Monoaminergic Regulation of Mecp2 Phosphorylation in Mouse Models of Psychiatric Disease
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Monoaminergic Regulation of MeCP2 Phosphorylation in Mouse Models of Psychiatric Disease Ashley N. Hutchinson Department of Pharmacology Duke University Date____________________ Approved: ________________________ Anne West, Supervisor ________________________ Marc Caron ________________________ Cynthia Kuhn ________________________ Donald McDonnell ________________________ Theodore Slotkin ________________________ William Wetsel Dissertation submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy in the Department of Pharmacology and Cancer Biology in the Graduate School of Duke University 2011 ABSTRACT Monoaminergic Regulation of MeCP2 Phosphorylation in Mouse Models of Psychiatric Disease Ashley N. Hutchinson Department of Pharmacology Duke University Date____________________ Approved: ________________________ Anne West, Supervisor ________________________ Marc Caron ________________________ Cynthia Kuhn ________________________ Donald McDonnell ________________________ Theodore Slotkin ________________________ William Wetsel An abstract of a dissertation submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy in the Department of Pharmacology and Cancer Biology in the Graduate School of Duke University 2011 Copyright by Ashley Hutchinson 2011 ABSTRACT Activation of monoaminergic receptors is essential to the mechanism by which psychostimulants and antidepressants induce changes in behavior. Although these drugs rapidly increase monoaminergic transmission, they need to be administered for several weeks or months in order to produce long-lasting alterations in behavior. This observation suggests that it is likely that molecular mechanisms downstream of receptor activation contribute to the effects of psychostimulants and antidepressants on behavior. Recently, we and others have demonstrated that the methyl-CpG-binding protein 2 (MeCP2) contributes to both neural and behavioral adaptations induced by repeated psychostimulant exposure (Deng et al, 2010, Im et al, 2010). Psychostimulants induce rapid and robust phosphorylation of MeCP2 at Ser421 (pMeCP2), a site that is thought to modulate MeCP2-dependent chromatin regulation (Cohen et al, 2011), and this phosphorylation event is selectively induced in the GABAergic interneurons of the nucleus accumbens (NAc). In order to understand the signaling pathways that contribute to the pattern of pMeCP2 we observe, I characterized the monoaminergic signaling pathways that regulate pMeCP2. I found that activation of dopamine (DA) and serotonin (5-HT) transmission is sufficient to induce pMeCP2. The novel finding that drugs that activate serotonergic signaling induce pMeCP2 suggests that pMeCP2 may be involved in serotonergic mediated behaviors. To determine the requirement of pMeCP2 in serotonergic mediated behaviors, I utilized mice that bear a knockin (KI) mutation that converts serine to alanine at 421 (S421A) (Cohen et al, 2011). After characterizing the behavioral phenotype of these mice, I conducted tests to assess anxiety- and depression-like behavior. I found that the iv KI mice do not display heightened anxiety in several assays. However, the KI mice exhibit depression-like behavior in the forced swim and tail suspension but show no differences compared to wild-type (WT) littermates in the sucrose preference test, suggesting that pMeCP2 may be implicated in the behavioral response to stressful stimuli. Because we are interested in examining the role of pMeCP2 in the behavioral adaptations to chronic monoaminergic signaling, I then put the KI mice and their WT littermates through chronic social defeat stress, a behavioral paradigm in which repeated exposure to aggressive mice causes social avoidance that is reversed by chronic but not acute antidepressant treatment. Although the WT mice show an increase in social interaction following chronic imipramine treatment, the KI mice fail to show a behavioral response to chronic treatment. These data suggest that pMeCP2 may be implicated in the antidepressant action of chronic imipramine. Finally, investigation of the brain regions in which pMeCP2 may be contributing to the behavioral response to chronic imipramine treatment revealed that chronic but not acute imipramine treatment induces pMeCP2 in the lateral habenula (LHb), a brain region involved in the behavioral response to stress and reward. Together, these data implicate a novel role for pMeCP2 in depression-like behavior and the behavioral response to chronic antidepressant treatment. v CONTENTS Abstract ................................................................................................................................ iv List of Tables ...................................................................................................................... xiii List of Figures .................................................................................................................... xiv Abbreviations and acronyms ............................................................................................. xxii Acknowledgements .......................................................................................................... xxiv 1. Introduction ....................................................................................................................... 1 1.1 Behavioral adaptations to chronic monoaminergic signaling. ........................ 1 1.1.1 The behavioral responses to repeated psychostimulant and antidepressant exposure .................................................................... 1 1.1.2 Epigenetic mechanisms of gene regulation ....................................... 3 1.1.3 The role of epigenetic mechanisms in the behavioral adaptations to repeated psychostimulant exposure .............................................. 5 1.1.4 Epigenetic mechanisms and the behavioral responses to repeated antidepressant treatment. ................................................................... 9 1.2 MeCP2: A transcriptional regulator linking DNA methylation and chromatin modifications ............................................................................... 12 1.2.1 MeCP2 is a methyl-DNA binding protein....................................... 12 1.2.2 Examination of the mechanisms by which MeCP2 regulates transcription .................................................................................... 13 1.3 Studies from mouse lines bearing mutations in MeCP2 ............................... 15 1.3.1 Mutations in MeCP2 cause the neurodevelopmental disorder Rett syndrome ......................................................................................... 15 1.3.2 Mouse models of RTT..................................................................... 16 1.3.3 Brain region- and cell-type-specific MeCP2 mutant mice. ............. 18 1.3.4 Utilizing mouse models of RTT to investigate the function of MeCP2 ............................................................................................ 19 vi 1.4 MeCP2 plays a role in the behavioral adaptations to repeated psychostimulant exposure .............................................................................. 21 1.4.1 MeCP2 expression in NAc contributes to neural and behavioral responses to psychostimulants ........................................................ 22 1.4.2 MeCP2 in the dorsal striatum mediates compulsive drug-taking behavior........................................................................................... 24 1.5 Psychostimulant exposure regulates MeCP2 phosphorylation. ..................... 26 1.5.1 Neuronal activity induces MeCP2 phosphorylation at Ser421. ...... 26 1.5.2 Functional consequences of pMeCP2 in vitro ................................ 28 1.5.3. The role of activity-dependent MeCP2 phosphorylation in vivo ... 29 1.5.4 pMeCP2 as a target of regulation by psychostimulants and DA signaling. ......................................................................................... 33 1.5.5 pMeCP2 levels correlate with degree of behavioral sensitization to repeated AMPH........................................................................... 34 1.6 Research objectives ........................................................................................ 35 2. Differential regulation of MeCP2 phosphorylation in the CNS by dopamine and serotonin..... .................................................................................................................... 37 2.1 Summary. ....................................................................................................... 37 2.2 Introduction .................................................................................................... 38 2.3 Materials and Methods. .................................................................................. 40 2.3.1 Animals ........................................................................................... 40 2.3.2 Immunofluorescent staining of brain sections ................................ 41 2.3.3 Image Analyses. .............................................................................. 42 2.3.4 Bilateral guide cannula surgery. ...................................................... 43 2.3.5 Intra-striatal infusions. .................................................................... 44 vii 2.3.6 Striatal cultures. .............................................................................. 44 2.3.7 Locomotor activity in the open field. .............................................