Formation of long term memory begins with the activation of many disparate signaling pathways that ultimately impinge on the cellular mechanisms regulating gene expression. We investigated whether mechanisms regulating chromatin structure were activated during the early stages of long term memory formation in the hippocampus. Specifically, we investigated hippocampal histone acetylation during the initial stages of consolidation of long term association memories in a contextual fear conditioning paradigm. Acetylation of histone H3 in area CA1 of the hippocampus was regulated in contextual fear conditioning, an effect dependent on activation of N-methyl-D-aspartic acid (NMDA) receptors and ERK, and blocked using a behavioral latent inhibition paradigm. Activation of NMDA receptors in area CA1 in vitro increased acetylation of histone H3, and this effect was blocked by inhibition of ERK signaling. Moreover, activation of ERK in area CA1 in vitro through either the protein kinase C or protein kinase A pathways, biochemical events known to be involved in long term memory formation, also increased histone H3 acetylation. Furthermore, we observed that elevating levels of histone acetylation through the use of the histone deacetylase inhibitors trichostatin A or sodium butyrate enhanced induction of long term potentiation at Schaffer-collateral synapses in area CA1 of the hippocampus, a candidate mechanism contributing to long term memory formation in vivo. In concert with our findings in vitro, injection of animals with sodium butyrate prior to contextual fear conditioning enhanced formation of long term memory. These results indicate that histone-associated heterochromatin undergoes changes in structure during the formation of long term memory. Mimicking memory-associated changes in heterochromatin enhances a cellular process thought to underlie long term memory formation, hippocampal long term potentiation, and memory formation itself.

Mutations in the X-linked methyl-CpG-binding protein 2 (MECP2), encoding a transcriptional repressor, cause Rett syndrome and a variety of related neurodevelopmental disorders. The vast majority of mutations associated with human disease are loss-of-function mutations, but precisely what aspect of MeCP2 function is responsible for these phenotypes remains unknown. We overexpressed wild-type human protein in transgenic mice using a large genomic clone containing the entire human MECP2 locus. Detailed neurobehavioral and electrophysiological studies in transgenic line MeCP2(Tg1), which expresses MeCP2 at approximately 2-fold wild-type levels, demonstrated onset of phenotypes around 10 weeks of age. Surprisingly, these mice displayed enhanced motor and contextual learning and enhanced synaptic plasticity in the hippocampus. After 20 weeks of age, however, these mice developed seizures, became hypoactive and approximately 30% of them died by 1 year of age. These data demonstrate that MeCP2 levels must be tightly regulated in vivo, and that even mild overexpression of this protein is detrimental. Furthermore, these results support the possibility that duplications or gain-of-function mutations in MECP2 might underlie some cases of X-linked delayed-onset neurobehavioral disorders.

Discoveries concerning the molecular mechanisms of cell differentiation and development have dictated the definition of a new sub-discipline of genetics known as epigenetics. Epigenetics refers to a set of self-perpetuating, post-translational modifications of DNA and nuclear proteins that produce lasting alterations in chromatin structure as a direct consequence, and lasting alterations in patterns of gene expression as an indirect consequence. The area of epigenetics is a burgeoning subfield of genetics in which there is considerable enthusiasm driving new discoveries. Neurobiologists have only recently begun to investigate the possible roles of epigenetic mechanisms in behaviour, physiology and neuropathology. Strikingly, the relevant data from the few extant neurobiology-related studies have already indicated a theme - epigenetic mechanisms probably have an important role in synaptic plasticity and memory formation.

Loss-of-function mutations or abnormal expression of the X-linked gene encoding methyl CpG binding protein 2 (MeCP2) cause a spectrum of postnatal neurodevelopmental disorders including Rett syndrome (RTT), nonsyndromic mental retardation, learning disability, and autism. Mice expressing a truncated allele of Mecp2 (Mecp2 308 ) reproduce the motor and social behavior abnormalities of RTT; however, it is not known whether learning deficits are present in these animals. We investigated learning and memory, neuronal morphology, and synaptic function in Mecp2 308 mice. Hippocampus-dependent spatial memory, contextual fear memory, and social memory were significantly impaired in Mecp2 308 mutant males (Mecp2 308/Y ). The morphology of dendritic arborizations, the biochemical composition of synaptosomes and postsynaptic densities, and brain-derived neurotrophic factor expression were not altered in these mice. However, reduced postsynaptic density cross-sectional length was identified in asymmetric synapses of area CA1 of the hippocampus. In the hippocampus of symptomatic Mecp2 308/Y mice, Schaffer-collateral synapses exhibited enhanced basal synaptic transmission and decreased paired-pulse facilitation, suggesting that neurotransmitter release was enhanced. Schaffer-collateral long-term potentiation (LTP) was impaired. LTP was also reduced in the motor and sensory regions of the neocortex. Finally, very early symptomatic Mecp2 308/Y mice had increased basal synaptic transmission and deficits in the induction of long-term depression. These data demonstrate a requirement for MeCP2 in learning and memory and suggest that functional and ultrastructural synaptic dysfunction is an early event in the pathogenesis of RTT.

DNA (cytosine-5) methylation represents one of the most widely used mechanisms of enduring cellular memory. Stable patterns of DNA methylation are established during development, resulting in creation of persisting cellular phenotypes. There is growing evidence that the nervous system has co-opted a number of cellular mechanisms used during development to subserve the formation of long term memory. In this study, we examined the role DNA (cytosine-5) methyltransferase (DNMT) activity might play in regulating the induction of synaptic plasticity. We found that the DNA within promoters for reelin and brain-derived neurotrophic factor, genes implicated in the induction of synaptic plasticity in the adult hippocampus, exhibited rapid and dramatic changes in cytosine methylation when DNMT activity was inhibited. Moreover, zebularine and 5-aza-2-deoxycytidine, inhibitors of DNMT activity, blocked the induction of long term potentiation at Schaffer collateral synapses. Activation of protein kinase C in the hippocampus decreased reelin promoter methylation and increased DNMT3A gene expression. Interestingly, DNMT activity is required for protein kinase C-induced increases in histone H3 acetylation. Considered together, these results suggest that DNMT activity is dynamically regulated in the adult nervous system and that DNMT may play a role in regulating the induction of synaptic plasticity in the mature CNS. DNA (cytosine-5) methyltransferases (DNMTs)5 are a family of enzymes that catalyze the methylation of cytosine residues (1-5). Many biological processes, including imprinting, differentiation, X-chromosome inactivation, and long term transcriptional regulation, involve cytosine methylation, a covalent modification of DNA (6, 7). Tissue-specific patterns of DNA methylation are established early during development as a consequence of cellular differentiation (8, 9). Expression and activity of DNMT is generally restricted to dividing cells and is very high during early development (5, 10 -13). In most cell types, DNMT expression diminishes greatly once terminal differentiation occurs (5, 10 -14).The mammalian brain consists primarily of postmitotic neurons and glial cells that possess relatively low proliferative potential. In addition, there are small populations of stem cells in various regions of the brain that have the potential to develop into new neurons (15). Therefore, reports that the adult central nervous system (CNS) possesses relatively high levels of DNMT mRNA and enzyme activity were surprising (5, 13, 16). Early studies into the function of DNMT in the brain suggested that this enzyme might be involved in DNA repair and neurodegeneration (16 -19). Important recent studies also have implicated misregulation of DNMT specifically or DNA methylation in general in such cognitive disorders as schizophrenia, Rett syndrome, and Fragile X mental retardation (20 -22).Epigenetics refers to a set of self-perpetuating post-translational modifications of DNA and nuclear proteins that produce lasting alterations in ch...