3.2.2. Histone Modifications
Histones Methylation
According to recent research, epigenetic mechanisms that mediate drug-induced transcriptional and behavioral changes induced by METH consumption are mostly caused by histone modification
[77][139]. Indeed, it was shown that METH induced H3 methylation through increased tri-methylation of histone H3 at lysine 4 (H3K4me3) at the promoter site of chemokine receptor 2 associated with behavioral sensitization in mice
[78][140].
Histones Acetylation
Similar to COC, METH induces alterations in the acetylation levels of histones H3 and H4, influencing the expression of various enzymes in different brain regions
[79][141]. In NAc, METH exposure leads to differential acetylation changes on various histone lysine residues by regulating the protein levels of histone deacetylases
[80][142]. Acute METH exposure results in decreased H3 acetylation (H3K9Ac and H3K18Ac) and increased H4 acetylation (H4K5Ac, H4K8Ac, and H4K16Ac)
[81][143]. Chronic METH causes a reduction in histone H4 acetylation (H4K5Ac, H4K12Ac, and H4K16Ac) at glutamate (GLUT) receptor promoters, impacting the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors, leading to oxidation, excitotoxicity, and neuroinflammation
[82][144].
Histones Ubiquitination
Neurotransmitter excitation and synaptic plasticity in brain disorders associated with drug addiction have been connected to the UPS, an enzymatic complex that controls proteolysis and turnover. Both pre- and post-synaptic neurons in the DA circuitry are impacted by UPS inhibition. Through endocytic internalization and degradation, the UPS reduces D1/D2-like DRs and Alphaamino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)
[83][150]. The UPS regulates the presynaptic release of Glu via both D1-like and D2-like receptors, which controls DA transmission
[84][151], also influencing postsynaptic plasticity
[85][86][152,153]. As a result, the DA and Glu signaling pathways interact with UPS substrates
[67][121]. METH, despite being an extremely powerful DA releaser, impairs UPS activity, which is largely due to dopamine. As a result, pre- and post-synaptic neurons in the dopamine circuitry are extremely vulnerable to UPS inhibition
[83][150]. Parkin promotes the ubiquitination of substrate proteins, which aids in their degradation.
3.3. ncRNAs
Similarly, as in COC addiction, epigenetic modifications in METH addiction involving ncRNA predominantly include miRNAs. Using the KEGG pathway analysis, miRNA-regulated genes were found to be involved in vesicular transport, METH addiction, the cyclic guanosine monophosphate cGMP-protein kinases G (PKG) signaling pathway, the dopaminergic synapse, and the GABAergic synapse
[87][157]. In genome-wide transcriptional profiling, the expression of multiple miRNAs is increased in the central amygdala alongside molecules related to METH addiction
[88][155]. METH increased the levels of miRNAs 237, 296, and 501 in the NAc. MiRNAs in the NAc regulate Wnt signaling and axon guidance genes
[89][158]. MiR-128 influenced METH-induced behavioral sensitization by altering synaptic plasticity-related molecules in the NAc
[90][159]. METH-induced locomotor sensitization is disrupted by Ago2-dependent miRNAs in the NAc. These Ago2/miR-3068-5p effects occur in conjunction with the glutamate receptor, GluN1/Grin1
[91][138]. METH can increase the expression of miRNAs in the striatum, which harmed motor coordination and reduced striatal volume and dendritic length
[92][160].
