2.3.1 MicroRNA
MicroRNAs can be detected in biological fluids, making them potential diagnostic biomarkers, which have been reported as biomarkers for the diagnosis of epilepsy. Meanwhile, microRNA, as an emerging therapeutic target, provides unique therapeutic advantages for some epilepsy with complex pathophysiological mechanisms by negatively regulating related proteins(Morris et al., 2021). Currently, many microRNAs are consistently dysregulated in epilepsy and affect seizures, which are also described as biomarkers for the diagnosis of TLE and predictive biomarkers for AED response(Mooney et al., 2016; De Benedittis et al., 2021) (Table1) .
Related studies have found that microRNAs are significantly upregulated in epilepsy. MicroRNA-20a-5p is involved in synaptic plasticity and silencing microRNA-20a-5p inhibits neuronal branching and axonal growth and prevents epileptogenesis by regulating RGMa-RhoA-mediated synaptic plasticity in the PTZ-induced epilepsy model(Feng et al., 2020). MicroRNA-21-5p reduces IL-6 levels, loss of hippocampal neurons and apoptosis by inhibiting STAT3 expression, which suggesting protective effects of microRNA-21-5p in hippocampal neurons of epileptic rats(Zhang et al., 2020). In KA-induced SE models, targeting of microRNA-21-5p also protects against seizure-induced injury by PTEN-mTOR (Tang et al., 2018). MicroRNA-23a is involved in hippocampal neuron injury, hippocampal oxidative damage and impairment of spatial memory in KA-induced TLE mice(Zhu et al., 2019b). Meanwhile, microRNA-23a can also regulate ADAM10, which contributes to epileptogenesis in pilocarpine-induced SE mice(Zhu et al., 2019a). MicroRNA-27a-3p regulates ion channel-related DEGs in multiple mTLE and downregulation of microRNA-27a-3p inhibits apoptosis of hippocampal neurons and inflammatory response by upregulating MAP2K4 in KA-induced epilepsy models(Lu et al., 2019; Su et al., 2022b). Inhibition of microRNA-103a regulates BDNF to improve neuron injury and inhibit activated astrocytes in pilocarpine-induced epilepsy rat models(Zheng et al., 2019). Depletion of microRNA-132 can reduce seizure-induced neuronal death in KA-induced epilepsy mice models and microRNA-132 can suppress BDNF/TrkB signaling to aggravate epileptiform discharges in the Mg2+-free treated hippocampal neuronal model of SE(Jimenez-Mateos et al., 2011; Xiang et al., 2015). Meanwhile, microRNA-132 also reduces the expression of pro-epileptogenic factors (COX-2, IL-1β, TGF-β2, CCL2 and MMP3) in human cultured astrocytes of TLE(Korotkov et al., 2020). Inhibition of microRNA-134 can effectively reduce the occurrence of spontaneous recurrent seizures and silencing microRNA-134 can produce neuroprotective, reducing the severity of seizures in KA-induced epilepsy mice model(Jimenez-Mateos et al., 2012; Morris et al., 2019). MicroRNA-134 inhibits the expression of cAMP-response element binding protein (CREB) and p-CREB to regulate synaptic plasticity in pilocarpine-induced epilepsy rat model(Zhu et al., 2015). Antagonizing microRNA-135a can reduce spontaneous recurrent seizures to affect synaptic function and plasticity by targeting Mef2a in KA-induced epilepsy mice model. And inhibition of microRNA-135a protects glial cells against apoptosis by regulating SIRT1-related signaling pathway in KA-induced BV2 microglia epilepsy model(Vangoor et al., 2019; Wang et al., 2021c). Inhibition of microRNA-141 can inhibit P53 to protect against apoptosis by SIRT1 expression in KA-induced epilepsy rat model(Liu et al., 2019a). MicroRNA-142 performs well in differentiating between drug-resistant and drug-sensitive TLE. Inhibition of microRNA-142 promotes mitochondrial autophagy and reduces hippocampal neuron damage by targeting PINK1 in pilocarpine-induced epilepsy rat model(Xiao et al., 2021). Downregulation of microRNA-145 improves the abilities of learning and memory by reducing apoptosis of hippocampal neurons in pilocarpine-induced epilepsy rat model(Zhao et al., 2019a). MicroRNA-146a is a powerful regulator of microglia-mediated inflammation in the chronic TLE(Aronica et al., 2010; Su et al., 2016). MicroRNA-146a-CFH-IL-1β loop circuit mediates the perpetuate inflammation of chronic TLE in KA-induced epilepsy rat model and antagonists targeting microRNA-146a can also protect against SE by regulating NF-κB pathway in pilocarpine-induced epilepsy rat model(Li et al., 2018c; Zhang et al., 2018a). In addition, microRNA-146a can ameliorate dysregulation of the MMP/TIMP proteolytic system in TSC(Broekaart