Investigation of the Expression Pattern and Functional Importance of the Gnasxl-Encoded Xlαs Protein of the Imprinted Gnas Locus
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Investigation of the Expression Pattern and Functional Importance of the Gnasxl-encoded XLαs protein of the Imprinted Gnas Locus Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor of Philosophy By Katie Louise Burton May 2012 ABSTRACT XLαs is a NH2-terminal splice variant of the stimulatory G-protein α- subunit Gsα. Both are encoded by the imprinted Gnas locus. Similar to Gsα, XLαs can couple 7-TM receptors to adenylate cyclase in cultured cells. Previously, a knock-out mouse specific for the Gnasxl transcript was generated (Gnasxlm+/p-) (PLAGGE et al. 2004). This mouse model exhibits a lean, hypermetabolic phenotype with elevated sympathetic nervous system (SNS) activity (XIE et al. 2006). Changes in phenotype between neonatal and adult stages (e.g. mortality; failure to thrive as neonates vs. healthy but hypermetabolic adults) are not yet fully understood. More recently a conditional gene trap knock-out mouse line (XLlacZGT) was established. Immunohistochemistry, XGal and immunofluorescence data were collected to document the changes in expression pattern of XLαs and determine possible signalling pathways affected by lack of XLαs in the brain; histological analysis of the lacZ-containing genetrap line revealed new sites of XLαs expression. A comparison of neonatal and adult brain expression patterns revealed that the lateral hypothalamus (LH), dorsomedial hypothalamus (DMH), arcuate nucleus (Arc), locus coeruleus and ventrolateral medulla express XLαs in both stages; the laterodorsal tegmental nucleus, hypoglossal and facial nucleus (motor nuclei important for feeding) express XLαs only in neonates; and expression in the amygdala and preoptic area are only found in adult brain. Colocalisation studies in the brain for XLαs, neuropeptides and other markers related to regulation of food intake and energy expenditure. Orexin partly colocalised with XLαs in the LH and DMH (22% orexin neurons XLαs positive); tyrosine hydroxylase/dopaminergic neurons colocalised with XLαs in the Arc (60% TH neurons XLαs positive); and phosphorylated S6, a component of the leptin signalling pathway, colocalised with XLαs in the Arc (30% XLαs neurons pS6 positive). MCH and CRF did not colocalise with XLαs. Changes in pS6/S6K1 and the indicators of ghrelin signalling in knock-out Arc neurons did not reach statistical significance. Analysis of the lacZ-genetrap line at neonatal stages revealed that XLαs is also expressed in spinal cord and peripheral tissues e.g. skeletal muscle, tongue muscle and blood vessel smooth muscle cells. Expression in muscle tissues, including blood vessel smooth muscle cells is silenced in adults, but the spinal cord remains positive for XLαs. XLαs expression pattern changes in the brain and peripheral tissues concur with changes in phenotype seen between neonatal and adult mice. pS6 is a good indicator of S6K1 activity, which influences leptin and insulin signalling. A decrease of S6K1 activity in Gnasxlm+/p- mice might explain their leptin sensitivity (Frontera et al. in prep). XLαs colocalises with orexigenic peptides – including orexin and NPY/AgRP neurons (Frontera et al. in prep) – in the hypothalamus; however, the function of XLαs in energy expenditure and SNS activity remains elusive. ii Acknowledgements ACKNOWLEDGEMENTS Firstly I would like to thank my supervisor Dr Antonius Plagge, without whom this project would not have been possible. I would also like to thank Nic Nunn and Stefan Krechowec for their help carrying out experiments in this project. I would also like to thank Anna Newlaczyl who completed an MRes project alongside my PhD project and carried out experiments which assisted in this project. I would like to thank Bettina Wilm and John Quayle for antibodies against blood vessel markers used in fluorescence experiments. Mum and Dad, thank you for always supporting me in my decision to carry on in further education and always encouraging me to do my best in whatever I choose. My little sister Kristy thanks for always making me laugh and being there when I need a chat. Karl, thank you for being there when I am feeling low and disheartened, thank you for listening to my problems and for helping me to pick myself up and carry on. I would also like to thank the Medical Research Council for supporting the project financially. iii Table of Contents LIST OF FIGURES ............................................................................... X LIST OF TABLES .............................................................................. XII ABBREVIATIONS .............................................................................XIII CHAPTER 1. INTRODUCTION ......................................................... 