<p>Plant natriuretic peptides (PNPs) are a novel class of extracellular, systemically mobile molecules that elicit a number of plant responses important in homeostasis and growth. Natriuretic peptides were first identified in vertebrates where they play a role in the regulation of salt and water balance. Subsequent experimental investigations have identified the presence of a natriuretic peptide hormone system in plants. While PNPs have been implicated in various physiological responses such as stomatal guard cell movements and regulation of net water uptake, its biological role has remained elusive. Here we have used co-expression and promoter content analysis tools to understand the biological role of the Arabidopsis thaliana PNP (AtPNP-A). The analysis of AtPNP-A and its co-expressed genes revealed that genes annotated as part of the systemic acquired resistance (SAR) pathway were over-represented, thus suggesting that AtPNP-A may function as a component of plant defense responses and specifically, SAR. The results further show that AtPNP-A shares many characteristics with pathogenesis related (PR) proteins in that its transcription is strongly induced in response to pathogen challenges, thus implying a newly described role for AtPNP-A in pathogen attack. Additional tissue expression analysis also indicated distinct localization of PNP activity in sepals and transcriptional meta-analysis showed that AtPNP-A may play a role in starch breakdown. Therefore, together with the finding that AtPNP-A plays a role in regulating phloem transport, we also hypothesize that AtPNP-A may play a role in phloem unloading in sepals to assist processes such as seed formation in plants. In plants, the second messenger, guanosine 3&rsquo / ,5&rsquo / -cyclic monophosphate (cGMP) mediates a whole range of important processes including salinity tolerance, disease resistance, drought tolerance and responses to light. Since PNPs regulate water and salt homeostasis via a cGMP-dependent signaling pathways, it is thus important to analyse the transcriptome induced by the second messenger (cGMP) in Arabidopsis thaliana to give a better understanding of its mechanism of action. This study was also supplemented by the analysis of the gibberellic acid (GA) dependent transcriptome, since cGMP also plays a role its transcription pathway. This data analysis, together with promoter content investigation, revealed that genes upregulated after cGMP treatment and down-regulated in the GA insensitive mutant (ga1-3) were enriched with a GA response element (GARE), while no GARE enrichment were observed in genes up-regulated in the ga1-3 mutant. These findings suggest that GARE is indicative of GA-induced and cGMP-dependent transcriptional up-regulation. Gene ontology analysis confirmed previous reports that cGMP is involved in ion homeostasis and indicated that the transcriptional cGMP response is bi-polar in the sense that both genes up- and down-regulated in response to cGMP is involved in cation transport. Additionally, ab initio analysis of genes transcriptionally dependent on cGMP identified CHX8 as a hub gene and promoter content of CHX8 co-expressed genes show enrichment of the GARE motif. The fact that CHX8 has its highest expression levels during male gametogenesis and pollen tube growth, together with our findings, suggest that GA-induced and cGMP- dependent genes may play a key role in ion and water homeostasis in the male gametophyte. Finally, we propose that the type of analysis undertaken here can yield new insights into gene regulation networks and inform experimental strategies to unravel complex transcription regulatory systems under different developmental and stimulus specific conditions.</p>

published_or_final_version / Zoology / Master / Master of Philosophy

No description available.

