2.3.2 lncRNA
LncRNAs are a class of long transcripts that do not have protein-coding
ability, which have become regulatory molecules widely involved in
biological processes(Villa et al., 2019). Increasing evidences indicate
that lncRNAs are associated with RNA processing, the control of nuclear
organization and transcriptional and post-transcriptional regulation of
gene expression (Yao et al., 2019). LncRNAs, as competitive endogenous
RNAs (ceRNAs), competitively suppress microRNAs to regulate
transcription of RNAs(Chen et al., 2021a). Moreover, lncRNAs are
involved in multiple biological processes, such as cell death,
immuno-inflammatory responses, proliferation, organogenesis, genomic
imprinting and chromatin remodeling (Fernandes et al., 2019). LncRNAs
have been also implicated in several human diseases, such as
neurological disorders, autoimmune disease, cardiovascular disease,
metabolic disease and cancer (Hu et al., 2018; Villa et al., 2019). In
the CNS, some lncRNAs may play key roles in neuronal function,
development and maintenance of memory, cognitive function and synaptic
plasticity(Wu et al., 2013). More and more studies believe that
regulation of lncRNAs is closely related to epilepsy, which may become
the prospect of new therapeutic interventions for epilepsy(Irwin et al.,
2021).
In epilepsy, many studies have reported that lncRNAs are dysregulated in
epilepsy and are involved in the pathological process of
epilepsy(Table2) (Villa et al., 2019). Abnormal expression of
lncRNAs has been found in both epileptic animal models and epileptic
patients(Lee et al., 2015; Jang et al., 2018; Xiao et al., 2018). In
pilocarpine-induced epileptic mouse model, the differentially expressed
lncRNAs are unique in each brain region(Jang et al., 2018). It has been
found that occurrence and progression of TLE is closely related to the
changed methylation profiles of lncRNAs. In this study, 384 lncRNAs are
significantly dysregulated in pilocarpine-induced epileptic model and
279 lncRNAs are significantly dysregulated in KA-induced epileptic
model(Lee et al., 2015). Hypermethylated lncRNAs are associated with
drug metabolism ion channel activity, GABAR activity and synaptic
transmission, suggesting that lncRNAs may be involved in the mechanism
of refractory mTLE(Xiao et al., 2018).
BDNF antisense RNA (BDNF-AS; BDNF-OS), is a lncRNA transcribed from the
opposite strand of BDNF (Lipovich et al., 2012). Study has found
that BDNF-AS levels are significantly decreased and BDNF expression is
significantly increased in human neocortex of intractable
epilepsy(Lipovich et al., 2012). Meanwhile, that BDNF-AS can negatively
regulate the expression of BDNF has also been confirmed in
vitro(Modarresi et al., 2012). Thus, regulation of BDNF by BDNF-OS may
be as a treatment for intractable epilepsy. New evidence has revealed
that lncRNAs mainly serve as ceRNA targeting microRNAs in regulating
neuronal apoptosis. LncRNA H19 suppresses microRMA-let-7b to regulate
hippocampal neuron apoptosis and lncRNA H19 also regulates JAK/STAT
signaling to promote activation of hippocampal glial cell in TLE rat
model(Han et al., 2018a; Han et al., 2018b). LncRNA FTX regulates
microRNA-21-5p/SOX7 axis to suppress apoptosis of hippocampal neurons in
a rat model of TLE(Li et al., 2019b). Meanwhile, LncRNA GAS5 inhibits
microRNA-219 to affect CaMKIIγ/NMDAR pathway and promote the progression
of epilepsy(Zhao et al., 2022). However, lncRNA GAS5 silencing regulates
microRNA-135a-5p to suppress the expression of KCNQ3, thereby preventing
the progression of epilepsy(Li et al., 2019a). LncRNA TUG1 may be a
biomarker of TLE diagnosis in children, and regulates miR-199a-3p to
affect cell activity and apoptosis of hippocampal neuron (Li et al.,
2021). LncRNA UCA1 regulates microRNA-495/Nrf2-ARE signal pathway to