20 1.1. GENOMIC IMPRINTING ...................................................................... 21 1.1.1. What is Genomic Imprinting? ................................................................... 21 1.1.2. The evolution of Genomic Imprinting ................................................... 24 1.1.3. Epigenetic Control of Genomic Imprinting .......................................... 25 1.1.4. Functions of Imprinted Genes. ................................................................. 29 1.1.5. Disorders Involving Imprinted Genes ................................................... 30 1.2. THE COMPLEX IMPRINTED GNAS LOCUS .......................................... 31 1.2.1. Transcripts of the Gnas locus .................................................................... 31 1.2.2. Knock-out Mouse Lines of the Gnas Locus ........................................... 35 1.2.3. Disorders Associated with the Gnas Locus .......................................... 38 1.3. CNS REGULATION OF ENERGY HOMEOSTASIS ................................. 38 1.4. AIMS OF THE PHD ............................................................................ 40 CHAPTER 2. MATERIALS AND METHODS...................................... 41 2.1. CHEMICALS AND REAGENTS.............................................................. 42 2.2. BUFFERS AND SOLUTIONS ................................................................ 44 2.2.1 Tissue Collection ............................................................................................. 44 2.2.2. Histology ........................................................................................................... 44 2.2.3. PAGE Gels ......................................................................................................... 45 2.2.4. Southern Blotting and In Situ Hybridisation ....................................... 47 2.2.5. Electrophoresis .............................................................................................. 48 2.2.6. Cloning ............................................................................................................... 48 2.3. MICE ................................................................................................. 50 CHAPTER 3. ANALYSIS OF EXON A20 SPLICING IN PROTEINS DERIVED FROM THE GNASXL TRANSCRIPT: XLΑS VS.XLN1 ........ 50 3.1. INTRODUCTION ................................................................................. 51 3.1.1. Alternative Splicing in Gnasxl: XLN1, Exon A20 and Exon 3 ......... 51 3.1.2. Aims .................................................................................................................... 52 3.2. MATERIALS AND METHODS .............................................................. 53 3.2.1. Tissue Collection ............................................................................................ 53 3.2.2. RNA Extraction ............................................................................................... 53 3.2.3. Reverse Transcription-PCR (RT-PCR) ................................................... 54 3.2.4. Ligation and Cloning into competent E. coli ....................................... 55 3.2.5. Cloning of PCR products using the TOPO TA Cloning® kit (Invitrogen) ..................................................................................................... 55 3.2.6. Preparation of Bacterial Mini-cultures ................................................. 55 3.2.7. TENS Mini-Prep for Extraction of DNA from Bacterial Mini- cultures ............................................................................................................. 56 3.2.8. Determination of colonies containing PCR product insertions by iv Table of Contents EtBr agarose gel electrophoresis ............................................................ 56 3.3. RESULTS ........................................................................................... 58 3.3.1. Analysis of Alternative Splicing of Gnasxl Exon A20 in Full-Length XLαs and XLN1 Protein .............................................................................. 58 3.3.2. Summary ........................................................................................................... 62 3.4. DISCUSSION ...................................................................................... 65 3.4.1. Alternative Splicing of Gnasxl Exon A20 ............................................... 65 3.4.2. Placement of the XLlacZGT Gene Trap .................................................. 65 CHAPTER 4. GNASXL EXPRESSION: A COMPARATIVE STUDY OF CHANGES BETWEEN NEONATAL AND ADULT DEVELOPMENTAL STAGES I. THE CENTRAL NERVOUS SYSTEM ................................. 69 4.1. INTRODUCTION ................................................................................