Once bound to methylated CpG sites, methyl-CpG-binding protein 2 (MeCP2) is thought to silence transcription of downstream genes by recruiting a histone deacetylase (HDAC). Mutations within the MeCP2 gene have been found to cause Rett syndrome, a disorder of arrested neuronal development. Using immunohistochemistry, we found that Mecp2, as well as the methyl-CpG-binding protein MBD1, were significantly induced in normal adult rat brain after repeated injections of fluoxetine or cocaine for 10 days (one injection per day). Mecp2 was not induced by repeated injections of 1-(2-bis(4-fluorphenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl)piperazine (GBR-12909) or nortriptyline. Together, the data indicate that the serotonergic system is predominantly involved. Using real-time reverse transcription-polymerase chain reaction experiments, MBD1 mRNA and both Mecp2_e1 and Mecp2_e2 transcripts were found to be induced by fluoxetine. Induction of the methylbinding proteins was accompanied with enhanced HDAC2 labeling intensity and mRNA synthesis in response to fluoxetine. In tandem, acetylated forms of histone H3 were found to be decreased. The effect was characterized in three serotonin projection areas, the caudate-putamen, the frontal cortex, and the dentate gyrus subregion of hippocampus. Our data highlight GABAergic neurons as major target cells expressing Mecp2 in response to the serotonin-elevating agents and suggest that serotonin signaling enhances gene silencing in postmitotic neurons.

No abstract

Stimulation of pheochromocytoma PC12 cells by cAMP-elevating agents caused the induction of the immediate early gene 3CH134, which encodes MAP kinase phosphatase-1 (MKP-1). Forskolin was as potent as serum in stimulating MKP-1 gene expression, whereas dibutyryl-cAMP and neuropeptide PACAP were less effective. Induction of the MKP-1 gene was accompanied by neo-synthesis of MKP-1 protein. MAP kinase activation was not involved in the cAMP-induced MKP-1 gene expression. The MAP kinase inactivation, that would result from MKP-1 induction in response to increased intracellular cAMP level, contributes to explain how hormones or neurotransmitters signaling through cAMP influence cell growth and differentiation. ß

The TIS8/egr-1 gene is a member of the class of immediate early genes. Originally discovered as a mitogen-induced gene, it can also be induced by synaptic activity. We report here the induction of the TIS8/egr-1 gene by the neurotransmitter 5-hydroxytryptamine (5-HT) in cultured rat phaeochromocytoma PC12 cells. Induction was maximal 40 min after addition of 5-HT to the cells, and declined very rapidly to reach the basal level after 90 min. The electrophoretic mobility-shift assay showed that induction of the TIS8/egr-1 gene by 5-HT was accompained by increased Egr-1 protein binding to its DNA consensus sequence. We found an overall correlation of 5-HT-induced egr-1 expression with that of c-fos expression. The kinetics of the ability of both gene products to bind to their respective DNA consensus sequence was also similar. The 5-HT-induced activation in Egr-1 binding was inhibited by ketanserin and mesulergine, indicating that 5-HT exerted its action via a 5-HT2 receptor subtype. The tyrosine protein kinase inhibitor genistein abolished induction of the TIS8/egr-1 gene, suggesting that tyrosine kinase activity is required for the induction of early genes by 5-HT. Genistein also inhibited 5-HT-induced Egr-1 binding activity. The increase in phosphotyrosine content of focal adhesion kinase we noticed upon addition of 5-HT to cells suggests that this cytoplasmic tyrosine protein kinase mediates the effect of 5-HT in eliciting early gene induction.

No abstract