4. Epitranscriptomics and Psychostimulant Addiction
Epigenetic modifications to DNA and histones regulate gene transcription, whereas epitranscriptomic post-transcriptional RNA modifications influence gene expression
[93][165]. The most common mRNA modifications are N1-methyladenosine (m
1A), N6-methyladenosine (m
6A), 5-methylcytosine (m
5C), pseudouridine (Ψ), and others
[94][166]. m
6A modification is one of the most abundant and reversible epitranscriptomic modifications, mediated by a set of proteins, which include methyltransferases (‘writers’), demethylases (‘erasers’), and m
6A-binding proteins (‘readers’)
[95][167]. m
6A methylation is associated with the control of mRNA metabolism, splicing, export, stability, translation, and degradation
[96][97][168,169]. Out of all tissues in the body, the brain has the highest abundance of m
6A methylation, which is developmentally decreasing
[96][168]. In the brain, m
6A methylation regulates neuronal transcripts and neuronal activity. Aside from its role in neuronal development
[98][30], m
6A modification is essential for the process of axon regeneration
[99][170]. Methyltransferases like-3 (METTL3) and like-14 (METTL14)
[100][101][102][171,172,173], along with other proteins needed for m
6A deposition, such as Wilms’ tumor 1-associating protein (WTAP)
[103][174] and RNA binding protein 15 (Rbm15)
[104][175], form a stable protein complex that catalyzes m
6A modification. Because they deposit RNA methylation modifications, these methyltransferases are collectively known as “writers”. Fat mass obesity-associated protein (FTO) and alkB homologue 5 (ALKBH5) are two demethylases from the family of α-ketoglutarate dependent dioxygenases that can reverse m
6A modification because it is dynamically regulated
[105][106][176,177]. Demethylases are known as “erasers” because they remove RNA methylation modifications. Posttranscriptional, site-specific adenosine-to-inosine base conversions, known as RNA editing, contribute to gene expression diversity and are catalyzed by Adenosine deaminases acting on RNA (ADARs)
[107][178]. Pseudouridine Synthase 7 (PUS7) is one of the major mRNA-modifying enzymes leading to pseudouridine (Ψ), a ubiquitous RNA modification
[108][179]. Both ADAR and PUS7 can lead to further mRNA modifications. m
6A modifications are in direct connection to the so-called m
6A “reader” proteins, which recognize the modified site. The proteins with YTH domains, which can specifically bind m
6A through their YTH domain, are the most well-studied m
6A readers. Fragile X mental retardation protein (FMRP) was also reported to be m
6A reader and plays critical roles in synaptic plasticity and neuronal development. The identification of methylated nucleosides (m
6A, m
5C, m
1A) is performed using immunoprecipitation and their variants using antibodies against methylated nucleosides or associated proteins methyltransferases, demethylases, and binding proteins. After fragmenting the RNA, fragments containing modified nucleosides are enriched before sequencing. The immunoprecipitate is analyzed using next-generation sequencing (NGS) to identify and map the modification
[109][180].
The connection between epigenetic regulation and m
6A RNA modification was associated with histone H3 trimethylation at Lys36 (H3K36me3), a marker for transcription elongation, which guides m
6A deposition globally connected through METTL14 (DOI: 10.1038/s41586-019-1016-7). m
6A modifications are mainly associated with neuronal plasticity in the brain, which is a consequence of learning and memory, and most of the literature is based on this line of research together with neurodegenerative disorders
[105][110][111][176,181,182]. Indeed, deficiency in m
6A-dependent pathways significantly impairs neuronal function including dopamine signaling and dopamine-dependent learning.
Lowering neuronal m
6A by overexpressing FTO or by adding m
6A inhibitor led to the induction of N-methyl-d-aspartate (NMDA) receptor 1 expression, elevated oxidative stress, and Ca
2+ influx, resulting in dopaminergic neuron apoptosis
[112][183]. In addition, it was shown that the overexpression of FTO delays the dephosphorylation of CREB, increases the expression of the CREB, and targets neuropeptide receptor 1 (NPY1R) and BDNF known to regulate food intake and energy homeostasis
[113][184]. FTO affects dopamine (D2)-dependent responses to reward learning in meso-striato-prefrontal regions, suggesting a mechanism by which genetic predisposition alters reward processing not only in obesity but also in other disorders with altered D2R-dependent impulse control, such as addiction
[114][185].