2.2.1 Histone acetylation
Histone acetylation modifying enzymes regulate transcription process by changing the status of histone acetylation in chromosomes(Riaz et al., 2015). Histone deacetylases (HDACs) and histone acetyltransferases (HATs) are necessary to maintain epigenetic regulation of gene expression by histone acetylation. HATs catalyze the reversible acetylation reaction at the ε-amino group of lysine residues. Chromatin relaxation and increased transcriptional activity of genes is associated with neutralization of the positive charge of lysine residues by histone acetylation. Meanwhile, HDACs can remove acetyl groups silencing the transcriptional activity of genes and leading to chromatin condensation(Figure2) (Wawruszak et al., 2021). HDACs are divided into four main classes: Class I (HDACs 1, 2, 3,8), Class IIa (HDAC4, 5, 7, 9) and Class IIb (HDAC6, 10), Class III (SIRT1-SIRT7), Class IV (HDAC11) (Wawruszak et al., 2021). HDACs (Class I, II and III) share sequence similarity and require Zn2+ for deacetylase activity. However, SIRT1-7(class III), show no sequence resemblance to members of the classical family and require NAD+ as the cofactor(Gregoretti et al., 2004; Whittle et al., 2007). In the HDAC family, classes I and II are most associated with epilepsy, and have been found to have the highest expression in the brain(Younus and Reddy, 2017). In the CNS, HDAC2 is thought to inhibit memory consolidation and synaptic plasticity by inhibiting HDAC acetylation and HDAC5 is also critical to memory formation(Younus and Reddy, 2017). HDAC6 plays a neuroprotective role by promoting autophagy of damaged proteins(Boyault et al., 2007). However, HDAC6 is also involved in regulating the function of microtubule networks and impairs neuronal axon transport(d’Ydewalle et al., 2012). In addition, the biological functions of HDACs are also involved in metabolism, protein degradation, angiogenesis, DNA damage, immune regulation, cell cycle and apoptosis.
In patients and animal models of epilepsy, changes of histone acetylation have attracted widespread attention and have been considered to be involved in epileptogenesis(Hauser et al., 2018; Boison and Rho, 2020). Relevant studies have shown that seizures promote deacetylation of histone H4 of GluR2, which is closely related to epileptogenesis and increased neuronal excitability(Huang et al., 2002; Tsankova et al., 2004; Chen et al., 2021b). In a model of Tuberous Sclerosis Complex (TSC) of mouse, histone H3 acetylation (H3K9Ac and H3K27Ac) levels are generally decreased in the hippocampal neurons. Inhibition of HDAC activity can restore histone H3 acetylation levels and improve abnormal synaptic plasticity and seizure threshold in TSC2, which suggesting HDAC inhibitors may be novel therapeutic strategies for TSC(Basu et al., 2019). Multiple studies reported increased H3 phosphorylation in pilocarpine and KA-induced seizures, which is believed to promote the underlying mechanism that induces histone acetylation(Younus and Reddy, 2017). Furthermore, increased histone H4 acetylation is also reported in pilocarpine, KA and electroconvulsive-induced epilepsy models(Huang et al., 2002; Tsankova et al., 2004; Sng et al., 2006). Altered histone acetylation at GluR2 and BDNF genes is an early event triggered by SE. Meanwhile, increased histone H4 acetylation is also linked to upregulated BDNF and c-FOS/c-JUN genes(Huang et al., 2002). Related studies have shown that the downregulation of c-FOS transcription may be achieved by histone H4 acetylation, while the upregulation of BDNF transcription may be related to the regulation of histone H3 acetylation(Tsankova et al., 2004).
Currently, several HDACs are upregulated during epilepsy. In the brain tissue of patients with DR-TLE, HDAC2 is upregulated, which is associated with reduced histone acetylation and gene expression. HDAC4 downregulates the gene expression of GABAA α1 subunit and AMPAR subunit GluA2, which suggesting that decreased GABAA α1 subunit and downregulated AMPAR subunit GluA2 are associated with histone deacetylation after seizures(Fonseca-Barriendos et al., 2021). In addition, mutations in the factor-induced-gene 4 (FIG4) gene are associated with multiple disorders including epilepsy. Defects of HDAC1 may also explain FIG4-associated disorders including epilepsy(Muraoka et al., 2021). Moreover, the transformation/transcription domain-associated protein (TRRAP) is a common component of many HAT complexes(Leduc et al., 2014). Some results demonstrate TRRAP-dependent histone acetylation plays an essential role in regulating neurogenesis and cell cycle(Tapias et al., 2014). Meanwhile, it has been found that mutations of TRRAP can also cause human neuropathies including epilepsy. In addition, TRRAP regulates microtubule dynamics through the SP1 signaling pathway to prevent neurodegeneration(Tapias et al., 2021).
