Neuroepigenetics is the study of how epigenetic changes to genes affect the nervous system. These changes may effect underlying conditions such as addiction, cognition, and neurological development.
Mechanisms
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 acetylation. Some of these mechanisms include ATP-dependent chromatin remodeling, LINE1, and prion protein-based modifications. Other silencing mechanisms include the recruitment of specialized proteins that methylate 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 histone methylation, affect arrangement of synapses, neuroplasticity, and play a key role in learning and memory.
Methylation
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 prosencephalon, where there was significantly greater amounts of neural degeneration and poor neural signaling 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 prosencephalon.
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 a 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 downregulate 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 upregulation or downregulation 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 signaling 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 exon 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 exon. Up-regulation or down-regulation of receptors through methylation leads to change in synaptic activity of the neuron.