higher in acetone-treated animals and was associated with an upregulation of apelinergic genes. Acetone-treated animals had reduced weight gain during treatment despite comparable food consumption to non-treated mice, suggesting transient effects on nutrient uptake. The mean duodenal and ileum villus height was reduced following exposure to acetone. We conclude that acetone treatment may mimic transient fasting, resulting in a subsequent improvement in glucose tolerance during pregnancy.

Ontology of the apelinergic system in mouse pancreas during pregnancy and relationship with ß-cell mass.

The apelin receptor (Aplnr) and its ligands, Apelin and Apela, contribute to metabolic control. The insulin resistance associated with pregnancy is accommodated by an expansion of pancreatic ß-cell mass (BCM) and increased insulin secretion, involving the proliferation of insulin-expressing, glucose transporter 2-low (Ins+Glut2LO) progenitor cells. We examined changes in the apelinergic system during normal mouse pregnancy and in pregnancies complicated by glucose intolerance with reduced BCM. Expression of Aplnr, Apelin and Apela was quantified in Ins+Glut2LO cells isolated from mouse pancreata and found to be significantly higher than in mature ß-cells by DNA microarray and qPCR. Apelin was localized to most ß-cells by immunohistochemistry although Aplnr was predominantly associated with Ins+Glut2LO cells. Aplnr-staining cells increased three- to four-fold during pregnancy being maximal at gestational days (GD) 9-12 but were significantly reduced in glucose intolerant mice. Apelin-13 increased ß-cell proliferation in isolated mouse islets and INS1E cells, but not glucose-stimulated insulin secretion. Glucose intolerant pregnant mice had significantly elevated serum Apelin levels at GD 9 associated with an increased presence of placental IL-6. Placental expression of the apelinergic axis remained unaltered, however. Results show that the apelinergic system is highly expressed in pancreatic ß-cell progenitors and may contribute to ß-cell proliferation in pregnancy.

Metabolic Adaptations to Pregnancy in Healthy and Gestational Diabetic Pregnancies: The Pancreas - Placenta Axis.

Normal pregnancy is associated with increased insulin resistance as a metabolic adaptation to the nutritional demands of the placenta and fetus, and this is amplified in obese mothers. Insulin resistance is normally compensated for by an adaptive increase in pancreatic ß-cell mass together with enhanced glucose-stimulated insulin release. Placentally-derived hormones and growth factors are central to the altered pancreatic morphology and function. A failure of ß-cells to undergo adaptive change after the first trimester has been linked with gestational diabetes. In the pregnant mouse, an increase in ß-cell replication contributes to a 2-3-fold increase in mass peaking in late gestation, depending on the proliferation of existing ß-cells, the differentiation of resident progenitor ß-cells, or islet cell transdifferentiation. Using mouse models and human studies placenta- and islet of Langerhans-derived molecules have been identified that are likely to contribute to the metabolic adaptations to pregnancy and whose physiology is altered in the obese, glucose-intolerant mother. Maternal obesity during pregnancy can create a pro-inflammatory environment that can disrupt the response of the ß-cells to the endocrine signals of pregnancy and limit the adaptive changes in ß-cell mass and function, resulting in an increased risk of gestational diabetes.

Increased alpha and beta cell mass during mouse pregnancy is not dependent on transdifferentiation.

Maternal pancreatic beta-cell mass (BCM) increases during pregnancy to compensate for relative insulin resistance. If BCM expansion is suboptimal, gestational diabetes mellitus can develop. Alpha-cell mass (ACM) also changes during pregnancy, but there is a lack of information about α-cell plasticity in pregnancy and whether α- to ß-cell transdifferentiation can occur. To investigate this, we used a mouse model of gestational glucose intolerance induced by feeding low-protein (LP) diet from conception until weaning and compared pregnant female offspring to control diet-fed animals. Control and LP pancreata were collected for immunohistochemical analysis and serum glucagon levels were measured. In order to lineage trace α- to ß-cell conversion, we utilized transgenic mice expressing yellow fluorescent protein behind the proglucagon gene promoter (Gcg-Cre/YFP) and collected pancreata for histology at various gestational timepoints. Alpha-cell proliferation increased significantly at gestational day (GD) 9.5 in control pregnancies resulting in an increased ACM at GD18.5, and this was significantly reduced in LP animals. Despite these changes, serum glucagon was higher in LP mice at GD18.5. Pregnant Gcg-Cre/YFP mice showed no increase in the abundance of insulin+YFP+glucagon- cells (phenotypic ß-cells). A second population of insulin+YFP+glucagon+ cells was identified which also did not alter during pregnancy. However, there was an altered anatomical distribution within islets with fewer insulin+YFP+glucagon- cells but more insulin+YFP+glucagon+ cells being present in the islet mantle at GD18.5. These findings demonstrate that dynamic changes in ACM occur during normal pregnancy and were altered in glucose-intolerant pregnancies.

Altered pancreas remodeling following glucose intolerance in pregnancy in mice.