In epilepsy, some drugs, including ADEs, also regulate histone acetylation. Blocking HDACs with specific inhibitors appears to be an effective therapeutic strategy for enhancing neuroprotection and interfering with epileptogenesis(Boison and Rho, 2020). Ketogenic diet (KD), as an important treatment for epilepsy, can enhance the production of β-hydroxybutyrate which is a HDAC inhibitor(Lusardi et al., 2015; Longo et al., 2019). Traumatic brain injury (TBI) is an important cause of epileptogenesis. Related study has found that inhibition of HDAC can improve learning and memory after TBI by combining with behavioral therapy (Dash et al., 2009). Meanwhile, HDAC inhibitor ITF2357 has neuroprotective effects and promotes neurological function recovery following TBI(Shein et al., 2009). In addition, HDAC inhibitor can also promote histone H3 acetylation and inhibt inflammatory response of microglia following TBI in rats(Zhang et al., 2008). Valproic acid (VPA), the most common AED, is known to suppress seizures by increasing levels of GABA in the brain. Meanwhile, VPA is also a well-known HDAC inhibitor(Lunke et al., 2021; Wawruszak et al., 2021). VPA regulates histone acetylation together with HATs, suggesting that epigenetic regulation of genes by VPA may be involved in the occurrence of neurological diseases(Hu et al., 2020b). Relevant study has shown that VPA may also participates in epileptogenesis by regulating HDACs(Hu et al., 2020b). The study has found that the inhibitory concentration of VPA on HDACs activity is about 0.3-1.0mM, which is within the therapeutic range of VPA in human epilepsy(Blaauboer et al., 2022). Both class I and class II HDACs are inhibited by VPA, with the highest potency for class I HDACs, especially HDAC1(Phiel et al., 2001; Blaauboer et al., 2022). The epigenetic effects of VPA mainly depend on inhibition of HDACs and regulation of BDNF(Ghiglieri et al., 2010). VPA and Other HDAC inhibitors can downregulate TrkB expression and BDNF/TrkB signaling(Dedoni et al., 2019). Meanwhile, VPA is also involved in inflammatory response by inhibiting of HDACs. VPA can mitigate traumatic spinal cord injury-induced inflammation response by HDAC3-dependent STAT1 and NF-κB pathway(Chen et al., 2018). In addition, sodium butyrate, a critical HDAC inhibitor in the aliphatic fatty acid family, can increase histones H3 and H4 acetylation in the hippocampus and cerebral cortex of mice(Younus and Reddy, 2017). Sodium butyrate can also improve the anticonvulsant activity of MK-801 (dizocilpine), an NMDAR antagonist(Deutsch et al., 2008). Sodium butyrate has been shown to modulate the effects of AED flurazepam to antagonize electrically precipitated seizures(Deutsch et al., 2009). Meanwhile, sodium butyrate significantly delays epileptogenesis, delays the development of seizures and reduces the severity of behavioral seizures in mice(Younus and Reddy, 2017). In the hippocampus kindling model of TLE, sodium butyrate significantly inhibited HDAC activity and retarded the development of limbic epileptogenesis. Meanwhile, inhibition of HDAC can significantly reduce persistent seizures, eliminate the epileptic state, and significantly reduce the sprouting mossy fiber(Reddy et al., 2018). Histone H3 and H4 acetylation levels are decreased in epileptic rats, while histone acetylation levels is significantly increased when treated with sodium butyrate or VPA alone in the brain, especially combined administration(Citraro et al., 2020). These findings suggest that sodium butyrate has a strong antiepileptic effect by regulating HDACs.
In a word, these findings support the underlying theory that HDAC inhibitors prevent epilepsy by interfering with epigenetic gene expression profiles. Importantly, histone acetylation modifications may have a crucial role in epileptogenesis and early treatment with HDAC inhibitors might be a possible strategy for preventing epileptogenesis.