<p>Plant natriuretic peptides (PNPs) are a novel class of extracellular, systemically mobile molecules that elicit a number of plant responses important in homeostasis and growth. Natriuretic peptides were first identified in vertebrates where they play a role in the regulation of salt and water balance. Subsequent experimental investigations have identified the presence of a natriuretic peptide hormone system in plants. While PNPs have been implicated in various physiological responses such as stomatal guard cell movements and regulation of net water uptake, its biological role has remained elusive. Here we have used co-expression and promoter content analysis tools to understand the biological role of the Arabidopsis thaliana PNP (AtPNP-A). The analysis of AtPNP-A and its co-expressed genes revealed that genes annotated as part of the systemic acquired resistance (SAR) pathway were over-represented, thus suggesting that AtPNP-A may function as a component of plant defense responses and specifically, SAR. The results further show that AtPNP-A shares many characteristics with pathogenesis related (PR) proteins in that its transcription is strongly induced in response to pathogen challenges, thus implying a newly described role for AtPNP-A in pathogen attack. Additional tissue expression analysis also indicated distinct localization of PNP activity in sepals and transcriptional meta-analysis showed that AtPNP-A may play a role in starch breakdown. Therefore, together with the finding that AtPNP-A plays a role in regulating phloem transport, we also hypothesize that AtPNP-A may play a role in phloem unloading in sepals to assist processes such as seed formation in plants. In plants, the second messenger, guanosine 3&rsquo / ,5&rsquo / -cyclic monophosphate (cGMP) mediates a whole range of important processes including salinity tolerance, disease resistance, drought tolerance and responses to light. Since PNPs regulate water and salt homeostasis via a cGMP-dependent signaling pathways, it is thus important to analyse the transcriptome induced by the second messenger (cGMP) in Arabidopsis thaliana to give a better understanding of its mechanism of action. This study was also supplemented by the analysis of the gibberellic acid (GA) dependent transcriptome, since cGMP also plays a role its transcription pathway. This data analysis, together with promoter content investigation, revealed that genes upregulated after cGMP treatment and down-regulated in the GA insensitive mutant (ga1-3) were enriched with a GA response element (GARE), while no GARE enrichment were observed in genes up-regulated in the ga1-3 mutant. These findings suggest that GARE is indicative of GA-induced and cGMP-dependent transcriptional up-regulation. Gene ontology analysis confirmed previous reports that cGMP is involved in ion homeostasis and indicated that the transcriptional cGMP response is bi-polar in the sense that both genes up- and down-regulated in response to cGMP is involved in cation transport. Additionally, ab initio analysis of genes transcriptionally dependent on cGMP identified CHX8 as a hub gene and promoter content of CHX8 co-expressed genes show enrichment of the GARE motif. The fact that CHX8 has its highest expression levels during male gametogenesis and pollen tube growth, together with our findings, suggest that GA-induced and cGMP- dependent genes may play a key role in ion and water homeostasis in the male gametophyte. Finally, we propose that the type of analysis undertaken here can yield new insights into gene regulation networks and inform experimental strategies to unravel complex transcription regulatory systems under different developmental and stimulus specific conditions.</p>

In Drosophila, neuropeptides have regulatory roles in development, growth, metabolism and reproduction. This study focused on GABA and the neuropeptides Drosophila tachykinin (DTK), short neuropeptide F (sNPF), adipokinetic hormone (AKH), corazonin (CRZ) and Drosophila insulin-like peptides (DILPs) as possible regulators of metabolic stress responses and homeostasis. We showed that metabotropic GABAB receptors (GBRs) are expressed on brain insulin producing cells (IPCs), suggesting an inhibitory regulation of these cells by GABA. Knockdown of GBR on IPCs shortened lifespan and stress resistance, altered carbohydrate and lipid metabolism at stress (paper I). We showed that three different neuropeptides; DTK, sNPF and ITP, are co-expressed in five pairs of adult neurosecretory cells (paper II). ITP-knock down was not studied yet, but sNPF- and DTK-knock down flies showed decreased stress resistance at desiccation and starvation and decreased water levels at desiccation, suggesting that these peptides are involved in water homeostasis during stress conditions. sNPF was previously shown to affect feeding, growth and DILP expression via the IPCs, but it was not known which sNPF-expressing neurons are responsible for these actions. We could identify a specific set of bilateral neurons (DLPs) that co-express sNPF and corazonin that target the IPCs. We showed that these peptides co-released from DLPs regulate DILP transcription and probably release in the adult Drosophila brain and thus have roles in regulation of stress resistance and metabolism (paper III). AKH signaling was previously shown to affect hemolymph carbohydrate levels and lipid stores in Drosophila. Insulin (DILP) signaling and AKH signaling are suggested to have opposing effects on lipid and sugar metabolism in Drosophila. We studied the possible functional relationship between these two systems; do they mutually regulate each other?  Our results suggest action of DILPs via the Insulin Receptor on the IPCs and the AKH producing cells, but we could not provide evidence for AKH action on IPCs or AKH cells (paper IV). / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Epub ahead of print. Paper 4: Manuscript.</p>