suppress seizure-induced brain injury and seizure, and lncRNA-UCA1 has
dynamic regulation effect on NF-κB in hippocampus of epilepsy rats(Wang
et al., 2017; Geng et al., 2018). Meanwhile, lncRNA UCA1 regulates
microRNA-375/SFRP1-mediated WNT/β-Catenin pathway to alleviate aberrant
hippocampal neurogenesis in KA-induced epilepsy model (Diao et al.,
2021). LncRNA UCA1 regulates microRNA-203-mediated MEF2C/NF-κB signaling
pathway to inhibit inflammation in epilepsy(Yu et al., 2020). LncRNA
ILF3-AS1 suppresses microRNA-212 to mediate epileptogenesis in the
hippocampus and targets microRNA-212 to promote the expression of
inflammatory cytokines and MMPs in TLE(Cai et al., 2020). Previous
studies have confirmed that lncRNA NEAT1 is responsive to neuronal
activity and is associated with hyperexcitability states. However,
LncRNA NEAT1 also targets microRNA-129-5p and regulates Notch signaling
to regulate inflammatory responses in epilepsy (Barry et al., 2017; Wan
and Yang, 2020). LncRNA XIST sponges miR-29c-3p and regulates NFAT5
expression to promote the secretion of inflammatory cytokines in
LPS-treated CTX-TNA2(Zhang et al., 2021). LncRNA Nespas, as a regulator
of microRNA-615-3p, inhibits the PI3K/Akt/mTOR pathway to suppress
apoptosis of epileptiform hippocampal neurons by upregulating
Psmd11(Feng et al., 2021). SP1 activated-lncRNA SNHG1 regulates
microRNA-154-5p/TLR5 axis to mediate the development of epilepsy(Zhao et
al., 2020). LncRNA ZFAS1 upregulates microRNA-421 to activate the
PI3K/AKT pathway, thereby inhibiting apoptosis and autophagy of
hippocampal neurons in epilepsy(Hu et al., 2020a). Meanwhile, lncRNA
ZFAS1 can also promote neuronal apoptosis and inflammation response,
thereby aggravating the development of epilepsy(He et al., 2021a). These
regulation mechanisms of lncRNAs in epilepsy and seizure-induced brain
injury by targeting microRNAs may provide new targets for biological
therapy of epilepsy. Silencing lncRNA PVT1 can downregulate WNT
signaling pathway to promote the expression of BDNF and suppress the
activation of hippocampal astrocytes in epileptic rats(Zhao et al.,
2019b). Related research has found that MALAT1 can regulate the density
of dendritic spines, and loss of lncRNA BC1 can reduce the convulsion
thresholds(Murugan and Boison, 2020). Downregulated lncRNA MALAT1
regulates the PI3K/Akt signaling pathway to protect hippocampal neurons
against excessive autophagy and apoptosis in rats with epilepsy(Wu and
Yi, 2018). In addition, lncRNA KCNH5-1 plays a key vital role in
developing TLE with hippocampal sclerosis (HS)(Wang et al., 2022).
In addition, circular RNAs (circRNAs) are a class of lncRNAs with a
closed loop structure that regulate gene expression, which abundant in
brain tissue. The abnormality of circRNAs may reflect the pathogenesis
of TLE, but the roles of circRNAs in epilepsy are still limited. It has
been found that circ-EFCAB2 and circ-DROSHA may be potential therapeutic
targets and biomarkers for TLE patients(Li et al., 2018b). Meanwhile,
circ-DROSHA can regulate microRNA-106b-5p/MEF2C axis to reduce the
neural damage in TLE cell model (Zheng et al., 2021). Circ-UBQLN1
upregulates microRNA-155-mediated SOX7, thereby inhibiting apoptosis and
oxidative stress and promoting proliferation of hippocampal neurons in
epilepsy(Zhu et al., 2021). Circ-ANKMY2 regulates the
microRNA-106b-5p/FOXP1 axis to affect TLE progression (Lin et al.,
2020). Circ-Hivep2 regulates microRNA-181a-5p/SOCS2 signaling to promote
microglia activation and inflammation in KA-induced epileptic mice model
(Xiaoying et al., 2020).
In a word, those results indicate the role for lncRNAs in modulating
neuronal activity and suggest a novel mechanistic link between an
activity-dependent lncRNA and epilepsy.
2.3.3N6-methyladenosine
(m6A) modification.