et al., 2020). Febrile seizure(FS)-related microRNA-148a-3p plays neuroprotective roles by increasing the proliferation of hippocampal neurons in Mg2+ -free medium treated TLE cell model(Yu et al., 2021). MicroRNA-155 is involved in epileptogenesis by the PI3K/Akt/mTOR signaling pathway(Duan et al., 2018). Meanwhile, inhibition of microRNA-155 attenuates MMP3 expression in cultured human astrocytes, increases the expression of BDNF and alleviates seizure severity in the pilocarpine-induced epilepsy, and attenuates KA-induced seizure by inhibiting microglia activation(Cai et al., 2016; Korotkov et al., 2018; Fu et al., 2019). MicroRNA-181b can inhibit P38/JNK signaling pathway by targeting TLR4, thereby reducing apoptosis and autophagy in KA-induced epilepsy rat model(Wang et al., 2019b). However, inhibition of microRNA-181a-5p also activates SIRT1 to reduce neuronal apoptosis, neuroinflammation, oxidative stress, cognitive dysfunction and activation of astrocyte and microglia in pilocarpine-induced epilepsy rat model(Kong et al., 2020). Downregulated microRNA-183 results in an inactivation of JAK/STAT signaling pathway by targeting Foxp1 to promote neuron proliferation and inhibit apoptosis of hippocampal neurons, thereby attenuating hippocampal neuron injury in pilocarpine-induced epilepsy rat model(Feng et al., 2019). MicroRNA-187-3p is upregulated and regulates KCNK10/TREK-2 potassium channel in electrical stimulation-induced SE(Haenisch et al., 2016). MicroRNA-194-5p regulates the proliferation and apoptosis of hippocampus neuron in children with TLE and Mg2+ -free medium treated epilepsy cell model (Niu et al., 2021). Targeting of microRNA-199a-5p protects against neuron damage by SIRT1-p53 cascade in pilocarpine-induced epilepsy rat model (Wang et al., 2016a). Downregulation of microRNA-200c-3p upregulates RECK and inactivates the AKT signaling to decrease apoptosis of hippocampal neuron in pilocarpine-induced epilepsy rat model(Du et al., 2019). MicroRNA-203 is targeted to Ppp2ca in both humans and mice, which can target Ppp2ca to increase seizure activity in the KA-induced SE model(Zhang et al., 2018b). And microRNA-203 antagomirs can targets glycine receptor-β(GLRB) to decrease the frequency of spontaneous seizures in pilocarpine-induced mice epilepsy(Lee et al., 2017). MicroRNA-219 regulates NMDARs in the amygdala and hippocampus of patients with mTLE and also suppresses seizure formation by regulating the CaMKII/NMDAR pathway in KA-induced epilepsy mice model(Zheng et al., 2016; Hamamoto et al., 2020). MicroRNA-223 also have the good performance in distinguishing drug-sensitive and drug-resistant TLE and microRNA-223 affects microglial autophagy by targeting ATG16L1 in TLE(De Benedittis et al., 2021; He et al., 2021b). MicroRNA-451 regulates GDNF expression to aggravate hippocampal neuronal apoptosis and seizure in KA-induced epilepsy mice model(Weng et al., 2020).
Related studies have also found that microRNAs are significantly downregulated in epilepsy. MicroRNA-15a targets GFAP to inhibit inflammation and apoptosis of hippocampal neurons by downregulating GFAP in pilocarpine-induced epilepsy rat model and Mg2+-free medium treated TLE cell model(Fan et al., 2020). Propofol regulates microRNA-15a-5p/GluN2B/ERK1/2 pathway to suppress apoptosis hippocampal neuronal apoptosis in Mg2+ -free medium treated epilepsy cell model (Liu et al., 2020). MicroRNA-22 inhibits neuroinflammatory signaling to protect against the development of epileptogenic brain networks. MicroRNA-22 prevents inflammation and development of epileptogenic focus by targeting P2X7R in the brain and microRNA-22 regulates aberrant neurogenesis and changes in neuronal morphology after SE in KA-induced epilepsy mice model(Jimenez-Mateos et al., 2015; Beamer et al., 2018; Almeida Silva et al., 2020). MicroRNA-25-3p targets OXSR1 to suppress oxidative stress and apoptosis of neurons, thereby suppressing epileptiform discharges in KA-induced epilepsy mice model(Li et al., 2020). MicroRNA-29a regulates seizure-induced cell death and inflammation in Mg2+-free medium treated epilepsy cell model(Wu et al., 2021). Activated microRNA-34a may lead to impaired corticogenesis in TSC during early brain development and inhibition of microRNA-34a can regulate apoptosis and Notch signaling to inhibit epileptiform discharges in Mg2+-free medium treated epilepsy cell model(Wang et al., 2019a; Korotkov et al., 2021). Meanwhile, microRNA-34c plays a negative role in seizure and cognitive function