Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

No description available.

published_or_final_version / Zoology / Doctoral / Doctor of Philosophy

published_or_final_version / Botany / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Medicine / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Biological Sciences / Doctoral / Doctor of Philosophy

Both estradiol and progeterone are proposed autocrine or paracrine regulators of ovarian function in primate species. However, specific receptors for these steroids have not been localized to individual compartments of the primate ovary. Using immunocytochemical techniques, estradiol receptors were detected in the germinal epithelium, but not other structures, of ovaries obtained from rhesus or cynomolgus monkeys during the follicular and luteal phases of the menstrual cycle. In contrast, progesterone receptors were present in stromal and interstitial tissue, the thecal layers of healthy and atretic follicles, as well as the functional corpus luteum. These results are consistent with the concept of a receptor-mediated role for progesterone, but not estrogen, within the predominant gametogenic and endocrine structures, e.g., the follicle and corpus luteum, of the primate ovary. The recent discovery of distinct cell types in the corpus luteum of domestic ungulates has revised concepts on the control of luteal function in these species. Studies were designed to test the hypothesis that the primate corpus luteum consists of cell subpopulations that differ in physical characteristics, function and regulation. Cells enzymatically-dispersed from the monkey corpus luteum at mid-luteal phase of the menstrual cycle differed in size (diameter) and the presence of the steroidogenic enzyme, 3β-hydroxysteroid dehydrogenase (3β-HSD). Analysis of dispersed cells for forward and 90° light scatter properties by flow cytometry revealed two distinct continua (Cα and Cβ). These continua were isolated using the sorting capabilities of the flow cytometer. Cα contained single cells of ≤ 15 μm and cell clusters; the cells were typically 3β-HSD-negative nonsteroidogenic. Cβ consisted of single cells that increased in size up to 40 μm and were 3β-HSD-positive. Cβ was divided into two regions (R₁ and R₃) and the cells isolated. R₁ cells were ≤ 15 μm whereas R₃ cells were ≥ 20 μm. Basal progesterone and estrogen production by R₃ cells was greater than that produced by R₁ cells (as determined by radioimmunoassay of the incubation media). Relative stimulation of progesterone production by hCG, cAMP or PGE₂ was not different between R₁ and R₃ luteal cells. These results support the hypothesis that the primate corpus luteum consists of distinct cell subpopulations which differ in size and steroidogenic capacity. However, the cell types which secrete progesterone are typically responsive to gonadotropin and PGE₂, possibly via a cAMP-mediated pathway.

The focus of this dissertation was leptin and the leptin receptor, and the role of these genes (OB and OB-R) in the development of obesity and type 2 diabetes in humans and Psammomys obesus, a polygenic rodent model of obesity and type 2 diabetes. Studies in humans showed that circulating leptin concentrations were positively associated with adiposity, and independently associated with circulating insulin and triglyceride concentrations. Analysis of two leptin receptor sequence polymorphisms in a Caucasian Australian population and a population of Nauruan males, with very high prevalence rates of obesity, showed no associations between sequence variation within the OB-R gene and obesity- or diabetes-related phenotypic measures. In addition, these two OB-R polymorphisms were not associated with longitudinal changes in body mass or composition in either of the populations examined. A unique analysis of the effects of multiple gene defects in the Nauruan population, demonstrated that the presence of sequence alterations in both the OB and OB-R genes were associated with insulin resistance. Psammomys obesus is regarded as an excellent rodent model in which to study the development of obesity and type 2 diabetes in humans. Examination of circulating leptin concentrations in Psammomys revealed that, as in humans, leptin concentrations were associated with adiposity, and independently associated with circulating insulin concentrations. This animal model was utilised to examine expression of OB-R, and the regulation of expression of this gene after dietary manipulation. OB-R is known to have several isoforms, and in particular, OB-RA and OB-RB gene expression were examined. OB-RB is the main signalling isoform of the leptin receptors. It has a long intracellular domain and has previously been shown to play an important role in energy balance and body weight regulation in rodents and humans. OB-RA is a much shorter isoform of OB-R, and although it lacks the long intracellular domain necessary to activate the JAK/STAT pathway, OB-RA is also capable of signalling, although to a lesser degree than OB-RB. OB-RA is found to be expressed almost ubiquitously throughout the body, and this isoform may be involved in transport of leptin into the cell, although its role remains unclear. OB-RA and OB-RB were both found to be expressed in a large number of tissues in Psammomys obesus. Interestingly, obese Psammomys were found to have lower levels of expression of OB-RA and OB-RB in the hypothalamus, compared to lean animals. This finding raises the possibility that decreased leptin signalling in the brain of obese, hyperleptinemic Psammomys obesus may contribute to the leptin resistance previously described in this animal model. However, the primary defect is unclear, as alternatively, increased circulating leptin concentrations may lead to down-regulation of leptin receptors. The effect of fasting on leptin concentrations and gene expression of OB-RA and OB-RB was also examined. A 24-hour fast resulted in no change in body weight, but a reduction in circulating leptin concentrations, and an increase in hypothalamic OB-RB gene expression in lean Psammomys. In obese animals, fasting again did not alter body weight, but resulted in an increase in both circulating leptin concentrations and hypothalamic OB-RB gene expression. In the liver, fasting resulted in a large increase in OB-RA gene expression in both lean and obese animals. These results highlighted the fact that regulation of leptin receptor gene expression in polygenic models of obesity and type 2 diabetes is complex, and not solely under the control of circulating leptin concentrations. Sucrose-feeding is an established method of inducing obesity and type 2 diabetes in rodents, and this experimental paradigm was utilised to examine the effects of longer term perturbations of energy balance on the leptin signalling pathway in Psammomys obesus. Addition of a 5% sucrose solution to the diet of lean and obese Psammomys resulted in increased body weight in both groups of animals, however only obese Psammomys showed increased fat mass and the development of type 2 diabetes. The changes in body mass and composition with sucrose-feeding were accompanied by decreased circulating leptin concentrations in both groups of animals, as well as a range of changes in leptin receptor gene expression. Sucrose-feeding increased hypothalamic OB-RB gene expression in obese Psammomys only, while in the liver there was evidence of a reduction in OB-RA and OB-RB gene expression in both lean and obese animals. The direct effects of sucrose on the leptin signalling pathway are unclear, however it is possible to speculate that the effect of sucrose to decrease leptin concentrations may have been involved in the exacerbation of obesity and the development of type 2 diabetes in obese Psammomys, From these studies, it appears that sequence variation in the OB and OB-R genes is unlikely to be a major factor in the etiology of obesity in human populations. The ability to examine regulation of expression of these genes in Psammomys obesus, however, has demonstrated that the effects of nutritional modifications on leptin receptor gene expression need closer attention. The role of the OB and OB-R genes in metabolism and the development of type 2 diabetes also warrants further examination, with particular attention on the differential effects of dietary modifications on leptin receptor gene expression across a range of tissues.

