Sean W Limesand

Islet replacement is a promising therapy for treating diabetes mellitus, but the supply of donor tissue for transplantation is limited. To overcome this limitation, endocrine tissue can be expanded, but this requires an understanding of normal developmental processes that regulate islet formation. In this study, we compare pancreas development in sheep and human, and provide evidence that an epithelial-mesenchymal transition (EMT) is involved in beta-cell differentiation and islet formation. Transcription factors know to regulate pancreas formation, pancreatic duodenal homeobox factor 1, neurogenin 3, NKX2-2, and NKX6-1, which were expressed in the appropriate spatial and temporal pattern to coordinate pancreatic bud outgrowth and direct endocrine cell specification in sheep. Immunofluorescence staining of the developing pancreas was used to co-localize insulin and epithelial proteins (cytokeratin, E-cadherin, and beta-catenin) or insulin and a mesenchymal protein (vimentin). In sheep, individual beta-cells become insulin-positive in the progenitor epithelium, then lose epithelial characteristics, and migrate out of the epithelial layer to form islets. As beta-cells exit the epithelial progenitor cell layer, they acquire mesenchymal characteristics, shown by their acquisition of vimentin. In situ hybridization expression analysis of the SNAIL family members of transcriptional repressors (SNAIL1, -2, and -3; listed as SNAI1, -2, -3 in the HUGO Database) showed that each of the SNAIL genes was expressed in the ductal epithelium during development, and SNAIL-1 and -2 were co-expressed with insulin. Our findings provide strong evidence that the movement of beta-cells from the pancreatic ductal epithelium involves an EMT.

Thyroid hormones are important regulators of growth and maturation before birth, although the extent to which their actions are mediated by insulin and the development of pancreatic beta cell mass is unknown. Hypothyroidism in fetal sheep induced by removal of the thyroid gland caused asymmetric organ growth, increased pancreatic beta cell mass and proliferation, and was associated with increased circulating concentrations of insulin and leptin. In isolated fetal sheep islets studied in vitro, thyroid hormones inhibited beta cell proliferation in a dose-dependent manner, while high concentrations of insulin and leptin stimulated proliferation. The developing pancreatic beta cell is therefore sensitive to thyroid hormone, insulin and leptin before birth, with possible consequences for pancreatic function in fetal and later life. The findings of this study highlight the importance of thyroid hormones during pregnancy for normal development of the fetal pancreas.

Insulin may stimulate its own insulin secretion and is a potent growth factor for the pancreatic β-cell. Complications of pregnancy, such as diabetes and intrauterine growth restriction, are associated with changes in fetal insulin concentrations, secretion, and β-cell mass. However, glucose concentrations are also abnormal in these conditions. The direct effect of chronic fetal hyperinsulinemia with euglycemia on fetal insulin secretion and β-cell mass has not been tested. We hypothesized that chronic fetal hyperinsulinemia with euglycemia would increase glucose-stimulated insulin secretion (GSIS) and β-cell mass in the ovine fetus. Singleton ovine fetuses were infused with iv insulin to produce high physiological insulin concentrations, or saline for 7-10 days. The hyperinsulinemic animals also received a direct glucose infusion to maintain euglycemia. GSIS, measured at 133 ± 1 days of gestation, was significantly attenuated in the hyperinsulinemic fetuses (P .05). There was no change in β-cell mass. The hyperinsulinemic fetuses also had decreased oxygen (P .05) and higher norepinephrine (1160 ± 438 vs 522 ± 106 pg/mL; P .005). Acute pharmacologic adrenergic blockade restored GSIS in the hyperinsulinemic-euglycemic fetuses, demonstrating that increased adrenergic signaling mediates decreased GSIS in these fetuses.

Intrauterine growth restriction (IUGR) reduces skeletal muscle mass in fetuses and offspring. Our objective was to determine whether myoblast dysfunction due to intrinsic cellular deficiencies or serum factors reduces myofibre hypertrophy in IUGR fetal sheep. At 134 days, IUGR fetuses weighed 67% less (P 0.05) than controls and had smaller (P 0.05) carcasses and semitendinosus myofibre areas. IUGR semitendinosus muscles had similar percentages of pax7-positive nuclei and pax7 mRNA but lower (P 0.05) percentages of myogenin-positive nuclei (7 ± 2% and 13 ± 2%), less myoD and myogenin mRNA, and fewer (P 0.05) proliferating myoblasts (PNCA-positive-pax7-positive) than controls (44 ± 2% vs. 52 ± 1%). Primary myoblasts were isolated from hindlimb muscles, and after 3 days in growth media (20% fetal bovine serum, FBS), myoblasts from IUGR fetuses had 34% fewer (P 0.05) myoD-positive cells than controls and replicated 20% less (P 0.05) during a 2 h BrdU pulse. IUGR myoblasts also replicated less (P 0.05) than controls during a BrdU pulse after 3 days in media containing 10% control or IUGR fetal sheep serum (FSS). Both myoblast types replicated less (P 0.05) with IUGR FSS-supplemented media compared to control FSS-supplemented media. In differentiation-promoting media (2% FBS), IUGR and control myoblasts had similar percentages of myogenin-positive nuclei after 5 days and formed similar-sized myotubes after 7 days. We conclude that intrinsic cellular deficiencies in IUGR myoblasts and factors in IUGR serum diminish myoblast proliferation and myofibre size in IUGR fetuses, but intrinsic myoblast deficiencies do not affect differentiation. Furthermore, the persistent reduction in IUGR myoblast replication shows adaptive deficiencies that explain poor muscle growth in IUGR newborn offspring.

PMID: 15013641;Abstract:

Facilitated glucose transporters (GLUTs) in the chorionic epithelium are primary conduits for glucose delivery to placental and fetal tissues. The objective of this study was to characterize GLUT8 in the ovine placenta and determine if differences in mRNA and protein concentrations occur in an ovine model of intrauterine growth restriction (IUGR). A GLUT8 partial mRNA was generated, which shares 95 per cent identity with bovine GLUT8 nucleotide sequence. Northern hybridization identified a 2.1 kilobase transcript. GLUT8 mRNA concentrations normalized to β-actin mRNA concentrations increased during late gestation. Western immunoblots with an affinity-purified anti-mouse GLUT8 antiscrum detected GLUT8 in late gestation ovine placenta plasma membranes. GLUT8 was immunolocalized to the chorionic epithelial layer and uterine epithelial cells from mid to late gestation. GLUT8 mRNA and protein concentrations at 135 days gestational age were decreased by 34.8 per cent and 21.8 per cent, respectively (P0.05), in an ovine placental insufficiency model of IUGR. Identification of GLUT8 in the ovine placenta indicates a potential role for GLUT8 in mediating glucose uptake within the placenta and transport to the fetus. Further studies are necessary to confirm this hypothesis and whether the observed decreases in GLUT8 in the PI-IUGR model might contribute, at least in part, to the placental glucose transport deficit that occurs in this model. © 2003 Elsevier Ltd. All rights reserved.