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a ubiquitous peptide hormone involved in the regulation of several physiological processes. This hormone is a member of the glucagon superfamily of hormones, which are primarily involved in regulation of metabolism and growth. PACAP shows the highest sequence conservation of this superfamily during evolution, suggesting a critical role for PACAP in species survival. Mice lacking PACAP (Adcyap 1) show high mortality during the postnatal period, as well as impaired reproduction in females. In this thesis I have characterized the reproductive phenotype in female mice lacking PACAP due to disruption (knockout) of the single copy pacap gene (Adcyapl ). Previous experiments in other laboratories have described reduced fertility in mice lacking PACAP or one of its receptors, the PAC] receptor. However, the mechanism by which PACAP acts to enhance reproduction has not yet been elucidated. The purpose of this study was to determine the site(s) of action of PACAP in the cascade of reproductive events from puberty to implantation. I hypothesized that PACAP was an important regulator of an early phase of reproduction, and that lack of PACAP would result in impaired reproductive success. Initially I examined puberty onset, estrous cycles, mating behaviour, and delivery of live offspring. Estrous cycles were evaluated through vaginal cytology, both in juvenile mice, to confirm that puberty had occurred, and in adult females, to determine if they maintained a normal 4-5 day estrous cycle. Mating behaviour was evaluated through a natural mating strategy, with daily assessment of the presence of a copulatory plug. The reproductive outcome of delivering live pups was characterized, and pup survival was followed. I then characterized PACAP null mice as to ovulation, ovarian histology and fertilization of eggs in vivo. Finally, I measured implantation rates in PACAP null females, as well as levels of the hormone prolactin, an essential regulator of early pregnancy. In the present study I found a number of reproductive functions that were normal without PACAP. Puberty onset was unaffected in knockout mice and they displayed regular 4-5 day estrous cycles. Also. PACAP null females mated when paired with a male of proven fertility. However, mating behaviour follows an unusual pattern in PACAP null mice, where 33% mate on more than one day during a week-long pairing unlike wild-type mice that would normally mate only once. Also, significantly fewer PACAP null females than wild-type females gave birth following mating: 21% and 100%, respectively. Ovulation and ovarian histology were normal in PACAP null females, as was fertilization of released eggs. However, only 13% of PACAP null females had implanted embryos 6.5 days after mating. The mechanism of impaired implantation may be a defect in prolactin secretion. Prolactin levels were significantly lower in PACAP null females than in wild-types following mating. Prolactin is an essential hormone for the support of early pregnancy. These results support the conclusion that PACAP acts as an important regulator of prolactin in reproduction. It is possible that PACAP in the hypothalamus is the dominant prolactin-releasing factor, which has been sought many years but never definitively identified.

A complex physiological system, composed of central and peripheral signals, balances energy intake and expenditure. Among these signals, the enteric and orexigenic hormone ghrelin is a regulator of energy balance with several uncharacterized functions. Although much research has accumulated regarding ghrelin's effects on metabolic parameters, little is known about its other behavioural and cognitive effects. Consequently, this study, using functional magnetic resonance imaging, showed that ghrelin administered intravenously to healthy volunteers increased the neural response to food pictures, as well as faces of fear and disgust, in brain areas regulating the hedonic and incentive evaluation of visual stimuli, such as the amygdala. Moreover, ghrelin exhibited memory enhancing effects for both food pictures and faces of fear and disgust. These findings suggest that ghrelin's activation of the amygdala may serve as a magnitude signal for value judgments of visually-presented food and non-food stimuli, thus engaging critical feeding, emotional and cognitive responses.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Also available in print.

Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 114-132) Also available in print.

Thesis (M. Phil.)--University of Hong Kong, 1998. / Includes bibliographical references (leaves 77-85).