NEUROEPIGENETICS

Neuroepigenetics is the study of how epigenetic:

changes to genes affect the nervous system. These changes may affect underlying conditions such as addiction, cognition, and neurological development. Neuroepigenetic mechanisms regulate gene expression in the neuron. Often, these changes take place due to recurring stimuli. Neuroepigenetic mechanisms involve proteins or protein pathways that regulate gene expression by adding, editing or reading epigenetic marks such as methylation or acetylating. Some of these mechanisms include ATP-dependent chromatin remodelling, LINE1, and prior protein-based modifications. Other silencing mechanisms include the recruitment of specialized proteins that methyl ate DNA such that the core promoter element is inaccessible to transcription factors and RNA polymerase. As a result, transcription is no longer possible. One such protein pathway is the REST co-repressor complex pathway. There are also several non-coding RNAs that regulate neural function at the epigenetic level. These mechanisms, along with neural his tone methylation, affect arrangement of synapses, neuroplasticity, and play a key role in learning and memory.

DNA methyltransferases (DNMTs) are involved in regulation of the electrophysiological landscape of the brain through methylation of CpGs. Several studies have shown that inhibition or depletion of DNMT1 activity during neural maturation leads to hypomethylation of the neurons by removing the cell's ability to maintain methylation marks in the chromatin. This gradual loss of methylation marks leads to changes in the expression of crucial developmental genes that may be dosage sensitive, leading to neural degeneration. This was observed in the mature neurons in the dorsal portion of the mouse pros encephalon, where there were significantly greater amounts of neural degeneration and poor neural signalling in the absence of DNMT1. Despite poor survival rates amongst the DNMT1-depleted neurons, some of the cells persisted throughout the lifespan of the organism. The surviving cells reaffirmed that the loss of DNMT1 led to hypomethylation in the neural cell genome. These cells also exhibited poor neural functioning. In fact, a global loss of neural functioning was also observed in these model organisms, with the greatest amounts neural degeneration occurring in the pros encephalon.

Other studies showed a trend for DNMT3a and DNMT3b. However, these DNMT's add new methyl marks on unmethylated DNA, unlike DNMT1. Like DNMT1, the loss of DNMT3a and 3b resulted in neuromuscular degeneration two months after birth, as well as poor survival rates amongst the progeny of the mutant cells, even though DNMT3a does not regularly function to maintain methylation marks. This conundrum was addressed by other studies which recorded rare loci in mature neurons where DNMT3a acted as maintenance DNMT. The Gfap locus, which codes for the formation and regulation of the cytoskeleton of astrocytes, is one such locus where this activity is observed. The gene is regularly methylated to down regulate glioma related cancers.

DNMT inhibition leads to decreased methylation and increased synaptic activity. Several studies show that the methylation-related increase or decrease in synaptic activity occurs due to the up regulation or down regulation of receptors at the neurological synapse. Such receptor regulation plays a major role in many important mechanisms, such as the 'fight or flight' response. The glucocorticoid receptor (GR) is the most studied of these receptors. During stressful circumstances, there is a signalling cascade that begins from the pituitary gland and terminates due to a negative feedback loop from the adrenal gland. In this loop, the increase in the levels of the stress response hormone results in the increase of GR. Increase in GR results in the decrease of cellular response to the hormone levels. It has been shown that methylation of the I7 axon within the GR locus leads to a lower level of basal GR expression in mice. These mice were more susceptible to high levels of stress as opposed to mice with lower levels of methylation at the I7 axon. Up-regulation or down-regulation of receptors through methylation leads to change in synaptic activity of the neuron.

CpG Islands (CGIs) are regulatory elements that can influence gene expression by allowing or interfering with transcription initiation or enhancer activity. CGIs are generally interspersed with the promoter regions of the genes they affect and may also affect more than one promoter region. In addition they may also include enhancer elements and be separate from the transcription start site. Hypermethylation at key CGIs can effectively silence expression of tumour suppressing genes and is common in gliomas. Tumour suppressing genes are those which inhibit a cell's progression towards cancer. These genes are commonly associated with important functions which regulate cell-cycle events. For example, PI3K and p53 pathways are affected by CGI promoter hypermethylation, this includes the promoters of the genes CDKN2/p16, RB, PTEN, TP53 and p14ARF. Importantly, glioblastomas are known to have high frequency of methylation at CGIs/promoter sites. For example, Epithelial Membrane Protein 3 (EMP3) is a gene which is involved in cell proliferation as well as cellular interactions. It is also thought to function as a tumour suppressor, and in glioblastomas is shown to be silenced via hypermethylation. Furthermore, introduction of the gene into EMP3-silenced neuroblasts results in reduced colony formation as well as suppressed tumour growth. In contrast, hypermethylation of promoter sites can also inhibit activity of oncogenes and prevent tumorigenesis. Such oncogenic pathways as the transformation growth factor (TGF)-beta signalling pathway stimulate cells to proliferate. In glioblastomas the over activity of this pathway is associated with aggressive forms of tumour growth. Hypermethylation of PDGF-B, the TGF-beta target, inhibits uncontrolled proliferation.

Global reduction in methylation is implicated in tumorigenesis. More specifically, wide spread CpG demethylation, contributing to global hypomethylation, is known to cause genomic instability leading to development of tumours. An important effect of this DNA modification is its transcriptional activation of oncogenes. For example, expression of MAGEA1 enhanced by hypomethylation interferes with p53 function.

Aberrant patterns of his tone modifications can also take place at specific loci and ultimately manipulate gene activity. In terms of CGI promoter sites, methylation and loss of acetylating occurs frequently at H3K9. Furthermore, H3K9 dimethylation and trimethylation are repressive marks which, along with bivalent differentially methylated domains, are hypothesized to make tumour suppressing genes more susceptible to silencing. Abnormal presence or lack of methylation in glioblastomas is strongly linked to genes which regulate apoptosis, DNA repair, cell proliferation, and tumour suppression.