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dc.contributor.advisorOwens, Julie Anne-
dc.contributor.advisorGatford, Kathy-
dc.contributor.advisorDe Blasio, Miles Jonathon-
dc.contributor.authorSulaiman, Siti Aishah-
dc.description.abstractLow birth weight or intrauterine growth restriction (IUGR) consistently predict increased risk of Type 2 diabetes (T2D) through impairment of glucose tolerance, insulin resistance and inadequate insulin secretion in humans (1, 2), as well as in many experimental studies in other species (3, 4). IUGR due to insufficient supply of fetal nutrients, decreased oxygen supply and elevated exposure to stress hormones are thought to ‘program’ the impairment of β-cell mass, function and plasticity which then contributes to development of diabetes later in life, as observed in humans (5-7) and animals (4, 8). Interestingly, administration of the glucagon-like-peptide 1 (GLP1) analogue exendin-4 to neonatal IUGR rats normalised subsequent β-cell mass and insulin secretion and prevented later development of T2D (9), thus providing a possible intervention strategy to prevent T2D following IUGR. However, there are differences in the timing of pancreatic and β-cell development between species and therefore in the developmental stages during exposure to IUGR and neonatal interventions. In humans and sheep, most pancreatic and β-cell development occurs before birth (10-17). In contrast, rodents undergo later development of β-cells than sheep or humans, with the majority of pancreatic remodelling occurred at postnatal ages (18-20). It is therefore necessary to test the efficacy of neonatal exendin-4 treatment in animal models such as sheep that share similar profile of pancreatic development and growth with humans (9, 21). Therefore, this thesis will address the effects of IUGR on β-cell mass and function, expression of their molecular determinants, as well as epigenetic modifications, and the possible involvement of altered circulating adiponectin abundance and expression in adipose tissue in the young lamb from birth to 16 d of age. The efficacy of neonatal exendin-4 treatment as a postnatal intervention to prevent these adverse effects of IUGR on metabolic outcomes will also be assessed. Here, natural twin pregnancies were used as a model of IUGR in progeny and unrestricted singleton lambs as the controls. In each twin set, sibling twin lambs with high and low birth weights were alternately allocated to either vehicle or exendin-4 treatment. Effects of IUGR due to twinning and of neonatal exendin-4 treatment of the twin lambs on neonatal growth, pancreatic β-cell in vivo and in vitro insulin secretory function, β-cell mass and islet expression of key regulatory genes including microRNAs, epigenetic regulators, and adiponectin, and on adiponectin abundance were analysed. IUGR due to twinning reduced size at birth and increased neonatal growth, without altering insulin sensitivity, in vivo insulin action, β-cell mass or islet mRNA expression of β-cell mass molecular determinants when compared to CON lambs. However, in vitro glucose-stimulated insulin secretion was increased in the IUGR twin lamb relative to controls (+420%, P = 0.081), consistent with up-regulation of islet mRNA expression of GCK in this group (+80%, P = 0.017), thus suggesting up-regulated β-cell function at this age. Interestingly, IUGR twin lambs also had increased islet mRNA expression of DNMT3B relative to CON lambs (+96%, P = 0.027), which is responsible for de novo DNA methylation (22, 23). Islet mRNA expression of GCK was positively correlated with that of DNMT3B in the IUGR twin group, suggesting that altered islet GCK mRNA expression and β-cell function after IUGR may occur in part via epigenetic changes that may persist throughout life. In conjunction with enhanced β-cell function, up-regulation of adiponectin mRNA expression in omental fat (+72%, P = 0.008) and increased circulating adiponectin levels (P = 0.012) were also observed in the IUGR twin lamb group. Omental adiponectin mRNA expression and circulating adiponectin correlated positively with insulin secretion and β-cell mass in combined control and IUGR twin lamb groups, suggesting that this adipokine may play a role in regulating neonatal insulin secretion. Daily exendin-4 treatment of IUGR twin lambs during neonatal life prevented accelerated neonatal growth or catch up growth (CUG) and fat accumulation (-57%, P < 0.001), and normalised in vitro insulin secretion and GCK and DNMT3B mRNA expression in their islets, relative to vehicle-treated IUGR twins. This may retain adaptive capacity of β-cell function for later life. Glucose tolerance of twin IUGR lambs was impaired during exendin-4 treatment (+156%, P = 0.003) reflecting decreased insulin sensitivity (-46%, P = 0.002) in this group, despite having normal in vivo insulin secretion. This may be due to central actions of exendin-4 to inhibit food intake and insulin sensitivity (24-26). β-cell mass in IUGR twin lambs treated with exendin-4 tended to be higher than in their IUGR counterparts (+28%, P = 0.083), and consistent with this, islet mRNA expression of IGF1 and IGF2R was increased in this group (+62%, P = 0.058 and +63%, P = 0.005 respectively) when compared to controls. Moreover, in the IUGR+Ex-4 lambs, islet mRNA expression of PDX1 correlated positively with that of IGF1R, while IGF1 mRNA expression correlated positively with β-cell volume density, which may suggest hyperplastic effects of the IGF axis on β-cell mass during exendin-4 treatment. Despite the profound reduction in visceral fat mass induced by neonatal exendin-4 treatment, circulating adiponectin concentrations were not reduced in exendin-4-treated lambs, possibly due to upregulation of adiponectin expression in subcutaneous fat in these animals (+91%, P = 0.007). Nevertheless, the reduction in fat accumulation and normalised in vitro β-cell action of IUGR lambs during neonatal exendin-4 treatment suggest that neonatal exendin-4 might have beneficial effects on insulin-regulated glucose homeostasis in later life. These outcomes also demonstrate the biological activity of exendin-4 for the first time in the sheep, at least in the context of individuals who had undergone growth-restriction before birth. In conclusion, IUGR due to twinning induced CUG, early life up-regulation of in vitro β-cell insulin secretion and islet expression of its determinant, GCK, but did not alter in vivo insulin action, glucose tolerance or β-cell mass in young lambs at 16 d of age. These metabolic and molecular changes may be partly mediated by increases in circulating adiponectin and its expression in omental fat, as part of an adipose tissue response during neonatal fat deposition. Consistent with our hypothesis, neonatal exendin-4 treatment prevented this IUGR-induced CUG and decreased visceral fat deposition, increased 2ⁿᵈ phase insulin secretion in vivo, normalised in vitro insulin secretion and islet expression of its determinant, GCK, at the end of treatment in the IUGR twin lambs. Although exendin-4 treatment only tended to increase β-cell mass in young IUGR lamb, the up-regulation of islet expression of β-cell mass determinants after 16 days of exendin-4 treatment may suggest beneficial effects of exendin-4 to subsequently expand β-cell mass. This may protect the exendin-4-treated IUGR individual from a need to increase β-cell function, and preserve the capacity of β-cells for later plasticity of insulin secretion in response to the development of insulin resistance with ageing. Hence, a long term investigation is required to address how these changes following IUGR and neonatal exendin-4 treatment at 16 d of age will affect β-cell function and mass and insulin action and their regulation in the IUGR sheep to adulthood.en
dc.titleEarly life determinants of Beta-cell function in the sheepen
dc.contributor.schoolSchool of Paediatrics and Reproductive Healthen
dc.provenanceThis electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
dc.description.dissertationThesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Paediatrics and Reproductive Health, 2015.en
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