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.