3. REST
NRSF, also known as REST, is a zinc finger transcription factor that is widely expressed in neuronal and non-neuronal cells(Soga et al., 2021; Su et al., 2022a). NRSF, a master regulator of the CNS, is the basis of neuronal differentiation, plasticity and survival, which is also involved in hyperexcitability, oxidative stress and neurodegeneration(Ghosh et al., 2021; Su et al., 2022a). REST is expressed in a wide range of brain regions, including the cerebral cortex and the hippocampus(Butler-Ryan and Wood, 2021). Relevant studies have found that the expression of REST is continuously upregulated in patients and animal models of epilepsy, so it has become the focus of epilepsy research(Butler-Ryan and Wood, 2021).
After binding to DNA, NRSF can bind to the neuron-restrictive silencer element (NRSE) to recruit co-repressors and then suppress transcription of NRSE downstream genes by epigenetic mechanisms(Su et al., 2022a). The N-terminal domain of NRSF can recruit the corepressor mSin3 by its paired amphipathic helix (PAH1) domain. mSin3 can recruit HDACs to nucleosomes, thereby promoting a chromatin repressive environment by histone deacetylation(Laherty et al., 1997; Nomura et al., 2005). Separately, the C-terminal domain can recruit REST corepressor 1 (CoREST), thereby recruiting chromatin modifying enzymes, including HDACs and HMT(Andrés et al., 1999; Yang et al., 2006). Meanwhile, CoREST contains two SANT domains that allow it to interact with histones(Yang et al., 2006). Finally, NRSF expression can be downregulated post-translationally by B-TrCP ubiquitination(Westbrook et al., 2008). In addition, REST-interacting LIM domain protein (RILP) is one of the chief nuclear importers of NRSF. The REST/NRSF plays an important role in nuclear translocation. RILP has been found to interact directly with ZFD5 of NRSF and is required for proper differentiation and maintenance of neuronal phenotypes(Shimojo and Hersh, 2006).
REST4, RILP, and CoREST play important roles in the regulation of NRSF activity. However, chromatin modifiers that leave repressive covalent modifications on histones and DNA can regulate the repressive function of NRSF(Thompson and Chan, 2018). Meanwhile, function of NRSF depends on recruitment of HDACs, HMT and DNA methylases, and NRSF also recruits mSin3a to its N-terminal region(Huang et al., 1999; Thompson and Chan, 2018). From there, mSin3a recruits HDACs that are essential for gene repression(Laherty et al., 1997). In addition, NRSF recruits G9a which appears to preferentially demethylate H3K9, and its activity does not overlap with HDAC repression from either mSin3a or CoREST(Roopra et al., 2004; Mulligan et al., 2008). Meanwhile, Chromodomain on Y-like (CDYL) bridges REST and HMT for gene repression and inhibition of cellular transformation(Mulligan et al., 2008). CoREST is seen as a recruiter for HDACs. In addition, CoREST can interact with methyl CpG binding protein 2 (MeCP2) or binds to methylated DNA to regulate long-term gene inhibition(Lunyak et al., 2002; Thompson and Chan, 2018).
In multiple models of epilepsy, the levels of REST mRNA and protein are consistently upregulated following seizures. In epilepsy patients, the levels of REST mRNA and protein are also overexpressed which correlates with the frequency of seizures(Navarrete-Modesto et al., 2019). In KA-induced seizures, the levels of REST mRNA and protein are increased in rat hippocampal and cortical neurons in vivo, with a downregulation of REST target genes, and REST protein peaks at 24 h after KA injection (Spencer et al., 2006; McClelland et al., 2011; Brennan et al., 2016; Carminati et al., 2019). In pilocarpine-induced epilepsy, the levels of REST mRNA and protein are upregulated 24 h after pilocarpine injection(Hu et al., 2011). Interestingly, REST protein expression is increased in PTZ-induced epilepsy and is resistant to kindling seizure(Chmielewska et al., 2020). Related studies have been found that REST can downregulate BDNF and TrkB to reduce excitability of neurons and protect against seizures, thereby playing a neuroprotective effect in the epilepsy brain(Butler-Ryan and Wood, 2021). Increased REST can also downregulate AMPAR subunit GluR2 and increase Ca2+ permeability, ultimately resulting in excitotoxicity, cell death and seizures(Butler-Ryan and Wood, 2021). In addition, REST, as an important regulator of epilepsy, can inhibit the expression of key neuronal genes KCC2 and GRIN2A(McClelland et al., 2011; McClelland et al., 2014). Those findings highlight an association between REST increase and protection against seizures.
Epileptic encephalopathies (EE) are severe epilepsy syndromes characterized by multiple seizure types, developmental delay and even regression. Increasingly, it is believed to be caused by de novo genetic mutations, including many identified mutations in chromodomain helicase DNA binding (CHD) protein family(Wilson et al., 2021). Related studies have been demonstrated that CHD2 directly binds to REST gene, and REST expression is decreased when CHD2 is silenced. REST-mediated neural differentiation is facilitated by CHD2 expression, which occurs by a direct association between the REST gene and CHD2 protein, rather than H3K4me-mediated processes(Shen et al., 2015). However, the interaction between CHD2 and NRSF in this context has yet to be investigated. In addition, some microRNAs such as microRNA-9, microRNA-124a, and microRNA-132 have been identified to target REST with direct roles in epigenetics(Wu and Xie, 2006). Related studies have found that the functional and structural effects of NRSF can regulate persistent memory impairment caused by developmental febrile epilepsy(Patterson et al., 2017). In progressive myoclonus epilepsy-ataxia syndrome, RILP mutations result in mislocalization of NRSF, thereby preventing the binding of RILP to NRSF and cause the accumulation of NRSF in the nucleus(Bassuk et al., 2008). NRSF and REST4 expression are increased during seizures with upregulated proconvulsant gene TAC3 (Gillies et al., 2009). It was found that increased REST4 expression may regulate NRSF to competitively inhibit the repression of NKB (Thompson and Chan, 2018). Abnormal regulation of potassium voltage-gated channel subfamily Q member 2 (KCNQ2), KCNQ3 and the ion channel genes SCN2A promotes the progression of infant epilepsy, and these genes are inhibited by NRSF(Mucha et al., 2010). In addition, NRSF regulates hyperpolarization-activated cyclic adenosine monophosphate gated channel type 1 (HCN1) channelopathy in TLE(McClelland et al., 2011). The dysregulation of NRSF seems to be implicated in epilepsy, and specific mechanisms are still lacking. Those findings highlight therapeutic potential of REST modulation through gene therapy in epilepsy patients.