Effects of hypothyroidism on pancreatic islet development and tissue insulin signalling pathways in the ovine fetus

Thyroid hormones are important regulators of fetal growth and maturation, although their mechanism of action and interactions with other hormones are unclear. The overall aim of the project was to elucidate the effects of hypothyroidism on the growth and development of the sheep fetus in late gestation. Specifically, the project investigated the extent to which the changes in fetal growth induced by thyroid hormone deficiency were mediated by changes in pancreatic islet development and insulin signalling in fetal tissues. In nineteen twin-bearing pregnant ewes at 105-110 days of gestation (dGA; term~145dGA) and under general anaesthesia, one fetus was thyroidectomised, while the other was sham-operated. At either 129 or 143dGA, umbilical blood samples and a variety of fetal tissues were collected after euthanasia. Hypothyroidism in utero did not affect fetal bodyweight but impaired skeletal growth and led to disproportionate patterns of organ growth. A 30-40% increase in pancreatic β-cell mass was observed in the thyroid deficient fetuses, compared to sham controls, and this was associated with increased plasma insulin and leptin concentrations. In studies using isolated fetal ovine pancreatic islets, β-cell proliferation in vitro was inhibited by T3 in a dose-dependent manner but was stimulated by the highest dose of insulin. Pancreatic β-cell proliferation was inhibited at low, and stimulated at high, leptin concentrations. Perirenal adipose tissue was enlarged in the hypothyroid fetuses due to an increase in the proportion of unilocular adipocytes, characteristic of white adipose tissue. The greater relative unilocular adipocyte mass was caused by hyperplasia in association with upregulation of the insulin signalling pathway. Kidneys of hypothyroid fetuses had no apparent changes in glomerular or tubular structure, but a greater water content may have accounted for the increased kidney mass seen in the thyroid deficient fetuses, compared to sham controls. No changes in insulin signalling or sodium transporter expression were seen in the kidneys of the hypothyroid fetuses. This research demonstrates that the thyroid hormones are required in the ovine fetus during late gestation for the normal development of the endocrine pancreas, adipose tissue and kidney. Alterations in organ development in response to hypothyroidism may have short and long term consequences for carbohydrate metabolism, obesity and renal function

Physiological control of Drosophila ovarian stem cell lineages

In order to support tissue function, adult stem cell activity must respond to organismal dietary status and whole-body physiology. The complex signaling networks impinging on stem cells, however, are not fully understood. The focus of this dissertation is how Drosophila ovarian stem cell lineages, which have well-characterized responses to diet, sense and respond to their physiological environment. In mammals, adipocytes have a key endocrine role, mediated in large part through secreted peptide hormones called adipokines. I describe an intrinsic requirement for the Drosophila homolog of a mammalian adipokine receptor, the adiponectin receptor (AdipoR), in germline stem cell (GSC) maintenance and demonstrate that overexpression of AdipoR in the germline mitigates age-associated GSC loss. I also explore the distinct, but overlapping, roles of the cellular energy sensor AMP-activated protein kinase (AMPK) in ovarian stem cell lineages. In addition to nutrient-dependent roles in cell proliferation and growth in these cells, AMPK intrinsically controls the function of cells in the somatic lineage by a diet-independent mechanism. In complementary studies, I address the regulatory role of the fat body in oogenesis. With Dr. Alissa Armstrong, a postdoctoral fellow in the lab, I characterize two mechanisms of amino acid sensing in adult adipocytes that affect distinct stages of oogenesis. Furthermore, I summarize my efforts toward identifying additional roles for the fat body in oogenesis, including AdipoR signaling and lipid storage in that tissue. I outline possible methods for the identification of the Drosophila adiponectin-like ligand. Finally, I provide a proof of principle for a screen to identify novel diet-dependent factors regulating Drosophila oogenesis using a laboratory strain of yeast. This dissertation sheds light on the dense physiological signaling networks regulating stem cells and their progeny and represents substantial progress toward understanding inter-organ communication