As the most abundant epigenetic modification of eukaryotic mRNA, m6A
methylation is considered to be the most common internal modification of
mRNAs and ncRNAs in organisms(Tao et al., 2022; Yang et al., 2022). The
occurrence of m6A methylation is controlled by a core methyltransferase
complex, including methyltransferase-like 3 and 14 (METTL3 and METTL14)
and wilms tumor 1-associated protein (WTAP)(You et al., 2022).
Meanwhile, two m6A demethylases (FTO and
ALKBH5)
can specifically eliminate the m6A sites from target mRNAs(You et al.,
2022). In addition, m6A-binding proteins mainly include YTH
domain-containing RNA-binding proteins(YTHDF1/2/3), which can
specifically recognize and bind to m6A-modified mRNA(Lei and Wang,
2022). Changes of m6A modification cause abnormal nervous system
functions, including brain tissue development retardation, synaptic
dysfunction, memory and cognitive function changes(Lei and Wang, 2022).
It has been reported that m6A modification is highly enriched in the
brain and plays an important role in CNS development and
neurodegenerative diseases involved in Parkinson’s disease (PD),
Alzheimer’s
disease (AD), epilepsy(Livneh et al., 2020).
In knockout of METTL3 mice, the cerebellum is severely atrophied, and
the weight of the whole brain and cerebellum is significantly reduced,
and the decrease of m6A modification causes apoptosis in the
cerebellum(Wang et al., 2018a). Deficient ALKBH5 can result in
disturbance of the m6A modification of genes related to cerebellar
development (Ma et al., 2018). The dysregulation of m6A modification
impairs synapse formation and function. Knockdown of FTO in axons
increases m6A modification of Growth associated protein 43 (GAP-43)
mRNA, thereby reducing translation of GAP-43 mRNA and inhibiting
axons(Yu et al., 2018). Meanwhile, some experiments have shown that the
loss of YTHDF1 or YTHDF3 leads to synaptic dysfunction(Merkurjev et al.,
2018). Those findings suggest that m6A modification is closely related
to brain tissue development.
METTL3-mediated
m6A mRNA modification enhances learning and memory ability(Zhang et al.,
2018d). Absent YTHDF1 in the hippocampus of adult mice, can lead to
learning and memory deficits, and re-expression of YTHDF1 can repair the
associated damage(Shi et al., 2018). Meanwhile, FTO is expressed in the
CA1 region of the hippocampus of mice and negative feedback regulates
the formation of memory(Walters et al., 2017). In addition,
METTL3-mediated m6A modification facilitated processing and maturation
of pri-microRNA-221 to upregulate microRNA-221-3p, thereby aggravating
cognitive deficits of rats(Niu et al., 2022). Those studies suggest
relationship between m6A modification and the formation of memory and
cognitive. In human AD samples, it has been observed that a high
concentration of METTL3 in insoluble fractions is correlated positively
with the concentration of the insoluble tau protein(Huang et al., 2020).
Meanwhile, FTO also promote the occurrence of AD by targeting
TSC1-mTOR-Tau signaling(Li et al., 2018a). In epilepsy, epileptogenesis
is closely related to NMDARs. However, overexpressed FTO in dopaminergic
neurons reduces the level of mRNA m6A modification, induces the
expression of NMDAR1, promotes oxidative stress and
Ca2+ influx, thus promoting degeneration or apoptosis
of neurons(Li et al., 2018a). Meanwhile, VPA can also induce the
expression of FTO and FTO knockdown eliminated the inhibitory effect of
VPA on MBD2 and Na1.3 expression in epilepsy(Tan et al., 2017). In
addition, based on the evidence that microRNA-134 and microRNA-146a are
involved in the pathogenesis of
epilepsy, both microRNA-134 and
microRNA-146a contain a potential m6A site in the seed region, which is
thought to play an important role in microRNA recognition of target
mRNA(Rowles et al., 2012). Those studies may suggest close relationship
between m6A modification and epilepsy. However, there’s not a lot of
research about m6A modification in epilepsy. In a word, the study of m6A
modification will be conducive to
reveal the underlying pathophysiological mechanism of neuropsychiatric
diseases. Meanwhile, regulation of the level of m6A modification in the
brain is an effective strategy for the treatment of CNS diseases,
including epilepsy and decline of epilepsy-related cognitive and memory.