The neurohypophysial peptide system is involved in modulating a variety of physiological, neurological, and behavioral responses in vertebrates. The principal forms of these peptides in non-mammalian tetrapods are vasotocin (VT) and mesotocin (MT). The studies described in this thesis used pharmacological, molecular, and biochemical techniques, along with phylogenetic analyses, to identify and characterize the mRNA sequences encoding the neurohypophysial peptide precursor proteins and their receptors in urodele amphibians. The cDNAs encoding preproVT and preproMT were amplified by PCR from the brains of two salamander species; the rough-skinned newt, Taricha granulosa, and the red-legged salamander, Plethodon shermani. The neurohypophysial peptides encoded by the identified Taricha cDNAs were VT and MT; the Plethodon cDNAs encoded VT and a novel MT-like peptide, [Val⁴]-MT. Phylogenetic analyses grouped both the Taricha and Plethodon preproVT and preproMT-like sequences with previously identified tetrapod preproVT-like and preproMT-like sequences, respectively. Additional analysis of the preproneurohypophysial sequences indicated that gene conversion (non-homologous crossing over of DNA sequences) appears to have occurred more frequently in mammals than in other tetrapod classes. The cDNAs encoding the VT receptor (VTR) and MT receptor (MTR) were amplified from the brain of T. granulosa by PCR. Sequence identity, and phylogenetic analysis, indicated that the Taricha MTR and VTR were most similar to MTR/OTRs and V[subscript 1a]-like VTRs, respectively. Distribution of PCR amplicons specific to the Taricha MTR and VTR matched previously reported tissue distributions of MTRs and VTRs in other vertebrates in every tissue but kidney, from which the Taricha primers were unable to amplify a cDNA product. Binding experiments of transiently expressed Taricha MTR indicated two binding states, and allowed the determination of ligand binding affinities for this receptor. Inositol phosphate accumulation assays demonstrated that the expressed Taricha MTR and VTR cDNA produced functional receptors, and allowed calculation of ligand potencies of activation and inhibition. Surprisingly, an antagonist frequently used in behavioral experiments to specifically block VTR activity, inhibited inositol phosphate accumulation in cells transfected with either the Taricha MTR or VTR. In conclusion, these studies report the first identified cDNA sequences encoding the preproVT, preproMT, MTR, and V[subscript 1a]-like VTR proteins from urodele amphibians. / Graduation date: 2005

Thesis (Ph.D.) in Biochemistry and Molecular Biology--University of Maine, 2009. / Includes vita. Includes bibliographical references (leaves 95-107).