Gestational diabetes mellitus increases the risk of dysglycemia postpartum, in part, due to pancreatic ß-cell dysfunction. However, no histological evidence exists comparing endocrine pancreas after healthy and glucose-intolerant pregnancies. This study sought to address this knowledge gap, in addition to exploring the contribution of an inflammatory environment to changes in endocrine pancreas after parturition. We used a previously established mouse model of gestational glucose intolerance induced by dietary low protein insult from conception until weaning. Pancreas and adipose samples were collected at 7, 30 and 90 days postpartum for histomorphometric and cytokine analyses, respectively. Glucose tolerance tests were performed prior to euthanasia and blood was collected via cardiac puncture. Pregnant female mice born to dams fed a low protein diet previously shown to develop glucose intolerance at late gestation relative to controls continued to be glucose intolerant until 1 month postpartum. However, glucose tolerance normalized by 3 months postpartum. Glucose intolerance at 7 days postpartum was associated with lower beta- and alpha-cell fractional areas and higher adipose levels of pro-inflammatory cytokine, interleukin-6. By 3 months postpartum, a compensatory increase in the number of small islets and a higher insulin to glucagon ratio likely enabled euglycemia to be attained in the previously glucose-intolerant mice. The results show that impairments in endocrine pancreas compensation in hyperglycemic pregnancy persist after parturition and contribute to prolonged glucose intolerance. These impairments may increase the susceptibility to development of future type 2 diabetes.

A mouse model of gestational glucose intolerance through exposure to a low protein diet during fetal and neonatal development.

KEY POINTS: Pancreatic ß-cell dysfunction is hypothesized to be the significant determinant of gestational diabetes pathogenesis, however pancreatic samples from patients are scarce. This study reports a novel mouse model of gestational glucose intolerance in pregnancy, originating from previous nutrition restriction in utero, in which glucose intolerance was restricted to late gestation as is seen in human gestational diabetes. Glucose intolerance was attributed to reduced ß-cell proliferation, leading to impaired gestational ß-cell mass expansion in maternal endocrine pancreas, in addition to reduced glucose-stimulated insulin secretion. This model reproduces some of the features of gestational diabetes and is suitable for testing safe therapeutic interventions that increase ß-cell mass during pregnancy and prevent or reverse gestational glucose intolerance. ABSTRACT: Gestational diabetes mellitus (GDM) is an increasingly prevalent form of diabetes that appears during pregnancy. Pathological studies link a failure to adaptively increase maternal pancreatic ß-cell mass (BCM) in pregnancy to GDM. Due to the scarcity of pancreatic samples from GDM patients, we sought to develop a novel mouse model for impaired gestational glucose tolerance. Mature female C57Bl/6 mouse offspring (F1) born to dams fed either a control (C) or low-protein (LP) diet during gestation and lactation were randomly allocated into two subsequent study groups: pregnant (CP, LPP) or non-pregnant (CNP, LPNP). Glucose tolerance tests were performed at gestational day (GD) 9, 12 and 18. Subsequently, pancreata were removed for fluorescence immunohistochemistry to assess α-cell mass (ACM), BCM and ß-cell proliferation. An additional group of animals was used to measure insulin secretion from isolated islets at GD18. LPP females displayed glucose intolerance compared to CP females at GD18 (P < 0.001). BCM increased threefold at GD18 in CP females. However, LPP females had reduced BCM expansion (P < 0.01) concurrent with reduced ß-cell proliferation at GD12 (P < 0.05). LPP females also had reduced ACM expansion at GD18 (P < 0.01). LPP islets had impaired glucose-stimulated insulin secretion in vitro compared to CP islets (P < 0.01). Therefore, impaired glucose tolerance during pregnancy is associated with a failure to adequately adapt BCM, as a result of reduced ß-cell proliferation, in addition to lower glucose-stimulated insulin secretion. This model could be used to evaluate novel interventions during pregnancy to increase BCM or function as a strategy to prevent/reverse GDM.

An increase in immature ß-cells lacking Glut2 precedes the expansion of ß-cell mass in the pregnant mouse.

A compensatory increase in ß-cell mass occurs during pregnancy to counter the associated insulin resistance, and a failure in adaptation is thought to contribute to gestational diabetes. Insulin-expressing but glucose-transporter-2-low (Ins+Glut2LO) progenitor cells are present in mouse and human pancreas, being predominantly located in extra-islet ß-cell clusters, and contribute to the regeneration of the endocrine pancreas following induced ablation. We therefore sought to investigate the contribution of Ins+Glut2LO cells to ß-cell mass expansion during pregnancy. Female C57Bl/6 mice were time mated and pancreata were collected at gestational days (GD) 6, 9, 12, 15, and 18, and postpartum D7 (n = 4/time-point) and compared to control (non-pregnant) animals. Beta cell mass, location, proliferation (Ki67+), and proportion of Ins+Glut2LO cells were measured using immunohistochemistry and bright field or confocal microscopy. Beta cell mass tripled by GD18 and ß-cell proliferation peaked at GD12 in islets (≥6 ß-cells) and small ß-cell clusters (1-5 ß-cells). The proportion and fraction of Ins+Glut2LO cells undergoing proliferation increased significantly at GD9 in both islets and clusters, preceding the increase in ß-cell mass and proliferation, and their proliferation within clusters persisted until GD15. The overall number of clusters increased significantly at GD9. Quantitative PCR showed a significant increase in Pdx1 presence at GD9 vs. GD18 or control pancreas, and Pdx1 was visualized by immunohistochemistry within both Ins+Glut2LO and Ins+Glut2HI cells within clusters. These results indicate that Ins+Glut2LO cells are likely to contribute to ß-cell mass expansion during pregnancy.