Schizophrenia is heritable, but even in monozygotic twins differences in susceptibly exists. What is causing this difference in genetically identical individuals? The objective of this thesis was... Show moreSchizophrenia is heritable, but even in monozygotic twins differences in susceptibly exists. What is causing this difference in genetically identical individuals? The objective of this thesis was to identify novel susceptibility genes and pathways for psychosis in a psychostimulant mouse model which is considered a model for psychosis. Genome-wide analysis of transcripts in the hippocampal CA1, driving mesocortical dopaminergic activity, which has a prominent role in schizophrenia, revealed differential expression of target genes of Myocyte Enhancer Factor 2 (MEF2) and Glucocorticoid Receptor (GR). This suggest that this gene network is involved in sensitivity to amphetamine. In primary hippocampal neurons, knockdown of MEF2 reduced the expression of c-Jun and abolished its regulation by GR. Moreover, activation of MEF2 by neuronal depolarization was attenuated by glucocorticoids, suggesting a mutual feedback regulation of these transcription factors. Finally, in vivo MEF2 and GR appeared to be active in the induction phase of amphetamine sensitization. Overall, the findings suggest that in the hippocampus activation of GR can modulate the role of MEF2 target genes in induction of behavioral sensitization. This finding points to the hippocampus as an exciting target for further studies on the role of MEF2 and GR in the precipitation of psychosis susceptibility. Show less
The glucocorticoid receptor (GR) and myocyte enhancer factor 2 (MEF2) are transcription factors involved in neuronal plasticity. c-JUN, a target gene of GR and MEF2, plays a role in regulating both... Show moreThe glucocorticoid receptor (GR) and myocyte enhancer factor 2 (MEF2) are transcription factors involved in neuronal plasticity. c-JUN, a target gene of GR and MEF2, plays a role in regulating both synaptic strength and synapse number. The aim of this study was to investigate the nature of this dual regulation of c-JUN by GR and MEF2 in a neuronal context. First, we showed that GR mediates the dexamethasone-induced suppression of c-JUN mRNA expression. Next, we observed that GR activation resulted in an increase in phosphorylation of MEF2, a post-translational modification known to change MEF2 from a transcriptional enhancer to a repressor. In addition, we observed an enhanced binding of MEF2 to genomic sites directly upstream of the c-JUN gene upon GR activation. Finally, in primary hippocampal neuronal cultures, knockdown of MEF2 not only reduced c-JUN expression levels but abolished GR regulation of c-JUN expression. This suggests that MEF2 is necessary for GR regulation of c-JUN. In conclusion, for the first time, we show that activated GR requires MEF2 to regulate c-JUN. At the same time, GR influences MEF2 activity and DNA binding. These results give novel insight into the molecular interplay of GR and MEF2 in the control of genes important for neuronal plasticity. Show less
Glucocorticoid (GC) hormones, released by the adrenals in response to stress, are key regulators of neuronal plasticity. In the brain, the hippocampus is a major target of GC, with abundant... Show moreGlucocorticoid (GC) hormones, released by the adrenals in response to stress, are key regulators of neuronal plasticity. In the brain, the hippocampus is a major target of GC, with abundant expression of the GC receptor. GC differentially affect the hippocampal transcriptome and consequently neuronal plasticity in a subregion-specific manner, with consequences for hippocampal information flow and memory formation. Here, we show that GC directly affect the mammalian target of rapamycin (mTOR) signaling pathway, which plays a central role in translational control and has long-lasting effects on the plasticity of specific brain circuits. We demonstrate that regulators of the mTOR pathway, DNA damage-induced transcript (DDIT) 4 and FK506-binding protein 51 are transcriptionally up-regulated by an acute GC challenge in the dentate gyrus (DG) subregion of the rat hippocampus, most likely via a GC-responseelement-drivenmechanism. Furthermore, two other mTOR pathway members, them TOR regulator DDIT4-like and the mTOR target DDIT3, are down-regulated by GC in the rat DG. Interestingly, the GC responsiveness of DDIT4 and DDIT3 was lost in animals with a recent history of chronic stress. Basal hippocampal mTOR protein levels were higher in animals exposed to chronic stress than in controls. Moreover, an acute GC challenge significantly reduced mTOR protein levels in the hippocampus of animals with a chronic stress history but not in unstressed controls. Based on these findings, we propose that direct regulation of the mTOR pathway by GC represents an important mechanism regulating neuronal plasticity in the rat DG, which changes after exposure to chronic stress. (Endocrinology 153: 4317-4327, 2012) Show less
Rationale Psychosis susceptibility is mediated in part by the dopaminergic neurotransmitter system. In humans, individual differences in vulnerability for psychosis are reflected in differential... Show moreRationale Psychosis susceptibility is mediated in part by the dopaminergic neurotransmitter system. In humans, individual differences in vulnerability for psychosis are reflected in differential sensitivity for psychostimulants such as amphetamine. We hypothesize that the same genes and pathways underlying behavioral sensitization in mice are also involved in the vulnerability to psychosis.Objectives The aim of the current study was to investigate which genes and pathways may contribute to behavioral sensitization in different dopaminergic output areas in the mouse brain.Methods We took advantage of the naturally occurring difference in psychostimulant sensitivity in DBA/2 mice and selected animals displaying extremes in behavioral sensitization to amphetamine. Subsequently, the dopamine output areas, prefrontal cortex, nucleus accumbens, and cornu ammonis 1 (CA1) area of the hippocampus, were isolated by laser microdissection and subjected to DNA microarray analysis 1 h after a challenge dose of amphetamine. Results A large number of genes with differential expression between high and low responders were identified, with no overlap between brain regions. Validation of these gene expression changes with real-time quantitative polymerase chain reaction demonstrated that the most robust and reproducible effects on gene expression were in the CA1 region of the hippocampus. Interestingly, many of the validated genes in CA1 are members of the cAMP response element (CRE) family and targets of the glucocorticoid receptor (GR) and myocyte enhancer factor 2 (Mef2) transcription factors.Conclusion We hypothesize that CRE, Mef2, and GR signaling form a transcription regulating network, which underlies differential amphetamine sensitivity, and therefore, may play an important role in susceptibility to psychosis. Show less