by regulating NMDARs and AMPARs in PTZ-induced epilepsy rat model(Huang et al., 2018). In addition, decreased microRNA-34c-5p enhances neuroinflammation to increase loss of hippocampal neuron in DRE from KA-induced epilepsy mice model and in children with DRE(Fu et al., 2020). MicroRNA-101a-3p inhibits apoptosis and autophagy by downregulating c-FOS in pilocarpine-induced epilepsy rat model and Mg2+-free medium treated epilepsy cell model(Geng et al., 2021). MicroRNA-124 inhibits some target genes to prevent upregulation of hippocampal NRSF, which participates in epilepsy and promotes the activation of hippocampal microglia and inflammatory cytokines(McClelland et al., 2014; Brennan et al., 2016). MicroRNA-124 suppresses seizure, regulates CREB1 activity, and inhibits neuronal firing with decreased expression of NMDAR in pilocarpine and PTZ-induced epilepsy rat model(Wang et al., 2016c). MicroRNA-125a-5p targets CaMK4 to alleviate dysfunction and inflammation in PTZ-induced epilepsy rat model(Liu et al., 2019b). Inhibition of microRNA-129-2-3p regulates GABRA1 to protect against refractory TLE in KA-treated primary hippocampal neurons and KA-induced epilepsy rat model(Wang et al., 2021a). MicroRNA-129-5p also targets HMGB1 to inhibit the development of autoimmune encephalomyelitis-related epilepsy by TLR4/NF-kB pathway(Liu et al., 2017a). MicroRNA-136 inhibits WNT/-Catenin signaling pathway to play a neuroprotective effect on pilocarpine-induced TLE rats (Cui and Zhang, 2022). Overexpression of microRNA-137 inhibits seizure activity in two different epilepsy mouse models (PTZ and pilocarpine) and suppresses neuronal excitability in Mg2+-free-induced brain slice model of epileptiform activity(Wang et al., 2018b). MicroRNA-139-5p negatively regulates GluN2A-NMDAR in pilocarpine-induced epilepsy rat model and TLE patients and upregulated microRNA-139-5p also regulates the Notch pathway to reduce spontaneous recurrent epileptiform discharge-induced apoptosis and oxidative stress in rat primary hippocampal neurons(Alsharafi et al., 2016; Zhao et al., 2021). MicroRNA-153 is downregulated in plasma and temporal cortex of mTLE patients and overexpression of microRNA-153 reduces HIF-1α expression in rat astrocytes of refractory epilepsy(Li et al., 2016). MicroRNA-204 regulates TrkB-ERK1/2-CREB signaling to inhibit epileptiform discharges in Mg2+-free medium cultured hippocampal neurons(Xiang et al., 2016). Dynamic changes of microRNA-211 expression is associated with epileptiform activity and cholinergic imbalances in murine forebrain(Bekenstein et al., 2017). MicroRNA-221-3p inhibits HIF-1α to suppress seizures and microglia activation in the VPA-resistant epilepsy of KA-induced epilepsy mice model(Fu et al., 2021). MicroRNA-322-5p regulates the TLR4/TRAF6/NF-κB axis to reduce neuronal inflammation in pilocarpine-induced epilepsy rat model(Zhou et al., 2022). MicroRNA-344a regulates seizure-induced apoptosis signaling pathways in PTZ-induced chronic epilepsy rat model (Liu et al., 2017b). Overexpressed microRNA-494 inactivates the NF-κB signaling pathway to reduce hippocampal neuron injury by inhibiting RIPK1 in pilocarpine-induced epilepsy rat model(Qi et al., 2020). MicroRNA-542-3p suppresses TLR4/NF-κB signaling pathway to reduce seizure-induced brain injury and the expression of P-gp in KA-treated primary hippocampal neurons and KA-induced epilepsy rat model(Yan et al., 2019). Oddly, microRNA-128 is significantly downregulated at various phases of TLE development in epilepsy rat models and TLE patients(Tan et al., 2013; Alsharafi and Xiao, 2015). However, it has been found that microRNA-128 is upregulated in KA-induced epilepsy rat model and promotes apoptosis by the SIRT1 cascade in PC12 cells(Chen et al., 2019). MicroRNA-128 can inhibit the expression of various ion channels and the signaling of ERK2 network that regulate neuronal excitability. Meanwhile, microRNA-128 also inhibits SNAP-25 and SYT1 expression to regulate epilepsy sensitivity in KA-induced epilepsy mice model(Wang et al., 2021b). In addition, microRNA-378, microRNA-575, microRNA-629-3p, microRNA-1202, microRNA-1225-5p, and microRNA-138-5p may also be diagnostic indicators and predicting surgical prognosis in human epilepsy(Gattás et al., 2022; Li et al., 2022; Ünalp et al., 2022).
In a word, these novel biomarkers may help to identify new epileptic treatment targets and contribute to improved epileptic patients’ quality of life through earlier diagnosis and a more precise prognosis.