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.